Youngsang Park

Gérald Ferblantier

Abdelilah Slaoui

Aziz Dinia

Hyeonwook Park

Salh Alhammadi

Woo Kyoung Kim
email: wkim@ynu.ac.kr

Yb-doped zinc tin oxide thin film and its application to Cu(InGa)Se 2 solar cells

Keywords:

The use of rare earth elements with semiconductor materials has attracted immense interest due to their unique properties. In this study, we investigated the characteristics of an ytterbium (Yb)-doped zinc tin oxide (Yb:ZTO) thin film and its application as a potential down-converter of Cu(InGa)Se 2 (CIGS) thin-film solar cells. Yb:ZTO thin films were deposited by reactive sputtering of Zn and Sn metal with oxygen flow. A few pieces of Yb were embedded in a Zn metal target; thus Yb elements were supplied during Zn sputtering. The relative composition of Zn and Sn was controlled by changing the sputtering power (10-70 W) of Sn, in relation to the fixed sputtering power for Zn (70 W). In addition, the substrate temperature was varied from room temperature to 400 °C. It was confirmed that a smaller amount of Sn with lower sputtering power led to more incorporation of Yb into ZTO. X-ray photoelectron spectroscopy analysis confirmed the incorporation of Yb into ZTO, and photoluminescence measurement demonstrated Yb emission. Grazing incidence X-ray diffraction indicated the shift of ZTO emission peaks induced by the difference in the composition of Zn and Sn. Finally, CIGS solar cells with an Yb:ZTO layer were fabricated. The results suggested that cells with the highest Yb photoluminescence emission showed the highest short-circuit current density and cell efficiency.

Introduction

Chalcopyrite Cu(InGa)Se 2 (CIGS)-based thin film photovoltaic cells have continuously broken the record of cell conversion efficiency over the last few decades, which is currently at 23.35% (Solar Frontier 2019) [START_REF] Colthorpe | Solar Frontier Achieves CIS Thin-Film Lab Efficiency Record of 23.35%[END_REF] superior to polycrystalline Si (22.3% [START_REF] Green | Solar cell efficiency tables (Version 53)[END_REF]) and thin film CdTe (21.0% [START_REF] Green | Solar cell efficiency tables (Version 53)[END_REF]). The typical stack structure of CIGS cells follows the pattern substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al/grid, and various novel approaches on the individual stacked layers and interfaces have been explored to achieve a further increase in the cell performance [START_REF] Lee | Non-vacuum deposition of CIGS absorber films for low-cost thin film solar cells[END_REF][START_REF] Chu | Semi-transparent thin film solar cells by a solution process[END_REF][START_REF] Lee | Improvement in CIGS solar cell efficiency using a micro-prism array integrated with sub-wavelength structures[END_REF][START_REF] Garris | Electrical characterization and comparison of CIGS solar cells made with different structures and fabrication techniques[END_REF][START_REF] Salomé | CdS and Zn1-xSnxOy buffer layers for CIGS solar cells[END_REF]. Photon conversion effects including up-conversion, down-conversion, and down-shifting using modified transparent conductive layers have been studied as promising approaches to enhance the current collection of photovoltaic cells [START_REF] Chen | Lanthanide nanomaterials with photon management characteristics for photovoltaic application[END_REF][START_REF] Bouras | Optical and structural properties of Nd doped SnO 2 powder fabricated by the sol-gel method[END_REF][START_REF] Bouras | Structural, optical and electrical properties of Nd-doped SnO 2 thin films fabricated by reactive magnetron sputtering for solar cell devices[END_REF]. The photon up-conversion converts two or more low energy (e.g., typically lower than the band gap energy of light absorber) photons to a single high energy photon with energy higher than the band gap energy of absorber. Down-conversion is the process to convert a high energy photon to multiple lower energy photons of which energy is still higher than the band gap energy. Both up-and down-conversion processes reduce the loss of light absorption and thus enhance the short circuit current density of solar cells [START_REF] Verma | Review on up/down conversion materials for solar cell application[END_REF].

Several rare earth elements have been explored as potential candidates for a photon converter, including Nd [START_REF] Park | Nd-Doped SnO 2 and ZnO for application in Cu(InGa)Se 2 solar cells[END_REF][START_REF] Liu | Near-infrared luminescence of Nd 3+ and Tm 3+ ions doped ZnO nanocrystals[END_REF], Tm [START_REF] Liu | Near-infrared luminescence of Nd 3+ and Tm 3+ ions doped ZnO nanocrystals[END_REF], Eu [START_REF] Du | Efficient energy transfer in monodisperse Eu-doped ZnO nanocrystals synthesized from metal acetylacetonates in high-boiling solvents[END_REF], Pr [START_REF] Li | Synthesis and optical properties of Pr 3 + -doped ZnO quantum dots[END_REF], Er [START_REF] Bai | Enhanced upconverted photoluminescence in Er 3 + and Yb 3+ codoped ZnO nanocrystals with and without Li + ions[END_REF], Yb [START_REF] Bai | Enhanced upconverted photoluminescence in Er 3 + and Yb 3+ codoped ZnO nanocrystals with and without Li + ions[END_REF][START_REF] Balestrieri | Structural and optical properties of Yb-doped ZnO films deposited by magnetron reactive sputtering for photon conversion[END_REF], and Dy [START_REF] Gu | Structure evaluation and highly enhanced luminescence of Dy 3+ -doped ZnO nanocrystals by Li + doping via combustion method[END_REF] for a ZnO matrix. Rare earth element-doped SnO 2 has been studied as well [START_REF] Bouras | Optical and structural properties of Nd doped SnO 2 powder fabricated by the sol-gel method[END_REF][START_REF] Bouras | Structural, optical and electrical properties of Nd-doped SnO 2 thin films fabricated by reactive magnetron sputtering for solar cell devices[END_REF][START_REF] Bastami | Synthesis of nanosized (Co, Nb, Sm)-doped SnO 2 powders using co-precipitation method[END_REF][START_REF] Psuja | Influence of concentration and sintering temperature on luminescence properties of Eu 3+ :SnO 2 nanocrystallites[END_REF]. However, the study on the Zn-Sn-O (ZTO) matrix was hardly reported. ZTO material is inexpensive and chemically stable. Furthermore, it can be deposited by abundant and non-toxic sources and thus considered a promising candidate as a substitute for indium-doped tin oxide and intrinsic ZnO transparent conductors. In addition, it possesses both the optical transparency of ZnO and the excellent electrical conductivity of SnO 2 [START_REF] Minami | Highly transparent and conductive zinc-stannate thin films prepared by RF magnetron sputtering[END_REF]. It is known that the structural and optical properties of ZTO can be controlled by adjusting the composition of Zn and Sn in ZTO [START_REF] Ko | Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach[END_REF]. Recently, Dimitrievska et al. tried to dope Eu 3+ ions into nanocrystalline Zn 2 SnO 4 powders using a mechano-chemical solid-state reaction method [START_REF] Dimitrievska | Eu 3+ Doped wide band gap Zn 2 SnO 4 semiconductor nanoparticles: structure and luminescence[END_REF]. In this paper, the ytterbium (Yb)-doped ZTO (Yb:ZTO) thin film was investigated. In particular, the effects of the relative composition of Zn/Sn and substrate temperature during the co-sputtering process on Yb:ZTO were measured and optimized processing parameters determined. In addition, the optimized Yb:ZTO layer was applied to Cu(InGa)Se 2 (CIGS) thin-film solar cells. To the best of our knowledge, this is the first work to incorporate Yb into ZTO host, and investigate their compositional effects on optical properties, and apply the Yb:ZTO to solar cells.

Materials and methods

Yb:ZTO thin films with a thickness of approximately 100 nm were prepared on 500 μm-thick p-type Si wafers with (100) orientation and a 1 mm-thick glass substrate by the radio-frequency (RF) reactive-magnetron sputtering method with flowing Ar and O 2 at 8 sccm and 3 sccm, respectively. The Yb-doped Zn (Yb:Zn) and pure Sn targets were simultaneously sputtered at the sputtering power of 70 W (fixed) and 10-70 W(varied), respectively. The Zn (or Yb:Zn) and Sn targets have a 2 inch-diameter disk shape. The Yb:Zn alloy target was prepared by implanting several pieces of small Yb pure metal (99.99%, 70 g each) onto a pure Zn target. The substrate temperature varied from 100 to 400 °C during initial experiments (to be optimized afterward.) The information on Sn sputtering power (10-70 W) and substrate temperature (100-400 °C) can be found in the sample ID. For example, sample S10T3 represents 10 W power and 300 °C substrate temperature. During the sputtering process, the working pressure was maintained at 3.4 mTorr (∼0.4533 Pa) and the substrate was located approximately 12 cm above the target. For lateral homogeneity of film thickness, the substrate was rotated. The deposition rate was varied from approximately 3 nm/min (for S10T3) to 20 nm/min (for S70T3) depending on the sputtering power of Sn target, which was estimated by measuring the thickness of film after each deposition. Two halogen lamps were located behind the sample holder for heating the substrate.

The Yb:ZTO thin film has been employed as a transparent conducting oxide to replace an i-ZnO layer in a conventional CIGS solar cell structure of glass/Mo/CIGS/CdS/i-ZnO/Al:ZnO/grid, as shown in Fig. 1. A p-type CIGS light absorbing layer with a thickness of 1.5-1.7 μm was fabricated by the reaction of CuGa/In bilayer metal precursors with H 2 Se and H 2 S gas. Bilayer CuGa/In precursors were deposited on Mo-coated glass by the sequential DC sputtering of a CuGa alloy (∼24 wt% Ga) and pure In targets, yielding a total CuGa/In thickness of 500 -600 nm and atomic composition of Cu/( Ga + In) = 0.85 -0.95 and Ga/(Ga + In) = 0.2 -0.3. The selenization of CuGa/In precursors by H 2 Se gas was performed at 480-500 °C for 25-35 min, and then followed by the sulfurization by H 2 S gas at 550-580 °C for 60-90 min.

The n-type buffer layer of CdS (∼70 nm) was deposited by chemical bath deposition. Yb:ZTO with a thickness of 100 nm was then deposited on CdS as a photon converter layer, followed by the DC sputter-deposition of n-type Al:ZnO (AZO) with a thickness of about 400 nm. Finally, a Ni/Ag front grid was deposited by electron beam evaporation at room temperature.

The crystallography of the as-deposited Yb:ZTO thin films was analyzed by grazing incidence X-ray diffraction (GI-XRD; PANalytical X'Pert PRO MPD) with CuKα 1 radiation (λ = 0.154056 nm) at an incidence angle of ω = 0.4°. The thickness of the thin films was measured using a spectroscopic ellipsometer (HORIBA Uvisel™ Lt M200 FGMS) in the range of 210-880 nm. The doping concentration of Yb in the ZTO thin films was estimated by X-ray photoelectron spectroscopy (XPS, K-Alpha) surface scans at operating conditions of 400 μm spot size, 200 eV pass energy, 0.1 eV step size, and the constant analyzer energy (CAE) mode. A photoluminescence (PL) measurement was carried out in the 300-1050 nm range of wavelength using a frequency-tripled neodymium-doped yttrium aluminum garnet (Nd-YAG) laser with a wavelength of 355 nm. PL signals enter into an optical fiber and then are analyzed by a multi-channel CCD. The transmittance of the thin film was measured by a spectrophotometer (U-Perkin-Elmer Lambda 950) in the wavelength range of 300-1300 nm. The current-voltage (I-V) characteristics of the CIGS solar cells were identified using a solar simulator (K3000 XE55 Solar Cell I-V Measurement System) with AM1.5 irradiation.

Results and discussion

Effect of ZTO composition

Fig. 2 presents the GI-XRD (ω = 0.4°) patterns of Yb:ZTO thin films deposited at different Sn sputtering powers of 10-70 W, with a fixed Yb:Zn alloy sputtering power of 70 W and substrate temperature of 300 °C. The XRD patterns of all samples except S10T3 look quite broad due to the probable formation of ZnO-SnO 2 or Zn-Sn-O alloys, which have been reported to be amorphous below approximately 450 °C [START_REF] Ko | Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach[END_REF][START_REF] Moriga | Transparent conducting amorphous Zn-Sn-O films deposited by simultaneous dc sputtering[END_REF]. However, S10T3 samples prepared using the lowest sputtering power (10 W) of the Sn target show sharp hexagonal ZnO (002) and (103) peaks by XRD analysis [START_REF] Olatunji | Tramadol hydrochloride delivery by regenerated cellulose nanofiber-TiO 2 -ZnO composites[END_REF][START_REF] Sehar | Superior dye degradation using SnO 2 -ZnO hybrid heterostructure catalysts[END_REF], as summarized in Table 1; they also have only 1.31 at.% Sn with Sn/(Zn + Sn) ∼ 0.02. Apparently, the increased Sn sputtering power resulted in the decreased intensity of the ZTO peak and its slight shift to a lower 2θ position, demonstrating the formation of more amorphous and higher Sn-containing ZTO. The low crystallinity of ZTO is partly attributable to the formation of stress by the different sizes of Zn and Sn ions [START_REF] Ko | Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach[END_REF]. In addition, increased Sn concentration may lead to the formation of SnO (JCPDS: 06-0395), SnO 2 (JCPDS: 41-1445), and other oxygen-rich ZTO compounds such as spinel Zn 2 SnO 4 (JCPDS: 24-1470) and ilmenite ZnSnO 3 (JCPDS: 52-1381) [START_REF] Moriga | Transparent conducting amorphous Zn-Sn-O films deposited by simultaneous dc sputtering[END_REF]. However, the Yb doping concentration decreased with increased Sn sputtering power.

The chemical composition of ZTO alloys has been estimated by XPS analysis. Fig. 3 presents peaks near 185 eV corresponding to the Yb 4d 5/2 core level of atoms bound to oxygen, confirming the formation of Yb-Zn-Sn-O structure rather than the elemental Yb of which bind energy is near 182-183 eV [START_REF] Briggs | Handbook of X-Ray Photoelectron Spectroscopy[END_REF]. Even though the Yb ion can exist in a 2 + oxidation state, it is hardly detected by XPS due to the very small difference in binding energy [START_REF] Flage-Larsen | Valence band study of thermoelectric Zintl-phase SrZn 2 Sb 2 and YbZn 2 Sb 2 : X-ray photoelectron spectroscopy and density functional theory[END_REF]. As shown in Fig. 3 and Table 1, the doping concentration of Yb in ZTO monotonically decreases with the sputtering power of the Sn target. In general, during doping, the crystalline structure of the host material is preferred to the amorphous structure because there are more available doping sites in crystalline materials than amorphous ones. For example, the wurtzite ZnO structure has empty tetrahedral and octahedral sites, which can be occupied by dopants, i.e., interstitial doping [START_REF] Hahn | Zinc oxide nanostructures and their applications[END_REF]. In addition, the grain boundary of a polycrystalline structure can be used as a penetration path for dopants [START_REF] Flage-Larsen | Valence band study of thermoelectric Zintl-phase SrZn 2 Sb 2 and YbZn 2 Sb 2 : X-ray photoelectron spectroscopy and density functional theory[END_REF]. However, the reduced crystallinity of the ZTO matrix by adding Sn, and thus increasing structural disorder, may restrict the incorporation of Yb.

The oxygen content approximately increases with increased Sn incorporation due to a higher oxidation state of Sn than Zn, i.e., SnO 2 vs. ZnO. Considering the stoichiometry of each film, S10T3 is close to ZnO; with increasing sputtering Sn power the film composition changes successively to Zn 3 SnO 4 , Zn 2 SnO 3 , ZnSnO 3 , and SnO 2 from the S20T3 to the S70T3 samples, respectively. From XPS results, it was confirmed that Yb was incorporated from the crystalline structure to amorphous structure. The optical activation of the Yb:ZTO thin films was investigated using PL spectroscopy. The PL spectra of Yb:ZTO thin films prepared by different Sn sputtering power were obtained using a 355 nm Nd:YAG laser. These are shown in Fig. 4. The strong and sharp peaks detected at 355 nm and 710 nm correspond to the laser source. A very weak intensity observed from the S10T3 sample near 380 nm in the UV region may result from band-to-band transitions. The broad peaks in the range of 500-850 nm of the visible light region identified from most of the samples are believed to be produced mainly by defect states. Even though their origin is still controversial, they are presumably associated with intrinsic defects such as oxygen vacancies and interstitial atoms [START_REF] Flage-Larsen | Valence band study of thermoelectric Zintl-phase SrZn 2 Sb 2 and YbZn 2 Sb 2 : X-ray photoelectron spectroscopy and density functional theory[END_REF]. For the S20T3 sample, a sharp and strong peak was detected near 975 nm; it is presumably associated with the radiative transition of the 2 F 5/2 excited state to the 2 F 7/2 ground state of the Yb 3+ ion. A satellite broad peak located at about 1010 nm is considered energy level splitting induced by the Stark effect in the Yb level. A similar observation was reported by Balestrieri et al. who studied a ZnO thin film Yb-doped by reactive sputtering [START_REF] Balestrieri | Structural and optical properties of Yb-doped ZnO films deposited by magnetron reactive sputtering for photon conversion[END_REF]. The Yb characteristic peak near 975 nm was also found in the S30T3 sample with a relatively weak intensity, but not detected in the other samples (S30T3 to S70T3) prepared with higher Sn sputtering power.

A more detailed comparison of peaks in the visible light region revealed that the peak intensity of the S10T3 and S20T3 samples was highest and similar to each other, and the intensity of the others decreased with increased Sn incorporation. This peak attenuation can be explained by several contributions. Firstly, the increase of Sn concentration reduces the crystallinity of ZTO, as shown in Fig. 2, and atomic disorder in an amorphous compound can produce the defect states causing non-radiative recombination [START_REF] Karmakar | Structural; morphological; optical and magnetic properties of Mn doped ferromagnetic ZnO thin film[END_REF][START_REF] Pita | Energy Transfer from Silicon Nanocrystals to Er 3+ Ions Embedded in Silicon Oxide Matrix, Nanocrysals[END_REF]. Similarly, Karmakar et al. reported that the disorder introduced in a Mn-doped ZnO thin film led to the decrease of crystallinity and PL intensity [START_REF] Karmakar | Structural; morphological; optical and magnetic properties of Mn doped ferromagnetic ZnO thin film[END_REF]. In our study, as the Sn concentration increases, the more amorphous condition of the ZTO thin film can generate a dangling bond state yielding the competitive pathways for transitions between energy levels, and thus restricting PL emission in the visible light region, as schematically represented in Fig. 5. Secondly, the higher Sn incorporation into the ZTO structure increases oxygen concentration in ZTO as well, and reduces oxygen vacancies, which are usually responsible for the PL process. Lastly, it is assumed that the expansion in the energy band gap of the ZTO film with increased Sn concentration can limit the band-to-band excitation causing the reduction of PL intensity. For example, the estimated energy band gap of the S30T3 and S40T3 samples (S30T3: 3.80 eV, S40T3: 3.70 eV) is larger than that of the laser (3.49 eV ≅ 355 nm) and thus the excitation from valence band to conduction band can be decreased. The drastic decrease of peak intensity in the S30T3 sample (with the highest band gap ∼ 3.8 eV) may be due to this reason.

As shown in Fig. 6, the S40T3 and S70T3 samples have a decreased band gap and increased oxygen vacancy, but showed lower PL intensity compared with S30T3, which suggests that the increased density of the dangling bond defect from an amorphous structure restricts the energy transfer. In general, the O1s peak can be deconvoluted into three peaks; a peak with the highest binding energy (∼532.3 eV) corresponds to oxygen in hydroxide, and one with middle energy (∼531.7 eV) is for oxygen in an oxygen deficient region. The peak with the lowest energy (∼530.3 eV) belongs to oxygen surrounded by Zn and Sn atoms. In this study, it was confirmed that O1s XPS spectra of all Yb:ZTO films were deconvoluted into only two characteristic peaks. The O m peak near low binding energy is commonly related to oxygen binding with metal such as Zn, Sn atom, and the O V peak near high binding energy is to oxygen in an oxygen deficient region [START_REF] Zhao | Impacts of Sn precursors on solution-processed amorphous zinc-tin oxide films and their transistors[END_REF]. The O V /(O m + O V ) ratio with Sn sputtering power is shown in Fig. 6. However, the lowest peak intensity was similarly observed from S70T3 samples having a lower energy band gap (S70T3: 3.10 eV) than the laser, which suggests that the effect of the band gap on the attenuation of the PL peak may not be dominant.

The Yb peak at 975 nm had the highest intensity in sample S20T3, and the intensity drastically decreased with increased Sn amount (S30T3 to S70T3). However, the sample with the smallest Sn amount (S10T3) showed much lower intensity compared with S20T3 presumably due to a concentration quenching effect. When the amount of optically active rare-earth ions increases to a certain level, the excitation energy is not transferred through a radiative transfer mechanism any longer; instead, it is transferred non-radiatively between dopant ions resulting in a reduced luminescence efficiency. Similar concentration quenching effects were also reported by other research groups in studies on Tb-doped SnO 2 [START_REF] Hild | Structural and photoluminescence properties of evaporated SnO 2 thin films doped with rare earths[END_REF] and Nd-doped SnO 2 [START_REF] Rinnert | Photoluminescence of Nd-doped SnO 2 thin films[END_REF] thin films. The attenuation of the Yb characteristic PL peak with increased Sn concentration can be attributed to the following reasons. Firstly, as similarly observed in the visible light region, more Sn incorporation into ZTO tends to increase the amorphous nature of the host compound and, thus, the density of dangling bond states to capture carriers, thereby restricting energy transfer from host to Yb ions. Secondly, the reduced density of internal defect states with increased Sn concentration can limit the PL efficiency of Yb, as illustrated in Fig. 5. It has been reported that the intrinsic or extrinsic defect states of the host material can enhance the energy transfer from the host to the rare earth ions [START_REF] Layek | Synthesis of rare-earth doped ZnO nanorods and their defect-dopant correlated enhanced visible-orange luminescence[END_REF]. Salarie et al. tried to enhance light emission using cooperative transitions with defect states produced by oxygen vacancies in the host material [START_REF] Salari | A significant enhancement in the photoluminescence emission of the Mg doped ZrO 2 thin film by tailoring the effect of oxygen vacancy[END_REF]. Thirdly, the reduced Yb amount with increased Sn concentration as analyzed by XPS (Table 1) might be another reason for the reduced Yb PL intensity. However, considering the small difference of Yb concentration, the effect of variation in the energy states of host materials, such as defect states and dangling bond states, will be more dominant than that of Yb concentration.

The PL results in Fig. 4 suggest that the down-conversion effect has been confirmed by the PL emission in both visible light and the near-infrared (NIR) region with the 355 nm incident UV laser. In ad dition, it should be noted that the scale of the y-axis for the Yb peak (at 975 nm) is 10 times larger than that for the peak in the visible light region, thus illustrating that the NIR PL emission of the Yb peak is much stronger than that in visible light. This suggests that ZTO can be a good host material for efficient energy transfer, and Yb light with a wavelength of 975 nm (∼1.27 eV) may be applicable for a CIGS-based (Eg ∼ 1.2 eV) thin film solar cell.

The optical transmittance of Yb:ZTO thin films prepared onto glass substrates was investigated in the range of 300-1300 nm wavelength. As shown in Fig. 7, most films, notably except S70T3, exhibited considerable transmittance of over 80% in the visible region. With the increase of Sn sputter power from 10 W to 30 W, the left absorption edge of the curve, i.e., the shortest wavelength region, shifted toward the left while it moved to the opposite direction with further increase of Sn sputter power from 30 W to 70 W. The left shift of the absorption edge with increased Sn sputter power (10-30 W) may be attributed to the increased energy band gap by the Burstein-Moss effect [START_REF] Young | Growth and characterization of radio frequency magnetron sputter-deposited zinc stannate, Zn 2 SnO 4 thin films[END_REF]; that is, the energy band gap of ZTO increases by adding Sn as an electron donor because Sn has four valence electrons compared to Zn having two valence electrons. In contrast, the right shift of the absorption edge in the higher Sn sputter power (30-70 W) can be explained by structural properties. As shown in Fig. 2, the enhanced incorporation of Sn into ZTO films by increasing Sn sputter power can make ZTO films more amorphous; thus, this leads to the formation of a localized band tail state, due to the structural disorder, yielding a reduced band gap and lower transmittance [START_REF] Ko | Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach[END_REF].

As shown in the inset of Fig. 7, the energy band gap of ZTO films was estimated by using the Tauc plots [START_REF] Tauc | optical properties and electronic structure of amorphous germanium[END_REF] based on the absorption edge of the transmittance curve, assuming the direct transition between valence and conduction bands. The estimated values of the energy band gap were in the range of 3.10-3.80 eV, which is similar to those reported by other research groups [START_REF] Young | Growth and characterization of radio frequency magnetron sputter-deposited zinc stannate, Zn 2 SnO 4 thin films[END_REF][START_REF] Chang | Growth of amorphous Zn-Sn-O thin films by RF sputtering for buffer layers of CuInSe 2 and SnS solar cells[END_REF]. Young et al. reported that crystalline ZTO thin films deposited by the sputtering technique showed a direct optical band gap in the range 3.3-3.9 eV, which was influenced by Burstein-Moss shift effect and structural dissimilarity of the ZTO thin films [START_REF] Young | Growth and characterization of radio frequency magnetron sputter-deposited zinc stannate, Zn 2 SnO 4 thin films[END_REF]. As mentioned in the previous paragraph, the band gap became wider with increased Sn sputter power from 10 to 30 W (Eg = 3.28-3.80 eV), and then reduced with further increase from 30 to 70 W (Eg = 3.80 to 3.10 eV).

Effect of substrate temperature

It has been reported that the deposition temperature has a strong influence on photoluminescence of rare-earth atoms in host materials such as ZnO and SnO 2 [START_REF] Bouras | Structural, optical and electrical properties of Nd-doped SnO 2 thin films fabricated by reactive magnetron sputtering for solar cell devices[END_REF][START_REF] Balestrieri | Structural and optical properties of Yb-doped ZnO films deposited by magnetron reactive sputtering for photon conversion[END_REF]. To investigate the effect of substrate temperature during the sputtering process of Yb:ZTO, the substrate temperature was varied from 100 to 400 °C while the sputtering power of Zn and Sn were held constant at 70 W and 20 W, respectively. Considering the subsequent fabrication of working solar cells, the maximum temperature was limited to 400 °C.

The structural and compositional properties of Yb:ZTO films were investigated by GI-XRD and XPS analyses. The results of the GI-XRD, shown in Fig. 8, revealed that there is no significant difference in shape and intensity of the characteristic ZTO peak at 2θ = 32-34°, which is not strong and thus nearly amorphous. It has also been reported that ZTO films maintain an amorphous nature up to 500-600 °C [START_REF] Lee | Characterization of zinc-tin-oxide films deposited by radio frequency magnetron sputtering at various substrate temperatures[END_REF][START_REF] Choi | Rapid thermal annealing effect on the characteristics of ZnSnO 3 films prepared by RF magnetron sputtering[END_REF]. Choi et al. suggested that the stability of the amorphous ZTO phase below 600 °C is attributed to the immiscible system of pseudo-binary ZnO and SnO 2 [START_REF] Choi | Rapid thermal annealing effect on the characteristics of ZnSnO 3 films prepared by RF magnetron sputtering[END_REF]. As temperature increased from 100 to 400 °C, the characteristic peak of ZTO near 2θ = 32-34°shifted slightly to a lower 2θ position, presumably due to atomic rearrangement. It can be assumed that the increased temperature can supply thermal energy for further rearrangement of atoms within the amorphous structure. Furthermore, higher temperature can cause atomic vacancies to be occupied, thus yielding the lattice expansion and the left shift of XRD peak. The composition estimated by XPS spectra of the Yb:ZTO films prepared at different substrate temperature, i.e., the deposition temperature, is summarized in Table 2. The overall composition of Sn/(Zn + Sn) for thin films deposited at a substrate temperature in the range of 100-400 °C was 0.239-0.253, indicating nearly a stoichiometric Zn 3 SnO 4 ternary phase.

As shown in Fig. 9 and summarized in Table 2, the incorporation of Yb into ZTO decreased at the highest substrate temperature of 400 °C, compared to 100 °C and 300 °C. This can be explained by the decrease of available doping sites in the ZTO host because the atomic rearrangement of active Zn and Sn atoms is enhanced at high temperature, the doping sites are preoccupied. Therefore, it can be assumed that a too-high temperature may not be beneficial to the incorporation of Yb into the ZTO compound under a crystallization phase, due to its immiscible nature (see Fig. 10).

As displayed in Fig. 11, PL measurements have been carried out to investigate the effect of deposition temperature on the photoluminescence properties of Yb:ZTO films. Apparently, an increase in temperature from 100 to 400 °C did not generate new peaks in the measured wavelength range of 300-1050 nm. As temperature increased from 100 to 400 °C, the defect band near the visible light region decreased. The intensity of the Yb PL peak at 975 nm was also reduced. This decrease in visible emission can be explained by sev eral contributions. As indicated in the previous XPS result, oxygen vacancies, which induce visible PL emission, were decreased at 300 °C, and thus the PL intensity decreased as well. In addition, the area of grain boundaries increased, due to the formation of ZnO grains. The grain boundaries usually cause non-radiative recombination; thus radiative emission was relatively decreased [START_REF] Matsumoto | Correlation between grain size and optical properties in zinc oxide thin films[END_REF]. The significant decrease at 400 °C may be attributed to the high band gap of the film (∼4.0 eV), which is larger than the laser excitation energy, thus reducing the excitation from the valence band. This increase in band gap will be discussed further in the next section. The decrease in the intensity of Yb PL emission is proportional to the amount of electrons in radiative defect transition. At 300 °C, while the number of excited electrons might be larger than that at 100 °C, due to a decreased band gap, it seems that energy transfer to the Yb atoms decreased because of a non-radiative defect center. At 400 °C, the number of excited electrons decreased further, resulting in the decreased transfer to the Yb atoms. Furthermore, the decreased number of Yb atoms at 400 °C could contribute to the decreased PL emission of Yb. The PL results indicate that despite the same number of Yb ions in the film, the properties of the ZTO host strongly influence its PL emission. From the PL measurement, a high PL emission is favorable in the ZTO host at a relatively low temperature within the amorphous state.

To investigate the effect of deposition temperature on transmittance of Yb:ZTO films, the transmittance was measured in the 300-1300 nm range, as displayed in Fig. 12. The values of the corresponding energy band gap were extracted by using a Tauc plot as in the figure inset. The transmittance of films with the deposition temperature of 100-400 °C showed over 80% in the entire wavelength region. It was revealed that the energy band gap of Yb:ZTO film slightly decreased (3.45-3.33 eV) or remained the same as the deposition temperature was raised from 100 to 300 °C, but it noticeably increased up to 4.0 eV as temperature reached 400 °C. This dramatic increase might be attributed to the increased oxygen vacancies with increased temperature. It was reported that oxygen vacancies in a TCO thin film act as donor-producing excess carriers, changing the energy band gap by means of the Burstein-Moss (BM) effect [START_REF] Tuna | High quality ITO thin films grown by dc and RF sputtering without oxygen[END_REF]. In addition, the increase in band gap of Yb:ZTO at a deposition temperature of 400 °C resulted in the shift of the transmittance plot to the low wavelength region, as coincidently shown in Fig. 12. It is well known that high deposition temperatures in semiconductor thin films can produce oxygen deficient films [START_REF] Tuna | High quality ITO thin films grown by dc and RF sputtering without oxygen[END_REF][START_REF] Hai-Bo | Investigation of oxygen vacancy and interstitial oxygen defects in ZnO films by photoluminescence and X-ray photoelectron spectroscopy[END_REF]. This is because oxygen desorption occurs in metal oxide materials at high temperatures due to the low formation energy of oxygen vacancies. At 300 °C, however, the formation of oxygen vacancies was restricted, presumably due to the formation of ZnO.

Application to solar cells

Based on the above study of ZTO properties by changing Sn sputter power and substrate temperature during co-sputtering of Sn and Zn in a flowing oxygen environment, we determined the potentially optimized conditions of 15 W Sn sputter power and 100 °C substrate temperature. These conditions were employed to deposit another set of Yb:ZTO films. As shown in Fig. 13, the Yb PL peak intensity of the Yb:ZTO film deposited at these sputtering conditions is greater than those of the other films in the study, suggesting better energy transfer efficiency. Therefore, these sputtering conditions were employed to deposit an Yb:ZTO film for the fabrication of a glass/Mo/CIGS/ CdS/Yb:ZTO:AZO solar cell. The current-voltage characteristics of this cell are compared with those of a conventional glass/Mo/CIGS/ CdS/i-ZnO/AZO solar cell in Fig. 14. It is interesting to note that the overall cell efficiency of CIGS cell with Yb:ZTO (η = 10.3%) is nearly equivalent to that of a conventional CIGS cell with i-ZnO (η = 11.2%). It seems that the slight increase in Jsc and Voc of the Yb:ZTO-CIGS cell was offset by the loss of fill factor (i.e., 51.2%-45.7%). Slight increase of V OC (i.e., 0.568-0.574 V) might be resulted from the increase of shunt resistance (R SH ) (i.e., 648 to 771 Ωcm 2 ). The improved Jsc (38.5-39.2 mA/cm 2 ) can be explained by down-conversion effect of Yb:ZTO layer leading to more absorption and collection of photons demonstrated by EQE results later.

The external quantum efficiency plot of Fig. 15 confirmed that the Yb:ZTO-CIGS cell demonstrated enhanced quantum efficiency, com pared to the conventional i-ZnO-CIGS cell. This is particularly so in the UV region (inset of Fig. 15), presumably because of the photon conversion effect. To make sure that the increase of quantum efficiency was due to the down-conversion effect but not the increased light absorption, the absorption by the Yb:ZTO and i-ZnO layers was compared by means of reflectance and transmittance measurements. It was observed that the reflectance and transmittance of the Yb:ZTO films were greater than those of i-ZnO film (see Supporting Data), thus supporting the claim that the enhanced quantum efficiency was not caused primarily by increased light absorption. It was also confirmed that the conductivity of a film was not improved by adopting Yb:ZTO, as noticed by the crossover of the EQE plot at the wavelength of 900-1000 nm. Therefore, the detailed study on the other reasons for enhanced EQE suggested that the Yb:ZTO layer acts as a photon converter. Further optimization of Yb doping, such as Yb concentration, deposition temperature, and film thickness may improve the optoelectronic properties of the Yb:ZTO film and the corresponding CIGS device performance.

Conclusion

Yb:ZTO films were prepared by reactive co-sputtering of Yb:Zn and Sn targets, and their crystallographic and optoelectronic properties were thoroughly investigated by changing Sn sputtering power (10-70 W) to determine ZTO composition, and substrate temperature (100-400 °C). It was demonstrated that Yb PL emission is significantly affected by the defect state in the ZTO host; it can be controlled by the amount of Sn in the ZTO and the substrate temperature during deposition. The results of the PL analysis suggested that the efficiency of energy transfer from host to Yb ions was highest for a sample prepared at a substrate temperature of 100 °C, in the range of 100-400 °C. As this study shows, the highest performance Yb:ZTO-CIGS solar cell has been fabricated by adopting Yb:ZTO deposited at the suggested optimum of 15 W Sn sputter power and 100 °C substrate temperature. The potential photon converting effect of Yb:ZnO film was supported by the observation of increased quantum efficiency without an increase of light absorption.

Fig. 1 .

 1 Fig. 1. Schematic diagram of CIGS solar cell structure: (left) conventional structure and (right) new structure with Yb:ZTO fabricated in this study.

Fig. 2 .

 2 Fig. 2. GI-XRD patterns of the Yb:ZTO thin films deposited by different Sn sputtering powers in the range of 10-70 W.

Fig. 3 .

 3 Fig. 3. Yb 4d 5/2 XPS spectra of Yb:ZTO films as a function of Yb:Sn sputtering power.

Fig. 4 .

 4 Fig. 4. PL spectra of Yb:ZTO films at different Sn sputtering power.

Fig. 5 .

 5 Fig. 5. Schematic energy band diagram of absorption and emission transitions in Yb:ZTO.

Fig. 6 .

 6 Fig. 6. Ratio of oxygen vacancy concentration estimated by O1s XPS spectra, and band gap energy (Eg) calculated by transmittance for Yb:ZTO films as a function of Sn sputtering power.

Fig. 7 .

 7 Fig. 7. UV-vis-NIR optical transmittance of Yb:ZTO thin films for different Sn sputtering power, and corresponding energy band gaps estimated by Tauc plot (inset).

Fig. 8 .

 8 Fig. 8. GI-XRD patterns of the Yb:ZTO thin films deposited at different substrate temperatures: 100 °C (T1), 300 °C (T3), and 400 °C (T4).

Fig. 9 .

 9 Fig. 9. Yb 4d 5/2 XPS spectra of Yb:ZTO films deposited at different substrate temperatures: 100 °C (T1), 300 °C (T3), and 400 °C (T4).

Fig. 10 .

 10 Fig. 10. Ratio of oxygen vacancy concentration (diamonds) estimated by O1s XPS spectra and band gap (circles) calculated by transmittance for Yb:ZTO films deposited at different substrate temperatures: 100 °C (T1), 300 °C (T3) and 400 °C (T4).

Fig. 11 .

 11 Fig. 11. PL spectra of Yb:ZTO films deposited at different substrate temperatures.

Fig. 12 .

 12 Fig. 12. UV-vis-NIR optical transmittance of Yb:ZTO thin films for different deposition temperatures, and corresponding energy band gaps estimated by Tauc plot (inset).

Fig. 13 .

 13 Fig. 13. PL spectra of Yb:ZTO films deposited at different sputter conditions: Sn sputter power of 10, 15, 20 W and substrate temperature of 100 and 300 °C.

Fig. 14 .

 14 Fig. 14. Current-voltage characteristics of Yb:ZTO-incorporated CIGS solar cell and reference (i-ZnO) cell.

Fig. 15 .

 15 Fig. 15. External quantum efficiency plot for Yb:ZTO incorporated CIGS solar cell and reference (i-ZnO) CIGS cell.

Table 1

 1 Composition estimated by XPS spectra of Yb:ZTO films deposited at different Sn sputtering power.

							Band
		Zn 2p	Sn 3d		O 1s	Yb 4d	gap
	Sample	(at.%)	(at.%)	Sn/(Zn + Sn)	(at.%)	(at.%)	(eV)
	S10T3	52.2	1.31	0.02	46.2	0.25	3.28
	S20T3	40.8	12.8	0.24	46.2	0.18	3.33
	S30T3	31.2	17.0	0.35	51.6	0.17	3.80
	S40T3	21.6	21.0	0.49	57.2	0.12	3.70
	S70T3	4.57	22.2	0.83	73.1	0.09	3.10

Table 2

 2 Composition estimated by XPS spectra of Yb:ZTO films deposited at different substrate temperatures.

		Substrate
	Sample	Temp. (°C)	Zn 2p (at.%)	Sn 3d (at.%)	Sn/(Zn + Sn)	O 1s (at.%)	Zn:Sn:O	Yb 4d (at.%)	Band gap (eV)
	S20T4	400	37.6	12.4	0.248	49.9	3 (+0.03):1:4 (+0.02)	0.16	4.00
	S20T3	300	40.8	12.8	0.239	46.2	3 (+0.18):1:4 (-0.40)	0.18	3.33
	S20T1	100	39.6	13.4	0.253	46.7	3 (+0.05):1:4 (-0.53)	0.18	3.45

Journal of Alloys and Compounds xxx (xxxx) xxx-xxx

Acknowledgment

This study was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A6A1031189) and "Human Resources Program in Energy Technology" of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20174030201760).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jallcom.2019.152360.


Karima Bouras
email: karima.bouras@cea.fr

Guy Schmerber

Gerald Ferblantier

Damien Aureau

Hyeonwook Park

Woo Kyoung Kim

Hervé Rinnert

Aziz Dinia

Abdelilah Slaoui

Silviu Colis
email: colis@ipcms.unistra.fr

Cu(InGa)Se 2 Solar Cell Efficiency Enhancement Using a Yb-Doped SnO x Photon Converting Layer

Keywords: CIGS solar cells, Yb-doped SnO x, down-shifting, energy transfer, sputtering, XPS

I. INTRODUCTION

One of the major issues governing the performances of solar cells is the mismatch between the incident solar radiation spectrum and the absorption of the cell [START_REF] Hirst | Fundamental losses in solar cells[END_REF][START_REF] Chen | Lanthanide nanomaterials with photon management characteristics for photovoltaic application[END_REF]. Because of the weak absorption coefficient of most of the semiconductor absorbers in the UV region, the energy supplied upon absorption of high-energy UV photons cannot be fully converted to electricity and most of energy is lost as heat. This process gives raise to the so-called thermalization losses. At the opposite side of the solar spectrum, infrared photons have energies generally smaller than the band gap of conventional semiconductors (1.1 eV for Si). The solar cell is almost transparent to these photons and additional energy is lost in this way. In order to overcome these issues, two solutions are most frequently investigated.

The first solution to limit the losses uses tandem (or multijunction) solar cells in which up to four p-n junctions with different bandgaps are stacked together in order to cover efficiently the solar spectrum. Moreover, when used under concentrated light, such systems may exceed a conversion efficiency of 45 % [START_REF] Dimroth | Four-junction wafer-bonded concentrator solar cells[END_REF]. A second approach, much less expensive, suggests the use of a photon conversion layer to improve light harvesting and therefore the cell efficiency. Thermalization and transparency losses can be thus considerably reduced [START_REF] Trupke | Improving solar cell efficiencies by down-conversion of high-energy photons[END_REF][START_REF] Trupke | Improving solar cell efficiencies by up-conversion of subband-gap light[END_REF][START_REF] Trupke | Efficiency enhancement of solar cells by luminescent up-conversion of sunlight[END_REF][START_REF] Ze'ev | Solar energy enhancement using down-converting particles: A rigorous approach[END_REF] as the incident solar spectrum matches the absorption of the cell. Rare earths embedded into selected hosts (most often oxides) are used for the different photon management processes: i) down-shifting (DS) or down-conversion (DC) when one high-energy UV photon is converted into one or two low-energy near-infrared photons, and ii) up-conversion (UC) when two lowenergy IR photons are converted into one visible photon which can be further absorbed by the solar cell [START_REF] Wang | Rare-earth ion doped upconversion materials for photovoltaic applications[END_REF][START_REF] Richards | Luminescent layers for enhanced silicon solar cell performance: Downconversion[END_REF][START_REF] Shalav | Luminescent layers for enhanced silicon solar cell performance: Up-conversion[END_REF][START_REF] Chen | Nanochemistry and nanomaterials for photovoltaics[END_REF][START_REF] Gavriluta | Tuning the chemical properties of europium complexes as downshifting agents for copper indium gallium selenide solar cells[END_REF]. Both conversion processes rely on the luminescence process. For this reason, lanthanides showing intense emission lines due to the intra-4f transitions are ideal dopants for photon management in the conversion layers.

To date, efficient lanthanides-based sensitization has been supplied via host matrices [START_REF] Bünzli | Chapter 261 -Lanthanides in Solar Energy Conversion[END_REF][START_REF] Lian | Rare earth ions doped phosphors for improving efficiencies of solar cells[END_REF][START_REF] Bünzli | New opportunities for lanthanide luminescence[END_REF][START_REF] Hemmer | Upconverting and NIR emitting rare earth based nanostructures for NIR-bioimaging[END_REF][START_REF] Bünzli | Lanthanide NIR luminescence for telecommunications, bioanalyses and solar energy conversion[END_REF][START_REF] Nyk | High contrast in vitro and in vivo photoluminescence bioimaging using near infrared to near infrared up-conversion in Tm 3+ and Yb 3+ doped fluoride nanophosphors[END_REF][START_REF] Kenyon | Recent developments in rare-earth doped materials for optoelectronics[END_REF]. Among the host matrices, oxides are very promising not only for hosting lanthanides but also for their easy integration into the solar cell devices. Due to their low phonon energy they provide a very low rate of non-radiative recombination, promoting the photoluminescence process. While numerous pioneering works have been published on spectral conversion using rare earth doped ZnO and ITO oxide thin films [START_REF] Fu | Emission of 1.53 µm originating from the lattice site of Er 3+ ions incorporated in TiO 2 nanocrystals[END_REF][START_REF] Liu | Optical properties of Nd 3+ ion-doped ZnO nanocrystals[END_REF][START_REF] Yin | Biocompatible folate modified Gd 3+ /Yb 3+doped ZnO nanoparticles for dualmodal MRI/CT imaging[END_REF][START_REF] Zhang | Preparation and photocatalytic activity of Nd doped ZnO nanoparticles[END_REF][START_REF] Balestrieri | Structural and optical properties of Yb-doped ZnO films deposited by magnetron reactive sputtering for photon conversion[END_REF][START_REF] Balestrieri | Luminescent properties and energy transfer in Pr 3+ doped and Pr 3+ -Yb 3+ co-doped ZnO thin films[END_REF][START_REF] Elleuch | Highly efficient NIR to visible upconversion in a ZnO:Er,Yb thin film deposited by a AACVD atmospheric pressure process[END_REF][START_REF] Gottardi | Effect of Nd 3+ incorporation on the microstructure and chemical structure of RF sputtered ZnO thin films[END_REF][START_REF] Liu | White upconversion of rare-earth doped ZnO nanocrystals and its dependence on size of crystal particles and content of Yb 3+ and Tm 3+[END_REF][START_REF] Soumahoro | Structural, optical, and electrical properties of Yb-doped ZnO thin films prepared by spray pyrolysis method[END_REF][START_REF] Wei | ,Yb,Gd particles designed for magnetic-fluorescent imaging and near-infrared light triggered photodynamic therapy[END_REF][START_REF] Du | Efficient energy transfer in monodisperse Eudoped ZnO nanocrystals synthesized from metal acetylacetonates in high-boiling solvents[END_REF][START_REF] Choi | Erbium concentration effects on the structural and photoluminescence properties of ZnO:Er films[END_REF], SnO 2 has been much less considered as a potential alternative host material. Nevertheless, SnO 2 has a gap of about 3.6 eV (bulk material), which offers a good absorption in the UV region while being transparent to visible and NIR photons. This is an asset for its integration in solar cells as conversion layers and should not affect the absorption of the cell. More interestingly, SnO 2 has the advantage of combining the above-mentioned optical properties with promising electrical ones. Owing to native oxygen vacancies that can be easily obtained in this system, tin oxide layers can exhibit a large n-type conductivity and carrier concentration [START_REF] Keller | Oxygen concentration as a combinatorial parameter: The effect of continuous oxygen vacancy variation on SnO 2 layer conductivity[END_REF]. From the conversion point of view, some earlier reports have shed light on the ability of SnO 2 to allow rare-earth doping, notably Nd, Yb, Er, Eu and Sm [START_REF] Chen | Improved ethanediol sensing with single Yb ions doped SnO 2 nanobelt[END_REF][START_REF] Bastami | Synthesis of nanosized (Co, Nb, Sm)-doped SnO 2 powders using co-precipitation method[END_REF][START_REF] Cao | Room temperature ferromagnetism in Sm-doped SnO 2 PLD film[END_REF][START_REF] Li | Preparation of Pr-doped SnO 2 hollow nanofibers by electrospinning method and their gas sensing properties[END_REF][START_REF] Hild | Structural and photoluminescence properties of evaporated SnO 2 thin films doped with rare earths[END_REF][START_REF] Bouras | Insight into photon conversion of Nd 3+ doped low temperature grown p and n type tin oxide thin films[END_REF][START_REF] Cojocaru | Nanoscale insights into doping behavior, particle size and surface effects in trivalent metal doped SnO[END_REF][START_REF] Dong | Photon upconversion in Yb 3+ -Tb 3+ and Yb 3+ -Eu 3+ activated core/shell nanoparticles with dual-band excitation[END_REF]. For all these dopants it was shown that the discrete intermediate levels induced within the gap of the host matrix were involved in intense emission lines.

Among the rare-earth dopants, Yb presents the simplest energy level structure and has been already used in spectral conversion as its NIR emission line matches perfectly with the band gap of the solar cell absorber [START_REF] Dong | Photon upconversion in Yb 3+ -Tb 3+ and Yb 3+ -Eu 3+ activated core/shell nanoparticles with dual-band excitation[END_REF][START_REF] Prorok | Energy migration up-conversion of Tb 3+ in Yb 3+ and Nd 3+ codoped active-core/active-shell colloidal nanoparticles[END_REF][START_REF] Nadort | Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties[END_REF][START_REF] Marciniak | A broadening temperature sensitivity range with a core-shell YbEr@YbNd double ratiometric optical nanothermometer[END_REF]. However, these systems suffer from many issues mainly related to the insertion of lanthanides into the host structure, as the ionic radii of lanthanides and cation from the host oxide are very different. As a consequence, the solubility of rare earth in the host matrix is rather small and oxygen vacancies are generally necessary to make the optical transitions in lanthanides allowed.

Very few reports exist on Yb-doped SnO 2 [START_REF] Bouzidi | Yb 3+ effect on the spectroscopic properties of Er-Yb codoped SnO 2 thin films[END_REF][START_REF] Callone | Synthesis improvement of Yb 3+ -activated SnO 2 nanocrystals[END_REF][START_REF] Wang | Preparation of Yb-doped SnO 2 hollow nanofibers with an enhanced ethanol-gas sensing performance by electrospinning[END_REF]. In this work, we report on the photon conversion of incident UV light to NIR photons in Yb-doped SnO x sputtered films by taking advantage of the efficient energy transfer from the SnO x host matrix to the Yb 3+ ions. The evolution of the structural, electrical and the conversion properties as a function of the deposition temperature is discussed. Finally, by replacing the n-type ZnO conduction layer in a standard Cu(InGa)Se 2 (CIGS) solar cell with a Yb-doped SnO x layer an increase of the cell quantum efficiency is demonstrated. Such an improvement can be understood on the basis of electrical properties improvement and on the down-conversion properties of the Yb-doped SnO x layer.

A. Yb:SnO x thin films deposition

Yb-doped SnO x thin films were deposited using an Orion 3 Radio Frequency (RF) reactive magnetron sputtering from AJA International Co. The deposition was performed on p-type Si(100), quartz substrates and finally on the CIGS/CdS junction of a solar cells. The doping of the SnO x films with Yb was carried out by placing Yb metal discs (99.99 % from Alfa Aesar Co.) on top of a metallic Sn target (99.99 % from Neyco Co.). During deposition, Ar and O 2 gas flows were fixed to 8 and 5 sccm, respectively. The working pressure and RF power were set at 3.4 mTorr and 50 W, respectively. The substrates are located in top configuration at 12 cm above the target. To insure a good film homogeneity, the substrate holder was in continuous rotation during the deposition process. In order to improve the crystalline quality and thus modulate the electrical and optical properties of the layer, the substrate temperature during the film growth was varied from 100 • C to 500 • C. The resulting layers have a thickness of about 100 nm. The layers were thoroughly analyzed before integration in solar cells.

B. CIGS-based solar cells fabrication

Molybdenum back contact (∼ 800 nm) was deposited onto glass substrates (2.5×2.5 cm 2 ) by DC magnetron sputtering. The CIGS absorber layer was grown above by molecular beam epitaxy (MBE) following a three-step procedure. In the first step, an (InGa) 2 Se 3 film was grown by co-evaporation of In, Ga, and Se at 400 • C during 30 min at evaporation rates of 0.19, 0.02, 0.5 nm/s, respectively. In a second step, Cu and Se co-evaporation (rates of 0.09 and 0.5 nm/s) was performed at 550 • C for 70 min. CIGS and Cu x Se are obtained with an excess of Cu and Se. For the final step, extra In, Ga and Se are supplied for 30 min to react with Cu x Se which resulted in the formation of a single phase CIGS layer with a thickness of about 2.5 µm.

A CdS buffer layer was further deposited on CIGS by chemical bath deposition using a bath of ammonia water, cadmium sulfate and thiourea solution. The solution temperature was increased to 65 • C under continuous stirring. The CIGS samples were immersed in the solution during 10 min which led to a CdS film of about 70 nm thick. The samples were then thoroughly rinsed by de-ionized water, dried under nitrogen gas and annealed at 100 • C for 15 min under vacuum. The Yb-doped SnO 2 layer was then deposited by sputtering at 100 • C with Ar:O 2 flow ratio of 8:5. The temperature was chosen in order to avoid damaging the cell absorber observed for larger deposition temperatures. The SnO x layer has a thickness of 20 nm, a resistivity of 9×10 -4 Ωcm and a carrier concentration of 6.5×10 20 cm -3 to ensure good transport properties. A ZnO:Al layer of about 100 nm was finally deposited atop by DC sputtering at 150 • C. In the reference cell, the SnO x :Yb layer from the front contact was replaced by a i-ZnO layer with a thickness of 80 nm and deposited at room temperature. The Al concentration in ZnO:Al was fixed at 2 at. %. Finally, both set of cells were completed by depositing Ni/Ag as grid using electron beam evaporation at room temperature.

C. Characterization techniques

The crystalline structure of the samples was analyzed by X-ray diffraction using a Rigaku SmartLab diffractometer equipped with a monochromatic source delivering a CuKα 1 incident beam (45 kV, 200 mA, 0.154056 nm). X-ray θ-2θ and reflectivity analyzes were performed in order to check the crystalline quality of the sample, the eventual presence of spurious phases, and determine the film thickness.

Insight into the chemical atomic environment of the different elements (Sn, O, and

Yb) and the stoichiometry of the films was obtained by X-ray photoelectron spectroscopy (XPS). The measurements were carried out using a Thermo Electron K-Alpha spectrometer equipped with a monochromatic Al-Kα X-ray source (1486.6 eV). The size of the analyzed spot was about 400 µm. The homogeneity of the sample was checked by measuring on several points on the surface.

The transparency of the films was investigated in the 200-850 nm range using a Perkin-Elmer Lambda 950 spectrophotometer. The photon conversion was checked by photoluminescence (PL) and photoluminescence excitation (PLE) measurements performed in the 360-1050 nm range. For the PL and PLE measurements, the excitation was provided by means of a Nd:YAG laser (355 nm) and a Xenon lamp, respectively. The PL signal was recorded using a cooled CCD camera.

The current-voltage characteristics of the cells were measured under a class AAA K201

McScience solar simulator (AM1.5 irradiation) to extract the photovoltaic parameters such as the open circuit voltage, the short circuit current and cell efficiency. External quantum efficiency (EQE) measurements were performed using a tungsten halogen lamp as light source and automated filter wheels (41 filters).

III. RESULTS AND DISCUSSION

A. Yb:SnO x thin films: effect of the deposition temperature

The structural properties of Yb-doped SnO 2 single films deposited on Si(100) substrates have been investigated by X-ray diffraction. All diffractogramms are typical of SnO 2 polycrystalline samples as already reported for other rare earth doped SnO 2 thin films [START_REF] Bouras | Structural, optical and electrical properties of Nddoped SnO 2 thin films fabricated by reactive magnetron sputtering for solar cell devices[END_REF]. No spurious phases such as SnO or Yb 2 O 3 could be detected in the resolution limit of the XRD technique. Fig. 1 shows the diffraction patterns recorded for a series of samples grown at different temperatures between 100 and 500 • C, while all the other deposition parameters remained unchanged. For visibility reasons and in order to have a better insight on the structural evolution induced by the growth temperature the patterns are presented only in the 25-40 • 2θ range where the (110) and (200) peaks of SnO 2 are clearly visible. For our samples these peaks are shifted towards smaller angles with respect to their position in undoped SnO 2 (dashed lines in fig. 1) indicating an increase of the lattice parameter due to doping. This can be easily understood if we keep in mind the larger ionic size of Yb 3+ ion (0.868 Å) with respect to that of Sn 4+ (0.69 Å) [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chaleogenides[END_REF]. Another consequence of the peak shift is that, at least partly, Yb ions substitute the Sn ones in the host matrix, most probably involving oxygen vacancies as well. The existence of other types of defects (interstitial ions and oxygen vacancies) is also suggested by the evolution of the diffraction peaks position with the deposition temperature. Indeed, as the temperature increases from 100 to 500 • C, the peaks shifts towards higher angles (horizontal arrows (→) in fig. 1) suggesting a more compact and therefore less defected structure. As a consequence the lattice parameter a calculated from the position of the (200) peak decreases progressively from 4.795 to 4.760 ± 0.005 Å, while the crystallite size (average value calculated using both (110) and (200) peaks) slowly increases from 18.5 to 20.5 ± 0.3 nm.

To further investigate the structural changes induced by the increase of the substrate temperature during deposition, XPS analyzes have been carried out. Changes of the atomic More details on the tin and oxygen environment can be obtained from the Sn 3d the deconvolution of both Sn and O core levels reveals the presence of two oxide phases, mainly SnO and SnO 2 with more or less comparable amounts [START_REF] Themlin | Characterization of tin oxides by x-ray-photoemission spectroscopy[END_REF][START_REF] Themlin | Resonantphotoemission study of SnO 2 : Cationic origin of the defect band-gap states[END_REF][START_REF] Lau | Oxidation of tin: an ESCA study[END_REF]. This apparent inconsistency is related to the fact that the kinetic energies of photoelectrons coming from O 1s and Sn 3d 5/2 are lower than the ones of photoelectrons coming for the valence band. It seems therefore possible that the two environments are mainly located at the extreme film surface and that the SnO:SnO 2 ratio calculated from the XPS data is more representative of the first nanometers of the films. This situation is also favored by the reversible dual valence of Sn and to the presence of the Yb dopant into the structure that substitutes partly Sn 4+ in SnO 2 . All films show almost same SnO:SnO 2 ratio, which is about 50:50 (table I). Although this ratio slightly varies with the deposition temperature, the changes are too small to be significant. It is therefore fair to conclude that the increase of the substrate temperature during the film growth seems to not deeply affect the atomic environment of the elements, and therefore their oxidation state. Table I also reports the oxygen / tin ratio (stoichiometry of the films) which is about 1.6 and decreases slightly when increasing the temperature. This is in agreement with the general behavior of oxides that tend to reduce upon heating.

Although this decrease is small, this result will help us understand the photoluminescence properties of the films that will be discussed later in this work.

A similar analysis has been also carried out on the XPS spectra corresponding to the Yb 4d 5/2 core level (fig. 4). At first sight, Yb atoms bounded to oxygen are present in all films whatever the deposition temperature is. This is witnessed by the position of Yb 4d 5/2 peak which is located at about 184.4 eV [START_REF] Rivera-López | X-ray photoelectron spectroscopy: surface and depth profiling studies of glasses doped with Nd and Yb ions[END_REF][START_REF] Schmidt | X-ray photoemission of YbInCu 4[END_REF]. Moreover, the Yb 4d 5/2 peak area is quite comparable for all samples, indicating the same density of optically active Yb 3+ ions.

Clearly, the increase of the deposition temperature does not seem to affect the Yb oxidation state. A slight shift towards high binding energies is noticed indicating a slight change in the atomic surroundings of Yb atoms. No Yb 2+ could be observed in the detection limit of the XPS technique.

The optical properties have been investigated by UV-visible transmission spectroscopy

(data not shown here). All the films are found to be transparent over the UV to NIR range with transparencies varying between 80 and 90%. The absorption edges are located In order to check the optical activity of the dopant, the PL spectra as a function of the deposition temperature were recorded on the Yb-doped SnO x films and reported in fig. 5.

The sample was illuminated with a 355 nm radiation provided by a Nd:YAG laser. Upon excitation, all samples exhibit PL over the UV-Vis-NIR range with different intensities.

The film grown at 100 • C shows a large UV-visible emission band as well as one intense NIR peak. Different studies on SnO 2 have already shown similar emission bands [START_REF] Gu | Photoluminescence properties of SnO 2 nanoparticles synthesized by sol-gel method[END_REF][START_REF] Gu | Luminescence of SnO 2 thin films prepared by spin-coating method[END_REF].

The emission band ranging between 380 and 650 nm was attributed to radiative transitions originating from defect states localized within the band gap of the host matrix. As for the NIR peak located at 973 nm it was assigned to radiative transition of 4f-electrons from the of radiative defects in the SnO x structure. The increase of the Yb 3+ PL (inset of fig. 5) can be understood in terms of atomic environment which becomes, due to temperature, less defected and therefore more favorable to radiative transitions in Yb. At 400 and 500 • C, the intensity of the defects emission strongly decreases while that of Yb increases about five times with respect to the one of the sample deposited at 100 • C. This is somehow awkward since the XPS data suggested a similar density of Yb 3+ ions that are optically active in all samples. This result can be however explained by either: i) a decrease of the defects density into the structure related to better crystalline quality, in this case the atomic environment of Yb being much more favorable for its emission), or ii) a transfer of the photon energy arising from transitions within the defects levels to Yb which will increase consequently its emission. It is also interesting to note that for deposition temperatures higher than 100 • C, the Yb emission is characterized by the appearance of a second peak at 1030 nm. This is rather surprising but can be explained by the degeneration of the energy levels of Yb 3+ induced by the crystal field effect of the host matrix. Such wide and intense NIR emission is of interest to solar cells.

In order to investigate the origin of the Yb emission in the NIR region and to understand the energy transfer process, further analyzes by PLE spectroscopy have been performed.

The PLE spectra of the Yb-doped SnO x films as a function of the deposition temperature are presented in fig. 6. The PL intensity of Yb 3+ ions at 973 nm was monitored while varying the excitation wavelength from 250 to 500 nm. The inset in fig. 6 shows a zoom of the PLE spectra for the samples exhibiting a weak PL intensity. As expected from the PL First, external quantum efficiency (EQE) measurements were performed to assess the down-shifting effect provided by the Yb:SnO x layer. The EQE spectra of both reference and functionalized CIGS cells are shown in fig. 7. As expected, a net gain in the UV region is noted, namely between 300 and 400 nm, due to the conversion of UV photons to NIR ones through down shifting. At 360 nm, the EQE response of the cell is increased to 17.5% against 9.2% for the reference cell. Interestingly, the EQE spectra also show an enhancement over the visible and NIR range. This behavior can be attributed to the improvement of electrical and optical properties of the cell when replacing the ZnO layer with Yb:SnO x . Although there is no direct proof in this sense, this effect involves more efficient passivation effect, a higher charge carrier collection (better conductivity) and/or a more appropriate reflectance.

To further investigate the performance improvement of the CIGS solar cell induced by the Yb:SnO x layer, current -voltage dependencies were recorded under standard illumination (AM1.5G). The J-V plots are shown in fig. 8 while the extracted photovoltaic parameters are displayed in table III. While the open-circuit voltage (V oc ) remains the same for both cells (about 0.55 V), the short circuit current density (J sc ) is increased by 0.57 mA/cm 2 as a consequence of the photon conversion provided by the Yb:SnO x layer. As for the fill factor (FF), its value increases by 2.4 % due to the good electrical properties of the conversion layers. More precisely, our SnO x and Yb:SnO x possess good transport properties compared to i-ZnO used for the reference cell. Indeed, the measured series resistance decreases from 3.68 Ωcm 2 for the reference cell to 3.30 Ωcm 2 for the Yb:SnO x based cell. As a consequence, the solar cell efficiency is enhanced by about 0.6 %, which is quite remarkable. Such result is an obvious proof of the potential of Yb:SnO x layers for solar cells performance enhancement and can constitute a viable low-cost solution for a wide range of solar cells where downshifting layers could be integrated. Finally note that the J sc values measured from the J-V curves (28.72

  FIG. 1: X-ray diffraction patterns of Yb-doped SnO 2 thin films deposited on Si(100) at different temperatures. The vertical dashed lines indicate the position of the (110) and (200) peaks of undoped SnO 2 . The horizontal arrows (→) indicate the sense of the peak shift while increasing the temperature.

FIG. 2 :

 2 FIG. 2: X-ray photoemission valence bands spectra of the Yb:SnO 2 thin films as a function of the deposition temperature.

FIG. 4 :

 4 FIG. 4: X-ray photoemission spectra showing the evolution of the Yb 4d 5/2 peak recorded on Yb-doped SnO 2 films grown at different deposition temperatures. The spectra recorded on the samples deposited at 300 and 500 • C were shifted along the vertical axis for visibility reasons.

FIG. 5 :

 5 FIG. 5: Photoluminescence spectra of the Yb-doped SnO x films as a function of the deposition temperature. The inset shows the evolution of PL intensity of Yb recorded at 973 nm.

FIG. 6 :

 6 FIG.6: Photoluminescence excitation spectra of the Yb-doped SnO x films as a function of the deposition temperature. The inset shows a magnification of the low-intensity spectra region.

2 )FIG. 8 :

 28 FIG. 8: Voltage dependence of the current density recorded under AM1.5G illumination on ZnO/CIGS (reference) and Yb:SnO x /CIGS solar cells.

  SnO x layers were obtained by RF magnetron sputtering at different temperatures ranging from 100 to 500 • C. The films are polycrystalline and strongly substoichiometric (x = 1.6). They showed a reduced resistivity in the 10 -3 Ωcm range and a large electron mobility that can reach 50.1 cm 2 /Vs. XPS and PL measurements indicated the presence of Yb 3+ ions that are optically active. An energy transfer occurring between the host SnO x and the Yb ions is demonstrated through PLE measurements. The Yb-doped SnO x optimized layers were integrated to CIGS solar cells to replace conventional ZnO. A net increase of the EQE response is noticed. In the UV region this is explained on the basis of down shifting process while in the NIR range this is due to electrical factors (passivation, resistivity, and mobility improvement). The electrical measurements show no change in V oc but an increase in the J sc and FF with about 0.56 mA/cm 2 and 2.4 %, respectively. Finally, the conversion efficiency of the CIGS-based cell is enhanced by about 0.6 %, which is extremely encouraging for future implementation of such low cost converting layers in photovoltaic technology.

TABLE I :

 I Variation of the SnO and SnO 2 content in Yb-doped SnO x films deposited at different

	temperatures (100, 300 and 500 • C), extracted from the deconvolution of Sn 3d and O 1s core levels
	spectra.
	Deposition	Sn 3d 5/2 line	O 1s line	Stoichiometry SnO x
	temperature	SnO 2	SnO	SnO 2	SnO	x = [O]/[Sn]
	100 • C	49.83	50.17	48.41	51.59	1.62 ± 0.01
	300 • C	51.32	48.68	53.13	46.87	1.58 ± 0.01
	500 • C	49.99	50.01	52.25	47.75	1.56 ± 0.01

TABLE II :

 II Resistivity, carrier concentration and mobility evolution in Yb-doped SnO x films deposited at different temperatures.

		80
		60
	EQE (%)	40	Yb:SnO x /CIGS cell
		20	ZnO/CIGS reference cell
		0
		400	600	800	1000	1200
			Wavelength (nm)
	FIG. 7: EQE spectra of a of ZnO/CIGS (reference) and Yb:SnO x /CIGS solar cells.
	Temperature	Concentration		Resistivity	Mobility
	( • C)	(cm -3 )			(Ω•cm)	(cm 2 /Vs)
	100	4.17×10 18			0.0530	28.2
	300	1.94×10 19			0.0064	50.1
	500	2.28×10 20			0.0018	14.6

TABLE III :

 III Electrical parameters (V oc , I sc , J sc , fill factor and efficiency) of ZnO/CIGS (reference) and Yb:SnO x /CIGS solar cells.

	Cell	V oc (V)	I sc (mA)	J sc (mA/cm 2 )	FF (%)	η (%)
	ZnO/CIGS (reference)	0.55	14.64	28.72	62.03	9.86
	Yb:SnO x /CIGS	0.55	14.93	29.28	64.43	10.45

  and 29.28 mA/cm 2 for the ZnO/CIGS and Yb:SnO x /CIGS cells, respectively -see tableIII) match very well the values determined from the spectral response(28.30 and 29.68 mA/cm 2 for the ZnO/CIGS and Yb:SnO x /CIGS cells, respectively). This supports the reliability of the measurements and that of the analyzed system. The small differences can be explained in terms of charge recombination, heating or passivation differences during the J-V and EQE experiments.

F 5/2 excited state to the 2 F 7/2 ground state in Yb

3+ . A similar intense and narrow peak was also reported in Yb-doped ZnO films[START_REF] Balestrieri | Structural and optical properties of Yb-doped ZnO films deposited by magnetron reactive sputtering for photon conversion[END_REF]. Increasing the substrate temperature during deposition to 300 • C did not affect much the emission coming from the host matrix but found to promote that originating from the Yb 3+ ions. Such behavior indicates a same density

Acknowledgments K.B. acknowledges the Région Alsace and the INSA of Strasbourg for financial support.