High-Efficiency Flexible Thin-Film Single-Crystal-Like GaAs Solar Cells Based on Cheap Metal Tape
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This study introduces the first flexible single-junction III-V photovoltaic solar cells (PVSCs) based on single-crystal-like GaAs thin films on a low-cost metal substrate by direct and continuous deposition, which can bypass expensive single crystal wafer fabrication. A promising SC device performance characteristic with an open-circuit voltage of 560 mV and short circuit current of 19.4 mA/cm2, resulting in a conversion efficiency of ~7.6%, is demonstrated for the first time. We developed a 2D numerical simulation code to study on the effects of low-angle grain boundaries (GB) in single-crystal-like GaAs thin-film on its photovoltaic performance. The 2D model is capable of investigating the effect of localized recombination centers in the material by defining different regions of defective low-angle grain boundaries and single crystalline intra-grains and shows very well-matched result with experiment. According to the modeling results, increasing the grain size of GaAs from 2 µm to 50 µm can improve efficiency of solar cells from 4.8% to 12.3%. The Voc of devices shows more sensitivity to the amount of grain boundary densities than other SC characteristic factors. The 2D model is also employed to study bulk passivation of GBs which shows that thin film single-crystal-like GaAs solar cells whit an efficiency of ~19.7% can be achievable even at small grain size of 2 µm if effective grain boundary passivation is applied. Therefore, we have employed various passivation techniques to increase the efficiency performance of actual single-crystal-like GaAs solar cells. The trioctylphosphine sulfide (TOP:S) solution treatment is the most effective technique for the single-crsytal-like GaAs SC passivation. The study on the effect of TOP:S treatment on the PV performance of newly developed single-crystal-like thin film GaAs showed that the SC dark saturation current after TOP:S treatment decreased from Jo ~210-3 A/cm2 to ~3.510-4 A/cm2 (at V = -500mV) and a remarkable improvement in efficiency performance of SCs was achieved with an increase of ~24%, 12.8%,14.5%, and 64% for Voc, Jsc, FF, and overall efficiency, respectively. The overall efficiency of flexile single-junction single-crystal-like GaAs thin film SC after TOP:S passivation become 13.5% which is about two times of the previous record for un-passivated SCs with efficiency of 7.6%.