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.

During the fabrication of a MOS-HEMT, the plasma-etching steps are critical because they can damage the GaN materials and lead to electrical degradation effects. To address these limitations, we studied the influence of plasma parameters on electrical degradation in an AlGaN/GaN heterostructure. In this work, the modifications induced by bias-voltage applied during the SiN capping layer opening are investigated. Physical (XRD, XRR, AFM and TEM) and chemical (XPS) characterizations have been performed to have a better understanding this plasma parameter on GaN damage. These results are correlated with Rsheet measurements to evaluate the electrical degradation in the heterostructure. Finally, degradation mechanisms and recommendation have been proposed to improve the plasma etching process.

During the fabrication of a MOS-HEMT, the plasma-etching steps are critical because they can damage the GaN materials and lead to electrical degradation effects. In this paper, we propose to evaluate GaN etching performances through comparing the Cl2-based atomic layer etching (ALE) process with He or Ar as the sputtering gas. The self-limiting synergy and process window of ALE has been investigated. Based on these results, we propose the reasons for the nonself-limiting behavior of the He ALE process. Both ALE processes were compared to a steady-state process by investigating roughness, and electrical measurements, in order to evaluate the induced damage.

The influence of chemical parameters on electrical degradation in an AlGaN/GaN heterostructure was investigated in order to improve performance in metal-oxide-semiconductor high-electron mobility transistor devices. We first examined the influence of plasma chemistry on electrical degradation by using different plasma chemistries for the SiN capping layer opening and comparing the results. The full standard chemistry was evaluated in order to determine the impact of each gas on the degradation. Rsheet and x-ray photoelectron microscopy characterizations and simulations were performed to better understand how light elements such as helium penetrate deeply into the heterostructure and degrade its electrical characteristics. The materials used as masks were also studied. A photoresist mask and a SiN mask were compared on an AlGaN/GaN heterostructure during plasma processing. Electrical degradation was always greater in the presence of a resist due to the decomposition of the resist under the plasma causing hydrogen to be released into the plasma. Simulation of hydrogen implantation in AlGaN was also performed to understand its impact on electrical performance.

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.

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.

Full recess architecture for GaN based High Electron Mobility Transistor (HEMT) enables high mobility and high density of electrons to be conserved without compromising on voltage threshold. To obtain such architecture, standard RIE plasma etching processes are not suitable due to electrical degradation effects and to the lack of well controlled etch depth. To address these limitations, several Atomic Layer Etching (ALE) processes have been developed in the past. In this work, we study an atomic layer etching (ALE) of GaN based on cyclic steps composed of O-2 plasma followed by BCl3 plasma. XPS analysis of the GaN's top surface after each step of the O-2-BCl3 process enabled to propose an etching mechanism and to track nitrogen depletion. The cyclic process conserves the surface stoichiometry. Finally, this cyclic process has been validated on patterns, showing a good morphology, a good etch depth control and a slightly lower electrical degradation compared to standard RIE process.

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.

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.

Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.

The development of lightweight and flexible photovoltaic devices is highly desirable for integration in new applications and to reduce the manufacturing cost of modules. In this context, a lot of effort is put into the development of Cu(In,Ga)Se2 (CIGS) based solar cells on flexible substrates as alternatives to the standard soda-lime glass substrates. Flexible metallic and polyimide foils are frequently used, but in this work we investigated another substrate material presenting advantageous properties: flexible ultra-thin glass. In this article we demonstrated CIGS solar cells with 11.2% efficiency grown on flexible glass as thin as 100μm. It was shown that the differences between solar cells fabricated on ultra-thin glass and standard cells fabricated on 1-mm-thick soda-lime glass lie in the lower Na supply in the first case. We also studied the evolution of solar cell performances when they are bent. This last point is rarely discussed in studies on CIGS on flexible substrates in spite of its importance for flexible device applications.

Sodium addition is necessary to reach high efficiencies with Cu(In,Ga)Se2 (CIGS) solar cells on metallic substrates. This can be achieved using DC-sputtered multilayer Mo back contacts including a sodium-doped layer (Mo:Na). In this study, 450 nm-thick Mo:Na layers were sputter-deposited on Ti foils using a working pressure ranging from 3.4 mTorr to 30 mTorr, and capped with 100 nm-thick, dense, pure Mo layers. The deposition pressure of the Mo:Na layers strongly affected their electrical resistance, optical reflectance, morphology, and crystal quality. The top Mo layer allowed controlling the electrical and optical properties of the back contact, but its own morphology was affected by the structure of the underlying Mo:Na layer. The Na concentration in the CIGS layer varied as a function of the deposition pressure of the Mo:Na layer, inducing changes in the absorber grain size and Ga distribution, as well as in the carrier density. Absolute conversion efficiency improvements over 3% were obtained using the sodium-doped back contacts on Ti foils, leading to a performance which was equivalent to the soda-lime glass reference.

The use of a Zn-based buffer layer for kesterite solar cells presents the double advantage of avoiding cadmium and reducing the amount of light absorbed in this layer. Cu2ZnSn(S,Se)4 solar cells with a ZnS(O,OH) buffer layer have been fabricated and power conversion efficiencies up to 5.8% after light soaking treatment have been measured. Cu2ZnSn(Se,S)4 solar cells with a CdS buffer layer have also been realized, leading to a power conversion efficiency up to 7.0%. The dynamics of the light soaking effect in the case of the ZnS(O,OH) buffer layer has been studied as well, and compared to the same effect on a cell with a Cu(In,Ga)Se2 absorber layer.