This study introduces a chalcogenide-based thin-film solar cell structure optimized for rear illumination, featuring a thinner, wide-bandgap Cu(In,Ga)S _2 chalcopyrite absorber. Matching the performance of traditional front-illuminated designs, this configuration paves the way for further photoelectrochemical advances thanks to its metallic top layer, which can serve as a versatile grafting platform that is resistant to their operating conditions.

A transmission electron microscopy study of epitaxial Cu(In,Ga)S 2 (CIGS) films coevaporated on GaP/Si(001), in either Cu‐rich or Cu‐poor conditions, is presented. The spatial distribution and the orientation of the different phases by means of electron diffraction are unveiled. From atomically resolved imaging of the CIGS film's atomic structure, it is concluded that different chalcopyrite domains, sharing cation antiphase symmetries of the cation sublattice, coexist in the films. At least three types of cation antiphase boundaries (CAPBs), which do or do not lead to a violation of the octet rule, depending on the propagation direction, are conceptualized. Even though it is observed that epitaxial CIGS is highly prone to cation antiphase disorder (CAPD), and it is found that the growth of CIGS in Cu‐rich conditions leads to a lower density of CAPBs, as compared to Cu‐poor growth conditions. This opens the question of the influence of CAPBs on CIGS electronic properties.

Recent progress in the development of chalcopyrite thin film-based solar cell onto transparent and conductive back contact led us to try to evaluate the feasibility, the performance and the limitations of wire connected 2T tandems, based exclusively on chalcopyrite technology. The performances of the investigated devices appear limited by too low transmission of photons of wavelength above 800 nm by the top cell; this poor transmission is primarily due to the use of heavily doped ITO back contact. It is also shown that increasing the current of the bottom cell has a severe drawback. In fact, the fill factor of the tandem is hindered, because then the device becomes limited by the top cell, which has rather low fill factor. As a conclusion of this work, one can suggest that the implementation of full chalcopyrite tandems relies on future progress in the performance of cells based on wide bandgap absorbers and deposited onto transparent back contacts.

This study examines the growth condition to obtain a single-phase Cu(In,Ga)S2 (CIGS) chalcopyrite film epitaxially grown by coevaporation on a GaP/Si(001) pseudo-substrate. In particular, we report the structural differences between KCN-etched Cu-rich and Cu-poor CIGS films coevaporated on GaP/Si(001) by 1-stage process. The Cu-poor CIGS film consists of at least three phases; the main crystal is found to be chalcopyrite-ordered, coexisting with In-rich CuIn5S8, and CuAu-ordered CuInS2, all sharing epitaxial relationships with each other and the GaP/Si(001) pseudo-substrate. On the other hand, the Cu-rich CIGS film is single-phase chalcopyrite and displays sharper X-ray diffraction peaks and a lower density of microtwin defects. The elimination of the secondary CuAu-ordered phase with Cu excess is demonstrated. In both films, the chalcopyrite crystal exclusively grows with its c-axis aligned with the out-of-plane direction of Si[001]. This study confirms prior findings on the thermodynamics of Cu–In-Ga-S and the stability of secondary phases.

The present article discusses the investigation of CuIn Ga S (CIGS) thin films for photovoltaic applications. For decades, a Cu-rich composition has been used to create solar cells with efficiencies of up to 13.5%; however, interest in chalcopyrite sulfide has recently been revived due to its high and adjustable bandgap, making it a serious candidate as a top cell in tandem configurations. Although chalcopyrite selenides share many properties with CIGS thin films, crucial differences have been reported. To further understand these materials, we studied more than 500 samples of absorbers and resulting solar cells. First, we found that the compositional window for obtaining single-phase CIGS thin films with a 3-stage co-evaporation process is very narrow. Second, we reported that a combination of low copper content and sodium addition during growth is required to maximize the Photoluminescence intensity ( i.e. to minimize the absorber-related open-circuit voltage losses). Finally, we showed that solar cell performance and stability depend not only on absorber quality but also on phenomena at interfaces (absorber/buffer and grain boundaries). Altogether, we formulate growth recommendations for the manufacture of stable CIGS/CdS solar cells with state-of-the-art efficiency.

The interest for pure sulfide Cu(In,Ga)S2 chalcopyrite thin films is increasing again because their optical properties make them relevant candidates to be applied as top cell absorbers in tandem structures. Nonetheless, their use as so is still hindered by the level of single-junction cells performance achieved so far, which are far below those demonstrated by selenide absorbers. Amongst the reasons at the origin of the limited efficiency of Cu(In,Ga)S2-based solar devices, one can mention the poor tolerance of S-chalcopyrite to Cu deficiency. In fact, Cu-poor Cu(In,Ga)S2 films contain CuIn5S8 thiospinel secondary phase which is harmful for device performance. In the present work, we investigate Cu(In,Ga)S2 thin films grown by a modified three-stage process making use of graded indium and gallium fluxes during the first stage. The resulting absorbers are single phase and made of large grains extended throughout the entire film thickness. We propose that such a morphology is a proof of the recrystallization of the entire film during the synthesis. Devices prepared from those films and buffered with bath deposited CdS demonstrate outstanding efficiency of 16.0%. Replacing CdS by Zn(O,S) buffer layer leads to increased open circuit voltage and short circuit current; however, performance become limited by lowered fill factor.

We propose to explore tandem junctions associating single crystalline silicon bottom cell (Eg = 1.12 eV) and wide bandgap (1.7 eV) CuIn0.75Ga0.25S2 (pure-sulfide CIGSu) top cell, using GaP intermediate layer. Our purpose is to grow CIGSu films under epitaxial conditions on GaP/Si(001) to improve the top cell efficiency, thanks to a reduction of the structural defects density detrimental for the cell performance, so that CIGSu/Si tandem cells can emerge as cost competitive for the next generation of PV modules. Record efficiency on standard AZO/ZnMgO/CdS/CIGSu/Mo/Glass solar cell and epitaxy of CIGSsu on GaP/Si are demonstrated.

In this study, the epitaxial growth of co-evaporated Cu(In,Ga)Se2 films (CIGSe) onto GaP/Si(001) pseudo-substrates, where the GaP thin layer is epitaxially grown by Molecular Beam Epitaxy (MBE), is investigated. Extensive structural characterisation of epi-CIGSe is carried out via X-ray diffraction as well as transmission electron microscopy. Sturdy evidence of an epitaxial growth of CIGSe on (GaP/Si)(001) is observed, with the propagation of twins originating from the GaP/Si interface, through the CIGSe/GaP interface. This work aims at paving the way for future CIGSe/GaP/Si structures for the development of tandem solar cells with a c-Si bottom cell, and a GaP interfacial buffer layer for band edge engineering, allowing for the monolithic epitaxial growth of high quality CIGSe as a thin film top cell absorber.