Bercx Marnik, Sarmadian Nasrin, Saniz Rolando, Partoens Bart, Lamoen Dirk
EMAT & CMT groups, Department of Physics, University of Antwerp, Campus Groenenborger, Groenenborgerlaan 171, 2020 Antwerp, Belgium.
Phys Chem Chem Phys. 2016 Jul 27;18(30):20542-9. doi: 10.1039/c6cp03468c.
Chalcopyrite semiconductors are of considerable interest for application as absorber layers in thin-film photovoltaic cells. When growing films of these compounds, however, they are often found to contain CuAu-like domains, a metastable phase of chalcopyrite. It has been reported that for CuInS2, the presence of the CuAu-like phase improves the short circuit current of the chalcopyrite-based photovoltaic cell. We investigate the thermodynamic stability of both phases for a selected list of I-III-VI2 materials using a first-principles density functional theory approach. For the CuIn-VI2 compounds, the difference in formation energy between the chalcopyrite and CuAu-like phase is found to be close to 2 meV per atom, indicating a high likelihood of the presence of CuAu-like domains. Next, we calculate the spectroscopic limited maximum efficiency (SLME) of the CuAu-like phase and compare the results with those of the corresponding chalcopyrite phase. We identify several candidates with a high efficiency, such as CuAu-like CuInS2, for which we obtain an SLME of 29% at a thickness of 500 nm. We observe that the SLME can have values above the Shockley-Queisser (SQ) limit, and show that this can occur because the SQ limit assumes the absorptivity to be a step function, thus overestimating the radiative recombination in the detailed balance approach. This means that it is possible to find higher theoretical efficiencies within this framework simply by calculating the J-V characteristic with an absorption spectrum. Finally, we expand our SLME analysis to indirect band gap absorbers by studying silicon, and find that the SLME quickly overestimates the reverse saturation current of indirect band gap materials, drastically lowering their calculated efficiency.
黄铜矿半导体作为薄膜光伏电池的吸收层具有相当大的应用价值。然而，在生长这些化合物的薄膜时，常常发现它们含有类CuAu畴，这是黄铜矿的一种亚稳相。据报道，对于CuInS2，类CuAu相的存在提高了基于黄铜矿的光伏电池的短路电流。我们使用第一性原理密度泛函理论方法研究了一系列选定的I-III-VI2材料中这两种相的热力学稳定性。对于CuIn-VI2化合物，发现黄铜矿相和类CuAu相之间的形成能差异接近每原子2毫电子伏，这表明存在类CuAu畴的可能性很大。接下来，我们计算了类CuAu相的光谱极限最大效率（SLME），并将结果与相应的黄铜矿相进行比较。我们确定了几种具有高效率的候选材料，如类CuAu的CuInS2，在500纳米厚度下我们得到的SLME为29%。我们观察到SLME的值可以高于肖克利-奎塞尔（SQ）极限，并表明这种情况会发生是因为SQ极限假设吸收率为阶跃函数，因此在详细平衡方法中高估了辐射复合。这意味着在这个框架内，只需通过计算具有吸收光谱的J-V特性就有可能找到更高的理论效率。最后，我们通过研究硅将SLME分析扩展到间接带隙吸收体，发现SLME很快高估了间接带隙材料的反向饱和电流，大幅降低了它们计算出的效率。
