Defects in Copper Indium Aluminum Diselenide Films and their Impact on Photovoltaic Device Performance
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Defects in Copper Indium Aluminum Diselenide Films and their Impact on Photovoltaic Device Performance Jennifer T. Heath1, J. David Cohen2, and William N. Shafarman3 1 Department of Physics, Linfield College, McMinnville, OR 97128, U.S.A. 2 Department of Physics, University of Oregon, Eugene, OR 97403, U.S.A. 3 Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A. ABSTRACT The electronic properties of polycrystalline CuIn1-xAlxSe2 (CIAS) films, which are incorporated as the absorber layer in photovoltaic devices, have been studied to better understand limitations on device performance. These studies have shown that compared to lower Al content films and to CuIn1-yGaySe2films, films with x ≥ 0.29 are relatively intrinsic, spatially nonuniform, and have broader bandtails characterized by much higher Urbach energies. This indicates that the CIAS films with x ≥ 0.29 are significantly more disordered than lower Al content CIAS or corresponding CIGS films, which likely negatively impacts the resulting photovoltaic device performance.
INTRODUCTION In order to create optimized single junction devices, devices that operate at higher VOC, or a wider bandgap layer for a tandem device, the bandgap of CuInSe2 has commonly been increased by alloying with Ga. Very high efficiency single junction devices, with bandgaps around 1.2 eV, have been obtained [1]. However, it is difficult to obtain high efficiency devices when Ga/(In+Ga) ≥ 0.4. Recently, alloys with Al instead of Ga have begun to be explored, and a cell with 16.9% efficiency has been demonstrated [2]. However, currently the performance of these CuIn1-xAlxSe2 (CIAS) devices deteriorates at higher Al fractions [3]. To better understand this behavior, we have begun to explore the electronic properties of such CIAS films.
DEVICE PREPARATION The photovoltaic devices for this study were prepared at the Institute of Energy Conversion, University of Delaware, using processes very similar to those developed to optimize CIGS devices [3,4]. Films were deposited using 4-source elemental evaporation on Mo-coated sodalime glass, at a substrate temperature of 530 ºC. The films were typically grown to about 2-3 µm of thickness. Throughout the growth process, the elemental fluxes were kept constant, so that there was no intentional grading of the film composition. The films were grown slightly Cu poor, with Cu/(In+Al) ≈ 0.8-0.9. The solar cells were completed by chemical bath deposition of 30-40 nm of CdS, followed by sputtered ZnO and ITO layers, and an evaporated Ni-Al grid. X-ray diffraction studies have verified that these CIAS films are single phase. Their elemental composition has been measured using energy dispersive x-ray spectroscopy. Devices for this study contained CIAS absorber layers with x = 0.13 to 0.48. The corresponding bandgaps, estimated from quantum efficiency measurements, ranged from 1.15 eV to 1.67 eV.
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