To my family

First I would like to thank my Thesis Director, Dr. Abdelilah Slaoui, for continual guidance and assistance throughout the duration of my Ph.D., and without whom this work would not have been possible. Even, its high support allowed my personal career growth by putting at disposal his work knowledge. I would like to thanks the members of jury Mimoun El Marrsi, Marie-Paule Besland, Nathalie Viart, Aziz Dinia, Thomas Fix, Jean-Luc Rehspringer, not only for have made through Strasbourg even if the chaos of the recent events but even for their interest in my work and suggestions and corrections to this manuscript which increase the value of this work.

I should give particular thanks to the contributions to this work to my supervisor Dr.Thomas Fix. He made vital contributions to most of the work and gave me invaluable help on a totally new research topic for me. His patience and wise allowed me to growth familiar with the research environment and pushed me to the end of these three years.

Another special thanks should be given to Prof. jean-Luc Rehspringer , for his invaluable help on chemical processes and for having putting his experience on the work in order to make big steps in the work. I should thank Professor Aziz Dinia, which allowed me to work independently in IPCMS, which collaboration allowed a huge breakthrough in the work.

Another special thank should go to Dr.Guy Schmerber, for his patience and understanding during my PhD project as well as willing to accomplish the work in very limited time laps. My PhD would have not be the same without his support.

A huge thanks to Cristophe Lefevre, which allowed me to work with him at the Synchrotron which allows a big step forward in this research project.

A big thanks will go to Damien Aureau and Mathieu Fregnaux for the XPS measurements that will put this work of 3 years under a new light; I would like to thank even Judith Driscoll which allowed a collaboration with the University of Cambridge where I have learnt more about the Kelvin Probe Force Microscopy.

A special thanks to the people who helped me during this work; in this sense I would like to thank Silviu Colis, Gilles Versini, Mircea Rastei, Bohdan Kundys, Daniel Stoeffler, Cristophe Lefevre, Cedric Leuvrey, Gérald Ferblantier, Dominique Muller, Sophie Barre and all the people who helped me during these 3 years.

A particular thought to Stephan Roques and Florian Mugler, which not only were always avaible for help but even with their sympathy made my life in the laboratory more easy and less stressing.

Eventually I would like to say a special thanks to the PhD and Post-Docs and Master's students who were with me in the laboratory especially I would like to thank Anatolie Graviluta, which is huge support and joyness during these 3 years. I should eventually mention all my Italian friends I met in Strasbourg which made my life way more easy and funny. I have found special people which I will be in contact for long and tat will always will make me feel a little bit more home wherever I will go.

Alessandro Quattropani

Table of contents

Photovoltaics: an overview

During the last decade, humankind have undergone a strong energetic challenge since the technological advance and the high rate of development of emergent countries, such as China, India and Brazil, have followed a huge energy request. Consequently, a high energy demand at low cost are requested to satisfy the global needs. On the other hand, the scarcity of fossil fuels and the recent global weather changes pushed scientist to look forward to implement solutions of green energy production. Among the renewable energy options, photovoltaic energy, consisting in converting the sun radiation to electricity, has been intensively studied because of the large amount of energy received every day by our planet from the sun. In fact, in only one hour the sun irradiates the earth with a quantity of energy which would satisfy the global needs per years. The photovoltaic technologies advanced a lot since the discovery by Einstein 1 of the photoelectric effect and the first solar cells developed at Bell's laboratory in 1950. For many years the commercialized silicon panel were too expensive to compete with fossil fuels 2 , thus the photovoltaic energy remains for years only exclusive for special applications such as energy generation for satellites or off-grid connected zones. In more recent years, thanks to research and advances in PV technologies and a large development of equipment and automation, a huge drop in costs occurred which make photovoltaics enough competitive with conventional energy production methods. This drop stimulated the research and development of the PV technology; it can be pointed out that in 2017, 98.9 GW of solar panels have been installed worldwide as compared to less than 20 GW in 2010. Fig. 1.1 summarizes this large increment by plotting the installed solar capacity between 6 big groups worldwide. It is noticeable how the huge development nowadays is linked especially to the emergent markets in China, and more generally, in the Asian-Pacific countries (APAC).

. The solar spectrum

In order to introduce the solar cell devices, it is important to get familiar with the photovoltaic technology. The energy is provided by the sun, which behaves as a black body at 5800 K and its emission band is optimal at a wavelength of 500 nm. The average solar irradiation outside of the atmosphere has a power of 1366 W/m 2 . When the solar radiation enters the atmosphere, it gets weighted by different factors: the different layers of the atmosphere, weather conditions well as the longitude and observation height. The gases which are composing the atmosphere (as Ozone (O3), carbon dioxide (CO2) and water vapour) absorbs the radiations close to their binding energy, so that the visible spectra presents some "features" as shown in fig. 1.2. Solar irradiation varies at different position on earth so that a reference called "Air Mass" (AM) has been established in order to compare the different irradiation in different part of the globe. Two standards are defined for terrestrial use. The standard spectrum for space applications is referred to as AM0. It has an integrated power of 1366.1 W/m 2 . The AM1.5 Global spectrum is designed for flat plate modules and has an integrated power of 1000 W/m 2 (100 mW/cm 2 ). The AM1.5 Direct (+circumsolar) spectrum is defined for solar concentrator work. It includes the direct beam from the sun plus the circumsolar component in a disk 2.5 degrees around the sun. The direct plus circumsolar spectrum has an integrated power density of 900 W/m 2 .

The absorption of the sun light

Semiconductors present an energy gap between the valence and the conduction bands which varies typically between 0.3 to 3 eV. When a photon with an energy higher than the bandgap reaches the semiconductor it can be absorbed by the material and an electron can be promoted to the conduction band, leaving in the valence band a pseudo-charge of opposite sign, the hole. radiative: This is the case, for example, for GaAs, which find different applications in optoelectronics as well as solar cell for space applications. In the case of Silicon, instead, the bandgap is indirect: in this case the valence band maximum and conduction band minimum are not coincident in the K space and the electronic transition are not vertical, so non-radiative because they imply a change in the wave vector of the electron. In this case, in order for a photon to be absorbed, either it has sufficient energy to get promoted to a relative minimum (and then will relax in the absolute minimum by thermalization) or the transition will occur with the help of a phonon absorbed (or emitted) by the electron (fig. 1.3.b).

The absorbing material

The solar cell device is based on an absorbing material which should be able to absorb the incident photons, create the hole-electron pairs as well as to separate them in order to collect them and consequently generate a current. Only few materials are able to strike as a solar cell absorber. The most important parameters for a solar cell needs are • Concentrations of doping atoms; this can be of two types; donor atoms which donate free electrons, ND, or acceptor atoms, which accept electrons, NA. The concentrations determine the width of a space-charge region of a junction.

• Mobility µ, and diffusion coefficient, D, of charge carriers that characterize carriers transport due to drift and diffusion, respectively.

• Lifetime τ, and diffusion length, L, of the excess carriers that characterize the recombination-generation processes • Bandgap energy, EG, absorption coefficient, α, and refractive index, n, which characterize the ability of a semiconductor to absorb visible and other radiation 1.1.3. Photovoltaic Effect Electron-hole separation is generally possible in solar cell devices thanks to a potential barrier present within the semi-conductor. The most common type of barrier are the homojunction (which is a p-n junction of the same semiconductor); heterojunction (p-n junction of two different materials) and Schottky barrier (metal/semiconductor). In the case of a silicon solar cell, the most used solution is a homojunction doped p and n, as shown in fig. 1.4: The generated photocarriers upon absorption and separation have a different behaviour that depends on the zone where they have been created:

• In the p or n zone, the minority carriers which reach the depletion zone are drifted from the electric field to the p-zone (the holes) and to the n-zone (the electrons) where they become majority carriers. A diffusion photo-current will be set into the system • In the depletion zone, the electron-hole pair created by incident photons are dissociated by the electric field; the electrons move to the n zone and the holes to the p zone. In this case a generated photo-current is set. These two currents give the total photocurrent Iph which is proportional to the light intensity and it opposes to the diode current Iobs which comes from the polarization of the material.

The resulting current will be:

I (V) = Iph-Iobs (V) (eq.1.1)
For a ideal solar cell, equation 1.1 takes this form: I (V) = Iph-Is (exp (qV/kT) -1) (eq.1.

2

)
where Is is the saturation current of the diode; q the elemental charge; k the Boltzmann's constant and T the Temperature.

Under light, the solar cell characteristic of an ideal solar cell is the addition of a current of intensity Iph and the characteristic of this diode in darkness. As a convention, the I/V curve is represented as the inversion of the current axis, so a typical Photovoltaic curve for a p-n junction based device looks like the curve in Figure 1.5: The important factors in the I-V curve are:

• Isc: the short-circuit current (obtained for V=0) is the current due to the generation and collection of light-generated carriers. For an ideal solar cell at most moderate resistive loss mechanisms, the short-circuit current and the light-generated current are identical. Therefore, the short-circuit current is the largest current which may be drawn from the solar cell.

• Voc: the open-circuit current (obtained for I=0) is the maximum voltage available from a solar cell. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias junction with the light-generated current.

• Im: the current obtained at the maximum power of the solar cell;

• Vm: the voltage obtained at the maximum power of the solar cell.

• The Fill Factor (FF): is the parameter which allows the determination of the efficiency of the solar cell together with the Voc and Isc. Graphically, the FF is a measure of the "squareness" of the solar cell and is also the area of the largest rectangle which will fit in the IV curve. The FF is the square represented in fig. 1.X and can be calculated as : FF= (VmIm)/(VocIsc) (eq.1.

3)

• The solar cell efficiency (η) which can be expressed as:

η= (maximum electrical power)/ (incident Solar power):

η= (VmIm)/φi = (FF VocIsc)/ φs (eq. 1.4) where φ i it's the light flux received by the solar cell for a given surface; These parameters vary with the light intensity, so the convention is to use standard conditions at AM 1.5 at a T=25 °C.

Efficiency limiting factors

The photons conversion is not perfect, there are several factors which limits the efficiency of the solar cell. These losses can be classified in two types: 1) losses due to the material; 2) losses due to the technology used.

a) Losses attributed to the material properties:

The main losses in a silicon based solar cell as an example are shown in fig. 1.6. The green part is that of the solar spectra effectively used by Silicon for electron-hole conversion. Photons with a wavelength higher than the one of the bandgap (λ>λg) do not have enough energy to promote an electron from the valence band to the conduction band and it is therefore lost (red part of the spectra in fig. 1.6). On the other hand, the photons with an energy higher than the one of the bandgap (purple and blue in the spectra) give rise to only one electron-hole pair and the rest of the energy is lost as heat (thermalization). Other losses are due to a non-perfect Fill Factor. As seen above, the I-V curve depends on the Boltzmann equation under an exponential form (exp (qV/kT) -1); the curve has not a rectangular shape and it is not higher than 0.89 in the best cases. These effect are due to a not perfect p-n junction, the resistance.

b) Losses attributed to the device fabrication:

One important loss in a silicon solar cell is the optical reflection. In fact, the refractive index of Silicon is different from the ones of the air so that at the interface air/Silicon some reflection would low the number of photons which reaches the absorbing material; another optical loss is the shadow effect due to metallic contacts that reflect the impinging sun light.

Finally, the absorption efficiency is important, especially for thin films (<100 µm) because a fraction of the photons which have enough energy get across the cell without being absorbed;

the efficiency of collection is instead linked to the number of carriers generated and the ones which are finally collected. These are related to the number of carriers which recombines before being collected and depends on the lifetime of minority carriers (average time between the generation and recombination of a minority carrier).

The Shockley-Queisser Limit

The simple solar cell is based on the PN junction concept. Shockley and Queisser 7 were the first to develop a model which took into consideration a single-junction solar cell, in order to establish the maximum theoretical efficiency of an ideal device. To do so, they started from some hypothesis:

• The sun behaves like a black body at a T=6000K;

• The solar cell behaves like a black body at T=300 K;

• The solar cell absorbs all the photons with an energy higher than its bandgap;

• Each absorbed photon generates an electron-hole pair which gets immediately dissociated at a rate of 100%;

• The recombination is only radiative;

• Each not recombined charge carrier is efficiently collected.

Using this model, the number of electrons generated in a solar cell is equal to the number of photons with an energy higher than the bandgap of the material minus the number of photons emitted by radiative recombination. In this way it is possible to calculate the maximum efficiency of a solar cell, which is 33.7% for a bandgap of 1.34 eV while it is 31%

for the Silicon with a bandgap of 1,12eV, In fig. Similar structures to CdTe Cell but not represented here is that using inorganic compound CIGS (copper-Indium-Gallium-diSelenide) as the absorbing material which has achieved solar efficiencies well above 20% 11 . The challenge to create solar cells materials with earthabundant and non-toxic elements pushed to the development of copper-zinc-tin-sulphide (CZTS) thin film compounds. CZTS is a quaternary compound with a direct bandgap of 1.5 eV and a large absorption 12 . Yet, the highest efficiency of these CZTS cells was found to be 12.6% in laboratory 13 . The main drawback of the CIGS and CZTS cells is the synthesis of the layers with the appropriate stoichiometry and free of secondary phases which often leads to inhomogeneous films and recombination centres Among other cell structures, GaAs based solar cells are established since a decade. This III-V material is well suited for photovoltaic applications as the electrons move faster within this semiconductor as compared to Silicon; moreover, it forms a single junction III-V semiconductor which is well suited in terms of absorption, non-radiative energy loss, minority carrier lifetime and mobility. However, the high fabrication cost of these cells did not allow a large implementation in the market despite an efficiency of 28.8% as a cell and 24.1% in modules. 1.9 shows also that new type of solar cells has emerged more recently. Among them, organic photovoltaics using semiconductor polymers or small molecules as absorbing materials. The main advantages are low temperature deposition, the use of coating methods, printable on several substrates as well as low weight Finally, the tandem organic solar cells were investigated to increase the 11% record efficiency of the single-junction one, and thus it was possible by using an interconnection layer to reach 12.5% efficiency in 2016 17 . Another important advantage of these solar cells is that the dye have a very high absorption coefficient 18,19 which can allow even to few nanometres to absorb enough light to reach a high efficiency, although they shows a low dielectric constant. Several works have addressed Bulk Heterojunction Solar cells (BHJ) and DSSC [20][21][22][23][24][25] structures and an efficiency of 12.9% has been achieved with DSSC 26 and 8% for BHJ 27 . Liquid junction QDSC using quantum dots has instead improved till 5% while a 7% efficiency has been reported for a solid state one 22,23 until development of a very recent colloidal quantum dots which showed a 13.4% efficiency 28 . One important problem of these cells is related to the stability of the absorber material, which should be overcome to widespread these cells in the market.

Materials for Photovoltaic

Recently, a new class of material which involves organic and inorganic Perovskites got renewed interest among researchers. As it will be further discussed in the next chapter, Perovskites are compounds with formula ABX3, where A and B are two cations (normally A has a larger ionic radii than B) and X is an anion, and they have shown an unprecedented increase in efficiency from 3% reported in 2009 29 to more than 22% this year 10,30,31 , which is comparable with the ones of CdTe and CIGS. What really strikes out in these cells is the large diffusion length and high mobility paired with a low recombination rate at surface and at the grain boundaries. Fig. 1.11. displays a schematic of a typical organic-inorganic perovskite cell device.

Fig.1.11. Schematic representation of an Organic-Inorganic perovskite solar cell

The results obtained for these devices are promising. However, several questions have to be answered before such cells penetrate the PV market. Firstly, the toxicity and environmentally dangerous lead can be detrimental for a wide use of the devices. Many researchers respond to this issue saying that the presence of lead is pretty small compared to other commonly used devices (i.e. cars batteries) and therefore this should not be a limiting factor. Secondly, the stability of the hybrid perovskites (as well as the organic compounds for Organic solar cells) is still a questionable point. Recent studies have pointed out the possible large stability of the material. Finally, the manufacturing steps of these cells are still mainly at the laboratory level, and it is far from an industrial pathway when compared to the other technologies.

The table 1.2. summarises the devices based on alternative PV technologies and their maximum reported efficiencies.

As reported in figure 1.8 (NREL chart above), multijunctions solar cells based on III-V materials for ultra-high efficiencies were also developed since many years. In theory, such devices formed by stacking 2 or more III-V semiconductors of different bandgaps can reach an optimal efficiency of about 60% under sun concentration. In practice, multjunctions solar cells with an efficiency approaching 46% has been reported from Fraunhofer Institute for Solar Energy Systems ISE, Germany However, the cell cost increases with increasing the number of junctions so they have found applications mainly in space power generation or under sun radiation concentration higher than 1000. This thesis' work is aiming at pursuing option (ii) and proposing inorganic perovskite oxides (POs) as absorbing material but with a completely different concept for charges separation and transport, thanks to the ferroelectric character of these POs.

Solar cell

Oxides materials for Photovoltaics

Oxide materials offers a wide range of different possible applications in electronics, memory, magnetoelectronics and even bioprocessing [35][36][37][38] . More recently, Oxides for photovoltaic is arising interest for their peculiar characteristics. In particular, Metal Oxides (MO) seems adequate to offer an alternative to the present photovoltaic structures, thanks to their chemical stability and non-toxicity and also easy fabrication. The most known MOs for photovoltaics are the transparent conducting oxides (TCO) which are already widely used in several solar cell structures as front or bottom electrodes thanks to their specific properties (bandgap, work function, carrier mobility and resistivity) which allows an appropriate choice to build a device. The most common MOs are ITO, FTO, ZnO, AZO, SnOx 35 . For example, in CdTe-based solar cells, SnO2 or ITO are frequently used as TCOs depending on the deposition temperature of the n-layer CdS. In case of lower temperatures used, FTO is used because of its better ratio between optical transmittance and sheet resistance. In most of the cases, the choice of the TCO is related to its work function which better matches the solar cell needs.

As for the MOs as solar absorbers, there are specific requirements that should be fulfilled: an oxide material with a bandgap compatible with the solar spectra as well as a high carriers' diffusion length within the material. Among potential PV oxide materials, Cu2O was the most investigated since 1970 . 39,40 This oxide has a bandgap of 2.1 eV. It is expected that a solar cell based on materials with such a high bandgap can reach a theoretical maximum efficiency of around 23% under AM 1.5 illumination, which is quite reasonable. In practice the best solar cells based on Cu2O have reached 6% 41 because of the difficulties in synthesizing very good material free of parasitic phases 42 .Therefore, such material can rather be envisaged as a high bandgap material for applications in multijunctions consisting in a stack of three or more junction. Furthermore it can find applications in semi-transparent PV solar cells.

CuO (Cupric Oxide) has been also investigated as a photovoltaic absorber. This is an intrinsic p-type semiconductor because of the Cu vacancies which forms spontaneously. Its bandgap varies between 1.2 and 1.8 even if there are some controversy about whether it is a direct or indirect [43][44][45][46] Within the family of functional metal oxides, Ferroelectric (FEOs) oxides are gaining more attention from the scientific community because of their peculiar characteristics that can be fitting the solar cells requirements. Moreover, it has been theoretically and experimentally

shown that it is possible to overcome the Shockley-Queisser limit 50 which is opening the door for a possible higher efficiency-single layer solar cell. Different Ferroelectric oxide compounds have been investigated in recent years such as BiFeO3 (BFO), BiMnO3 (BMO), BaTiO3 (BTO), LiNbO3 (LNO), Pb(Zr,Ti)O3 (PZT) and Bi2FeCrO6 (BFCO).

One of the main problem for FEOs absorbers is the high bandgap found for most of these materials. As discussed earlier, an optimal gap of 1.34 eV is the target for an optimal matching with the solar spectra and to approach a high efficiency device. Bismuth Ferrite based oxide, one of the most studied material for Ferroelectric solar cell, shows instead a gap of 2.7 eV.

The mechanism will be further discussed in chapter 2 but the low light absorption did not allow BFO based device to overcome 10% efficiency (fig. 1.12).

Figure 1.12. Bandgap and relative device's efficiency for solar cells 51 In Fig. 1.13 are reported some of the Inorganic oxide compounds by their bandgap and consequently the expected solar cell-based efficiencies according to SQ limit. The ferroelectric oxides exhibit a strong potential as materials to match the solar spectrum absorption. The recent efficiencies BMO/BMn2O5 come from the possibility to tune the bandgap. The same was done by introducing a controlled amount of Cr within the structure of BFO; this allowed Nechache et al. 52 to fabricate for he first time a solar cell with a remarkable 8.1% efficiency.

Yet, there are several questions (growth models, order vs disorder, photovoltaic versus

1.3.Summary

In this chapter, we first reported briefly on the principle of photovoltaic effect and discussed some basic principles. Then we presented an overview of the present photovoltaic materials and technologies, ranging from silicon based solar cells to thin films. The best efficiencies as recorded in literature are given and some technical and/or ecological limitations are given.

Finally, the metal oxides for photovoltaics were introduced. The potential of ferroelectric oxides as absorbing materials for solar cells was discussed.

The next chapter will focus more deeply on the properties of the perovskite oxide and their potential for photovoltaics conversion.

There are several types of oxide materials that differentiates often by the number of cations, which results in different structures and compositions. The most typical structure of complex or mixed oxides consists simply in two or more different cations with different oxidation states, ionic radii and coordination numbers. This diversity creates a large variety of complex structures which reflects on different properties of these oxides. Among the most known structures are the spinel structures (AB2O4) which are well considered for their magnetic properties. In the most typical case, the A site is a six-folded coordinated ion while a sixfoldcoordinate cation occupies the B-site. Eventually, in the most complex cases, a mixing of ion and cation in the A and B site occurs. For this reason, these materials result in a large variety of complex structures and different properties. In this chapter, we will report on oxide compounds, especially those with perovskite crystallographic structure, looking to their characteristics and how they can find application in photovoltaics.

Structure of perovskite oxides

Perovskites are a family of materials with typical chemical composition ABO3 where A and B are two different cations. They were named so after the discovery of the first crystal, CaTiO3, in Ural Mountains in Russia. The chemical composition is similar to that of the ilmenite: in the ilmenites the A and B atoms are approximately of the same size (e.g. LiSbO3) and occupy the octahedral site. Yet, ilmenites have different structures as compared to the perovskite ones.

The family of Perovskites evolves a large number of structures. A typical structure consists in a large cation on the A site that is 12 coordinate and a smaller 6 coordinated ion on the B site.

As shown in figure 2.1, the ideal perovskite is a cubic lattice. However, due to different atoms that can take part in the structure, most of the perovskite compounds are distorted and presents a lower symmetry (e.g. orthorhombic or hexagonal), or just a slight distortion from the perfect cubic lattice.

.

FIG.2.1.

Simple cubic double perovskite with general formula ABX3, A is the cation surrounded by a network of corner-sharing BX6 octahedra 1 The deviation from the ideal structure is estimated through a factor of tolerance t which is taken out from the perovskite oxide ABO3 regarded as a purely ionic. In this way, the relation between the radii of the A atom, the B atom and the Oxygen is:

rA+rO=√𝟐 (rB+rO) (eq. 2.1)
It is then possible to express the deviation from the ideal cubic structure by tIR 2 : tIR= (rA+rO)/ √𝟐(rB+rO) (eq.2.

2)

The tolerance factor for maintaining a perovskite structure should be between 0.80 and 1.10.

For smaller or larger t, the same compound would turn to the ilmenite structure.

In fig. 2.2 are shown all the chemical elements which can form a perovskite structure. As can be seen, most of the elements, except the noble gases and few others, can form a perovskite.

The stability of these structures relies not only on the ratio between the A and the B atoms but even on their nature. In fact, for the B atom, the bonds are highly covalent. In this sense, is not surprising that an oxide such as BaGeO3 does not fit the perovskite structure in ambient condition, as the preferred coordination number for Ge is 4. An alternative is to form a perovskite BaGeO3 structure by high pressure synthesis, as higher Ge coordination number starts to be preferred 3 .

Fig.2.2.

A map of the elements which occupy the A, B, and/or X sites within the 576 compounds experimentally characterized as perovskite or non-perovskite at ambient conditions and reported in [4][5][6] . The data set spans 77 elements, with 49 elements appearing at the A site, 67 at the B site, and 5 at the X site.

It is clear that the perovskite structure can be formed by a great number of atoms and therefore many materials are reported. In the

Typical properties of perovskite oxides

As it has been mentioned above, the perovskite oxides can show a large variety of chemical composition and a variety of structures. It is thus not surprising to find that they possess a large number of properties and different applications. There are some applications which have been known since a long time such as ferroelectricity which can be considered somehow the stone of these family of perovskites. This is for instance the case for BaTiO3. Another wellknown property is the superconductivity like for YBa2Cu3O7. Table 2.2 summarizes the main applications of some of the most known perovskites.

The double perovskite

Cation substitution is one of the ways to tune the properties of the perovskite oxide compound. The cation can take place both at A or B site and it can involve different amount of substitution. In the case of a substitution of half of the B site with a different cation, there is a possibility of ordered growth double perovskite of composition A2B'B''O6 where B' and B'' are the two different cations at the B site.

A schematic representation of the double perovskite is given in fig. 2.3.

The alternation of B and B' cations allow the creation of ordered or disordered structures, adding a degree of complexity in the structure and properties of these compounds, that can be engineered to obtain the desired properties.

Ordering in double perovskite

The degree of order or disorder inside the double perovskite is of great importance as it controls many of the final properties of the resulting material. For example, the degree of order inside Sr2FeMoO6 strongly affects the magnetoresistance as well as the half-metallic properties of the film. 9,10 . Most of the double perovskites crystallize with a rock-salt order (more than 650 compounds have been reported to grow in this way) or just disordered (100 compounds). A third possibility of ordering resides on a "layered" order. It has been postulated that this type of order is only possible in a very narrow range of balancing properties: only 5 compounds have been reported to crystallize in this way [11][12][13] . The disorder inside the double perovskite can occur in different ways. The most common one is the antisite (AS) disorder, in which one cation just takes the place of the other inside the crystal structure. This type of order' defects happens mostly during the crystal growth because of the configurational entropy. Another common type of defect is the anti-phase boundary (APB). In this case, two ordered zone are divided by two inverted B' and B'' sites.

A schema of ordered and disordered crystal structures is given in fig. 2.4. antiphase boundary. 14 Since the ordering inside the double perovskite is essential for their applications, is important to understand how to study it. Most often, the study of ordering inside the double perovskite is done by diffraction analysis. The ordering and the doubling of the cell generate superstructures peaks which can be easily detected. On the other hand, because of distortion, octahedron tilting and cation ordering can make the results vary. Moreover, if B' and B'' are cations with close number of electrons, it can cause other difficulties in the ordering studies, and consequently it will be hard to distinguish between the two atoms. These problems have already been found in some double perovskite like La2NiMnO6 15 , Ba2RTaO6 16 ( R= lanthanide) and in some compounds in which B' and B'' have the same number of electrons like A 3+ 2Ca 2+ Ti 4+ O6; A 2+ 2Sc 3+ V 5+ O6 17,18 .

The degree of order inside the crystal depends on several factors and it's not straightforward to understand whether the film would crystallize in an ordered or disordered way. In fact, most of these compounds are expected to be highly disordered, as a first approximation, at least for energetic consideration 19 . Surprisingly however, many reports on these compounds show that they crystallize in a highly ordered state. One of the suggested reasons of ordering inside the double perovskite lies on the B-site cation' difference oxidation state, ΔZB=|ZB '' -ZB'|, where Z is the oxidation state of the B cation. Generally speaking, it is possible to say that when ΔZB<2, the compounds crystallize in a disordered way, while if ΔZB>2 the compounds are ordered 4,10,[20][21][22] . Qualitatively speaking, it is possible to explain this effect by an unfavourable situation while placing two high charged B cations one close to the other. More specifically, a high increment of the Madelung energy (which calculates the electrostatic repulsion of an ion in a crystal by approximating the ion to a point charge) 23 is caused by an increased charge difference 24 . It has been calculated that the Madelung energy ordering is proportional to ΔZB 2 which means that the ordering energy can become really important by a small change in the oxidation state. In that case, the repulsion energy overcomes easily the entropic energy and the crystal orders during the growth. 25,26 The second common reason that provokes cation order is the difference in the radius between the cations. The more is the difference, which reflects in a larger factor ΔrB=|rB '' -rB ' | (where r is the radius of the B cation), the more probable is the ordered growth of the crystal. Making the similar assumption as before, it is possible to consider that larger is the difference ΔrB, more easily the lattice strain can overcome the entropic energy and so the cations are ordered. 26 The effect of ordering by changing the ionic radius of the two cations has been calculated in some different compounds containing M 3+ /Nb 5+ by Galasso and Darby in 1962 27 . From their work, it has been established that a difference in the radii higher than 7% is already enough for cationic ordering. Vasala and Karpinnen, in their double perovskite review 14 , summarized the ordering of the double perovskite by ionic radii as follows:

ΔZB Compound Ordering 0 A 2+ 2B' 4+ B'' 4+ O6 A 3+ 2B' 3+ B'' 3+ O6
Disordered with ΔrB<0.17 Å, ordered above Disordered 2

A 2+ 2B' 3+ B'' 5+ O6 A 3+ 2B' 4+ B'' 6+ O6 A 3+ 2B' 2+ B'' 4+ O6 Disordered Ordered
Most of the compounds are at least partially ordered 4

A 2+ 2B' 2+ B'' 6+ O6 A 3+ 2B' 1+ B'' 5+ O6
Mostly highly ordered Ordered

6 A 2+ 2B' 1+ B'' 7+ O6 Ordered Table 2.3.
Ordering of known double perovskite by the difference of the oxidation state of B' and B''

cations 14

Multiferroic perovskite oxide for Photovoltaics

As mentioned above some of the perovskite oxides can exhibit ferroelectric and ferromagnetic properties that can be interesting for the photovoltaic conversion. Before developing further, let's remind some basics of ferroelectricity, ferromagnetisms and finally multiferroicity effects.

Ferroelectricity

Ferroelectricity is a property that some materials have for which their spontaneous The first discovery of this effect was made in 1921 in single-crystal materials (called Rochelle salt) and subsequently in other materials like perovskite ones (BaTiO3 (BTO) in 1940). In perovskites, 2 phases are possible: the paraelectric one when the crystal is centro-symmetric and the ferroelectric one where the atoms are displaced along a specific direction showing a net dipole moment which responds to an external polarization. This shifting can occur for different causes. In the case of BFO, as an example, it occurs because the Bismuth in the perovskite structure presents a lone pairs (valence pair of electrons not shared with another atom) on the outer shell of the atom, which are high sensitive to the polarization.

Ferromagnetism

Ferromagnetism is a mechanism where an uncharged material forms permanent magnets. It is given by a certain number of atoms whose spins are aligned one to each other so that they create a net magnetic moment. Different types of magnetism are known in physics: paramagnetic effect is shown; in this case, the material is weakly attracted by an external magnetic field and forms a little induced internal magnetic field as its spins tends to align to the external field. When the atom's spins are ordered antiparallel and can cancel each other there will be an antiferromagnetic order like in figure 2.6 c; when the order is antiferro but the spin are not sufficient to cancel each other there is a ferrimagnet behaviour as in figure

2.6 d.

Multiferroicity

Multiferroics are a class of materials which shows at least two of the three order parameter (electrical, magnetical and mechanical) at the same time [29][30][31] . These type of materials, in which it would be for example possible to control magnetism and coupling are of strong interest for different potential application [32][33][34] . Fig. 2.7 gives examples of multiferroic behavior.

Most of the compounds known which show a Multiferroic behaviour have a Curie temperature well below room temperature. The Curie temperature is the temperature at which a ferroelectric material loses its ferroelectric behaviour and starts to act as a paramagnetic one.

For example, these MF materials have been investigated for tunnelling magnetoresistance (TMR) sensors or magnetic memories applications, which can in the late case be written electrically and read magnetically.

FiG.2.7.

Summary of all the functionalities that multiferroic materials may offer. E and P stand for electric field and electric polarization, H and M stand for magnetic field and magnetization, and σ and εstand for stress and deformation, respectively 35 It is generally difficult to obtain a multiferroic material since there are severe restrictions on the synthesis 30 . Therefore, most of them are complex oxide, as shown in fig. 2.8.

Thus, BiFeO3 (BFO) 33 is a perovskite which has been studied for years since it exhibits ferroelectricity (Tc= 1083 K) 36 and ferromagnetism ( TN= 683 K) 37 at room temperature. Other compounds have been synthesized and demonstrated to have multiferroicity in this condition.

Perovskites like BaTiO3 38 or compounds like KBBNO 39 have also shown promising properties.

Among other oxides present at the interface of the red and green circles, there is the double perovskite Bi2FeCrO6 (or BFCO) that has attracted a great interest for several reasons which will be discussed below. For instance, Nechache et al. 40 were able to exploit this material for Photovoltaic (PV) applications to get an unprecedent high solar conversion efficiency of 8.1%.

Fig.2.8:

In the large circle are represented the insulating oxides of interest; within them there are those polarizable (green ellipse) and magnetically polarizable (orange ellipse).

Inside the insert circles, depending on the ellipse, there are Ferroelectrics and Ferro/Ferrimagnets. At the intersections, there are different multiferroics 41

The case of BiFeO3 and Bi2FeCrO6

The closer structure to the one of Bi2FeCrO6 (labelled often BFCO) is BiFeO3 (labelled BFO).

The structure of BFO is characterized by two distorted perovskite which are connected along diagonal or the pseudocubic It has been predicted to bring, in this long-range order configuration, a magnetic moment of 2 µB per formula unit (ca.160 emu/cm 3 ) at 110K and a ferroelectric polarization of 80 µC/cm 2 .

It is well known that in order to have a multiferroic material, the two ferroic orders should be brought from 2 different atoms in the crystalline structure.

For BFCO, the ferroelectricity is due to the 6s 2 lone pairs of the Bi 3+ atoms (like in BiMnO3 or BiFeO3) while the ferromagnetism is linked with Fe 3+ and Cr 3+ ordering inside the crystal.

Taking into account the Kanamori-Goodenough rules 43,44 , ferromagnetism appears when:

• Two transition metal (TM) are ordered in a B'-O-B'' rocksalt structure

• One of these two TM presents eg electrons while the other don't.

In the case of BFCO, a Fe 3+ with configuration t2g 3 eg 2 coupled with a Cr 3+ that presents a configuration t2g 3 eg 0 should produce a ferromagnetic crystal, while a coupling between Fe-Fe and Cr-Cr should results in antiferromagnetic coupling. This effect was well described by Villafuerte-Castrejon et al. 45 A schematic representation of magnetic domains in the BFCO crystal is given in fig. 2.10. Fig. 2.12. Ferroelectric Properties of BFCO Films, from 48 The black solid line is the polarization after subtracting the leakage contribution. It is worth noticing the reached value of the polarization of 55µC/cm 2 which is close to the theoretical value as predicted by the calculations of Baettig et al. 49 This behaviour was well explained by means of long-range molecular electric field, such as that already found in BiMnO3 31 , Bi2NiMnO6 50 and Bi2CoMnO6 51

Tuning the Bandgap

There are great interests to look for new ways to engineer oxide crystals in order to obtain the desired bandgap while maintaining the ferroelectric behaviour as it open paths for several applications, including photovoltaics. For the latest, the limited PV performances of the oxides is mostly due to their high bandgaps (generally above 3 eV). These wide bandgap exceed by far the optimal bandgap value that gives the maximum efficiency for an ideal solar cell 52 .

Lowering the bandgap of the oxides is a must. This happen to occur in double-perovskite oxides. In these materials, with a composition A2B'B''O6 , B' and B'' alternates in B'O6 and B''O6 octahedra. 49,[START_REF] Ležaić | High-temperature multiferroicity and strong magnetocrystalline anisotropy in 3 d -5 d double perovskites[END_REF] Just as multiferroic perovskites, ferromagnetism (or antiferromagnetism) is driven by the octahedral B'O6 site while the Ferroelectricity is due to the distortion of the structure by the large A cation. In this case, however, the bandgap is controlled by the interaction of B' and B'' cations passing by O atoms, as in the case of Bi2FeMnO6 [START_REF] Choi | Interface-Coupled BiFeO 3 /BiMnO 3 Superlattices with Magnetic Transition Temperature up to 410[END_REF] . In other words, manipulating the order of the perovskite allows to obtain perovskites with the same stoichiometric composition but of different bandgaps.

Different double perovskites have been proved to have ferroelectric effect (Bi2MMnO6 [START_REF] Bhattacharjee | First principles calculations of magnetism, dielectric properties and spin-phonon coupling in double perovskite Bi 2 CoMnO 6[END_REF][START_REF] Zhao | Density functional investigations on electronic structures, magnetic ordering and ferroelectric phase transition in multiferroic Bi 2 NiMnO 6[END_REF] (M=Ni,Co) ; La2NiMnO6 57 ). Among them, only Bi2FeCrO6 (BFCO) has been established as the one with the more interesting characteristic of bandgap engineering and carriers mobility. .

Let's first give some insights on the potential of the ferroelectric perovskite (FEP) based solar cells.

Photovoltaic Effect in Ferroelectric Perovskite

The ferroelectric photovoltaic effect was discovered about half a century ago in a variety of ferroelectric photovoltaic materials without a centre of symmetry in which a steady photovoltaic response (photovoltage and photocurrent) can be generated along the polarization direction [START_REF] Chynoweth | Surface Space-Charge Layers in Barium Titanate[END_REF]59 . The photovoltaic effect in ferroelectric perovskite oxides is fundamentally different from the conventional p-n junction based one. In ferroelectrics, only a single material is needed to separate the charges instead of relying on two different doped regions in the same material (homojunction) or in different materials (heterojunction). The main effect relies on the non centro-symmetry of the crystal, i.e. breaking of the strong inversion symmetry, which allows these FE materials to set up a strong internal field which allows charges' separation. What is even more interesting is that the ferroelectric behaviour allows the control of the internal electric fields and injection barriers, mechanism on which Photovoltaic effect relies [START_REF] Lopez-Varo | Physical aspects of ferroelectric semiconductors for photovoltaic solar energy conversion[END_REF] . In fact, the ferroelectricity generates an internal-induced electric field which can separate the photo-excited charges induced by the light absorption. This effect is even enhanced by the already existing electric field which is created by the Schottky barrier at the interface Ferroelectric/electrode. Another effect which has been usually noticed in FE oxides is the generation of a very high photovoltage (higher than their bandgap in some cases) but low photocurrent. Understanding the FE mechanism is extremely important for the fabrication of a solar cell based on these oxides. to be originating from bulk photovoltaic effect. When such material is illuminated, the electrons take one direction without crossing other materials as in a common p-n semiconductor. This effect, linked to the asymmetric nature of the crystal, was discussed a lot by researchers who tried to explain it with some models. Firstly it was explain by means of asymmetric carrier model [START_REF] Butler | Ferroelectric materials for solar energy conversion: photoferroics revisited[END_REF][START_REF] Belinicher | The photogalvanic effect in media lacking a center of symmetry[END_REF] which is briefly described in fig 2 .14 a where the random diffusion creates a net photocurrent [START_REF] Jalaja | Ferroelectrics And Multiferroics For Next Generation Photovoltaics[END_REF] . This model, proposed by Butler et Shurman. [START_REF] Belinicher | The photogalvanic effect in media lacking a center of symmetry[END_REF] , assumes that in a medium which contains randomly located wedges, which are identically oriented, will give rise to a net current (local and short living) only because of inelastic scattering probability calculations. E1 is less than the potential Ф1, the electron would be trapped in the potential well while if it is higher than the potential Ф2, the electron diverges isotropically and the net current will be lower. If the incoming energy is between the potential Ф1 and Ф2, the electron is free to reach the end of the well. Eventually, the last possibility would be the tunnelling through the potential barrier Ф2 . [START_REF] Lines | Principles and Applications of Ferroelectrics and Related Materials[END_REF] The precedent models were not able to predict formally the high voltage shown by Ferroelectric materials. Instead, it was proposed another theory, the shift current, which has been proved to be a good model to predict the large photovoltage found in non-centro symmetric materials [START_REF] Chynoweth | Surface Space-Charge Layers in Barium Titanate[END_REF][START_REF] Fridkin | Bulk photovoltaic effect in noncentrosymmetric crystals[END_REF][START_REF] Fridkin | The possible mechanism for the bulk photovoltaic effect and optical damage in ferroelectrics[END_REF] . This theory was developed after the discovery of BPVE effect in BaTiO3 crystals [START_REF] Koch | Anomalous photovoltage in BaTiO 3[END_REF] . In this crystal, in fact, a photocurrent dependence on the light polarization and photon energy was found as well as a BPVE but not strictly linked to the polarization [START_REF] Sturman | The photovoltaic and photorefractive effects in noncentrosymmetric materials[END_REF] . In these systems, the non-centrosymmetry of the crystal creates an asymmetry in the momentum distribution of light generated carriers which creates a steady state current. The electrons are excited by the interaction with the monochromatic light changing their momentum, which allows the current to flow from the asymmetry of the potential. The shift current is created by the "shift" of the electron's position at each photon absorbed due to a real-space shift of carriers during interband excitation. The change in position of electrons creates then an internal electric field which is the main contributor to the high photovoltage shown in these materials. After the first discovery in BaTiO3 crystal, the shift current BPVE has been observed in many other materials systems including ferroelectrics [START_REF] Auston | Optical Rectification by Impurities in Polar Crystals[END_REF][START_REF] Glass | High-voltage bulk photovoltaic effect and the photorefractive process in LiNbO 3[END_REF][START_REF] Somma | High-Field Terahertz Bulk Photovoltaic Effect in Lithium Niobate[END_REF] , quantum wells [START_REF] Bieler | Shift currents from symmetry reduction and Coulomb effects in (110)-orientated GaAs ∕ Al 0.3 Ga 0.7 As quantum wells[END_REF] , organic crystals [START_REF] Ogden | Bulk photovoltaic effect in polyvinylidene fluoride[END_REF] and two-dimensional interfaces [START_REF] Nakamura | Interface band profiles of Mott-insulator / Nb : SrTiO 3 heterojunctions as investigated by optical spectroscopy[END_REF] . After these discovery, Young and Rappe 75 reformulated the theory to enable the calculation from the first-principle and provided the first comparison of experimental BPVE data with shift current theory. In the subsequent first-principle studies on the shift current in ferroelectric materials, it has been demonstrated how this is the main contributor to the BPVE effect [START_REF] Nastos | Optical rectification and current injection in unbiased semiconductors[END_REF][START_REF] Young | First-Principles Calculation of the Bulk Photovoltaic Effect in Bismuth Ferrite[END_REF][78][START_REF] Wang | Substantial bulk photovoltaic effect enhancement via nanolayering[END_REF][START_REF] Wang | First-principles calculation of the bulk photovoltaic effect in KNbO 3 and (K,Ba)(Ni,Nb) O 3 -δ[END_REF] There are other effects which take part in the ferroelectric solar cell working mechanism. For example, under a higher voltage, the polarization can be in principle reversed. From the device side, it has been proved [START_REF] Yang | Above-bandgap voltages from ferroelectric photovoltaic devices[END_REF][START_REF] Wang | Switchable diode effect and ferroelectric resistive switching in epitaxial BiFeO3 thin films[END_REF] that controlling the polarization with an external field allows electrical tuning of charge transport and hence unidirectional electric conduction. It is therefore interesting to see what other parameters can influence the polarization.

Domain Walls

Photovoltaic in Ferroelectric materials took a new light after an interesting experiment in 2009 on BiFeO 3 single crystal which exhibit a switchable behaviour [START_REF] Choi | Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO 3[END_REF] as well as on BFO thin film [START_REF] Yang | Above-bandgap voltages from ferroelectric photovoltaic devices[END_REF] .

As it is possible to see from fig. In order to put new light on this experiment and others [START_REF] Catalan | Domain wall nanoelectronics[END_REF][START_REF] Sluka | Free-electron gas at charged domain walls in insulating BaTiO3[END_REF][START_REF] Guyonnet | Conduction at Domain Walls in Insulating Pb(Zr0.2Ti0.8)O3 Thin Films[END_REF] , a new theory linked on domain walls has been established. Domain walls are an extension of few microns inside the FE crystal linked to the spontaneous polarization vector. Each domain in the Ferroelectric, following the domain wall theory, is separated from another by a space of 1-2 nm called "Domain wall" [START_REF] Ho Sung | Single ferroelectric-domain photovoltaic switch based on lateral BiFeO3 cells[END_REF] .

In these crystal, there is a high probability to produce an excess of charges which accumulates at the interface because of the divergence of the polarization. The explanation given to this phenomena is that a drop in the potential occurs across the domain wall of the crystal, mostly because of the rigid rotation of the polarization, as shown in fig. 2.18. 88 In the case reported above, BFO oxide is known to have domain walls at 71° and 109°. It has been shown that the drop in the polarization was largest for the 180° DWs (example b in fig. 2.13), therefore there was no PV effect when metal contacts are disposed perpendicularly to the DWs. The drop was less for 109° DWs (which has been proved to have PV effect [START_REF] Seidel | Nanoscale Probing of High Photovoltages at 109° Domain Walls[END_REF] ) while the least drop is given for 71° DWs. A model was proposed [START_REF] Yang | Above-bandgap voltages from ferroelectric photovoltaic devices[END_REF][START_REF] Seidel | Efficient Photovoltaic Current Generation at Ferroelectric Domain Walls[END_REF] based on the assumption that the drop of the potential creates an high electric field in the DW which is able to separate the photogenerated carriers. On the other hand, the charges will experience a lot of recombination in the domain as the electric field is not strong enough. Therefore, it makes sense to compare a Domain Wall with a p-n junction: the DW separate the charges which accumulates into the interface from where they will diffuse.

Ferroelectric Photovoltaic Evaluation

For ferroelectric absorbers (like BFO in fig. 2.13), a general phenomenology has been set, which differentiates from the one of a typical p-n junction. In this case (whether the polarization is pointing up or down), the current density scales linearly with the voltage so that it can be expressed as:

J= -Jph + σF (eq.2.1)
Where F is the internal electric field, Jph the photocurrent and σ the total conductivity, which is a combination of dark conductivity σd and photoconductivity σph:

σ= σd + σph (eq.2.2)
In the equation 2.1, the internal electric field F, is directly linked to the applied Voltage and internal Voltage, so that the equation 2.3 can be written like:

J= -Jph + σ(V -Vbi ) / d (eq.2.3)
where Vbi is the built-in voltage due to the polarization and d is the distance between the electrodes. Thus, the open circuit voltage can be extracted at J=0:

Voc= Vbi + (Jphd)/ σ (eq.2.4)
The short circuit current depends on the polarization history as well as it scales linearly with the photogeneration rate G:

Jsc = -K.G -Jd (eq.2.5)
And from the previous equations:

Jd= σdVbi/d and Jph= K.G -σphVbi/d (eq.2.6)

The final equation for current density is therefore:

J= -Jd -K.G + σV/d (eq.2.7)
These relations are valid for many ferroelectrics such as BaTiO3 [START_REF] Spanier | Power conversion efficiency exceeding the Shockley-Queisser limit in a ferroelectric insulator[END_REF] .

From these equations, we can get some insight of the photovoltaic response of Ferroelectrics:

• (eq.2.8)

depending if the polarization P is pointing toward the electrode (-) or away from it (+) [START_REF] Pintilie | Metal-ferroelectric-metal heterostructures with Schottky contacts. I. Influence of the ferroelectric properties[END_REF][START_REF] Pintilie | Metal-ferroelectric-metal structures with Schottky contacts. II. Analysis of the experimental current-voltage and capacitance-voltage characteristics of Pb(Zr,Ti)O3 thin films[END_REF] ; εr is the dielectric constant of the ferroelectric and ϑ the spatial extent of the spatial layer. As a consequence, the original built-in voltage Vp linked to the different metal work functions is modified by the polarization at each spatial layer. It is known that the built-in potential of the Schottky barrier in the equilibrium state is given by the difference of work function between the metal Φm and the one of the semiconductor Φsc:

Vp= (1/q)(Φm -Φsc) (eq.2.9)

Where q is the positive elemental charge. This formula considers the vacuum level alignment after the formation of the interface.

When the dipole is formed in a metal-semiconductor interface, eq.2.9 changes 96 to:

Vp'= (1/q)(Φ m -Δ -Φ sc ) = Vp -Δ/q (eq.2.10)
Where Δ is the energy step formed at the metal-semiconductor interface.

ΔΦP=± (P/ε0εr) ϑ

In the case of a Ferroelectric solar cell with a Schottky barrier, the voltage will change according to eq.1:

Vp'= Vp ± (Ps/ε0εr) ϑ (eq.2.11)

Where as the change in the barrier can be even caused by oxygen vacancies in the layers or some photoinduced effect at the interface. Therefore, in order to have a working FE device, one should take in account all these factors to study the workfunctions of the material and the electrodes aiming at an efficient charge extraction being sure that the effect is not related to other factors.

Current Switching

What's really interesting in Ferroelectrics is the possibility to control the direction of the forward current by changing the polarization direction. Current switching can occur in different manners. One is intimately linked with the Schottky junction at the interface ferroelectric/metal [START_REF] Blom | Ferroelectric Schottky Diode[END_REF][START_REF] Ahn | Electric field effect in correlated oxide systems[END_REF][START_REF] Yang | Photovoltaic effects in BiFeO3[END_REF] . In fact, as already mentioned before, choosing a contact with a proper work function, a Schottky barrier is then formed. Changing the polarization direction changes the injection through the barrier and the polarity of the diode can be reversed.

Normally one can play with the barrier height at the interface in order to achieve current switchability at a certain coercive field. If the barrier is too high, switching will hardly happen.

Another switching process can occur because of internal ferroelectric polarization. In this case, one of the best example has been given by Lee at al. [START_REF] Lee | Polarity control of carrier injection at ferroelectric/metal interfaces for electrically switchable diode and photovoltaic effects[END_REF] in 2011, who demonstrated that the switching is linked with ferroelectricity. They were using a metal-ferroelectric-metal system (MFM) consisting of 400nm BFO sandwiched between a top Pt electrode and a bottom SRO deposited on STO substrate. In this device, it was shown that the photovoltaic current perfectly superimposed to the polarization hysteresis loop, as it can be seen from fig. 2.22. The switching effect was also reported in other papers, which regarded not only BFO [START_REF] Wang | Switchable diode effect and ferroelectric resistive switching in epitaxial BiFeO3 thin films[END_REF] but also other ferroelectric material like WO3-doped PLZT 100 . Thanks to these effects, a practical switching memory has been created in the same way than for BFO, where a current density of 0.1 μA/cm 2 was reversed after illumination of 10 mW/cm 2 .

Other effects

An internal electric-field is generated not only by the electrodes at the barrier but also from oxygen vacancies. It has been shown for example [START_REF] Lee | Polarity control of carrier injection at ferroelectric/metal interfaces for electrically switchable diode and photovoltaic effects[END_REF] that in Pt/BiFeO3/SrRuO3 thin film, it was possible to reach switchability only by removing oxygen vacancies when applying a bias higher than the coercive field. Clearly, defects can be detrimental or helpful for the device: oxygen vacancies can in fact increase conductivity, increase absorption or create a screening field 101 .

As an example, in a BiTiO3/Si 102 device, it has been showed how it was possible to obtain a noticeable photocurrent only with samples grown in poor oxygen atmosphere. This has been explained with the formation of a p-n junction at the interface between the space charged layer of oxygen vacancies in BTO and the p-doped Silicon.

Summary

In this chapter, we have presented the structure of the perovskite oxides, their properties and potential applications. We have highlight how different factors can engender order or disorder within the crystal and how this is reflected on the properties of the material. We have also shown why the properties of multiferroic oxides can be possibly a path for the next generation of solar cells.

Moreover, we have given an overview of the bulk photovoltaic effect. We have shown how the Domain Walls theory fits well on Ferroelectric oxides to prove the effect. We have in particular highlights what are the conditions to maximize the efficiency of the solar cell based on FE oxides. The main parameters seem to be the thickness of the material, the oxygen vacancies and the type of electrodes. The conditions of current switching were also discussed.

In the following chapters, we will describe the properties of BFCO double perovskite as grown by two different methods. Chapter 3 will focus on the properties of BFCO made using a wet method (sol-gel) while chapter 4 will deal with BFCO grown by a physical method, namely the pulsed laser deposition technique. The structural, optical and electrical properties of the formed BFCO films will thoroughly presented.

Chapter 3. Properties of BFCO deposited by sol-gel technique

Among the chemical tools to fabricate materials especially as thin films, the sol-gel solution method is the most popular one. It is a low-cost technique which is commonly used for synthesising different compounds and it has been proved to be suited for the synthesis of magnetic and multiferroic compounds [1][2][3][4][5] . In particular, the synthesis of compounds such as BiFeO3 and BiCrO3 by sol-gel was already demonstrated but to date, there are not many information regarding the sol gel synthesis of BFCO films. The main problem resides on the fact that many Bi-based perovskites with the formula Bi[B]O 3 requires usually high pressure in order to obtain their perovskite form 6 . In principle, BiFeO3 is one of Bi perovskite which does not require high pressure. It is therefore of great importance and challenging to obtain a single phase using the sol-gel solution and annealing under atmospheric pressure. In fact, the chemical stability of BFO is quite controversial: some groups showed a stability until 930 °C 7,8 and after it decompose into Bi2O3 and Bi2Fe4O9 9 ; Others reported that it is only stable under 830 °C 10 or even at lower temperatures 11 . Thus, it has been pointed out from Lisnevskaya et al. 11 that the reaction proceeds at 80% conversion into BFO at above 700 °C but only keeping this reaction at this temperature many hours. Near 100% conversion have been obtained by firing at 850 °C for 5-10 min while longer treatment times lead to the formation of the two products mentioned above 12 . Complementary studies have also addressed the synthesis of these ceramic by sol gel method [13][14][15] and most of them have demonstrated difficulties for a 100% conversion into BFO. Since a BFO perovskite is already very difficult to obtain by sol-gel, it is expected to be more challenging for the double perovskite BFCO.

In this work, we have prepared the sol-gel solution containing the appropriate weight of elements of the BFCO; then the spun solution was thermally heated using either a conventional furnace (CTA or conventional thermal annealing) or a lamps assisted furnace (RTA or rapid thermal annealing) following the methodology published by Murari et al. 16 for BiFe0.95Cr0.05O3 material.

Sol-gel solutions preparation:

Three solutions of the different metal chelates in propionic acid ( Bi 3+ , Cr 2+ and Fe 3+ ) were provided by RBnano company. The concentration of the solutions depends on chelates solubility and are ranging from 0.5mol/l to 1 mol/l. To prepare the final sol-gel solution we calculate the amount of each solution to be mixed and assuming densities close to 1 we weight the quantities of each solutions and poor then in the same recipient. The final mixture is used for spin coating and few grams are dried and preheated at 400°C to remove all organic moieties. This amorphous powder is used for DTA-TGA measurements.

Powder preparation and Thermo-Gravimetrical Analysis-Differential Thermal Analysis

Before starting sol gel deposition, some analyses have been carried on a powdered precursor produced by solution drying and preheating at 400 °C in order to eliminate the organic parts.

It leads to an amorphous precursor of BFCO. The powder was synthesized by solid state method starting from Bi2O3; Fe2O3 and Cr2O3 taken in stoichiometric quantity; an excess of 10% of Bismuth was added in order to compensate the loss due to its volatility. The powders were mixed up and then grounded down into a mortar. Subsequently Thermo-Gravimetrical Analysis-Differential Thermal Analysis (TGA-DTA) were performed on the resulting powder. This assumption is made as between the possible reducing atoms in the powder (Fe and Cr), Cr is the easier to reduct at this temperature (assuming the presence of some native carbon which can catalyse the reduction). If this assumption holds true, small gain of weight which starts at 580 °C can be related to a change of chromium valence from Cr II to Cr III (and a gain of oxygen which increase the weight) where CrO oxidises in Cr2O3 and the reaction of BFCO phase formation takes place. The reaction holds on until the temperature increases around 750 °C and a degradation reaction occurs (notable as a fast loss of weight) corresponding to the sublimation of Bi2O3 close to melting temperature: From these preliminary analysis, it is possible to confirm that our powder is forming the phase at lower temperatures compared to the one predicted for BFO from Lisnevskaya et al. 11 , which predicted a phase formation in a range of Temperature between 800 °C and 850 °C. In order to get a better idea on the reactions taking place in the compounds, XRD analysis on the powder were performed. In figure 3.2 are shown the XRDs made on the powder of BFCO at different temperatures: at 500 °C, when the BFCO phase formation should not have yet started; at 650°C corresponds to when the reaction have just started and at 750°C when the reaction is completed. The powders annealed at 500°C and 650°Cr were kept in the furnace with a heating rate of 5°C/min and kept in the oven to cool down. This treatment could have been a problem at 750 °C, as the ramp could have started some lower Temperature reactions without giving them the time to go to completion. To avoid reactions which could have interfeared, the powder was introduced in the furnace directly at a set temperature of 750°C and cooled down to room temperature which takes 6h The first remark from figure 3.2 is that the phase of Bismuth oxide (Bi2O3) is always present in the powder at very high intensity. Iron and chromium oxides signatures are well present at 500°C, while they start to lower at 650°C and are very weak at 750°C. The BFCO phase within the powder is identified by taking into consideration the presence of the peak around 80°, which is also present for BFCO made by PLD deposition as it will be shown later. The double perovskite phase starts to appear at 650°C when the reaction has just initiated and it is also present at 750°C but the peak is at low intensity as the reaction ended and degradation reaction begins. This behaviour can be assigned to a long time of reaction (6h in this case). The main issue is still the formation of parasitic phases as witnessed by the presence of several peaks in Figure 3 By these analysis we even tried to understand more about the intermediate reactions of BFCO phase formation as well as its development by increasing the temperature. Figure 3.3 shows that a lower time of heating at 700°C still results in the formation of different phases in the powder.. It is worth to notice the presence of an amorphous phase visible at the low angle region of the diffraction spectra, which is more marked at 720 °C than at 700 °C. JCPDS files are shown at the bottom of the figures in order to prove the presence of different compouds in the films. The most probable phase will be the Bi2 CrO6 with a doping of Fe which made a shift to lower angles. We speculate that this is the consequence of some non-complete reaction, as the reaction ending point was not reached at 720 °C. It can be partially concluded from these investigations that our experimental procedure did not allow to grow BFCO with a single phase of BFCO, as several parasitic phases are still present. Such results are in agreement with those of Lisnevskaya et al. 6 who claim that thermal annealing under only high pressures favours the formation of single phase compounds. Yet, we have pursue our studies to attempt forming BFCO thin films by the sol-gel method starting from the synthesized powders as a precursor.

Bi2O3 ➔ 2BiO + ½ O2

BFCO layer deposition

The Sol-gel solutions were obtained from oxide powders with a jellying agent. A 10% excess of Bismuth was added to the solution in order to compensate the expected weight loss due to its volatility at high temperature. The as-formed solutions were stirred at 80 °C and cooled down, then deposited by spin coating on silicon or quartz substrates. Subsequently, the solutions were spun at different speed (1500, 2500, 3500 or 4500 rpm) for 60s. Then 2 different types of annealing have been performed, commont thermal annealing (CTA) and rapid thermal annealing (RTA). In the CTA annealing, the BFCO films on substrates were thermally treated in a conventional furnace in air at increasing temperature. The annealing time was varied from 1 h to 6 hours, depending on the experiment. In case of treatment in a RTA furnace, the samples were heated at 750 °C for 2 or 3 minutes under Nitrogen/ Oxygen flow (80:20 or 70:30).

CTA of sol-gel BFCO solutions a) Films preparation

Prior to the deposition the <100> silicon substrates used for this experiment were cleaned in a HF/H2O (1:10) solution for 90 s in order to eliminate the native SiO2; then the silicon surface was etched with an oxygen plasma for 30 s to create Si-O zones and facilitates the wetting.

Subsequently, the substrates were placed in a spinning equipment. On the other hand, the solutions were stirred at 80 °C and then cooled down. Later, few drops were deposited on the Si substrates and spun at a speed of 1500 rpms for 60s. The substrates with the fresh layer on top were preheated at 350 °C for 10 minutes in order to remove the organic compounds.

Afterwards, they were CTA heated at temperatures from 500 to 800°C for 1 h as indicated in Table 3.1.

Table 3.1: BFCO films prepared by sol gel on Silicon substrate and CTA annealed in air

As for the film thickness, a deposition process of a film at a speed of 1500 rotation per minute (rpm) for 60 seconds and annealed at 700°C results in an average 90 nm thick film, as shown in SEM cross-section later (fig. 3.8). This is an important information when it comes to the final application such as a photovoltaic device which requires an appropriate thickness for an optimal photon conversion.

3.3.2.Structural properties of the films

The XRD spectra of the BFCO sol-gel films on silicon and CTA treated in air as prepared above are reported in figure 3.4. In these spectra, most of the peaks were not labelled as the peaks exhibit low intensities. Yet, it is possible to make a comparison with figures 3.2 and 3.3 which concern the XRD spectra of the raw powder BFCO compounds.

As it can be seen from figure 3.4, only Bi2O3 phase is formed at 500 °C. A peak at 17.5° starts to rise at 550 °C but disappears completely at 800 °C. Taking in account the TGA analysis, this peak can be related to a secondary phase appearing during the formation of BFCO phase and disappearing when the degradation reaction starts. On the other hand, a secondary phase related to degradation reaction is visible at angle 22.5°. At 700°C a low intensity peak starts to appear and it become more intense until forming a doublet at 800 °C. Finally, a low intensity peak is observed at 78° for samples annealed at 700 and 750°C, which may indicate the formation a BFCO phase. This is consistent with the TGA analysis reported above which revealed that the BFCO phase can be formed in the range of temperature 700°C-750°C. To get more insights, Raman Spectroscopy analysis were recorded in the range 100 -700 cm - 1 for BFCO sol-gel films deposited on silicon substrates and annealed at different temperatures. The spectra are given in Figure 3.5 a. The typical peak of the crystalline silicon is clearly visible at 520 cm -1 for all samples. The spectra reveal that several features are present for the samples annealed at 700 and 750°C while none are found for the samples treated at 500°C and 800°C. For a comparison purpose, we show the raman spectra recorded on the BFCO deposited by PLD (fig. 3.5 b) and a zoom of the raman on sol-gel in the zone (100-700 cm -1 ). We also show the Raman spectra reported by Yin et al. 17 The Raman analysis of our samples confirm somehow the XRD results and the results from above. At 550 °C the reaction has still not taken place, so that no BFCO phase is formed; at 700 and 750 °C the BFO and BFCO phases are present (peak around 100 cm -1 ). Yet, the presence of several peaks, which have not been identified at this stage, can probably be attributed to the formation of several other phases than the BFO and BFCO ones. The peaks in the film annealed at 750 °C are more intense in respect to the ones at 700 °C; this is a confirmation that the reaction has gone to completion, as suggested from TGA analysis. At 800 °C, the degradation reaction took place in the film and only low intensity peaks are present in the Raman spectra.

b) Optical properties of the films

To determine the optical properties of the BFCO sol gel films, the solutions were spun on quartz substrates. Prior to deposition, the substrates were cleaned in a bath of H2O/HF (10:1) for 90 s in order to eliminate any trace of compounds on the surface that could alter the coating. Optical absorption spectra of the films annealed at 700°C and 800°C are shown in figure 3.6. A huge difference between the two absorption spectra is found. For BFCO annealed at 700 °C, a shoulder appears at a wavelength of about ≈400 nm. Nechache et al. 18 have attributed this behaviour to charge transfer excitation between Cr and Fe mixed d orbital Hubbard transitions that occur in ordered regions of the sample. This suggestion agrees with the spin-density functional theory calculations which has been demonstrated by the absorption spectra of the similar double perovskite La2FeCrO6 19 . Though this effect is still not well understood, it can be considered as a good proof of the formation of a BFCO phase in the film annealed at 700 °C. This corroborated since by the fact that the feature is not anymore visible at 800 °C, temperature at which no BFCO phase is expected to be formed. The absorption spectra were also used to extract the materials bandgap. The Tauc plot 20 method was used for this purpose. The coefficient α has been calculated following the steps:

A= log (Io/I) (eq. 1) and I = Ioe αL (eq.2) that gives:

α=2.303 (A/L) (eq.3)

Where A is the Absorption of the film, I the intensity of the transmitted light, IO the initial intensity and L the optical path.

The plot is shown in Figure 3.7 for the sample annealed at 700°C. The bandgap of the grown film was extracted taking in consideration the direct permitted transition for BFCO 18 . The linearization of the curve allowed to extract two bandgaps of 2.64 eV and 3.14 eV. The bandgap of 2.64 eV is close to that of BiFeO 3 (BFO) and BiCrO 3 (BCO) as reported by Nechache et al. 18 . The higher bandgap is speculated to be related to BFCO. Indeed, Pignolet et al. 21 has notified that a high bandgap can be related to a disordered phase of BFCO, in which Fe and Cr cations are randomly distributed at the B sites of the simple perovskite structure, corresponding to a solid solution of two perovskites, namely BFO and BCO. Thus, the high extracted bandgap (around 3 eV) 18 can be explained. the surface. This effect can be explained as a consequence of fresh layers of sol-gel solution will lose about 60% of their weight and high volume reduction. This weight loss can occur only from the top surface, while the volume reduction produces cracked surface as it can be seen on dry clay sediments. In contrast, the surface of the film annealed at 700 °C is flat and does not present large grains which is good news for a future application in photovoltaics, as presence of cracks would generate short-circuits. In the composition mode, the SEM tool differentiates between the different phases (which indicate different oxide compounds in our case) thanks to a scale of grey. Thus, dark grains are related to low atomic weight elements (iron and chromium) while bright grains are a signature of high atomic weight elements (e.g.

Bismuth).

In the SEM image in composition mode of the film annealed at 550 °C, the scale of grey indicates different average atomic weights, which can suggest the presence of different phases in the film which can be related to bismuth oxide (well identified in the XRD spectra) and also the presence of iron and chromium oxides, as the BFCO compound is not formed yet.

The interpretation is more complex for the film annealed at 700 °C, since the reaction of phase formation has started, so that many phases are present in the film, which are not only related to simple metal oxides but also to BFO, BCO, BFCO with different stoichiometry. 3.2. It is found that although the bismuth was used in an excess of 10%, its high volatility conducted to a bismuth' deficiency in both films. The same effect is observed for the chromium and iron elements. Oxygen is usually difficult to detect accurately with RBS method. Yet, the measured data shows a high oxygen intrusion during the reaction for both samples.

RBS

d) Reproducibility

In order to confirm the reproducibility of the experiments, a new solution of BFCO was prepared by sol gel technique and deposited on silicon substrates or quartz. Figure 3.9 compares the diffraction spectra of the sol-gel BFCO films as prepared from the first solution (blue) and from the second one (orange), deposited on silicon substrates and heated at 700°C.

The XRD signatures are quite similar for the two solutions, meaning that the structural quality of the films is the same. On the other hand, Figure 3.10 exhibits the absorbance spectra of the former and new sol-gel BFCO films as deposited on quartz and heated at 700 °C. Despite a slight difference in the absolute absorbance values for the UV wavelengths, the two spectra match very well and both exhibit a bump at about 400nm. The slight variation in the absorbance can be attributed to a difference in thickness of the films upon deposition and annealing.

From these results, we can be confident about the reproducibility of the sol-gel BFCO films. For additional investigations, RBS analysis were performed on these samples. Figure 3.12 plots the experimental spectra and the modeled (simulation) one. The last considers a film of Bi2FeCrO6 of 200 nm free of defect on the surface Additional analysis should be carried out, and especially using deposition on substrates such as FTO to avoid contamination by silicon from the substrate. Complementary analysis will be needed to further understand the complexity of the crystal.

Conclusions

In this chapter, we present our investigations attempting to synthesize BFCO films with a single phase using the sol-gel method. First, TGA analysis carried out on BFCO powders has shown that the reaction which forms BFCO phase starts at 640 °C and end at 750 °C. Lower annealing temperatures do not allow the formation of the double perovskite while higher thermal treatments lead to low quality products. In a second step, BFCO films were deposited from sol-gel solutions and thermally annealed in conventional (CTA) or lamps assisted (RTA) furnaces.

We have shown that heating the BFCO films in a CTA furnace led to uniform deposited films but also to the presence of too many phases as witnessed by XRD and SEM analysis. The determination of a bandgap value of the films which is found to be closer to the one of BFO confirms that it is still challenging to obtain a single phase BFCO material by sol-gel/CTA method.

We have also performed RTA on sol-gel BFCO films grown in different conditions of composition. Although some conditions such as those of samples BFCO 8 and BFCO 9 has revealed some proportion of BFCO single phase, most of the layers contains parasitic phases, in particular the Bismuth oxide phase. The presence of Bismuth Oxide within the films could be in principle detrimental to the fabrication of a photovoltaic device as it can influence the conduction of the material. Yet, as far as it is not a ferroelectric compound, it should not lower the multiferroic properties of the double perovskite. Chapter 4.

Properties of BFCO grown by the Pulsed Laser Deposition Technique

Pulsed Laser Deposition (PLD) is one of the most used technique [1][2][3] for the epitaxial growth of inorganic oxide. PLD is employed to deposit different materials on various substrates which give birth to ordered films that show many interesting properties, from super-conduction [4][5][6] to magnetism [7][8][9] ; In particular, several works already reported the possibility to deposit inorganic perovskites by PLD in order to obtain Ferroelectric films [10][11][12] and which exhibit the BPVE effect. In the case of BFO, the substrate of choice is normally SrTiO 3 (STO) and Nb:STO (NBSTO) thanks to the matching of their lattices and helps the epitaxial growth. On the other hand, BFCO films were reported to exhibit an epitaxial growth on STO and NBSTO substrates 13,14 . Although with very limited studies, ferroelectricity and photovoltaic effects were shown for BFCO films made by PLD 15,16 .

In this chapter, we will report extensively on the growth of BFCO films by PLD. We will present the operational parameters which tune the oxides bandgap. We will show how the variation of the bandgap can be related to the degree of order within the crystal. This will be confirmed by theoretical modelling. We will also investigate the charges transport within the BFCO grown films, and in particular will show how it is be possible to modulate the current by playing with the memory of the polarization. Finally, we will present the performances of our BFCO based solar cells and will discuss the potentials and limitations of such devices.

Experimental procedure

Several BFCO films were grown by Pulsed Laser Deposition (PLD) on SrTiO3 (STO) and 0.5% doped Nb:SrTiO3 (NbSTO) (001) oriented-substrates. The pellet for the ablation was prepared by the solid-state method starting from the powder of Bismuth, Chromium and Iron Oxides.

The powder was first mixed in right amount (stoichiometric quantity of Chromium and Iron oxides; a 10% excess of Bismuth oxides was added in order to compensate the Bismuth loss during the deposition due to its volatility. In order to produce the pellet, we proceed to a standard sintering procedure. The mixture was first pre-calcined at 800 °C for 12h; afterwards it was grounded down and calcined at 820 °C for 24h. The fine powder obtained has been pressed into a pellet and heated at 840 °C for 24 hours. The pellet was found to be 83% of the theoretical density of the material. The pellet was afterwards aligned in front of a STO substrate in a PLD system and a vacuum of 10 -7 -10 -8 mbar was obtained with a turbomolecular pump.

The films were deposited using a KrF laser (248nm) with an energy of 26 mJ (corresponding in our technique to a fluency of 1,03 J/cm 2 for a laser spot of 2 mm 2 ) obtained with an attenuator.

We adjusted the oxygen partial pressure, the deposition temperature, the laser fluence and the laser repetition rate in order to optimize the films for our application. In order to get a uniform deposition, the carousel on which the substrate was positioned was rotating during the experiment. After deposition, the samples were cooled down at 5 °C/min in oxygen atmosphere.

X-Ray

Effects of the deposition conditions on structural properties

The experimental parameters for depositing the films were the temperature, the oxygen partial pressure and the laser energy. In the following, the properties of the grown films are reported versus the deposition parameters. The main purpose is to identify the best operational conditions to obtain an epitaxial BFCO film, without secondary phases. is appearing together with a small peak at 57°. At 26 mJ, BFCO phase is present and no other phases are visible within the detection limits of our instrument. By increasing the laser energy (40 mJ, 2.0 J/cm 2 ), the phase at 27,5° is even higher than for 20 mJ case and in addition the peak at 57° is more clear and another phase around 30° is present. Further analysis on this phase should be performed to characterize it properly. From Figure 4.3, the best condition seems to be the BFCO deposition at 26 mJ. lattice parameters between the film (0.3930 nm) and the substrate (0.3906 nm). These were intentionally chosen in order to have a good epitaxial growth of BFCO on STO. These patterns also suggest that the films are grown epitaxially with out-of-plane (OP) lattice parameter c of the BFCO pseudo cubic phase oriented along the normal to the surface of the STO substrate.

No secondary phases could be observed within the resolution limit of the X-ray diffraction technique. It should be noticed that although it is known that superstructures peaks are characteristics of the presence of cation ordering inside the double perovskite, these peaks can also have another origin that can be cationic displacements which is common in ferroelectric oxides. 18 . 7 also shows that the vertical correlation growth (or crystallite size, calculated by the Debye-Scherrer formula) is decreasing while the thickness decreases. Note that the crystallite size is always much smaller than the thickness. Another feature to notice is that the lateral correlation length is larger for thinner films. These results suggest that even if the film grows epitaxially on the substrate, it does not present a columnar growth mode and a larger number of defects is present in the film as the deposition frequency increases. Moreover, it is possible to assume that the substrate induces a strain into the films which starts to relax at higher thicknesses. In order to confirm the effect of the laser repetition rate on the deposition, we performed new deposition at a lower temperature, namely at 680 °C. The XRD spectra are shown in figure 4.8. No clear difference is observed between the two conditions. We can therefore consider that the repetition rate has the same effect on the sample whatever is the deposition temperature. The bandgap values of the BFCO films grow by PLD were extracted from ellipsometry measurements and fitting models. The results are showed in fig. 4.9. The figures of merit related to chi-squared values were always better than 0.2. It should be noticed that the chisquared is a value to measure the goodness of the fit. In ellipsometry experiment, a value χ 2 < 0.5 is considered a good fit. It is found that the bandgap increases from 1.9 to 2.6 eV when the deposition rate increases from 1 to 10 Hz while the crystallite size increases up to 6Hz and then slightly decreases. This observed effect has been already reported 13 and has been explained as Jahn-Teller distortion of the structure, taking advantage of the inverse relation between deposition rate and crystallite size in this type of materials. In this way, the distance between the atom B', B" and the Oxygen in the double perovskite are modified allowing us to engineer parameters like, for example, the bandgap and consequently the absorption of the material. Another effect that can produce changes in bandgap is the presence of defects in the material. The anti-site defect, which have been already introduced in chapter 2, can be usually identified by X-ray data. The defects concentration is proportional to the ratio of the Surprisingly, not a clear evolution is found for the ratio Isp/I444 with increasing the repetition rate. This effect can be explained by suggestions taken from Shabadi 18 which reports that the SP of the structure are not a reliable factor of merit of Fe/Cr ordering as Fe and Cr have low scattering contrasts. Moreover, as already discussed in chapter 2, a higher value of ΔZ, where Z is the chemical valence, is known to bring order to the structure. It is therefore acceptable to conclude that an ordering in the structure should be accompanied by a change in the valence from Fe 3+ / Cr 3+ to Fe 2+ / Cr 4+ . Since the XRD analysis did not help that much to rise insights regarding the ordering within the crystal, we investigate the magnetic properties of the grown films. In fact, as discussed in chapter 2, cationic disorder inside the double perovskite should reduce strongly the magnetization because of antiferromagnetic interactions (Fe-O-Fe and Cr-O-Cr) appearing in the system. Fig. 4.10 gives the magnetization loops as recorded for Bi2FeCrO6 films at room temperature and laser repetition rates of 2, 6 and 10 Hz. Hz.

Is it possible to notice that in our samples the magnetization (below 10 emu/cm 3 , ≈ 0.12 μB) is much smaller than the one expected by other reported films (≈ 150 emu/cm 3 ) 19 . As discussed above, as a lowering in magnetization is related to a lower order in the crystal, we can make two assumption:

• Increasing the repetition rate lowers the ordering in our samples;

• Our samples present a low ordering degree.

We have shown how the increment of the repetition rate was related to an increment of the bandgap so we supposed that there should have been a correlation between disorder structure and bandgap change which allows a proper engineering of the bandgap by changing the repetition rate of deposition. underline that a reduced absorption, which translates in a reduced number of light photons interacting within the film, affects directly the number of electron/holes couple formed and therefore generates a reduced photovoltaic current. There is a compromise to be found for thin films as they offer a high conductivity on one hand but a lower generation of electrons on the other hand. A trade-off is therefore mandatory. From fig. 4.12 we can also notice the "double peaked" shape of the absorption curve for the 100 nm thick film. The same behaviour is still visible at 50 nm while it disappears at 7.5 nm. This effect can be linked to an electronic exchange between Fe and Cr in the crystal, as discussed in chapter 2, or to ordered and disordered zones in the crystal which have different bandgaps as we will discuss later on in this chapter.

The HSE06 function 25,26 is used in order to extract the bandgap from the calculation without having to play with empirical parameters as in the GGA+U approach 27,28 . Moreover, we have checked that the bandgap obtained with the HSE06 function is recovered with the GGA+U method with so large Ueff values (around 7 eV) that the electronic structure is strongly distorted. All calculations are performed with a cutoff energy of 400 eV and k-point samplings of 4x4x4 for the trigonal cell and 5x5x2 for the hexagonal cell. After full relaxation, the lattice parameters and the internal structural positions of each atom were extracted and the values are reported in Table 4.2. By building the hexagonal unit cells, we kept the atomic position fixed regardless the site of Fe and Cr was occupied by a Fe or a Cr atom. We built the "perfectly ordered" structure by using a hexagonal cell where Fe occupies the Fe site and Cr occupies the Cr site. In this way, the cell presents an alternate configuration of Fe and Cr (0001) planes. We then used the notation FCFCFC where These DOS results present several common features: i) as HSE 06 was used, the results got affected by a strong localization which makes the Fe occupied energy states ranging always between -8 and -6.5 eV;

Atom

ii) the bottom of the conduction band is defined by the edged structure of the Fe unoccupied states;

iii)

The range of Cr occupied states varies mainly between -1.5 and 0 eV and they are weakly hybridized with the Fe states;

iv)

The minority spin energy band gap is included in the majority spin band gap. For the "perfectly ordered" configuration, which has the lowest total energy among all four cases, with all Fe (respectively Cr) sites being occupied by Fe (Cr) atoms, they carry a positive analysis" 29 by using a code developed by Henkelman et al. 30,31 which uses the minimum of potential energy between two atoms to separate them and calculate their charge independently. In this case the charge brought from each atom (Fe and Cr) was nearly equal (≈ 1.9 |e| for both of them) and did not change even if the sequence F-C changed. Based on some already reported works on Fe3O4 and CaFeO3 systems 31 , we concluded that this method is not helpful for our model. We have found a better model in the work of Wang et al 32 which calculate the charge ordering by using the Born effective charge (BEC) carried on by each atom. By using this model, we were not bounded to a defined volume as we took in account the variation of the polarization around the considered atom. We first tried to make the calculation using HSE06 functional but it was too much computer time consuming so we used the approximation GGA+U with U=4eV. We were able to properly describe the occupied states and then the charge density even if the bandgap was found to be low (i.e. 1.6eV). This effect can be related to the fact that the polarization and the Born effective charge are calculated by summing over the occupied states. Furthermore, the BEC are obtained using a linear-response approach as implemented in VASP 33 and the polarization is calculated in the framework of the modern theory of polarization.

The charges obtained for the four considered configurations are given in table.4.4.

In the "perfectly ordered" structure, the Fe BEC are found 0.80 |e| larger than the Cr ones in clear contrast with the results of the Bader charge analysis.

For the three "disordered" configurations, depending on the sequence, there are huge ΔQ*zz variations:

i) For Fe atoms in a Cr-Fe-Cr, stacking values of ΔQ*zz are found to be equal to -0.05 and -0.02 |e| while for Cr in a Fe-Cr-Fe stacking they are found to be equal to +0.01 and 0.04 |e|.

ii) For Fe atoms in Fe-Fe-Cr or Cr-Fe-Fe stacking, the values are found to be equal to -0.19,-0.20,-0.09,-0.22,-0.18 and -0.17 Subsequently, we performed capacitance-voltage measurements. Firstly, we performed the measurement in darkness (fig. 4.22 a) by applying increased voltage loop each time. We started with 0/1V/-1V/1V loop (labeled as "1V" in the figure), then we made a second measure with 0/2V/-2V/2V loop (noted as "2V"). We used the same procedure for 3V and 4V. The curve of 1V is not exhibiting a ferroelectric effect since the capacitance data shows the conventional "banana loop" typical of insulating material (cf. chapter 2). As we increase the voltage, a ferroelectric shaped curve starts to appear. The effect is better visible for the 4V loop' curve especially around 4 V where the capacitance is highly increasing with a small change in the applied voltage. Since we obtained the best response at 4V, we performed the same loop under dark and under illumination using a laser radiation (fig. In order to avoid any artefacts linked to hot carriers during illumination, we investigated the effect of temperature on the sample. Thus, we first illuminate the sample with a laser at 365 nm for 4 minutes, then we looked to the sample with a thermic camera in order to record the temperature variation. As it can be seen in fig. 4.27, the irradiated zones (yellow-red zones)

only shows a ΔT<2°C in respect to the non-irradiated ones (blue zone). The sample was afterwards heated in a cryostat and it was found that such heating induces a negligible contribution to the pyro-current and voltage changes compared to that observed after light illumination. These results give us confidence that hot carriers are not playing any role during our measurements. It is obvious that the highest photovoltage is obtained for a laser excitation at 365nm, which corresponds to a high optical absorption range of BFCO. A value of about 500 mV is recorded.

Exposure to white light illumination, which is used usually for photovoltaic characterization, resulted in a 200 mV lower Voc in respect to the one found for the exposure at 365 nm. These results should be taken into account while doing light excitation measurements.

In order to study the effect of the light on BFCO sample, we performed I-V measurements in darkness and under a light emitting diode (LED) of 365 nm with 30 nm of spectral linewidth.

Since the photovoltaic effect in ferroelectric materials can depend on the polarization state of the material, we first depolarized it in darkness by an oscillating damped voltage procedure and then measured the I-V characteristic under a 365nm LED with 153 mW/cm 2 . The sample remains in the depolarized state as we collect current from zero to +8V. Afterwards, the voltage was swept from +8V to -8V (marked as positive to negative (P→N) sweep) followed by negative to positive (N→P) sweep from -8V to +8V. The resulting curves are reported in figure 4.29. Ferroelectric history dependence becomes important in the photovoltaic response as shown in the evolution of the short circuit current at 0V. The current Isc can be effectively tuned between 0.9 and 2.2 μA depending on the poling sweep. Another important result to notice is that there is an effect even in depolarized state which can suggest a role of the domain walls in the photovoltaic response. The depolarized state can be advantageous compared to the P→N sweep in this case. It is important to understand this hysteretic behaviour in order to reach high PV performance. The curves show that it is possible to change the current behaviour of the device by applying a voltage higher than the coercive field of the ferroelectric material (here 8.5 V) as already described above. However, the Isc and Voc (give in table 4.5 for -8. Only the sample deposited for 20 s has an acceptable transmittance as a top contact. The other two samples absorb more than 50% of the incident light and are not of use in our case.

The thickness of the as-deposited films has been deduced from XRR measurements. The sample deposited for 30 s was found to be 3 nm thick while the ones of 50 s is 5 nm thick. The film deposited for 20 s was too thin to be detected by the equipment. It is guessed to be around 2 nm thick. Although the measured short circuit current is one order of magnitude higher than in the case of ITO/BFCO device, the limited Isc value is probably due to the high light absorption of the Pt top contact. More surprising is the low Voc. In fact, as the Pt work function is higher than BFCO (around 5.7 eV) one should expect the Schottky barrier formed at interface to increase the internal electric field and therefore to generate higher V oc . On the other hand, the high conductivity of Pt can cause the formation of interfacial charges with opposite sign in respect to the internal polarization of the film. The presence of such charges is detrimental for the device itself, as explained in chapter 2. Consequently, both low Isc and Voc are expected.

Furthermore, the internal resistance due to interfacial charge formation can be responsible of the low Fill Factor of the device, and also reduces the current and the voltage. The I-V curve of the Au/BFCO/Nb:STO device shows a quite reasonably high Voc , around 330 mV, which is larger than the ones found for ITO/BFCO/Nb:STO and Pt/BFCO/Nb:STO devices.

However, the photo-generated current is still too low to reach a good efficiency for the cell.

Similar to the Pt case, the high conductivity of Au and therefore the presence of interfacial charges are probably responsible of the low Fill Factor. The devices using 50nm and 98 nm thick BFCO did not show any better photovoltaic performances as compared to previous devices. In contrast, the sample with a 7.5 nm BFCO exhibits a higher Voc of 620mV and a good Fill Factor of almost 64%. It is most likely that the lower height barrier was able to generate a huge polarization inside the film and therefore a high Voc. The good FF can be related to the low resistance of the thinner film. On the other hand, the low photo-generated current is probably due to a weak light absorption by the thin BFCO absorber.

Conclusions

In this chapter, we have presented the structural, optical and electrical properties of BFCO films synthesized by the pulsed laser deposition technique. The synthesis resulted in materials oxides with a high degree of crystallinity. Especially, we were able to obtain an epitaxial growth of BFCO on STO and Nb:STO along the axis (100) without any secondary phase in the limit of detection of the machine. We have proven the formation of superstructures along the axis (111) and shown that no other phases were present. Thanks to the operational deposition parameters, especially the laser repetition rate, we were able to tune the bandgap of the BFCO films from 1.94 eV to 2.67 eV, which make such oxide materials suitable for several applications. These experimental results were confirmed by theoretical study, in which we demonstrated that a change in the ordering disposition Fe-Cr have an influence on the bandgap of he material. Moreover we have proven that these changes have an impact even on the magnetic properties of the sample, as shown in SQUID measure and theoretical study.

We have demonstrated the ferroelectricity of the deposited films and shown how the polarization memory has a huge impact on the photovoltaic properties. Moreover, we have shown how photovoltaic measurements should be done carefully as the light has an impact on the polarization of the sample, possibly because of light generated carriers which got trapped in the interface metal/semiconductor and influence the internal electric field. We performed electrical studies on the device ITO/BFCO/Nb:STO and proved that it is possible to achieve current switching. We have studied the photovoltaic properties of the PLD-BFCO films.

We

Conclusions and perspectives

This thesis work is a contribution to the studies of structural, optical and electrical properties of BFCO double perovskite films and their implementation as absorbers in photovoltaic devices. The main objective is to produce a new type of solar cell, based on a ferroelectric absorber, without the need of a p-n junction which would be able in the near future to compete with the conventional devices.

We investigated the properties of BFCO synthesized by sol-gel technique and by PLD techniques. We demonstrate for the first time the formation of a double perovskite by sol-gel technique while published works reported the synthesis of BFCO by this wet method only when doping the BFO perovskite. The kinetic of the reaction by TGA analysis showed that the BFCO phase formation reactions takes place between 580 °C and 750 °C. We investigated the evolution of the phase formation at different temperatures in a conventional furnace, and showed the formation of multicrystalline phase films. On the other hand, we applied the rapid thermal annealing process on sol-gel BFCO synthesis which looks promising. Indeed, By an optimization of the N2/O2 ratio as annealing ambient and optimizing the annealing time, we were able to lower the presence of the parasitic phases leading to films containing only bismuth oxide and BFCO phases. In particular, the best results were obtained for the film annealed for 2 minutes in an atmosphere N2/O2: 70/30. Future works should focus on the optimization of the RTA annealing process aiming to avoid the formation of the bismuth oxide phase leading to a multiphase BFCO film. This optimization will allow to develop a BFCO based device by wet technique by depositing the BFCO solution on different bottom contact.

The last part of this work was dedicated to the growth and characterization of BFCO films deposited by pulsed laser deposition. We were able to obtain an epitaxial growth of BFCO on STO and Nb:STO along the axis (100) without any secondary phase in the limit of detection of the machine. By varying the deposition parameters, namely the laser repetition rate, we changed the bandgap from 1.96 eV to 2.64 eV. We demonstrated that the change in bandgap can be attributed to the ordering degree inside the double perovskite. Theoretical calculation of the Fe-Cr stacking inside the crystal has confirmed the importance of the ordering of the atoms in the structure of the BFCO films. The electrical properties of the BFCO films were evaluated through measurements of current and capacitance versus the voltage and measures of the internal polarization of the film. Thanks to I-V analysis made before and after positive and negative polarization, we have shown that the short-circuit current depends on the memory of polarization within the BFCO films. The effect of the illumination on the polarization was confirmed by both electrical and PFM measurements. We have reported for the first time how light exposure increases the capacitance of the film and lowers the internal polarization. We have given an explanation to this phenomenon by assuming an electron charge trapping in the interface metal/semiconductor. Finally, we have fabricated photovoltaic devices using BFCO films and we have experienced different top contacts. We demonstrated that using a top contact with a high carrier mobility , such as Pt and Au, can be detrimental for the quality of the solar cells as the generation of an interfacial dipole can interfere with the internal polarization. We have also tried transparent oxide contacts with different workfunctions and we have proved that AZO creates a lower Schottky junction at the interface metal/semiconductor compared to ITO and WO3. As a result, the AZO/BFCO solar cell device exhibits a good Voc of 620 mV and a high FF of 64%. However, we have found that the cell performances are limited by the low photovoltaic current most probably because of the lower light absorption within the 10 nm thick film. Future works should focus on the effect of the Schottky barrier on the internal polarization in order to optimize the photovoltaic properties of the device. Moreover it would be interesting to dope the double perovskite to possibly increase the conductivity of the film without affecting the ferroelectric properties of BFCO. Finally, a tandem cell by depositing BFCO with different bandgaps should be made in order to fabricate competitive ferroelectric oxides in the photovoltaic market.

Résumé de thèse en français :

Le marché photovoltaïque, utilisant des modules qui convertissent directement le rayonnement solaire en électricité, est en pleine expansion depuis 20 ans, avec une production mondiale approchant 400GW en 2017. L'essentiel de cette production, presque 95%, est à base de modules en silicium monocristallin ou multi cristallin, le reste est à base de semi-conducteurs en couches minces tels que les composés CdTe (tellure de cadmium), CIGS

Chapter 1 .

 1 Photovoltaics: an overview ....................................................................................................................

Fig. 1 . 1 .

 11 Fig.1.1.Worldwide solar panel energy installation from 2000 to 2017 3

Fig. 1 . 2 .

 12 Fig.1.2. Solar Spectrum at different irradiation energy. Modified from 4

Fig. 1 . 3 .

 13 Fig.1.3. Direct (a) and indirect (b) transition bandgap

Fig. 1 . 4 .

 14 Fig.1.4. Band diagram of a p-n junction solar cell 5

Fig. 1 . 5 .

 15 Fig.1.5. Conventional representation of a photovoltaic I-V curve for a p-n junction based device 6

Fig. 1 . 6 .

 16 Fig.1.6. Solar Spectrum and loss into a Si based solar cell device 4

  1.7 are shown (a) the curve calculated by Shockley and Queisser in their article (considering the sun as a black body at 6000 K) and (b) the one modified for a realistic solar spectrum. 1.7. a) curve calculated by Shockley and Queisser in their article 7 (considering the sun a 6000 K) and (b) curve modified for a realistic solar spectrum 8 .

Fig. 1 .

 1 Fig. 1.8 gives a generic view of the present photovoltaic device technologies based on the primary active materials. The established technologies are wafer-based solar cells (crystalline silicon and III-V compounds) or thin films (a-Silicon and Chalcogenides). Also represented are the emerging solar cells technologies such as those based on organic (polymers, small molecules), on hybrid materials (DSSC, perovskite) or using nanomaterials (dots, wires…).

Fig. 1 . 8 .

 18 Fig.1.8. Solar cells classification based on the active material. Modified from 9

Fig. 1 . 9 .

 19 Fig.1.9.Highest efficiencies devices reported from 1975 till now 10

Figure 1 .Fig. 1 .Fig. 1 .

 111 Figure 1.10.a shows a schematic of a conventional solar cell structure based on singlecrystalline or multicrystalline silicon wafers, 150 to 250 m thick. The structure exhibits a conventional P-N homojunction where P region is a boron-doped silicon substrate and the N region is a phosphorous-doped silicon emitter. The backside contact is a screen printedAluminium layer while the front contact is silver based screen printed through a mask. This structure is the most used to make modules and represents about 90% of the market share.

Fig. 1 . 10 .

 110 Fig.1.10. Schematic of (a) a conventional silicon solar cell; (b) an amorphous silicon solar cell, (c) a CdS-CdTe solar cell 9

Figure 1 . 13 .

 113 Figure 1.13.Bandgap of some metal oxides used in Photovoltaic Devices and reported maximum efficiency of the device

Fig. 2 . 3 .

 23 Fig.2.3. A2BB'X6, in the rock salt double perovskite structure where the A cations are surrounded by an alternating network of BX6 and B'X6 octahedra 8 .

Fig. 2 . 4 .

 24 Fig.2.4. B-site cation disorder schemes: (a) completely ordered, (b) antisite disorder and (c)

FiG. 2 . 5 .

 25 FiG.2.5. Typical curve of Polarization vs the Electric field for a Ferroelectric material. It is possible to distinguish between the Remnant polarization (Pr), the saturated Polarization (Ps) and the coercive field (Ec)28

FerromagnetismFiG. 2 . 6 .

 26 FiG.2.6. Spins representation in different material: a) Ferromagnetic; b) Paramagnetic; c) Antiferromagnetic; d) Ferrimagnetic

Fig. 2 . 9 .

 29 Fig.2.9. Schematic representation of BFCO double perovskite. From 42

Figure 4 . 5 .

 45 Figure 4.5. Sketch of the crystalline structure of BFCO which underlines the equivalency of the hexagonal and pseudo-cubic notification. The black lines are meant to visually enhance the structure.

Fig. 2 . 10 . 47 .Fig. 2 .

 210472 Fig.2.10. Representation of magnetic domains in the crystal, linked to Fe 3+ and Cr 3+ position45

Figure 2 . 13 .

 213 Figure 2.13. Separation of charges mechanism in FE solar cell

Figure 2 . 14 .

 214 Figure 2.14. Sketch of the two hypothetical mechanism for BPVE source

Figure. 2 .

 2 15 shows a schematic illustration of the phenomenon. The electron in position A can shift right side in the position B by the absorption of one photon (red arrow), then go from B to C by releasing many photons (black arrow) which creates a net shift current.

Figure 2 . 15 .

 215 Figure 2.15. Sketch of the mechanism of the shift current

  Figure 2.16. a) Bulk photovoltaic effect in BFO with electrodes parallel to domain walls; b) No photovoltaic effect with electrodes perpendicular to the domain walls source

Fig. 2 . 17 .

 217 Fig.2.17. Architectures of the domain-wall: a) PFM image of the 71° domain walls. The inset shows X-Ray rocking curves along the two orthogonal axes which confirms the quality of the films. b) Schematic representation of the DW system at 71°. The small arrows marks the components of the polarization (in plane and out of plane) while the big arrows shows the net polarization in the sample. C) and d) show the same for 109° DW 81

Fig. 2 .

 2 Fig.2.18. operation system for a Ferroelectric solar cell: the diagram shows the Conduction band (CB) the valence band (VB) and the bandgap. hv is the energy of an incident photonwhich excites the elementary charge (e). The polarization P is shown in the bottom. As it is possible to see , there is a band-bending in the domain wall.[START_REF] Huang | Solar energy: Ferroelectric photovoltaics[END_REF]

Fig. 2 . 19 .

 219 Fig.2.19. Diagram of the energy level for a Metal-Insulator. Energy diagram for a MIM after contact of electrode materials, showing the built-in voltage given by the difference in work functions between both metals 60

Figure 2 . 20 .

 220 Figure 2.20. Separation of charges mechanism in FE solar cell 60

Fig. 2 . 21 .

 221 Fig.2.21. Diagram of the energy level for a Metal-Insulator. Energy diagram for a MIM after the application of the polarization (a) ; diagram considering the polarization decrease due to electrode response to polarization (b)[START_REF] Lopez-Varo | Physical aspects of ferroelectric semiconductors for photovoltaic solar energy conversion[END_REF]

  Ps takes into consideration the energy step and the charge Reaching an efficient electron extraction in the device is a fundamental question to create a performing photovoltaic device. As shown, one can in principle imagine to control the polarization induced electric field to control the charge extraction in the device. On the other hand, how the Schottky barrier influence the ferroelectric behaviour is still an open question,

Fig. 2 . 22 .

 222 Fig.2.22. The photovoltaic current direction which follows the hysteresis loop in Pt/BFO/SRO device source

Figure 3 .

 3 Figure 3.1 shows TGA-DTA done on few milligrams of BFCO powdered precursor. Around 360°C a first reaction that regards residual organic compounds occurs. Between 360°C and 460°C, an endothermic reaction happens associated with a loss of weight. This can be due to the formation of CrO from reaction of native Cr2O3 with residual organic moieties

Fig. 3 . 1 :

 31 Fig.3.1: Fast TGA made on BFCO powder

Fig. 3 . 2 :

 32 Fig.3.2: XRD of BFCO powder annealed at 500 °C (red); 650 °C (blue): 750 °C (green)

. 2 .

 2 Figure 3.3 a. and b.:

Fig. 3 . 3 :

 33 Fig.3.3: a) XRD of BFCO powder annealed at 700 °C for 60 min; and the JCPDS file of theoretical compounds.

Fig. 3 . 4 :

 34 Fig.3.4: X-Ray diffraction spectra of BFCO films deposited by sol gel and heated at different temperatures in air ambient

  of BFO and BFCO films grown on Fluorine Tin Oxide (FTO) (fig.3.5 d). Their BFCO film contains also only a low amount of Chromium (BiFe 0.97 Cr 0.03 O 3 ). The presence of peaks in the region around 100-200 cm -1 is indicative of a signature of BFO-BFCO phases.

Fig. 3 . 5 :

 35 Fig.3.5: (a) Raman spectra taken from BFCO films deposited on silicon at different Temperatures; (b) Raman spectra obtained for BFCO deposited by PLD (chapt.4); (c) Zoom in the zone 100-700 cm -1 of the Raman made on sol-gel samples (d) Raman spectra of BiFeO3 and BiFe0.97Cr0.03O3 published by Yin et al.17

Fig. 3 . 6 :

 36 Fig.3.6: Absorption spectra of BFCO deposited at room temperature by sol gel on Quartz substrate and annealed at 700 °C (blue) and 800 °C (green) for 90 nm thick samples.

Fig. 3 . 7 :

 37 Fig.3.7: Tauc Plot of Direct allowed transition for BFCO deposited at 700 °C.

Fig. 3 . 8 :

 38 Fig.3.8: SEM analysis for BFCO films heated at 550 °C and 700 °C

Fig. 3 . 9 :Fig. 3 . 10 :Fig. 3 . 11 :

 39310311 Fig.3.9: XRD spectra of BFCO deposited by sol gel and heated at 700 °C with the first solution (blue) and the second one (orange)

Fig. 3 . 12 :

 312 Fig.3.12: RBS spectra recorded for sol-gel BFCO films with different stoichiometry deposited under N2/O2: 80/20 heated by RTA

  Diffraction (XRD) and X-Ray Reflectivity (XRR) measurements were performed by means of a Rigaku SmartLab diffractometer equipped with a monochromatic source (Ge(220x2) delivering a CuKα1 incident beam (45 kV, 200 mA, λ=0.154056 nm), in order to determine with accuracy phase purity, crystallite size and film thickness. Absorption measurements were obtained by a Perkin Elmer Lambda 950 UV/Vis Spectrometer while the optical bandgaps were extracted from spectroscopic ellipsometry (Horiba Uvisel) measurements. For the last, the dispersion model used is Adachi-New Fourouhi17 . The magnetic properties were analyzed at room temperature using a MPMS SQUID-VSM (Quantum Design) magnetometer allowing a maximum field of 7 T. Piezoresponse Force Microscopy (PFM) measurements were performed on a Bruker Icon QNM using a 0.01-0.025 Ohm-cm Antimony (n) doped Si covered with conductive Diamond and a.c. voltage at 20 kHz.

4. 2 . 1 .Fig. 4 . 1 .

 2141 Fig.4.1.XRD of BFCO deposited at different Temperatures

Fig. 4 . 2 .

 42 Fig.4.2.XRD of BFCO deposited at 10 -2 and 10 -3 mbar

Figure 4 . 3 .

 43 Figure 4.3. Θ-2θ scan of BFCO deposited by PLD at 20, 26 and 40 mJ

Figure 4 . 4 .

 44 Figure 4.4. Θ-2θ scan of BFCO deposited by PLD for different repetition rates ranging from 10 Hz to 1 Hz.

Figure 4 . 6 .

 46 Figure 4.6. a) Φ scan of BFCO along the [111] direction b) θ-2θ scan of BFCO along the axis [111] The thickness of the grown films at different laser repetition rate was estimated by X-Ray reflectivity (XRR) while the vertical and lateral coherence length were obtained by the θ-2θ (shown in fig.4.4) and rocking curves. The results are shown in fig.4.7. It is observed an increase of the thickness of the film when lowering the repetition rate. It should be noticedthat the film grown at 1Hz was deposited at 7500 shots, instead of 30.000 otherwise the deposition duration would have been extremely long (around 8h). As a consequence, the thickness of this 1Hz film does not match with that of the others. The films do not present a columnar growth and show a lower concentration of defects as the repetition rate increases.

Fig 4 .

 4 Fig 4.7 also shows that the vertical correlation growth (or crystallite size, calculated by the

Fig. 4 . 7 .

 47 Fig.4.7. Variation of film thickness, vertical correlation length and lateral correlation length as extracted from reflectivity, θ-2θ and rocking curve X-ray diffraction measurements respectively.

Fig. 4 . 8 .

 48 Fig.4.8.XRD of BFCO films at different frequencies.

Fig. 4 . 9 .

 49 Fig.4.9. Bandgap variation with the laser repetition rate. The values are defined as the ratio between the intensity of the superstructure peak (SP) and the (444) peak of BFCO (zoomed in the inset)

Fig. 4 .

 4 Fig.4.10 Room Temperature magnetization loop recorded for BFCO deposited at 2, 6, and 10

4. 2 . 5 .

 25 Thickness effect BFCO films were deposited by PLD by varying the number of laser shots. For this series, the other parameters were kept constant: oxygen pressure of 10 -2 mbar, a deposition temperature of 750°C, a laser energy of 26 mJ and a frequency of 2Hz. A larger number of shots is expected to give rise to thicker films. The θ-2θ spectra of the grown BFCO samples are shown in fig.4.11. It should be noticed that for the film of 7.5 nm, the phase of BFCO is visible only in the most intense peak, which is the (200) one. It is possible to notice a "shoulder" which identify the BFFCO phase.

Figure 4 .Fig. 4 . 12 :

 4412 Figure 4.11. θ-2θ scan of BFCO films 98 (black), 49 (red) and 7.5 (blue) nm thick

2 .

 2 Structural data at reduced coordinates of the R3 symmetry of BFCO trigonal unit cell with a = 0.55135nm and α = 59.821°, obtained from the full relaxation calculation.

  F and C correspond to Fe and Cr (0001) planes in the stacking respectively. We have built three other slightly disordered configurations which are CFFCFC, FFCCFC and FFFCCC. The ordered configuration as well as the three disordered ones are presented in fig.4.14.

Figure 4 .

 4 Figure 4.14. a) Ideal "fully ordered" and b-d) "disordered" crystalline structure used for ab initio calculation. "F" and "C" stands for Fe and Cr atomic planes respectively. The black lines represent the hexagonal unit cell.

Figure 4 . 15 .

 415 Figure 4.15. Total density of states (DOS)-Fe and Cr atoms projected-for the "perfectly ordered" configuration (FCFCFC) in the hexagonal cell. Positive and negative DOS corresponds to majority and minority spin states respectively.

Figure 4 . 16 .

 416 Figure 4.16. Total density of states (DOS)-Fe and Cr atoms projected-for the three "disordered" configuration in the hexagonal cell. Positive and negative DOS corresponds to majority and minority spin states respectively.

  |e| and for Cr atoms in Cr-Cr-fe or Cr-Cr-Fe stacking they are found to be +0.43, +0.15, +0.25, +0.26, +0.25 and +0.14 |e|. iii) Finally, for the Fe atom in a Fe-Fe-Fe stacking the values are -0.33 |e| and for Cr in Cr-Cr-Cr are +0.40 |e|.

Fig. 4 .Fig. 4 . 18 :

 4418 Fig.4.17: a) sketch of the Ferroelectric based device; b) zoom on the ITO contact and glued copper wire

4. 4 . 2 .Fig. 4 . 19 :Table 4 . 5 :Fig. 4 . 20 .

 4241945420 Fig.4.19: KPFM of ITO and BFCO

Figure 4 .

 4 21 plots an I-V curve between -8V and 8V recorded in darkness conditions. At +6V, a clear peak originating from FE polarization reversal is clearly visible despite the high leakage contribution. This effect is more obvious in the inset of fig.4.21. It should be noticed that the ferroelectric peak does not appear for negative voltages due to a high ohmic conductivity. The FE coercive field of the order of 64 MV was extracted from the I-V characteristic, which is in a good agreement with the previous study19

Figure 4 . 21 :

 421 Figure 4.21: I-V loop of a Bi2FeCrO6 film under dark.

  4.22 b). It is clear that the capacitance is enhanced under light illumination which is indicative that the generation of electro-hole pairs influences the polarization state of the BFCO sample. The capacitance is in fact related to the internal polarization due to the ferroelectric effect. An enhancement of the ferroelectricity translates into a change in the internal polarization, which can be favorable to enhance the electron/hole pair separation in the photovoltaic device

Figure 4 . 22 :

 422 Figure 4.22: (a) C-V loop of a Bi2FeCrO6 film under dark at 1, 2, 3 and 4V; (b) C-V loop under dark and light at 4V

Figure 4 .FIG. 4 . 23 :

 4423 FIG.4.23: (a) PFM phase image after a writing process on the surface and (b) Ferroelectric loop for BFCO The figure shows the ability to switch the current by applying a voltage higher than the coercitive field, which confirms the previous measurements. The amplitude and phase (which change behaviour from the different steps in PFM) (fig.4.23.b) proves that the writing in PFM mode is not related to a ferroelectric artefact. Subsequently we performed conductive AFM (c-AFM) measurements on the polarized square zone of fig.4.24. We used a 2V charged AFM point to collect the current from the polarized sample.

Figure 4 .

 4 Figure 4.26 a) Impedance scan for BFCO at 98 nm measured under darkness and light and b) the same for BFCO 49 nm

Figure 4 . 27 .

 427 Figure 4.27. Thermic camera image of the BFCO film illuminated by a laser at 365 nm during 4 minutes The whole ITO/BFCO/NSTO device was also exposed to different light excitation wavelengths and the corresponding photovoltage values were measured. All LEDs where calibrated against power to provide the same light intensity (≈ 8W/m 2 ). The results are reported in figure 4.28.

Figure 4 . 28 .

 428 Figure 4.28. Photovoltage measured during illumination of an ITO/BFCO//NSTO device at different wavelengths.

Fig. 4 . 29 .

 429 Fig.4.29. Current-voltage measurements on a BFCO sample, in darkness and under 365 nm light irradiation

Figure 4 . 30 .

 430 Figure 4.30. Transient and remnant effect of the BFCO sample

Figure 4 . 31 .

 431 Figure 4.31. PFM poling images before and after exposure to 365 nm light

Figure 4 . 32 .

 432 Figure 4.32. Relative change in voltage induced by 365 nm illumination plotted versus light intensity

Figure 4 .

 4 Figure 4.33. I-V under dark and illumination for the device ITO/BFCO/Nb:STO. The sample has been polarized by applying a positive (+8.5V) or negative (-8.5V) voltage prior the measurements.

Table 4 . 5 .Fig. 4 . 34 .

 45434 Fig.4.34. Sketch of the energy levels of the device Pt/BFCO/NBSTO

Figure 4 . 35 plots

 435 the transmittance of the Pt/glass substrates. As expected, lower is the time of deposition, thinner is the Pt film and higher is its transmittance.

Figure 4 . 35 .

 435 Figure 4.35. Transmittance measurements of Platinum sputtered on glass during 3 different time: 50 s (blue); 30 s (green); 20 s (red).

Figure 4 . 36 .

 436 Figure 4.36. XRR measurements of Platinum sputtered on glass during 3 different time:

Finally,

  Pt/BFCO/NbSTO devices with 2 nm thick Pt layer and a 98 nm thick BFCO film were produced. Their I-V curves were recorded under dark and solar light illumination. The results are plotted in figure 4.37. Once more, the device did not reach acceptable Isc and Voc values.

Figure 4 .Table 4 . 5 .Fig. 4 . 38 .

 445438 Figure 4.37. I-V curves of the Pt/BFCO/Nb:STO device under dark and light illumination

Figure 4 .

 4 Figure 4.39.I-V curve of the Au/BFCO/Nb:STO device under darkness and illumination

Fig. 4 .

 4 Fig.4.40 Sketch of the energy levels of the device WO3/BFCO/NBSTO

Figure 4 . 41 .

 441 Figure 4.41.XRR of WO3 film deposited by sublimation on Quartz

Figure 4 .

 4 Figure 4.42 a) trasmittance measure of WO3 film deposited on quartz; b) absorbance spectra of the same film compared with the bulk absorption one

Figure 4 .Table 4 . 7 .

 447 Figure 4.43. I-V curve of the WO3/BFCO/Nb:STO device under darkness and illumination .

Fig. 4 .FirstFig. 4 . 45 :

 4445 Fig.4.44 Schema of the energy levels of the device AZO/BFCO/NBSTO

Figure 4 .

 4 Figure 4.46.I-V curve of the AZO/BFCO/Nb:STO device under darkness and illumination

(

  Cuivre-Indium-Gallium-Sélénium (et/ou souffre)) ou du silicium amorphe. D'autres matériaux constituant la troisième génération de cellules/modules photovoltaïque sont organiques, donc à base de polymères ou de petites molécules. Plus récemment, les cellules solaires pérovskite hybrides ont fait leur entrée dans le domaine du photovoltaïque, en particulier parce que leur rendement de conversion supérieur à 20% associé à leur faible coût de fabrication les rend prometteurs. Les inconvénients majeurs à ce jour de cette nouvelle catégorie de cellules sont leur instabilité sous rayonnement (très forte dégradation) et la présence du plomb dans le composé principal qui est dangereux pour l'environnement. De nouvelles voies (nouveaux matériaux et/ou concepts novateurs) sont donc à trouver pour développer des cellules avec un rendement de conversion convenable et un coût raisonnable.

Fig. 1 :Fig. 3 :Fig. 4 :

 134 Fig.1 : Schéma de nouvelles technologies photovoltaïques

Figure 5 :Fig. 6 .Fig7: 50 Figure 8 :

 56508 Figure 5 : Θ-2θ scan du BFCO déposé par PLD Un Ф scan et un Θ-2θ le long de l'axe (111) ont été réalisés pour montrer la présence de superstructure du BFCO. Les valeurs de la bande interdite des films BFCO dépsoés par PLD ont été extraites des mesures d'ellipsométrie. Les résultats sont présentés sur la fig.4.9. Les facteurs de mérite liées aux valeurs du chi carré étaient toujours meilleures que 0,2. On constate que la bande interdite augmente de 1,9 à 2,6 eV lorsque la vitesse de dépôt augmente de 1 à 10 Hz:

Figure 9 :

 9 Figure 9: Boucle I-V d'un film Bi2FeCrO6 sous l'obscurité. Afin d'étudier l'effet de la lumière sur l'échantillon BFCO, nous avons effectué des mesures I-V sous obscurité et illuminé par une diode électroluminescente (DEL) de 365 nm avec une largeur de raie spectrale de 30 nm. L'effet photovoltaïque dans les matériaux ferroélectriques pouvant dépendre de l'état de polarisation du matériau, nous l'avons d'abord dépolarisé dans l'obscurité puis avons mesuré la caractéristique I-V sous une illumination LED 365 nm à 153 mW / cm2. L'échantillon reste dans l'état dépolarisé pendant que nous collectons le courant de zéro à + 8V. Ensuite, la tension a été balayée de + 8V à -8V (marquée comme positive à négative (P → N)) suivie de balayage négatif à positif (N → P) de -8V à +8V (Fig.4.31.). Les courbes résultantes sont reportées dans la figure 10. La dépendance à l'histoire ferroélectrique devient importante dans la réponse photovoltaïque, comme le montre l'évolution du courant de court-circuit à 0V. l'Isc peut être modulé efficacement entre 0,9 et 2,2 μA en fonction du balayage de polarisation.

Fig. 10 :

 10 Fig.10 : Mesures de courant-tension sur un échantillon BFCO, dans l'obscurité et sous irradiation lumineuse à 365 nm

Figure 11 :

 11 Figure 11 : Effet transitoire et résiduel de l'échantillon de BFCO

Figure 12 :

 12 Figure 12 : Images de polarisation PFM avant et après exposition à la lumière à 365 nm

Figure 13 :Figure 15 :

 1315 Figure 13 : Variation relative de la tension induite par un éclairage à 365 nm en fonction de l'intensité lumineuse

Table 1 .

 1 1 highlights the established PV technologies and the corresponding maximum efficiencies for the upper mentioned structures.

	Solar cell	Maximum PCE	References
	c-Si	26.6%	Yoshikawa et al. 14
	a-Si	13.4% (multijunction)	LG solar
	CdTe	21%(lab)/17.5%(module)	First Solar
	CIGS	20.3%	Jackson et.al. 15
	GaAs	28.8%(lab)/24.4%(module)	Matsubara et al. 16
	CZTSSe	12.6%	Wang et al. 13

Table 1 .

 1

1. Established technology solar cells and their maximum reported efficiencies

Figure

Table 1 . 2

 12

	Maximum PCE	References

. Alternative solar cells technologies and their maximum reported efficiencies

Based on this short review of PV technologies and on data of figure 1.8 (NREL), It can be pointed out that there were continuous investigations over years either (i) by continuously improving the existing technologies by reducing the losses (optical and electrical) or by developing new cell architectures (PERC, PERT, Bifacial, HTJ…), or (ii) by proposing new absorbing semiconductors (organics, hybrids, QDs…) but still using the "junction" concept for charge carriers' separation.

Perovskites in A 3+ B 3+ O3 d A-O (Å) d B-O (Å) Tolerance factor (t)

			table below are given some of them which
	exhibit different numbers of oxidation, consequently affecting the crystallographic structure
	as well as the quantity of oxygen related defects and therefore the final properties. The table
	gives the ideal A-O, B-O bond distances (dA-O; dB-O) and the ionic radii for several oxides.
	BiFeO3	2.607	2.015	0.915
	GdFeO3	2.578	2.015	0.904
	LaAlO3	2.685	1.876	1.012
	DyScO3	2.514	2.105	0.844
	NdGaO3	2.618	1.986	0.932
	BiMnO 3	2.607	2.016	0.914

Perovskites in A 2+ B 4+ O3 d A-O (Å) d B-O (Å) Tolerance factor (t)

	BaTiO3	2.948	1.965	1.061
	SrRuO3	2.781	1.984	0.991
	SrTiO3	2.781	1.965	1.001
	PbTiO3	2.775	1.965	0.999
	CaTiO3	2.630	1.965	0.964

Perovskites in A + B 5+ O3 d A-O (Å) d B-O (Å) Tolerance factor (t)

	KNbO3	3.051	1.971	1.091
	NaWO3	2.722	1.975	0.983
	KUO3	3.051	2.142	1.007

Table 2 .1.Ideal A

 2

	-O, B-O bond distances and ionic radii calculated tolerance factor (t) for
	different perovskites of A 3+ B 3+ O3 group, A 2+ B 4+ O3 group and A + B 5+ O3 group respectively 4

Typical oxide Main Property BaTiO3, PdTiO3 Ferromagnetic Pb(Zr,Ti)O3, (Bi,Na)TiO3 Piezoelectricity SrFeO3, LaCrO3, LaNiO3 Electrical conductivity La0.9Sr0.1CuO3, HgBa2Ca2Cu2O8 Superconductivity CaTiO3, BaZrO3, BaCeO3 Ion conductivity LaFeO3, LaMnO3 Magnetic property LaCoO3, BaCuO3 Catalytic La0.6Sr0.4CoO3, La0.8Ca0.2MnO3 Electrode Table 2.2.Typical properties

  of perovskite oxides and some examples7

Table 3 .2: Composition

 3

		Bi	Cr	Fe	O
	550	1.7 (2)	0.7 (1)	0.6 (1)	12 (6)
	700	1.8 (2)	0.8 (1)	0.6 (1)	13 (6)

of the BixFeyCrzOt films heated at 550 °C and 700 °C. In the brackets is indicated the expected value.

  21. Nechache, R., Harnagea, C. & Pignolet, A. Multiferroic properties-structure relationships in epitaxial Bi 2 FeCrO 6 thin films: recent developments. J. Phys. Condens. Matter 24, 096001 (2012).

Table 4 .6.Photovoltaic parameters

 4 of the device Au/BFCO/NBSTO under Sun illumination A new serie of devices with a WO3 layer as top contact was prepared. The purpose is to use a material with a higher work function and a lower conductivity than Pt, which should enable to avoid the interfacial charges problem.

	Voc (V)	Isc (nA)	FF
	0.33	4.8	0.22

Table 4 . 8 .

 48 Photovoltaic parameters of the device AZO/BFCO/NBSTO under Sun illumination

  have varied the top contact type, namely ITO, Pt, Au, WO3, looking for the best charges extraction. We have shown that the best solar cell device is AZO/BFCO/NBSTO for which the AZO/BFCO height barrier is the lowest. An open circuit voltage of more than 600 mV and a high fill factor of 0,64 were measured. However, the photogenerated current is still very low,

	Voc (V)	Isc (nA)	FF
	0.62	78.7	0.64

in the nano-ampere range, which is indicative of either a low optical absorption or carriers' recombination traps or both. Such results are encouraging for photovoltaic applications but much further studies are needed towards a high efficient device.

RTA of sol gel BFCO films a) Films preparation

The Rapid Thermal Annealing (RTA) process uses several halogen lamps as a heating source. It is characterized by a thermal profile consisting of a high heating rate (up to 150°C/s), a short plateau time (<120 sec) and a high cooling down rate (about 80°C/s). The annealing is carried out under an N2/O2 ambient. Table 3.3 presents the experimental conditions of the RTA process as applied to our sol-gel BFCO films prepared from different solutions : BFCO with the right stoichiometry, BFCO with an excess of iron and BFCO with an excess of chromium. All films consist in deposition of 2 layers on the silicon substrate. Each layer was spun at 2000 rpm for 60 s and sintered at 350 °C for 10 min in air. After deposition, the films were annealed by RTA for 120 s at 750 °C under different ambients.

Table 3.3: BFCO samples deposited by sol gel heated by RTA a) Structural properties

The XRD measurements were recorded using the equipment D8 (Bruker) which is usually used for powders. In order to protect the detector, the signal of silicon was cutted. Figure 3.11 shows the XRD spectra of the samples deposited under N2 /O2 : 80/20.

A new set of sol-gel BFCO samples was prepared and RTA annealed under a higher flow of oxygen than above, namely N2/O2 : 70/30. The X-Ray Diffractions of these samples were recorded using Rigaku X-Ray Diffractometer (Bruker) with a source of Cu α : 0.154 nm. The The XRD data show that the annealing under stronger oxidizing atmosphere inhibits the formation of a BFCO phase in the samples and results rather in the formation of the Bismuth oxide phase and other 1-2 phases which are most probably oxide compounds of iron or chromium.

AB Initio Calculations for the Order effect

To get more insight on the ordering effect within the structure and it is correlation with the electronic bandgap of the BFCO grown films, ab initio calculations were performed thanks to the collaboration with D. Stoeffler at CNRS-IPCMS. We adopted the R3 symmetry as previous reported work 20,21 , which corresponds to a space group 146 for the BFCO unit cell. The primitive cell of BFCO was built starting from the trigonal cell of the BiFeO3 (fig. 4 All calculations were performed using the Vienna Ab initio Simulation Package (VASP 5.4) 22,23 allowing accurate determination of the electronic structure and investigation of the magnetic properties of the considered system. It uses the so-called augmented plane waves and it is based on the projector augmented wave (PAW) 24 method using pseudopotentials to determine the wave function outside the augmentation region.

(negative) magnetic moment of 4.21 μB (-2.76 μB) giving a total magnetic moment per cell of 6 μB (Table 2). For the "disordered" configurations, the Fe atom on a Cr site carries a negative magnetic moment and, reciprocally, the Cr atom on a Fe site carries a positive magnetic moment, so the total magnetic moment per cell is reduced to 2 μB (Table 4.3). The sign of the magnetic moment changes from one magnetic atomic layer to the next one whatever the nature of the magnetic atom is, which indicates that only antiparallel aligned magnetic moments are obtained for neighboring Fe/Fe, Cr/Cr, and Fe/Cr atoms. These results strongly support the reduced magnetization observed in fig. 4 The difference in the calculated bandgap is clearly less than the one we reported above for the experimental values (Fig. 4.9). The observed difference can be explained as follows: i) in our calculation the hybridization Fe-Cr is weak, and ii) we used an hexagonal cell which allows the "disorder" only in planes perpendicular to the c direction, thus limiting the disorder degree in the model. Yet, these theoretical calculations give a good support to the experimental results.

As we have seen, the degree of disorder inside the crystal depends on the valence between Fe and Cr. We have investigated this effect by means of theoretical calculations by computing the charges carried on by each atom. First, we performed the so-called "Bader charge |e| by lowering the grade of "disorder" in the crystal until the value of 0.07 for the extreme case of FFFCCC. In other words, the higher the disorder, the lower the difference in the charge between Fe and Cr, which is in agreement with the rules discussed in chapter 2. It should be noticed that the polarization is not affected in this last calculation as it has been found always about 0.57 μC/cm 2 .

Electrical properties of the PLD BFCO films 4.4.1. Samples preparation

In order to perform the electrical measurements, a 100 nm thick film of transparent ITO layer was sputtered at room temperature on the BFCO films, providing both good electrical conductivity and high optical transparency (figure 4.17 a). The thickness was confirmed by the XRR measurements which gives a roughness of about 2 nm. The 1x1 mm 2 area of the electrode was then contacted with a copper wire using a conductive epoxy (figure 4.17 b). The currentvoltage measurements were then performed using a precision LCR Meter (Agilent E4980A).

The electrical measurements were performed on BFCO produced in the following conditions: Mots-clé : Oxydes, pérovskites, ferroélectricité, cellules solaires, BPVE, BFCO

Résumé en anglais

In this work, we have produced Bi2FeCrO6 oxides (BFCO) by sol-gel technique and pulsed laser deposition (PLD). By sol-gel, precursors in solution were prepared, which are then deposited by centrifugation on silicon or quartz substrates. The numerous structural (XRD) and optical images (SEM, TEM) analyses carried out on these BFCO films show that the films are fairly homogeneous but exhibit many parasitic phases, which they can be partly eliminated by rapid thermal annealing. Finally, we present the first results obtained on BFCO-SG perovskite devices. On the other hand, BFCO films were deposited on STO and Nb:STO substrates. Their structural, optical and electrical properties are presented. High-quality epitaxial growth and pure-phase films are demonstrated by X-ray diffraction. We show that the band gap of the PLD-BFCO films can be tuned from 1, 9 to 2.6 eV thanks to the variation of growth conditions. Theoretical calculations has confirmed the observed behavior and highlight the importance of the ordering phase. The ferroelectric properties of the PLD films are studied by the piezoresponse force microscopy. Illumination is shown to have a strong effect on polarization. We show that the polarization memory affects the photovoltaic response. Finally, devices based on BFCO are manufactured and their photovoltaic properties are analyzed.

Keywords: Oxides, perovskites, ferroelectricity, solar cells, BPVE, BFCO