• Energy Research

Electrodeposition followed by rapid thermal annealing is a favorable process for industrial solar thin film fabrication. In this regard, we attempt to develop a process for earth-abundant pure sulfide kesterite solar cells fabrication of Cu2ZnSnS4 (CZTS) at pre-industrial scale (15 × 15 cm2). Synthesis at a large scale is a challenging issue that we attempt to address discussing the choice of annealing parameters. We found out that in our system a low background pressure is needed to ensure a good vapors distribution. However, S, SnS, Zn volatilities are enhanced, making difficult the control of the composition. Annealing temperature profile has a strong influence on the final absorber composition. The introduction of an intermediate reactive pre-alloying step previously to the high temperature annealing is shown to help to obtain more compact absorbers, leading to a maximum power conversion efficiency of 2.4% for a 0.435 cm2 CZTS device.

Cu2ZnSnSe4 solar cells were synthesized by electrodeposition of metal stack precursors followed by selenization, a high potential process for industry, leading to conversion efficiencies above 5%. An additional selenium-capping layer deposited on the precursor before annealing showed improved uniformity and morphology of CZTSe layers compared to other selenization routes. Two different atmospheric annealing systems were used: a closed graphite box in a tubular furnace and a three-chamber dynamic rapid thermal processing furnace. The RTP system gave larger grains and more compact layers, whereas CZTSe selenized in tube furnace had smaller grains and a higher series resistance. Both annealing systems gave best cells power conversion efficiencies over 5%. We will discuss the device photoelectric properties and their relation to material structures and processing.

AbstractDuring the elaboration of standard CISEL™cells, electroplated CuInSe2 precursors undergo a rapid thermal processing (RTP) in a sulfur-containing atmosphere to promote grain growth and enable sulfurization of the precursor. The aim of this work is to show how structural and morphological properties of the CuIn(S,Se)2-based solar cells can be modified with RTP parameters, namely temperature, heating rate, and sulfur addition. X-ray diffractograms show that the preferential (112) orientation of the electrodeposited CuInSe2 precursor is maintained after annealing but the coefficient of crystallographic texture can be modified with specific RTP parameters. It is also shown that the quantity of sulfur incorporated in the chalcopyrite lattice can be controlled and reaches almost pure CuInS2 according to the sulfur quantity used during the RTP. Another effect of the RTP annealing is to form a Mo(S,Se)2 layer which can lead to a quasi-ohmic contact between the molybdenum and the absorber. The properties of the Mo(S,Se)2 buffer layer are also studied according to the process parameters and an increase of the annealing temperature or of the sulfur concentration tends to increase the thickness of this layer.

Abstract In this work, Cu(In,Ga)Se2 (CIGS) absorbers with thicknesses ranging from 2 µm to 370 nm were prepared by a two-step process using electrodeposition of Cu-In-Ga followed by annealing under a pure Se atmosphere. Based on compositional characterizations, it is shown that in order to decrease the thickness of the precursors, it is not enough to reduce only the deposition time of Cu-In-Ga layers without working on the composition and deposition parameters of the thin films. After the optimization of annealing conditions, the properties of the absorbers and solar cells with three different thicknesses (2 µm, 0.7 µm and 0.37 µm) were compared. It is shown that, in spite of the decreasing thickness, hence a decrease in JSC, the VOC of ultrathin CIGS electrodeposited solar cells can be improved due to an increase in the Ga content of the electrodeposited absorbers. Without deliberate light trapping and anti-reflecting coating from the very thin absorber layer of 0.37 µm, an efficiency of 8.7% with VOC of 685 mV, JSC of 19 mA/cm2 and FF of 67%, was achieved.

Abstract Cu–In electrodeposited layers were annealed using rapid thermal processing (RTP) in a reactive atmosphere containing sulfur vapors. The CuInS 2 formation mechanism during sulfurization of electrodeposited precursors proceeds mainly through direct sulfurization of the metallic Cu–In alloy, forming spinel CuIn 5 S 8 and chalcopyrite CuInS 2 ternary phases. During the heating step, the Cu–In metallic alloy gets richer in copper as the temperature increases and transforms from CuIn 2 to Cu 11 In 9 , then Cu 16 In 9 and finally to Cu 7 In 3 . The use of rapidly cooled samples stopped after different durations of the process along with ex-situ XRD analysis enabled us to differentiate the Cu 16 In 9 and Cu 7 In 3 phases. Finally, the efficiency of the solar cells made with the two-step electrodeposition and RTP low-cost process reaches 11% (active area 0.421 cm 2 ), which is close to the results obtained for cells made with PVD precursors.

Earth-abundant kesterite Cu2ZnSnSe4 material is a promising candidate for the mass production of low-cost thin film solar cells. However, the synthesis of single-phase kesterite films is especially challenging, since the kesterite single-phase region in the equilibrium phase diagram is very narrow. In this study, the metal composition was varied within the Cu-poor composition range in order to study the presence of Sn-Se secondary phases. Both SnSe and SnSe2 are found in copper-poor CZTSe absorbers where Zn/Sn < 1; in addition, these phases are also found when Zn/Sn > 1 because the studied composition range is actually copper-poor zinc-rich and tin-rich. The Sn-Se secondary phases can be detected using X-ray diffraction, a bulk detection method. They are also detected at the absorber's surface by SEM and Raman spectroscopy. Therefore, when the Sn-Se phases are present, at least a part of them is located at the absorber's surface, which is highly detrimental to device performance. Acting as shunting paths, they reduce the device power conversion efficiency and demonstrate an apparent quantum efficiency effect under reverse bias. Removal of these phases from the surface by chemical etching greatly reduces their detrimental influence.

AbstractCu2ZnSnSe4 solar cell absorbers are synthesized by large‐area electrodeposition of metal stack precursors followed by selenization. A champion solar cell exhibits 8.2% power conversion efficiency, a new record for Cu2ZnSnSe4 solar cells prepared from electrodeposited metallic precursors. Significant improvements of device performance are achieved by the application of two etching procedures and buffer layer optimization. These results validate electrodeposition as a credible alternative to vacuum processes (sputtering, co‐evaporation) for earth‐abundant thin‐film solar cell fabrication at low cost. Copyright © 2015 John Wiley & Sons, Ltd.

Abstract This work describes the use of quasi-resonant Raman scattering measurements for the assessment of chemical composition and nanocrystalline phases in CuInS2 based photovoltaic technologies. Raman spectra measured in S-rich CuIn(S,Se)2 layers at a fixed wavelength of 785 nm show a strong increase in the intensity of the peaks that are related to the quasi-resonant excitation of the corresponding vibrational modes. The spectra measured at these conditions are characterised by the presence of seven bands that have been identified with four first order peaks in the 200–400 cm−1 spectral region and three second order peaks in the 550–750 cm−1 spectral region. These spectra are strongly sensitive to changes in the composition of S-rich CuIn(Se,S)2 alloys. On the other hand, the strong increase in the intensity of the peaks allows the development of in-situ measurements for real time process monitoring. As an example of this application, Raman spectra have been measured at real time conditions during the growth of colloidal CuInS2 nanocrystals that are being developed for the fabrication of low cost solar cells. The data obtained corroborate the potential of quasi-resonant Raman scattering measurements for the development of ex-situ and in-situ quality control and process monitoring tools in thin film chalcopyrite photovoltaic technologies.