domain: spi.nano chim phys spi

text:
Victor Malgras

Andrew Nattestad

Jung Ho Kim

Shi Xue Dou

Yusuke Yamauchi

Understanding chemically processed solar cells based on quantum dots

Keywords: solar cells, photovoltaics, quantum dots, lead sulfide, heterojunction

Photovoltaic energy conversion is one of the best alternatives to fossil fuel combustion. Petroleum resources are now close to depletion and their combustion is known to be responsible for the release of a considerable amount of greenhouse gases and carcinogenic airborne particles. Novel third-generation solar cells include a vast range of device designs and materials aiming to overcome the factors limiting the current technologies. Among them, quantum dot-based devices showed promising potential both as sensitizers and as colloidal nanoparticle films. A good example is the p-type PbS colloidal quantum dots (CQDs) forming a heterojunction with a n-type wide-band-gap semiconductor such as TiO 2 or ZnO. The confinement in these nanostructures is also expected to result in marginal mechanisms, such as the collection of hot carrier and generation of multiple exciton, which would increase the theoretical conversion efficiency limit. Ultimately, this technology could also lead to the assembly of a tandem-type cell with CQD films absorbing in different regions of the solar spectrum.

Historical aspects

A.E. Becquerel first observed the photovoltaic effect for the first time in 1839 by detecting small currents when silver chloride was illuminated [START_REF] Becquerel | Mémoire sur les effets électrique produits sous l'influence des rayons solaires [Essay on electric effects occuring under the influence of solar radiation[END_REF]; but it was only in 1883 when C. Fritts deposited selenium on a thin layer of gold that the junction solar cell was first produced, albeit with an efficiency below 1%. The early 20 th century is marked by significant advances in crystallography (P. Curie), solid state physics (J.J.

Thomson, P. Drude, P. Debye, F. Bloch) and statistical physics (A. Einstein, M. Plank, L. Boltzmann), which provided the necessary knowledge to understand semiconductorjunction-based photovoltaic devices. Various architectures were attempted before D.

Chapin developed a doped (diffused) silicon p-n junction based solar cell in Bell Laboratories in 1954 following R. Ohl's patent. The device, with an efficiency of around 6%, announced the first generation of commercially relevant solar cells. Most contemporary solar panels are still built on this crystalline silicon p-n junction technology attaining an efficiency of 26.3% (commercially available 21.5%) [START_REF] Green | Solar cell efficiency table (version 49)[END_REF].

Combined with the invention of the transistor in 1947 (J. Bardeen, W. Shockley, and W. Brattain), which replaced vacuum tube technology by scalable electronics, the demand for manufactured semiconductors raised significantly. The price of silicon based solar cells dropped from USD 76.67/watt in 1977 to USD 0.60/watt in 2015, making the sun a competitive energy source, substituting for coal and other fossil fuels [START_REF]Transparent Cost Database -OpenEI[END_REF].

Nevertheless, researchers are still aiming to improve stability (life span, heat/moisture resistance), recyclability and especially conversion efficiency and fabrication costs.

For multiple reasons, researchers had to look in other directions, as this technology started to reveal certain limitations. W. Shockley and H. Queisser calculated in 1961 a theoretical limit specific to this type of single junction in bulk semiconductor solar cells restricting the efficiency to 33.7% (for 1 sun illumination) [START_REF] Shockley | Detailed balance limit of efficiency of p-n junction solar cells[END_REF]. Moreover, typical silicon purification lines require 650 °C baking processes [START_REF] Becker | Microstructure and photovoltaic performance of polycrystalline silicon thin films on temperature-stable ZnO:Al layers[END_REF] which are responsible for most of the energy cost of production. The National Renewable Energy Laboratory (NREL) keeps a detailed track of the certified efficiencies of various photovoltaic technologies which have appeared since 1975 (Figure 1). The 2 nd generation of solar cells was aimed towards ecologically sustainable solutions and tried to decrease the amount of matter involved in the architecture of the device by using strongly light-absorbing materials such as 2-4 μm copper-indium-gallium-selenide (CIGS) thin films, which efficiently harvest most of the light in the 400-800 nm range. This technology can now achieve 21.7% conversion efficiency [START_REF] Jackson | Properties of Cu(In,Ga)Se 2 solar cells with new record efficiencies up to 21.7%[END_REF]. The 2 nd generation also includes organic and dye-sensitized solar cells which are assembled through relatively simple and low-cost processes and able to reach efficiencies close to 12% [START_REF] Green | Solar cell efficiency table (version 49)[END_REF].

The latter attracted considerable attention because of their do it yourself potential (simple technological manufacturing and low material purity requirements). These devices suffer from relatively short life-spans and instability, due to the use of molecular absorbers and liquid electrolytes, which make the devices hard to encapsulate. More recent research tends to address this drawback by using solid-state hole transporting materials [START_REF] Bach | Solid-state dye-sensitized mesoporous TiO 2 solar cells with high photon-to-electron conversion efficiencies[END_REF], ionic liquids [START_REF] Kuang | Stable dye-sensitized solar cells based on organic chromophores and ionic liquid electrolyte[END_REF], or photonic crystal [START_REF] Chung | All-solid-state dye-sensitized solar cells with high efficiency[END_REF]. The 3 rd generation solar cells target various strategies to overcome the Shockley-Queisser limit. The present record comes from tandem cells with 46% efficiency (using a concentrator), resulting from the stacking of several p-n junctions made from elements optimized to absorb specific regions of the solar spectrum. Unfortunately, such technology requires metalorganic vapour phase deposition techniques, which increase the cost of production by several orders of magnitude, thus making it only suitable for aerospace applications.

Another approach consists of using quantum dots (QDs) as light absorbers.

Under a specific size, certain binary crystals show significant changes in their optoelectronic behaviour, making them an attractive option for photovoltaic technologies. The interest for quantum dot-based solar cells started when A. J. Nozik assumed in 2001 that marginal phenomena such as hot carrier collection and multiple exciton generation could significantly improve solar cell performances and thus, overcome the Shockley-Queisser limit [START_REF] Nozik | Spectroscopy and hot electron relaxation dynamics in semiconductor quantum wells and quantum dots[END_REF][START_REF] Nozik | Quantum dot solar cells[END_REF]. Different methods exist to synthesize these nanocrystals, such as vapour-liquid-solid, molecular beam epitaxy, electron beam lithography, successive ionic layer adsorption and reaction, and the synthesis of colloidal quantum dots (CQDs) through nucleation processes.

The former three are physical syntheses and require highly controlled atmosphere, high voltage, and/or high vacuum, which hinder their widespread application. The other methods, known as chemical syntheses, are relatively cheap to set up, but require significant optimization in order to obtain controlled size and size distribution. Moreover, one has to replace the long organic ligand used for the synthesis process, capping the colloidal QDs to prevent agglomeration, by smaller molecules.

There is a great deal of research which is currently aiming to improve this ligand exchange method and thus improve the performance and stability of the device. There are three main designs that have been investigated to achieve proper photovoltaic devices: the Schottky junction, the quantum dot sensitizer and the depleted heterojunction. The last architecture has recently achieved 10.7% efficiency through the use of hybrid passivation methods [START_REF] Yoon | High-efficiency colloidal quantum dot photovoltaics via robust self-assembled monolayers[END_REF]. This review presents a brief survey of the typical principles of operation of solar cells and narrows down to place colloidal quantum dot-based device in their technological context.

Operation of solar cells

Solar cells can be seen as diodes in which the generation current can be greatly increased due to the ability of the material to absorb photons, thus exciting electrons which will contribute to the typical thermally generated current.

Solar spectrum and solar simulator

Many factors can affect spectral irradiance distribution, such as the latitude, time of the year, and time of the day, as well as with the weather conditions, (clouds, humidity, wind, etc.). In order to define a standard sun used to compare the efficiency of photovoltaic devices, one can refer to the air-mass (AM) index that relates to different conditions. AM 0 corresponds to the solar spectrum above the edge of the atmosphere and AM 1.0, 1.5, and 2.0 express the solar irradiance from the sun after passing through the atmosphere with angles of 0, 48.2 and 60.1°, respectively (see Figure 2). This gaseous mass is composed of various compounds which absorb a significant proportion of the light intensity (up to 23%). AM 0 is therefore only suitable for extraterrestrial applications (e.g. satellites) while the others provide insight on the input power a solar cell can absorb in a day. AM 1.0 is exact only for devices installed in equatorial or tropical regions at the zenith. Most of the Earth's population living further from the equator in temperate zones where the light path across the atmosphere is longer, AM 1.5 represent a much more relevant standard. Some other factors include the albedo of the surroundings (diffuse reflectivity of a surface). For these reasons, most solar simulators use a xenon arc lamp with appropriate filters mimicking the AM 1.5 spectrum.

p-n junction under illumination and the Shockley-Queisser limit

Under dark conditions, the generated current comes from thermally activated charge carriers. Photons conveying more energy than phonons (lattice vibrations, < 100 meV), their contribution to the generation current can quickly become significant. Indeed, most of the solar spectrum is spread between 250 nm (4.96 eV) and 4000 nm (0.31 eV), divided between ultraviolet (UV), visible, and infrared (IR) light.

As described by Shockley and Queisser in 1961 [START_REF] Shockley | Detailed balance limit of efficiency of p-n junction solar cells[END_REF], the mechanisms responsible for conversion and extraction limit the efficiency of standard solar cell to 33.7%. First, photons with energy lower than the forbidden bandgap of the material ( ) will be diffracted, reflected, or transmitted through the junction. This phenomenon is accountable for the loss of 19% of the solar energy in a typical standard crystalline silicon solar cell with (see Figure 3). Secondly, in the case where a photon transfers an energy higher than to an electron, the latter will be excited to a higher energy level to further thermalize to the bottom of the conduction band ( ) by releasing a phonon with an energy (analogously for holes) with . This other mechanism is responsible for 33% solar power loss.

Finally, phenomena such as the radiation of the photovoltaic device (black body radiation) and radiative recombination (detailed balance principle) also account for another ~15% loss of the incoming solar energy.

In a solar cell under short circuit conditions (Figure 4(a)), the diffusion flow remains unchanged, but most of the photogenerated charges drift along the electric field. The short-circuit current density, , is maximum and corresponds to the photogenerated charges diffusing towards the depletion region to be driven along by the junction polarity (the built-in potential).

This typical current density-voltage relation is the principal figure of merit to assess the performance of a solar cell and is further discussed in the following section.

Solar cells characteristics: ideal vs. real

The principal information regarding the performance of a solar cell resides in the current densityvoltage ( ) characteristic. The short-circuit current density ( ) is the maximum current that can be collected from the device and reflects the output of a broad set of properties such as photo-absorption, injection/diffusion, junction engineering, and defect/impurity levels. In general, will depend on:

 the ability of the active material to strongly absorb light;

 how fast is the injection from the absorbing material to the transport material compared to the back-surface recombination process;

 the potential distribution through the cell containing the least barriers/well which could act as recombination centres.

For optimally engineered solar cells, the short-circuit current density can be expressed as:

(

where and are the charge generation rate (includes absorption spectrum and injection rate), the depletion region's width, and the diffusion lengths of minority carriers (electrons and holes), respectively.

Figure 5 shows a typical curve and highlights the most relevant parameters. The maximum power, with coordinates ( ), which can be obtained by plotting the power curve , determines under which regime the solar cell should operate in order to optimize its output. From this value, one can calculate the fill factor (FF):

(4)

provides insight on the squareness of the solar cell response is. Higher fill factors are attributed to more ideal devices: they result in higher and more stable power as the input conditions fluctuate. Finally, the device efficiency represents the ratio between the maximum power output and the power input:

(5) with for the 1.5 AM standard.

Quantum dots: properties and synthesis

Quantum dots are small particles (or nanocrystals) with electronic properties which differ from those of their bulk counterpart due to their reduced size. This section compiles their properties of interest in the context of solar cells, the different methods of synthesis, and their role in different photovoltaic device architectures.

Confinement in quantum dots

The so-called particle-in-a-box model is the most comprehensive example to introduce confinement in quantum mechanics, describing how the energetic configuration of a particle depends on the size and shape of the space it is confined in. The energy levels available for a particle of mass , confined in a box of size , can be obtained by solving the Schrödinger equation for the single dimension case with infinite potential boundaries [START_REF] Griffiths | Introduction to quantum mechanics[END_REF]:

(6)
where , and are respectively the reduced Planck constant, the wavenumber and an integer justifying mathematically the terms discrete (or quantized) energy levels. The energy between E n and E n+1 increases as the size of the box L decreases and becomes negligible at macroscopic levels.

QDs are semiconducting nanocrystals that are small enough to be considered as potential wells (similar to the particle-in-a-box) within which electrons undergo confinement regime. This regime is considered to be strong when the size of this threedimensional box becomes smaller than the theoretical distance between embedded electron and hole, the exciton Bohr radius :

with , , , and being the hydrogen atom's Bohr radius, the relative permittivity constant of the material, the electron-hole reduced mass, and the free electron mass, respectively. For example, in the case of lead sulfide with and , we obtain . This value is a first approximation, as it does not take into consideration effects due to the dielectric properties of the crystal. Other factors, will also be responsible for modifying the boundary potential seen by the confined charge in a nanocrystal and thus, affecting the energy states distribution, such as shape symmetries (or asymmetries), surface reconstruction and additional chemical interactions.

Tunable bandgap

In the quantum confinement regime, variations in band edge energy level positions will become significant. Louis E. Brus [START_REF] Brus | Electron-electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state[END_REF] first reported the effective mass model to evaluate the bandgap of QDs: [START_REF] Bach | Solid-state dye-sensitized mesoporous TiO 2 solar cells with high photon-to-electron conversion efficiencies[END_REF] where and are the radius and the dielectric constant of the QD, respectively. Lead chalcogenides, however, have relatively high dielectric constants and small bandgaps (see Table 1). The model deviates from real experimental data for crystal sizes under 10 nm (see Figure 6) as the common approximations employed to solve the Schrödinger equation do not hold anymore. Wang et al. developed a hyperbolic model [START_REF] Wang | PbS in polymers -from molecules to bulk solids[END_REF] overcoming this discrepancy and rewrote the equation as:

Another method was later proposed, using a complex 4-band model [START_REF] Kang | Electronic structure and optical properties of PbS and PbSe quantum dots[END_REF][START_REF] Hyun | Electron injection from colloidal PbS quantum dots into titanium dioxide nanoparticles[END_REF] using the Hamiltonian: [START_REF] Chung | All-solid-state dye-sensitized solar cells with high efficiency[END_REF] where is the energy level relative to vacuum, is the energy level from the calculation, and is the electron affinity of the bulk.

When it comes to band structure engineering of electronic devices and specifically designing the desired properties of homo-or heterojunctions, tuning the bandgap has significant advantages. For example, many photovoltaic devices require the use of type II semiconductor junctions (staggered gap) for effective charge injection, transport, and collection.

Electron-hole pairs, excitons

In bulk junction semiconductor devices, electron-hole pairs are formed by considering the final state of an excited electron as being in the conduction band, leaving a hole behind in the valence band and having both charges swept away from each other by the electric field in the depleted region. In reality, however, this only hold for macroscopic materials where the long-range periodicity of the lattice ensures that the electronic properties remain locally the same, wherever the charges are located. In nanostructured devices, many new parameters must be taken into account: crystal boundaries, shape effect, and interface tunnelling. They can be seen as defects introducing perturbations which result in potential wells, barriers and mid-gap states.

For example, in crystals with high dielectric constants, the Coulomb interactions between electrons and holes can be screened. Therefore, they become weakly bound and form a quasi-particle called an exciton. Its energy state can be calculated by solving the Schrödinger equation for the hydrogen atom, replacing the mass of the proton by the effective mass of the hole from the material considered. In bulk materials, the excitonic levels are located in the bandgap near the conduction band (see Figure 7), reflecting the Coulomb interaction. Excitons have no net charge, but can travel in a medium until they receive enough energy to split. In bulk materials, this can be as low as the thermal vibrations at room temperature ( ). In isolated quantum dots (such as PbS quantum dots), it is well established that there is no free charge state, because since the excited electrons and holes are spatially confined, therefore and the whole transitional spectrum is entirely dictated by excitonic levels [START_REF] Yang | Strong electronic coupling and ultrafast electron transfer between PbS quantum dots and TiO 2 nanocrystalline films[END_REF][START_REF] Gao | Charge trapping in bright and dark states of coupled PbS quantum dot films[END_REF][START_REF] Fernée | Highly efficient luminescence from a hybrid state found in strongly quantum confined PbS nanocrystals[END_REF][START_REF] Lewis | Unconventional gap state of trapped exciton in lead sulfide quantum dots[END_REF][START_REF] Wanger | The dominant role of exciton quenching in PbS quantum-dot-based photovoltaic devices[END_REF].

Relaxation dynamics, hot carriers, and multiple exciton generation

When an electron absorbs a photon with an energy which is above the energy of the lowest exciton energy (generally referred as 1S h -1S e ), various pathways can occur:

(1) the electron may thermalize to its lowest state by dissipating energy as lattice vibration through electron-phonon interaction or Auger process;

(2) the excess energy can be transferred to one or more electrons through reverse Auger process, resulting to multiple exciton generation (MEG);

(3) the electron-hole quasi-particle can split, leaving a highly energetic charge (hot carrier) that must be extracted rapidly before recombining.

Pathway [START_REF] Becquerel | Mémoire sur les effets électrique produits sous l'influence des rayons solaires [Essay on electric effects occuring under the influence of solar radiation[END_REF] results in an obvious energy loss (Shockley-Queisser limit) and is still the most probably fate of excited charges because of its extremely fast occurrence in bulk semiconductors (~ ps). Theoretical models have shown that these processes can be slowed down when the photogenerated carrier density is increased up to ~10 18 cm -3 , thus inducing a hot phonon bottleneck due the non-equilibrium distribution of longitudinal optical (LO) phonons (pathways ( 2) and ( 3)) [START_REF] Bockelmann | Phonon scattering and energy relaxation in two-, one-, and zero-dimensional electron gases[END_REF][START_REF] Benisty | Intrinsic mechanism for the poor luminescence properties of quantum-box systems[END_REF]. Later on, Nozik and co-workers showcased the potential of this effect if applied in optoelectronic devices [START_REF] Pelouch | Investigation of hot-carrier relaxation in quantum-well and bulk GaAs at high carrier densities[END_REF][START_REF] Rosenwaks | Hot-carrier cooling in GaAs quantum-wells vs bulk[END_REF][START_REF] Pelouch | Comparison of hot-carrier relaxation in quantum-wells and bulk GaAs at high carrier densities[END_REF]. This process is, however, still limited by crystal momentum which must be conserved during the transitions. Therefore, MEG was only be observed in bulk semiconductors for [START_REF] Beard | Third generation photovoltaics based on multiple exciton generation in quantum confined semiconductors[END_REF]. For QD-based devices, however, research groups report more and more promising results demonstrating enhanced photoconversion efficiencies through MEG, even under AM 1.5 conditions [START_REF] Semonin | Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell[END_REF][START_REF] Jellicoe | Lead telluride quantum dot solar cells displaying external quantum efficiencies exceeding 120%[END_REF][START_REF] Davis | Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120%[END_REF].

Because of their small size, QDs do not suffer from the limitations related to conservation of momentum which is generally inherent to long-range periodicity (Figure 8). These assumptions arise from the idea that if the energy separation between two discrete exciton levels is higher than the fundamental phonon energy, multiplephonon processes would be necessary in order for the charge to relax to the lowest level. These mechanisms are significantly slower than single phonon interactions, and their relaxation time could then be estimated from: [START_REF] Nozik | Spectroscopy and hot electron relaxation dynamics in semiconductor quantum wells and quantum dots[END_REF] where is the hot carrier cooling time, is the phonon frequency and is the energy These properties , if fully exploited in solar cells technology [START_REF] Nozik | Quantum dot solar cells[END_REF][START_REF] Beard | Third generation photovoltaics based on multiple exciton generation in quantum confined semiconductors[END_REF], are expected to enhance the Shockley-Queisser limit (Figure 9) from 33.7% to 45% (for MEG) [START_REF] Hanna | Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers[END_REF] and to 67% (for hot carriers collection) [START_REF] König | Hot carrier solar cells: principles, materials and design[END_REF]. Until now, however, these measurements were only successfully performed on individual nanocrystals under controlled conditions. The challenge of incorporating QDs into a photovoltaic device while taking advantage from MEG [START_REF] Semonin | Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell[END_REF][START_REF] Beard | Variations in the quantum efficiency of multiple exciton generation for a series of chemically treated PbSe nanocrystal films[END_REF][START_REF] Luther | Multiple exciton generation in films of electronically coupled PbSe quantum dots[END_REF][START_REF] Lee | Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays[END_REF] or hot carrier [START_REF] Takeda | Practical factors lowering conversion efficiency of hot carrier solar cells[END_REF][START_REF] Nozik | Semiconductor quantum dots and quantum dot arrays and applications of multiple exciton generation to thirdgeneration photovoltaic solar cells[END_REF] mechanisms is still attracting a lot of attention.

Synthesis

Different methods have been investigated to produce QDs of different materials, with various shapes, sizes and size distributions for a multitude of applications. Physicalchemical vapour deposition techniques generally involve the growth/formation of the materials directly on a substrate, giving an improved control over the size and spatial distribution. They are especially appropriate for the fabrication of superlattices which amplify quantum electronic confinement properties [START_REF] Okada | Increase in photocurrent by optical transitions via intermediate quantum states in direct-doped InAs/GaNAs straincompensated quantum dot solar cell[END_REF][START_REF] Robinson | Spontaneous superlattice formation in nanorods through partial cation exchange[END_REF][START_REF] Yu | Fundamentals of semiconductors: physics and materials properties[END_REF]. On the other hand, wet chemical techniques provide good alternative routes to producing QDs in a colloidal suspension (CQDs) with 3D quantum confinement characteristic. These methods typically use standard glassware with temperatures below 300 ºC, significantly reducing the production costs.

Physical vapour deposition

A standard method to grow a 3D structure through vapour phase deposition is the Stranski-Krastanov growth [START_REF] Matthews | Experimental investigation of the effect of wetting-layer states on the gain-current characteristic of quantum-dot lasers[END_REF][START_REF] Bailey | Open-circuit voltage improvement of InAs/GaAs quantum-dot solar cells using reduced InAs coverage[END_REF][START_REF] Raviswaran | Evolution of coherent islands during strained-layer Volmer-Weber growth of Si on Ge (111)[END_REF]. By depositing several monolayers of semiconductors with a strong lattice mismatch, epitaxial growth can be initiated in a layer-by-layer fashion and to coherently grow forms 3D islands (Figures 10(a)-(f)) [START_REF] Benyoucef | Direct growth of III-V quantum dots on silicon substrates: structural and optical properties[END_REF].

Another widely reported technique is the vapour-liquid-solid (VLS) deposition [START_REF] Schmidt | Growth, Thermodynamics, and electrical properties of silicon nanowires[END_REF][START_REF] Schmidt | UHV chemical vapour deposition of silicon nanowires[END_REF][START_REF] Trentler | Solution-liquid-solid growth of crystalline Ill-V semiconductors: an analogy to vapor-liquid-solid growth[END_REF][START_REF] Westwater | Growth of silicon nanowires via gold/silane vapor-liquid-solid reaction[END_REF][START_REF] Wu | Direct observation of vapor-liquid-solid nanowire growth[END_REF], where a thin film of gold (1-10 nm) is typically deposited onto a silicon wafer [START_REF] Kramer | Solar cells using quantum funnels[END_REF] and heated above the Au/Si eutectic point to form droplets of Au-Si alloy on the surface of the substrate. The sample is then placed inside a vacuum chamber with a flow of reactive gas mixture (typically SiCl 4 :H 2 ) at 800 ºC. The gas is absorbed into the droplets, which act as a catalyst to lower the activation energy of normal vapour solid growth, until supersaturation is reached. This is followed by the excess Si atoms to be automatically driven down to the substrate, leading to anisotropic growth (see Figures 10(g)-(i)).

Successive ionic layer adsorption and reaction (SILAR)

Vogel et al. reported the sensitization of wide-band-gap semiconductors with various binary sulfides semiconductor nanoparticles through chemical bath deposition [START_REF] Vogel | Quantum-sized PbS, CdS, Ag 2 S, Sb 2 S 3 and Bi 2 S 3 particles as sensitizers for various nanoporous wide-bandgap semiconductors[END_REF]. The method was further renamed later on as SILAR in order to prevent the confusion with other types of chemical bath deposition techniques. The technique consists in the successive immersion of the substrate in aqueous solution of salts (e.g., lead nitrate followed by sodium sulfide). The deposition can be controlled by varying the immersion time, the number of repetitions, the type of salt or the concentration. The number of seeds deposited during the first cycle remains a limiting factor, however, as any subsequent steps will only feed the pre-existing crystallites. The direct growth on the substrate has the advantages of increasing the cohesion of the sensitizer and thus improving electron injection. This method, which, for instance, is used for the fabrication of quantum dot sensitized solar cells, suffers still from certain drawbacks which will be discussed in section 4.1.1 [START_REF] Kamat | Quantum dot solar cells: Semiconductor nanocrystals as light harvesters[END_REF].

Colloidal growth synthesis

After Faraday synthesized gold colloidal nanoparticles in 1857 [START_REF] Faraday | The Bakerian lecture: experimental relations of gold (and other metals) to light[END_REF], many other chemical routes were developed in order to obtain similar nanostructures with a wide range of binary compositions. A typical method involves the combination of two (or more) precursors, generally from groups II/VI or IV/VI, in a hot solvent containing carefully selected coordinating molecules under vigorous stirring. At the start, a multitude A of nucleation centres are formed, initiating the growth of particles through Ostwald ripening. The role of the coordinating ligands is to set a critical crystal size, after which, the sterically hindered growth leads to a narrowed size distribution [START_REF] Peng | Kinetics of II-VI and III-V colloidal semiconductor nanocrystal growth: "Focusing" of size distributions[END_REF][START_REF] Qu | In situ observation of the nucleation and growth of CdSe nanocrystals[END_REF][START_REF] Xie | Nucleation kinetics vs chemical kinetics in the initial formation of semiconductor nanocrystals[END_REF].

The mean size can be empirically controlled through parameters such as the precursor ratio, ligand concentration, temperature and reaction time. These techniques developed and modernized by Murray et al. in IBM's laboratory remain, even nowadays, the standard recipes for the synthesis of cadmium and lead chalcogenides [START_REF] Murray | Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites[END_REF][START_REF] Murray | Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies[END_REF][START_REF] Murray | Colloidal synthesis of nanocrystals and nanocrystal superlattices[END_REF].

Typically, tri-n-octylphosphine (TOP) is used to dissolve the chalcogen (S, Se, Te)

precursor, and tri-n-octylphosphine oxide (TOPO) act as the coordinating ligand (Figure 11(a)) [START_REF] Jin | Amphiphilic p-sulfonatocalix[4]arene-coated CdSe/ZnS quantum dots for the optical detection of the neurotransmitter acetylcholine[END_REF]. They also introduced the combination of lead oleatebis(trimethylsilyl)sulfide precursor to produce PbS quantum dots in hot diphenyl ether (boiling point ~260 ºC). Nowadays, researchers are generally adopting the Hines and

Scholes method [START_REF] Hines | Colloidal PbS nanocrystals with size-tunable nearinfrared emission: Observation of post-synthesis self-narrowing of the particle size distribution[END_REF], in which toxic diphenyl ether is replaced by 1-octadecene (boiling point 315 ºC). The synthesis must be followed by appropriate washing to extract the quantum dots from the reaction solution and remove unreacted ligands and precursors.

The final product remains capped with oleate (or TOPO) molecules, making it stable in non-polar solvents, hence the name colloidal quantum dots. Parameters such as injection temperature and reaction time can be accurately controlled to produce a wide range of nanoparticle sizes, and thus a wide range of bandgaps with different absorption cut-off and emission wavelengths (Figure 11(b)).

Quantum dots for photovoltaic application

QDs show unique optoelectronic properties due to their extreme confinement, including a high extinction coefficient allowing thin layers to absorb a significant portion of incident photons. There has been considerable research aiming to design devices with the purpose of optimizing photo-absorption and charge transport/collection while maintaining a high voltage output. Researchers use various architectures as scaffolds to observe the effects of new materials and new treatments, and to study the fundamentals of electronic transport.

Typical device architectures

Here, three different architectures are reviewed (Figure 12): the quantum dot sensitized solar cell, the colloidal quantum dot Schottky junction solar cell and the colloidal quantum dot heterojunction solar cell [START_REF] Emin | Colloidal quantum dot solar cells[END_REF][START_REF] Pattantyus-Abraham | Depletedheterojunction colloidal quantum dot solar cells[END_REF]. Other strategies, not covered in the current review, have more recently been investigated, such as hybrid cells blending colloidal quantum dots with polymers [START_REF] Huynh | Hybrid nanorod-polymer solar cells[END_REF][START_REF] Ren | Inorganic-organic hybrid solar cell: bridging quantum dots to conjugated polymer nanowires[END_REF][START_REF] Liu | Employing end-functional polythiophene to control the morphology of nanocrystal-polymer composites in hybrid solar cells[END_REF][START_REF] Mcdonald | Solution-processed PbS quantum dot infrared photodetectors and photovoltaics[END_REF], fullerenes [START_REF] Ryan | Nongeminate recombination dynamics-device voltage relationship in hybrid PbS quantum dot/C60 solar cells[END_REF][START_REF] Zhou | Efficiency enhancement for bulk-heterojunction hybrid solar cells based on acid treated CdSe quantum dots and low bandgap polymer PCPDTBT[END_REF], graphene [START_REF] Li | An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics[END_REF][START_REF] Guo | Layered graphene/quantum dots for photovoltaic devices[END_REF], or carbon nanotubes [START_REF] Landi | CdSe quantum dot-single wall carbon nanotube complexes for polymeric solar cells[END_REF].

Quantum dot sensitized solar cell (QDSC)

Inspired by their organic counterparts (dye-sensitized solar cells, DSCs), the inorganic sensitizers from QDSCs are generally grown through SILAR deposition and are selected for being strongly light-absorbing in the visible range. The operation mechanism can be briefly summarized as: i) a photon is absorbed in a QD, generating an exciton; ii) the electron and hole dissociate at the interface with a TiO 2 particle; iii) the electron is injected into the TiO 2 , resulting in the oxidation of the QD sensitizer, and iv) is transported to the working electrode, which is typically a transparent conductive oxide (TCO); v) the hole recombines with an electron from a redox medium and regenerates the ground state; vi) Finally the system is at equilibrium once the oxidized electrolyte diffuses to the counter electrode where it is reduced.

Typical electrolytes in DSCs use the iodide/triiodide redox couple; it is however a reactive source of corrosion for the QD sensitizers. Other compositions, including polysulfides dissolved in methanol [START_REF] Lee | Efficient polysulfide electrolyte for CdS quantum dotsensitized solar cells[END_REF], cobalt complexes [START_REF] Lee | Regenerative PbS and CdS quantum dot sensitized solar cells with a cobalt complex as hole mediator[END_REF] or solid state hole conductors such as (2,2(,7,7(-tetrakis-(N,N-di-pmethoxyphenylamine) 9,9(spirobifluorene) (spiro-OMeTAD) [START_REF] Lee | PbS and CdS quantum dot-sensitized solid-state solar cells:"Old concepts, new results[END_REF], have shown more stable performance. As in DSCs, the main detrimental pathway for photogenerated carriers to recombine in QDSCs is from the TiO 2 conduction band into the redox couple from the electrolyte [START_REF] Mora-Seró | Recombination in quantum dot sensitized solar cells[END_REF][START_REF] Tenne | Improved efficiency of CdSe photoanodes by photoelectrochemical etching[END_REF]. This can be explained from the low coverage efficiency during the SILAR deposition. Generally, a ZnS coating efficiently screens these back-recombination mechanisms, but it can also introduce new monoenergetic surface states affecting the fill factor [START_REF] Shen | Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells[END_REF]. Various other strategies are still being investigated [START_REF] Radich | Cu 2 S reduced graphene oxide composite for high-efficiency quantum dot solar cells. Overcoming the redox limitations of S 2-/Sn 2at the counter electrode[END_REF][START_REF] Lee | Highly efficient CdSe-sensitized TiO2 photoelectrode for quantum-dot-sensitized solar cell applications[END_REF][START_REF] González-Pedro | Modeling high-efficiency quantum dot sensitized solar cells[END_REF][START_REF] Diguna | High efficiency of CdSe quantum-dotsensitized TiO 2 inverse opal solar cells[END_REF][START_REF] Rühle | Quantum-dot-sensitized solar cells[END_REF]. The latest devices sensitized with PbS quantum dots showed short-circuit current as high as 20.8 mA/cm 2 , leading to an overall efficiency of 8.2% [START_REF] Zhao | Boosting power conversion efficiencies of quantum-dot-sensitized solar cells beyond 8% by recombination control[END_REF].

CQD Schottky junction solar cell (SJSC)

The CQD SJSC was the first of its kind to achieve efficiencies beyond 1% from CQDs [START_REF] Luther | Schottky solar cells based on colloidal nanocrystal films[END_REF]. The architecture is based on overlaying a TCO with large work function (such as indium-doped tin oxide) with a film of p-type CQDs to form an Ohmic contact. This is followed by evaporating a metal with a shallow work function (aluminium, magnesium)

to generate an appropriate band-bending suitable to extract electron while screening holes.

This attractive strategy had, however, a few limitations. The short diffusion length in these films limits their thickness to 200 nm, which is too thin to absorb a sufficient portion of the incident radiation. Increasing the thickness of the device above this critical limit causes charge recombination to become a substantial problem. Also, the Fermi level can be easily pinned by defect states at the metal/semiconductor interface, affecting the overall open-circuit voltage. Nevertheless, optimization of the material synthesis, post-treatments, and assembly [START_REF] Strasfeld | Imaging Schottky barriers and ohmic contacts in PbS quantum dot devices[END_REF], along with carefully engineered hole-selective contacts, allowed Piliego et al. to produce devices with an efficiency of 5.2% [START_REF] Piliego | 5.2% efficient PbS nanocrystal Schottky solar cells[END_REF].

CQD depleted heterojunction solar cell (DHJSC)

This architecture has similar aspects to the CQD SJSC, except that it has an additional n-type layer of wide-band-gap semiconductor particles (TiO 2 , ZnO) between the TCO electrode and the CQD layer to secure electron transport. The back contact is typically made of a metal with a large work function (such as Au or Pt).

As compared to the SJSC, the mild depletion region of heterojunction provides more efficient electron-hole dissociation, and because it is located on the illuminated side, carrier separation happens faster. Back electron transfers from the oxide to the CQD layer can be effectively suppressed by the built-in field. Finally, a higher opencircuit voltage can be achieved because of the large difference between the Fermi level of the TiO 2 and the work function of the counter electrode. The first DHJSC [START_REF] Pattantyus-Abraham | Depletedheterojunction colloidal quantum dot solar cells[END_REF] was reported in 2010 with an efficiency of 5.1%, far beyond the records previously achieved by other architectures at the time. This high performance was partly due to optimized parameters such as: CQD synthesis, size selection, ligand exchange and film thickness (both for TiO 2 and PbS CQDs). Further improvement, including controlled oxide doping and inorganic passivation, enhanced the performance of these solar cells up to 7.4% [START_REF] Liu | Systematic optimization of quantum junction colloidal quantum dot solar cells[END_REF][START_REF] Liu | Electron acceptor materials engineering in colloidal quantum dot solar cells[END_REF][START_REF] Tang | Colloidal-quantum-dot photovoltaics using atomic-ligand passivation[END_REF][START_REF] Ip | Hybrid passivated colloidal quantum dot solids[END_REF]. Ultimately, strategies such as replacing the wide-band-gap oxide by ndoped CQD film and stacking films with different QD size were considered to fabricate promising tandem structures to increase the absorption range [START_REF] Ning | Graded doping for enhanced colloidal quantum dot photovoltaics[END_REF][START_REF] Tang | Quantum junction solar cells[END_REF][START_REF] Kramer | Solar cells using quantum funnels[END_REF][START_REF] Wang | Tandem colloidal quantum dot solar cells employing a graded recombination layer[END_REF].

The role of the ligands

Ligands are ions or molecules coordinating with a metal atom [START_REF] Cotton | Advanced Inorganic Chemistry[END_REF]. In the context of nanocrystal chemistry, ligands form a bond with surface atoms where, by definition, the periodicity is interrupted and fulfils three main roles: passivation, functionalization, and steric spacing.

The term passivation literally implies that a material is made less reactive to its environment. The surface of nanoparticles can be unstable due to strains, uncontrolled reconstruction, or unbalanced charge. These reactive sites are ready to bond with any adventitious moieties so as to lower its surface energy. The most common contaminants are the oxides formed from oxygen and moisture in air. These species generally have a detrimental impact on the particle properties by adding new localized surface states (generally mid-gap states) to the overall crystal energy structure. These levels can not only pin the Fermi level down (and thus lower the open-circuit voltage), but also act as deep traps and recombination centers. Surface passivation usually involves the introduction of ligand molecules (or ions) to coordinate with these unstable sites. An efficient passivation will induce a minimum change in the energy state distribution, while preventing other adventitious contaminants from being adsorbed.

The term functionalization has a broad meaning, as it includes any modification in the physical or chemical reactivity of the material. For instance, in the context of biotechnology, CQDs can be functionalized to improve their biocompatibility [START_REF] Mazumder | Review: biofunctionalized quantum dots in biology and medicine[END_REF] or can act as a fluorescent chromophore binding to specific cells or proteins [START_REF] Algar | Beyond labels: a review of the application of quantum dots as integrated components of assays, bioprobes, and biosensors utilizing optical transduction[END_REF]. For PbS CQD-based solar cells, the nanoparticles are coated with oleate ligands having long non-polar hydrocarbon chains. This has the effect to neutralize the apparent surface charge, thus giving the material the ability to be suspended in non-polar solvents (e.g., alkanes, toluene, chloroform). This enable the ability of CQDs to be deposited on substrates through spin-coating, dip-coating or even potential screen-and inkjet-printing techniques [START_REF] Konstantatos | Colloidal quantum dot optoelectronics and photovoltaics[END_REF].

In order for QDs to retain their confined optoelectronic properties, they must maintain a certain degree of isolation, thus preventing the electron wave function from delocalising in neighbouring nanocrystals. The loss of confinement leads to uncontrolled and non-uniform energy level reconfiguration; the first exciton transition is reduced and the electronic landscape regains its continuous character (from the bulk).

On the other hand, electrons from completely remote nanoparticleshave a very low hopping probability and thus suffer from low conductivity. After being cast on a solid surface, ligands of different lengths provide various steric spacing between the QDs and a balance must be found between quantum confinement and electronic conductivity. For this reason, CQD films are generally made through a layer-by-layer process, where in each cycle consists in exchanging long chain ligands for shorter ones (see Figures 13(a)

and (b)). Ligand exchange can roughly be categorized into two groups: organic and inorganic.

Organic ligand exchange

Replacing oleate molecules with short ionic dithiol ligands such as 1,2-ethanedithiol (EDT) and 1,3-benzenedithiol (BDT) is a promising strategy to improve connectivity in PbS CQD films [START_REF] Moody | PbS nanocrystals functionalized with a short-chain, ionic, dithiol ligand[END_REF][START_REF] Luther | Structural, optical and electrical properties of self-assembled films of PbSe nanocrystals treated with 1,2-ethanedithiol[END_REF][START_REF] Barkhouse | Thiols passivate recombination centers in colloidal quantum dots leading to enhanced photovoltaic device efficiency[END_REF][START_REF] Szendrei | PbS nanocrystal solar cells with high efficiency and fill factor[END_REF][START_REF] Xu | Interconnected TiO2 nanowire networks for PbS quantum dot solar cell applications[END_REF][START_REF] Padilla | Thermal properties of TiO 2 /PbS nanoparticle solar cells[END_REF]. Some groups, however, have reported poor resistance to ambient atmospheric conditions [START_REF] Klem | Impact of dithiol treatment and air annealing on the conductivity, mobility, and hole density in PbS colloidal quantum dot solids[END_REF][START_REF] Inerbaev | Quantum chemistry of quantum dots: Effects of ligands and oxidation[END_REF]. Later on, optimized ligand exchanged employing 3-mercaptopropionic acid (MPA) seemed to lead to better stability, improved mobility, and the resulting film was less influenced by possible trap states for similar interparticle spacing (Figures 13(c) and (d)) [START_REF] Voznyy | A charge-orbital balance picture of doping in colloidal quantum dot solids[END_REF]. Jeong et al. suggested that the greater chemical diversity of MPA (thiol + carboxylic groups) in comparison to EDT can be responsible for passivating a broader distribution of surface states [START_REF] Jeong | Enhanced mobility-lifetime products in PbS colloidal quantum dot photovoltaics[END_REF].

Through modelling and X-ray photoelectron spectroscopy analysis, it was found however that the ligand coverage efficiency, and thus the stability of the particle surface, could be hindered by hydroxylation mechanisms [START_REF] Zherebetskyy | Hydroxylation of the surface of PbS nanocrystals passivated with oleic acid[END_REF][START_REF] Malgras | The effect of surface passivation on the structure of sulphur-rich PbS colloidal quantum dots for photovoltaic application[END_REF]. It was also reported that substituting the oleate group for oleylamine [START_REF] Tang | Quantum dot photovoltaics in the extreme quantum confinement regime: The surface-chemical origins of exceptional air-and light-stability[END_REF] or octylamine [START_REF] Koleilat | Efficient, stable infrared photovoltaics based on solution-cast colloidal quantum dots[END_REF] through a 3day solution-phase ligand exchange prior to the MPA or EDT solid phase exchange promoted a more effective replacement and improved passivation. More recently, Giansante et al. reported a complete study of PbS CQDs passivated with various types of short conjugated ligands and were able to enhance their broadband light absorption while maintaining their stability [START_REF] Giansante | Darker-than-Black" bS Quantum Dots: Enhancing Optical Absorption of Colloidal Semiconductor Nanocrystals via Short Conjugated Ligands[END_REF].

Inorganic ligand exchange

Because of their bulkiness as well as their vulnerability to thermal degradation and oxidation, other researchers have aimed to substitute organic ligands for their inorganic analogues. Talapin's group started by using Sn 2 S 6 4-ions to cap various types of quantum dots (CdSe, CdTe, CdS, Bi 2 S 3 , Au, Pd) and extended this work further with a wider range of inorganic ligands such as HS -, Se 2-, HSe -, Te 2-, HTe -, TeS

Transport in CQD depleted heterojunction solar cells

Recently, the term selective contact has been considered as a better description for the role of these collecting junctions, compared to heterojunctions. Mora-Sero et al. clearly observed how the choice of material can literally screen the charges: fluorine doped tin oxide, Au or poly(3,4-ethylenedioxythiophene) (PEDOT) for holes; TiO 2 or ZnO for electrons [START_REF] Mora-Sero | Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics[END_REF]. It was also reported that major back surface recombination mechanisms can be suppressed by simply adding hole [START_REF] Malgras | Trap-assisted transport and non-uniform charge distribution in sulfur-rich PbS colloidal quantum dot-based solar cells with selective contacts[END_REF] or electron [START_REF] Yuan | Synergistic doping of fullerene electron transport layer and colloidal quantum dot solids enhances solar cell performance[END_REF] selective contacts. The appropriate selection of materials and doping techniques have been at the center of considerable attention when it comes to engineering the interface between PbS CQDs and the electron selective contact [START_REF] Maraghechi | The donor-supply electrode enhances performance in colloidal quantum dot solar cells[END_REF][START_REF] Ehrler | Preventing interfacial recombination in colloidal quantum dot solar cells by doping the metal oxide[END_REF][START_REF] Willis | The transitional heterojunction behavior of PbS/ZnO colloidal quantum dot solar cells[END_REF][START_REF] Wang | Enhanced open-circuit voltage in visible quantum dot photovoltaics by engineering of carrier-collecting electrodes[END_REF][START_REF] Jean | ZnO nanowire arrays for enhanced photocurrent in PbS quantum dot solar cells[END_REF]. Hole collection was successfully improved by using LiF in Schottky devices [START_REF] Debnath | Ambient-processed colloidal quantum dot solar cells via individual pre-encapsulation of nanoparticles[END_REF][START_REF] Tang | Schottky quantum dot solar cells stable in air under solar illumination[END_REF][START_REF] Johnston | Schottky-quantum dot photovoltaics for efficient infrared power conversion[END_REF][START_REF] Etgar | Light energy conversion by mesoscopic PbS quantum dots/TiO 2 heterojunction solar cells[END_REF], while DHJSC shows better results using MoO X (see Figure 15

Concluding remarks and outlook

A tremendous effort has been deployed to analyse and exploit the properties of nanostructures such as quantum dots in order to assess their applicability in the field of photovoltaic and other optoelectronic devices. While theoretical speculations and calculations place these materials at the centre of the third-generation solar cells, recent research output tends to demonstrate that many unpredictable issues arise from the implementation of such structures inside devices. The considerable work targeting material synthesis and device engineering, however, is gradually circumventing these hindrances, opening the door to a potential solar cell technology which could be entirely fabricated through chemical processes and thus, at lesser costs. For example, recent attempts have been aiming to hybridize PbS quantum dots with methylammonium lead halide perovskite, and achieved an unprecedented efficiency of 10.6% [START_REF] Lan | Passivation using molecular halides increases quantum dot solar cell performance[END_REF][START_REF] Yang | Colloidal quantum dot photovoltaics enhanced by perovskite shelling[END_REF][START_REF] Lan | 6% certified colloidal quantum dot solar cells via solvent-polarity-engineered halide passivation[END_REF].

Nowadays, extensive research aims at nanostructuring a wide range of materials [START_REF] Zhang | Highly efficient, transparent and stable semitransparent colloidal quantum dot solar cells: a combined numerical modeling and experimental approach[END_REF][START_REF] Cappelluti | Impact of doping on InAs/GaAs quantumdot solar cells: A numerical study on photovoltaic and photoluminescence behavior[END_REF][START_REF] So | Solid-state colloidal CuInS 2 quantum dot solar cells enabled by bulk heterojunctions[END_REF], including the promising lead halide perovskite [START_REF] Gonzalez-Carrero | The luminescence of CH3NH3PbBr3 perovskite nanoparticles crests the summit and their photostability under wet conditions is enhanced[END_REF][START_REF] Swarnkar | Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics[END_REF][START_REF] Malgras | Observation of quantum confinement in monodisperse methylammonium lead halide perovskite nanocrystals embedded in mesoporous silica[END_REF][START_REF] Malgras | Hybrid methylammonium lead halide perovskite nanocrystals confined in gyroidal silica templates[END_REF], to further improve the efficiencies and versatility of nanocrystal-based optoelectronic devices. Copyright 2013 American Chemical Society. with various sizes emitting at various wavelength. Reprinted with permission from [START_REF] Lim | The nanolight revolution is coming[END_REF].

Copyright 2016 Nature Publishing Group.

  level separation. According to Equation (11), strongly quantized levels (> 0.2 eV) would extend the relaxation time to ~100 ps. Using ultra-fast transient absorption spectroscopy or time-resolved photoluminescence decay [33-35], Schaller et al. observed different decaying components, which were associated to single-and multipleexcitons.

  Figure14(a))[START_REF] Kovalenko | Expanding the chemical versatility of colloidal nanocrystals capped with molecular metal chalcogenide ligands[END_REF][START_REF] Kovalenko | Colloidal nanocrystals with molecular metal chalcogenide surface ligands[END_REF][START_REF] Nag | Metal-free inorganic ligands for colloidal nanocrystals: S 2-, HS -, Se 2-, HSe -, Te 2-, HTe -, TeS 3 2-, OH -, and NH 2 as surface ligands[END_REF]. In a similar vein, Yang et al. tuned the external quantum efficiency by supressing Auger recombination through adjusting the composition of the outer and intermediate shells of core-shells structure[START_REF] Yang | High-efficiency light-emitting devices based on quantum dots with tailored nanostructures[END_REF]. Supran et al. also improved shortwave-infrared device performance by engineering PbS-CdS core-shell CQD in a type IV LED (organic-CQD-inorganic structure)[START_REF] Supran | High-performance shortwave-infrared light-emitting devices using core-shell (PbS-CdS) colloidal quantum dots[END_REF]. Other promising methods include atomic chlorine ligand passivation (Figure14(b)), leading to improved electronic transport[START_REF] Bae | Highly effective surface passivation of PbSe quantum dots through reaction with molecular chlorine[END_REF][START_REF] Zanella | Atomic ligand passivation of colloidal nanocrystal films via their reaction with propyltrichlorosilane[END_REF]. After completely removing the oleate ligand using ammonium sulphide, Zhang et al. reported that the remaining QDs were selfpassivated and interconnected through metal-sulfide bonding[START_REF] Zhang | Surfactant ligand removal and rational fabrication of inorganically connected quantum dots[END_REF]. Cate et al. observed the activation of carrier multiplication after infilling PbSe films with Al 2 O 3 or Al 2 O 3 /ZnO by atomic layer deposition using microwave conductivity transients[START_REF] St | Phonons do not assist carrier multiplication in PbSe quantum dot solids[END_REF].Kinder et al. assembled various solar cells that could reach an efficiency of 2.4% from a matrix of PbS QDs encapsulated in a CdS matrix, creating a quasi-superlattice[START_REF] Kinder | Fabrication of all-inorganic nanocrystals solids through matrix encapsulation of nanocrystal arrays[END_REF].In their inorganic halide ligand passivation method, Tang et al. successfully improved passivation due to surface sulfur dangling bonds by treating the quantum dots in a mixture of tetradecylphosphonic acid, CdCl 2 , and oleylamine (60 ºC, 5 min) (Figure14(c))[START_REF] Tang | Colloidal-quantum-dot photovoltaics using atomic-ligand passivation[END_REF]. This improved the stability and size distribution by removing certain surface defects. The spin-coating process took place in a glove box where each sublayer was then post-treated with solutions of cetyltrimethylammonium bromide (Br -), hexadecyltrimethylammonium chloride (Cl -), or tetrabutylammonium iodide (I -). These treatments improved the device performance significantly[START_REF] Ning | All-inorganic colloidal quantum dot photovoltaics employing solution-phase halide passivation[END_REF][START_REF] Zhitomirsky | N-type colloidal-quantum-dot solids for photovoltaics[END_REF]. Thon et al. also studied the evolution of mid-gap trap-states after such a passivation by ab initio calculations[START_REF] Thon | Role of bond adaptability in the passivation of colloidal quantum dot solids[END_REF]. Ip et al. performed further optimization, leading to a solar cell with 7.4% efficiency[START_REF] Ip | Hybrid passivated colloidal quantum dot solids[END_REF].

  (a))[START_REF] Brown | Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using a MoO3 interfacial layer[END_REF][START_REF] Gao | n-type transition metal oxide as a hole extraction layer in PbS quantum dot solar cells[END_REF]. Gao et al. also reported that hole injection could be controlled through Schottky barrier engineering[START_REF] Gao | Quantum dot size dependent J-V characteristics in heterojunction ZnO/PbS quantum dot solar cells[END_REF]. This could be achieved by align the work function of the metal with the energy bands of the PbS CQDs with specific sizes.Bakulin et al. observed that charge immobilization and poor charge separationwere caused by the presence of trap states with various depths (0.3-0.5 eV) below the conduction band (see Figure15(b))[START_REF] Bakulin | Charge trapping dynamics in PbS colloidal quantum dot photovoltaic devices[END_REF]. Using 1D and 3D models and taking into consideration the geometry of the device and its photoluminescence response, Zhitomirsky calculated the lifetime, trap density, mobility, and diffusion coefficient[START_REF] Zhitomirsky | Measuring charge carrier diffusion in coupled colloidal quantum dot solids[END_REF]. He found that state-of-the-art devices have an effective diffusion length of 80 nm.Recently, Whitham et al. demonstrated that charge localization can be greatlysuppressed by reducing the level of disorder in CQD films, by epitaxially connecting ordered PbSe nanocrystals[START_REF] Whitham | Charge transport and localization in atomically coherent quantum dot solids[END_REF].Surface passivation can have a considerable impact on transport in CQD films which is substantially mediated by intraband (in-gap) states[START_REF] Guyot-Sionnest | Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands[END_REF]. Using an optical field-effect transistor configuration, Nagpal and Klimov described the existence of a mid-gap band with different levels of participation to charge transport, depending on whether the device is in the dark or under illumination (Figures15(c)-(e))[START_REF] Nagpal | Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films[END_REF]. Using similar methods, Stadler et al. employed sub-threshold analysis to determine the trap distribution and map the density of state distribution in CQD films[START_REF] Stadler | Joint mapping mobility and trap density in colloidal quantum dots solids[END_REF].

Figure 1 .Figure 2 .

 12 Figure 1. NREL efficiency chart, emphasizing on the recent development in quantum dot solar cells. Adapted from the NREL efficiency chart [6]

Figure 3 .

 3 Figure 3. AM 1.5 (blue) solar power and proportion which is actually absorbed by a standard crystalline silicon solar cell (purple). The orange dashed line represents the energy carried per photon at a specific wavelength.

Figure 4 .

 4 Figure 4. Illustration of the band diagrams at a p-n junction under short-circuit (a) and open-circuit (b) conditions. Vbi, EFV and E

Figure 6 .

 6 Figure 6. First excitonic energy dependence on the crystal diameter of the effective mass model (dotted curve), the hyperbolic model (dashed curve), and the 4-band model (solid curve). Symbols are experimental data from various publications. Reprinted with permission from [17]. Copyright 1997 Optical Society of America.

Figure 7 .

 7 Figure 7. Schematic representation of excitonic levels located within the bandgap.

Figure 8 .

 8 Figure 8. Fast relaxation in continuous energy levels ( ) and conservation of crystal momentum ( ) in (a) bulk semiconductors versus (b) MEG in nanocrystals. Reprinted with permission from [29]. Copyright 2013 American Chemical Society.

Figure 9 .

 9 Figure 9. Theoretical improvement of the Shockley-Queisser limit due to the MEG efficiency P. PCE: Photoconversion efficiency. Reprinted with permission from [29].

Figure 10 .

 10 Figure 10. InAs islands grown at different V/III ratios [a) 15, b) 25, c) 35] and different temperatures [d) 400 °C, e) 450 °C, f) 500 °C]. Reprinted with permission from [50]. Copyright 2013 Institute of Physics. (g) Schematic of VLS mechanism. Reprinted with permission from [51]. Copyright 2010 American Chemical Society. Low (h) and high resolution (i) transmission electron microscopy images of Si nanowires. Reprinted with permission from [52]. Copyright 2005 Hanser.

Figure 11 .

 11 Figure 11. a) QDs capped with tri-n-octylphosphine oxide. Reprinted with permission from [65]. Copyright 2005 Royal Society of Chemistry. b) ZnCdSeS quantum dots

Figure 12 .

 12 Figure 12. Schematic description of three quantum dot-based solar cells along with band diagrams illustrating the charge dynamic within the device. Reprinted with permission from [69]. ITO stands for indium tin oxide. Copyright 2011 American Chemical Society.

Figure 13 .

 13 Figure 13. Representation of steric spacing between CQDs when using (a) oleic acid or (b) 3-mercaptopropionic acid. Colour codes are as follows: oxygen: red, carbon: grey, hydrogen: white and sulfur: yellow. Scanning transmission electron microscopy images of CQDs after MPA (c) and EDT (d) ligand exchange. Reprinted with permission from [115]. Copyright 2012 American Chemical Society.

Figure 14 .

 14 Figure 14. a) S 2-inorganic capping. Reprinted with permission from [123]. Copyright 2011 American Chemical Society. b) Atomic-chlorine passivation. Reprinted with permission from [126]. Copyright 2012 American Chemical Society. c) Atomic-ligand

Figure 15 .

 15 Figure 15. a) Cross-sectional scanning electron microscopy image and band structure of a device with MoO X selective contact. TMO stands for transition metal oxide, NC for nanocrystal, ITO for indium tin oxide, CB for conduction band and VB for valence band. Reprinted with permission from [147]. Copyright 2011 American Chemical Society. b) Push pump photocurrent method to probe trap states. Reprinted with permission from [149]. Copyright 2013 American Chemical Society. Mid-gap band (MGB) conduction (c) in the dark and (d) under illumination along with simulated density of states. Reprinted with permission from [153]. Copyright 2011 Nature Publishing Group. e) Band diagram showing a schematic density of states in the quantum dot film on the right. Reprinted with permission from [154]. Copyright 2013 American Chemical Society.

Table 1 .

 1 Dielectric constants ( ), bandgaps ( ) and reduced masses for electron () and holes ( ) of lead chalcogenides PbS, PbSe and PbTe.

		PbS	PbSe PbTe
		17	23	33
	(eV)	0.37 0.27	0.32
		0.080 0.040 0.024
		0.075 0.034 0.022

Acknowledgements

V.M. is an overseas researcher under Postdoctoral Fellowship of the Japan Society for the Promotion of Science (JSPS). A.N. would like to thank the Australian Renewable Energy Network (ARENA, project 6-F020) and the Australian Research Council (DE160100504) for funding as well as the ARC Centre of Excellence for Electromaterials (ACES) for support in the preparation of this manuscript.


Hybrid Methylammonium Lead Halide Perovskite Nanocrystals Confined in Gyroidal Silica Templates

Victor Malgras

a , Joel Henzie a , Toshiaki Takei a , Yusuke Yamauchi a

We successfully synthesized hybrid methylammonium lead halide nanocrystals confined in a gyroidal mesoporous silica template. The nanocrystals adopted the same crystal structure as their bulk perovskite counterparts, highlighting the ability to control the bandgap by tuning the pore-size of the template. Besides providing more accessible spaces for the materials to diffuse within the channels, the 3D mesh of the gyroidal template might also allow easier escape of the solvent and pinning of the particles at different points, thus promoting the formation of isolated nanoparticles over larger structures (e.g., nanowires, nanorods).

Lead halide-based perovskite quantum dots have drawn considerable attention in recent years. [1][2][3][4][5][6][7] In addition to the wellknown properties of bulk perovskites, these nanocrystals have unique attributes that make them attractive for various optoelectronic applications.

8-13

In particular, bromide-based perovskite nanocrystals have already shown outstanding emissive performance with quantum yields approaching 100%. 14 Typically, lead bromide perovskite quantum dots are synthesized by colloidal methods, which offer the possibility to functionalize the surface of the nanocrystals. The primary yield, however, consists of a mixture of nanoplatelets (product of interest) and more bulky structures, which need to be removed to achieve an adequately narrow size distribution. Most studies are based on quantum confinement in one dimension, 2,15 with considerable influence being imparted by the ligand group. 3,14,16,17 Growing nanocrystals inside an ordered mesoporous host matrix is an alternative, ligand-free route to examine the real effects of size reduction on the optical properties of nanoparticles. The mesoporous template offers a simple procedure to arrange and organize 0D and 1D nano-objects in a three-dimensional (3D) space. We previously demonstrated the ability to synthesize lead halide perovskite nanocrystals by growing them inside the pores of hexagonally ordered 1-D mesoporous silica templates. 18 In the present work, we extended this procedure to grow methylammonium lead bromide (MAPbBr 3 ) and iodide (MAPbI 3 ) perovskite nanocrystals in KIT-6 mesoporous templates. KIT-6 silica templates have a complex 3D gyroidal topology with a cubic Ia¯,3d space group (Scheme 1). We show that the optical properties of the nanocrystals are highly dependent on the pore size of the template, while maintaining the overall bulk-like crystal structure.

Scheme 1 Illustration of a) the empty KIT-6 silica template with channels adopting a gyroidal arrangement and of b) the complementary available space within the template. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

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The silica templates were synthesized according to the previously reported method, 19 with the average pore size and poreto-pore distance determined by the temperature of the hydrothermal reaction (T H ). 1. The SAXS patterns provide direct evidence of expansion of the mesoporous lattice with increasing temperature (Figure 1a). The gradual shift of the 211 peak toward lower angles indicates that the distance between the pores is increasing with higher T H . The detailed experimental method to synthesize MAPbBr 3 and MAPbI 3 perovskite nanocrystals inside the silica host (i.e. MAPbBr 3 @KIT-6 and MAPbI 3 @KIT-6, respectively) can be found in the ESI. It was of primary importance to determine if the KIT-6 maintains its 3D arrangement and does not collapse upon impregnating them with the guest materials. This was confirmed from the SAXS patterns, which remained mostly unchanged after filling the template, suggesting that the impregnation had a negligible effect on the integrity of the 3D ordering of the pores. The powder-XRD data reveals that the crystal structure of MAPbBr 3 nanocrystals matches the typical Pm¯,3m cubic structure of the bulk (Figure 1b). Similarly, the crystal phase of MAPbI 3 perovskites can be assigned to the I4cm tetragonal structure of their bulk counterparts (Figure 1c). Despite the strong contribution of amorphous silica (broad peak centered around 22°), the lower intensity and broader peaks observed for the smaller pores suggest a strong dependence of the particle size on the average pore diameter of the template. The absence of sharp peaks for the templates with smaller pores suggests that the Please do not adjust margins Please do not adjust margins formation of bulk perovskite at the surface of the silica porous particles is inexistent or negligible.

Figure 2a shows a TEM image of a typical empty KIT-6 template (here using T H = 60 °C) viewed along the [110] direction. The silica template is uniform and has long range order. 20 After infiltrating the template with perovskite, nanocrystals with a uniform size distribution can be observe throughout the sample. The nanocrystals are well-aligned inside the gyroidally arranged channels (see STEM image, Figure 2b). Single particles of both MAPbBr 3 and MAPbI 3 perovskite were imaged with HR-TEM (Figure 2c,d). Special care was taken to minimize the exposure of the crystal to the electron beam so that the structure was not damaged. In general, the size of the nanocrystals measured in TEM was similar to the average pore size of the template determined by BET (within the standard deviation). The perovskite crystal structure was further supported by measuring the lattice fringe distances (see Figure S2). The crystal is oriented in the [200] direction for MAPbBr 3 (d 200 = 2.94 ± 0.02 Å) and in the [220] direction for MAPbI 3 (d 220 = 3.1 ± 0.02 Å). Previous work determined that the perovskite lattice undergoes minor distortions when confined in small pores, however with a negligible effect on the absorption/reflection properties.

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In the UV-Vis-NIR reflectance spectra (Figure 3a), a clear band edge is present for each sample, providing a simple reading of the bangaps as the pore size is reduced. MAPbI 3 perovskite displays a gradual shift towards shorter wavelengths as the pore size decreases from 11.9 to 5.1 nm, highlighting the strong dependence of the optical response on the effective crystal size. The photoluminescence (PL) peak is displaced in a similar fashion, maintaining a Stokes shift of approximately 80 meV. The band edges of the MAPbBr 3 perovskite exhibit some inflections. This type of shoulder could be the signature of bulk perovskite or bigger crystals. Due to the gyroidal ordering of the mesoporous silica template, the local volume of the channel can vary in the vicinity of intersections, leaving enough space for crystals to grow further. This can give rise to a bimodal particle size distribution. Nevertheless, both reflectance band edges and PL peaks exhibit a clear blue shift for smaller pore sizes. Figure 3b highlights the pore size dependence of the bandgaps determined from the cut-off wavelenght for each band edge (also summarized in Table 1). It is presumed that the gradual shift is mostly influence by a combination of weak quantum confinement and surface effects.

To conclude, methylammonium lead halide perovskite nanocrystals were successfully grown inside a mesoporous silica host with gyroidally-arranged channels. The pores are tightly packed and expose a considerable surface area, which can favour efficient solvent removal while providing sufficient pinning site to promote the formation of isolated particles instead of bigger structures. Expanding the field of perovskite nanostructures hosted in mesoporous templates to KIT-6 geometry can be critical to improve material diffusion, which can be possibly blocked in the case 1D channels. The systematic study of the crystal structure and optical properties showed a consistent change as the particle size is reduced. On one hand, the crystal structure does not seem to be significantly distorted by the template regardless of the pore size. On the other hand, the clear shift of the reflection band edge suggests a strong dependence of the bandgap on the average particle size. This preliminary work represents another stride toward chemically integrated and interconnected 3D nanoarchitectures with strong potential for optoelectronic device applications.

Notes and references

Figure 1 a

 1 Figure 1 a) Pore size distribution of the KIT-6 prepared under various hydrothermal temperatures, calculated from adsorption-desorption isotherms using the BJH method. The data points are fitted with Gaussians (dotted lines). b) SAXS patterns for each template, highlighting the gradual shift of the 211 feature towards lower angles. The black dotted lines correspond to the patterns of the respective templates after impregnating with the hybrid perovskite nanocrystals. Powder-XRD patterns for each template containing MAPbBr 3 (c) and MAPbI 3 (d). The lower pattern (black line) is plotted from the calculated pattern of the bulk counter for each perovskite.

Figure 2

 2 Figure 2 TEM analysis of the perovskite nanocrystals synthesized in KIT-6 templates (using T H = 60°C). a) TEM image of the empty KIT-6 silica template. b) STEM image of the KIT-6 silica template after MAPbBr 3 perovskite impregnation. HR-TEM images of single particles of MAPbBr 3 (c) and MAPbI 3 (d) exhibiting crystal lattice fringes.

Figure 3 a

 3 Figure 3 a) UV-Vis-NIR reflectance and PL spectra for both MAPbBr 3 and MAPbI 3 perovskite impregnated in each KIT-6 silica template. b) Bandgaps as a function of the average pore size of the templates. The bandgaps were determined from the cut-off of the reflectance band edge. The dashed lines correspond to the bandgaps measured for polycrystalline bulk samples.

Table 1 a

 1 ) Summary of the pore size and bandgaps (E G ) for each sample.