Bulk CsPbBr3 is an emerging semiconductor that has shown unprecedented increase in performance over the last decade for optoelectronic applications. However, further development of devices based on CsPbBr3 is hampered by their poor electrical stability under operation. Migration and accumulation of native ions (Cs+, Pb2+, and Br-) under electric fields has been suggested by many groups to be responsible for the observed device instabilities, although direct experimental evidence of ionic motion during operation has been seldom reported. In our study, ion migration has been probed by grazing incidence x-ray fluorescence (GIXRF) in CsPbBr3 polycrystalline layers grown in vapor phase. Our findings indicate that both Cs and Br experience ionic migration under electric field, suggesting that these ions are responsible for the measured current instability in our devices. In the timescale of a few hours, the ionic drifting rates of mobile Cs and Br under the top electrode were found to be similar, as high as ∼ 20 ppm h−1 V−1 mm. This work paves the way for a better understanding of ion motion issues that play a key role in the optoelectronic properties of CsPbBr3 devices.

The all-inorganic metal halide perovskite CsPbBr3 is a well-known semiconductor which attracts attention from many researchers in the field of optoelectronics. Although CsPbBr3 in the form of single crystals offers the best charge transport properties, their high cost of fabrication makes them less suitable for large-scale applications. Polycrystalline films, on the other hand, can fill this market gap, and numerous solution- and vapor-based techniques exist to produce them. However, no superior solution has been found yet, and we propose the less explored close space sublimation (CSS), a fast and scalable physical vapor deposition technique, as a viable processing technique for manufacturing CsPbBr3 films. We show the fabrication of high-quality single-phase CsPbBr3 films with homogeneous film thicknesses over an area of 3.14 cm(2). By optimizing the deposition temperature, a growth rate of 10 mu m/min can be reached with a material utilization of up to 98%. In addition, the necessary CsPbBr3 source material is synthesized via ball milling; hence, the whole process is solvent-free. This work aims at rationalizing the CSS process and offering a comprehensive study of the impact of the deposition conditions on the growth of CsPbBr3.

Kesterite‐based solar cells suffer from a large open‐circuit voltage deficit, which largely arises from carrier recombination at the buffer interface. In this study, we compare two strategies to passivate the absorber surface in order to fabricate devices with power conversion efficiency higher than 10% and an open‐circuit voltage deficit as low as 306 mV. These two strategies consist of annealing in air or performing a chemical etching of the absorbers before buffer deposition. They lead similarly to a significant reduction of the interface recombination but as well to a shortening of the minority carrier diffusion length from 1 μm to less than 500 nm. This latter effect limits the short‐circuit current and fill factor of the devices but is largely compensated by the open‐circuit voltage gain of more than 100 mV. For the absorber air annealing, which is the simplest solution to implement, absolute photoluminescence measurements reveal that the voltage gain is directly linked to a drop in the nonradiative losses in the absorber and to a small reduction of the band tailing. It is demonstrated that the removal of detrimental secondary phases at the surface of the absorber due to oxidation at elevated temperature and etching in the basic CdS solution is responsible for these improved opto‐electronic properties. On the contrary, the apparent Cu‐depletion observed after air annealing is totally recovered after the chemical bath and cannot be responsible for the improved performances.

Replacing the CdS buffer layer with a ZnSnO one in Cu2ZnSnS4-based solar cells allows both to improve the device performances and to avoid using toxic Cd. Additionally, using a sputtered buffer layer is a major asset for solar cells fabricated by physical vapor deposition processes. In this study, ZnSnO layers are deposited by sputtering of a single metal oxide target. Structural and optical properties of the layers deposited on Si or glass are first described. The possibility of modifying the ZnSnO metallic composition by adjusting the deposition power is demonstrated. Attempts to improve the optoelectronic properties of the ZnSnO layers with Ar:O-2 or Ar:SF6 reactive sputtering are shown as well. These ZnSnO buffer layers are transferred in Mo/CZTS/ZnSnO/ZnO:Al solar cells. After postdeposition thermal treatment and optimization of the deposition condition (notably with the use of Ar:O-2 or Ar:SF6 reactive sputtering), a solar cell with a power conversion efficiency of 5.2% is demonstrated. It is 0.6% absolute higher than the reference solar cell with a CdS buffer layer. To avoid absorber damaging and achieve high performances, deposition power must be as low as possible. A two-stage sputtering process is used to conciliate both the absorber surface preservation and a reasonable deposition time. Last, photovoltaic properties of optimized CZTS- and CZTSe-based solar cells with ZnSnO buffer layers are compared.

We report on the precise determination, in Ag2ZnSnSe4 epitaxial layer, of both the band gap E-g and the characteristic Urbach energy U that describes the density of localized, defect states in the gap. Various origins for these defect band tail states have been considered, together with the corresponding modeling for their density of states, in order to fit the whole of the optical spectral data. The interest of the methodology developed here is to account quantitatively not only for the absorption and steady-state photoluminescence data but also for the time-resolved photoluminescence spectra. We compare the different origins of localized band tail states to select the standard textbook, Urbach tail model that corresponds to short-range band gap fluctuations. Such an approach is different from the one most often used to evaluate the energy extent of the localized states, which is the Stokes shift between the energies of the photoluminescence emission and the absorption threshold. The advantage of the present method is that no arbitrary choice of the low power excitation has to be done to select the photoluminescence emission spectrum and its peak energy. Thanks to this systematic study of both photoluminescence excitation and time-resolved photoluminescence spectra at low temperature (6 K), the values E-g = 1226 +/- 5 meV and U = 20 +/- 3 meV are found for this promising absorber for thin films photovoltaics.

Cu2ZnSn(S,Se)4 (CZTSSE)‐based solar cell performances are limited by band tailing due to a large amount of CuZn antisite defects. Partially replacing the Cu atoms by larger Ag ones can significantly reduce the prevalence of these defects, which are particularly detrimental close to the front interface. Herein, the possibility of synthesizing (Cu1‐xAgx)2ZnSnSe4 absorbers with various Ag contents by vacuum‐based processes is demonstrated. Although the synthesis of high‐quality materials is demonstrated, their use in thin film photovoltaic devices does not exhibit performance improvement compared with efficient pure CZTSSE‐based solar cells. Moreover, the comparison with literature data reopens the debate of the beneficial effect of homogeneous Ag alloying in kesterite. On the contrary, a new method is proposed to fabricate graded (Cu1‐xAgx)2ZnSnSe4 absorbers with increased Ag content at the interfaces. The solar cells with graded absorbers exhibit better performances than the reference Ag‐free ones. Particularly, improved current collection at the back contact and slight reduction of the front interface recombination are demonstrated.

Cu2ZnSnS4‐based solar cells suffer from limited power conversion efficiency (PCE) and relative small grain size compared to selenium containing absorbers. Introduction of Na in Cu2ZnSnS4 absorbers either during the synthesis or after this step is used to improve device performances and to determine whether its effect is based on structural properties improvement (grain size enhancement, better crystallization) or on opto‐electronic properties improvement (defect passivation). In both cases, presence of Na in the absorber notably improves current and voltage of the solar cells, but the effect is more pronounced when Na is present during synthesis. Quantum efficiency analysis shows that these improvements can be related to longer minority carrier diffusion length and reduced absorber/buffer interface recombination. Introducing Na in the process mostly leads to preferential (112) orientation of the crystal which is clearly correlated with better device performances. Otherwise, the performance limitation due to small grain size is discarded by the joint use of Sb and Na, which has a significant impact on grain size but does not affect solar cells efficiency.

We review the present state-of-the-art within back and front contacts in kesterite thin film solar cells, as well as the current challenges. At the back contact, molybdenum (Mo) is generally used, and thick Mo(S, Se)(2) films of up to several hundred nanometers are seen in record devices, in particular for selenium-rich kesterite. The electrical properties ofMo(S, Se)(2) can vary strongly depending on orientation and indiffusion of elements from the device stack, and there are indications that the back contact properties are less ideal in the sulfide as compared to the selenide case. However, the electronic interface structure of this contact is generally not well-studied and thus poorly understood, and more measurements are needed for a conclusive statement. Transparent back contacts is a relatively new topic attracting attention as crucial component in bifacial and multijunction solar cells. Front illuminated efficiencies of up to6% have so far been achieved by adding interlayers that are not always fully transparent. For the front contact, a favorable energy level alignment at the kesterite/CdS interface can be confirmed for kesterite absorbers with an intermediate [S]/([S]+[Se]) composition. This agrees with the fact that kesterite absorbers of this composition reach highest efficiencies when CdS buffer layers are employed, while alternative buffer materials with larger band gap, such as Cd1-xZnxS o rZn(1-x)Sn(x)O(y), result in higher efficiencies than devices with CdS buffers when sulfurrich kesterite absorbers are used. Etching of the kesterite absorber surface, and annealing in air or inert atmosphere before or after buffer layer deposition, has shown strong impact on device performance. Heterojunction annealing to promote interdiffusion was used for the highest performing sulfide kesterite device and air-annealing was reported important for selenium-rich record solar cells.

We report the influence of the AlN interlayer thickness (0-15 nm) on the photovoltaic properties of Al0.37In0.63N on Si heterojunction solar cells deposited by radio frequency sputtering. The poor junction band alignment and the presence of a 2-3 nm thick amorphous layer at the interface mitigates the response in devices fabricated by direct deposition of n-AlInN on p-Si(111). Adding a 4-nm-thick AlN buffer layer improves the AlInN crystalline quality and the interface alignment leading to devices with a conversion efficiency of 1.5% under 1-sun AM1.5G illumination. For thicker buffers the performance lessens due to inefficient tunnel transport through the AlN. These results demonstrate the feasibility of using In-rich AlInN alloys deposited by radio frequency sputtering as novel electron-selective contacts to Si-heterojunction solar cells.

The influence of the AlInN thickness (65-145 nm) on the photovoltaic characteristics of In-rich n-AlxIn1-xN (x approximate to 0.38-0.42) on p-Si(111) heterojunctions deposited by radio frequency sputtering has been reported. All samples show a closely packed columnar morphology with a root mean-squared surface roughness below 3.7 nm and an apparent optical bandgap energy of approximate to 2.0 eV. Dark current density-voltage curves of the solar cell devices based on the developed AlInN/Si(111) heterojunction reveal shunt and series resistances in the range of 1.3-5.0 k omega and 7.7-16.2 omega depending on the AlInN thickness, respectively. Their photovoltaic performance shows an enhancement with the AlInN thickness, with an increase of the short circuit current and conversion efficiency from 16 to 19 mA cm(-2) and from 1.8 to 2.5% under one-sun AM1.5G illumination. At the same time, the open circuit voltage and the fill factor remain at approximate to 0.34-0.40 V and approximate to 30-37%, respectively. These effects are due to the enhanced optical transmittance of the AlInN layer in the wavelength range in which the maximum of the Si spectral photoresponse occurs, in agreement with the increased responsivity of the devices at 950 nm of 450 mA W-1. These results demonstrate the feasibility of using In-rich AlInN alloys deposited by radio frequency sputtering as n-type layer of AlInN/p-type Si heterojunction solar cells.

Intense research has been carried out in the past few years to improve efficiencies and understand limitations in kesterite-based solar cells. Despite notable efforts to determine and list the different failure modes affecting the photovoltaic properties of these devices, very few works have tried to quantify and classify the effects of these failure modes. In this study, an exhaustive literature review has first been conducted to determine the different causes leading to limited efficiencies in kesterite devices, with an additional focus on cadmium-free and critical raw material-free devices. Second, an original approach has been employed to quantify the impact of these failure modes on solar cells, based on the evaluation of feedback from 18 scientific experts working on kesterite technology. The result of this survey is analyzed, which allows us to determine what should be the research priority for the community to improve efficiencies and drive kesterite technology to the market.

In this study, we detail a Cu2ZnSnSe4 based solar cell fabrication process based on the selenization of metallic precursor stacks with elemental Se. 9.4% efficient devices without antireflection coating have been obtained. First, reproducibility issues of the process are carefully shown and discussed. It is demonstrated that device performances are strongly impacted by the precise control of the precursor composition. Then, starting from this robust process, a review of existing strategies to improve kesterite efficiencies is conducted. A significant increase in efficiency (+1.4% absolute efficiency and +50 mV VOC) is obtained with absorber surface treatment and post-annealing, while no effect of Ge incorporation in the precursor stack is observed. This contradictory result to most of the recent publications raises the question of the universality of this strategy to improve kesterite solar cell performance. Finding a universal activation step to boost kesterite efficiencies and bring it to the market remains a crucial need for the community.

This study shows the fabrication of CIGS solar cells with various absorber thicknesses synthesized with two different methods: the classical 3-stages coevaporation process and a 1-stage coevaporation process. Structural characteristics and photovoltaics properties as function of absorber thickness are described. 1-stage coevaporation process gives lower efficiencies for CIGS solar cells with nominal absorber thickness (~2000nm) but performs similarly or even better when the absorber thickness is decreased down to 600nm. Periodically textured glass substrates with different feature sizes have been fabricated and used to increase current in 1-stage coevaporated CIGS solar cells. A current improvement up to 4.1mA·cm−2 (6% relative) has been obtained leading to an increase in efficiency up to 5% relative. A maximum increase has been found when texturation size is similar to CIGS absorber thickness.

Cu(In, Ga)Se2 (CIGS) based thin film solar cells have been extensively studied and today, power conversion efficiencies higher than 20% have been demonstrated on both rigid and flexible substrates. However, very little is known about the mechanical resistance of flexible CIGS solar cells under flexion. Here we report an original study on the mechanical properties of CIGS solar cells fabricated on 100µm-thick ultra-thin glass substrates. The Young's modulus and hardness of Mo and CIGS thin films are measured by nanoindentation, a technique well adapted to the characterization of thin film materials. Young's modulus values of 289GPa and 70GPa are obtained for the Mo and the CIGS layers respectively, as well as a CIGS hardness of 3.4GPa. These values, combined with an analytical model, allow calculating the strain induced in thin film during the flexion of solar cells fabricated on ultra-thin glass substrate as well as on polyimide substrate. Thereby, we show that using a substrate with a low thickness and a low Young's modulus enables to lower the thin films strain during the flexion of cells.

CZTSSe(Cu2ZnSn(S1-xSex)4) material is a promising candidate to replace CIGS (Cu(In,Ga)Se2) in thin film solar cells technology as it uses only earth abundant constituents. Particularly, sulfur-based kesterite (CZTS) are of prime interest due to their wide bandgap (1.5eV) and the absence of toxic element. Despite their similar properties, CZTSSe based solar cell only achieved 12.6% efficiency [1] (9.2% for pure sulfur CZTS solar cells [2]), which is far from 22.6% efficiency achieved by CIGS [3]. Absorber doping with Alkali (Na, K) and crystallization control with Sb are some of the reasons explaining the high performances of CIGS solar cells [2]-[3], due to the defect passivation and surfactant ability of those dopants.