Impact of nonparabolic electronic band structure on the optical and transport properties of photovoltaic materials
Impact of nonparabolic electronic band structure on the optical and transport properties of photovoltaic materials
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DOI：
10.1103/physrevb.99.085207
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发表时间：
2018-11
影响因子：
3.7
通讯作者：
Lucy D. Whalley;J. Frost;B. Morgan;A. Walsh
中科院分区：
文献类型：
--
作者：
Lucy D. Whalley;J. Frost;B. Morgan;A. Walsh
对于光伏器件中使用的半导体，有效质量近似允许根据第一原理计算计算重要的材料特性，包括光学特性（例如激子结合能）、缺陷特性（例如供体和受体水平）和传输特性（例如载流子迁移率）。半导体的导带和价带通常近似为围绕其极值的抛物线，这给出了简单的理论描述，但忽略了实际材料的复杂性。在这项工作中，我们使用密度泛函理论来评估能带非抛物线对四种常见薄膜光伏材料（GaAs、CdTe、Cu$_2$ZnSnS$_4$ 和 CH$_3$NH$_3$PbI$_3$）在与实际应用相关的温度和载流子密度下的影响。首先，我们计算带边缘的有效质量。我们比较有限差分、未加权最小二乘法和热加权最小二乘法。我们发现热加权最小二乘法降低了对采样密度选择的敏感性。其次，我们采用凯恩准线性色散来量化非抛物线的程度，并比较不同电子结构理论的结果，以考虑自旋轨道耦合和电子交换的影响。最后，我们重点关注卤化物钙钛矿 CH$_3$NH$_3$PbI$_3$ 作为模型系统，以评估非抛物面性对高载流子浓度下计算的电子传输和光学性能的影响。我们发现，在浓度为 10$^{20}$ cm$^-3$ 时，光学有效质量相对于低载流子浓度值增加了两倍，而极化子迁移率则降低了三倍。我们的工作表明，应该对具有显着带非抛物线性的其他半导体的预测光学和传输特性进行类似的调整。
For semiconductors used in photovoltaic devices, the effective mass approximation allows calculation of important material properties from first-principles calculations, including optical properties (e.g. exciton binding energies), defect properties (e.g. donor and acceptor levels) and transport properties (e.g. carrier mobilities). The conduction and valence bands of semiconductors are commonly approximated as parabolic around their extrema, which gives a simple theoretical description, but ignores the complexity of real materials. In this work, we use density functional theory to assess the impact of band non-parabolicity on four common thin-film photovoltaic materials - GaAs, CdTe, Cu$_2$ZnSnS$_4$ and CH$_3$NH$_3$PbI$_3$ - at temperatures and carrier densities relevant for real-world applications. First, we calculate the effective mass at the band edges. We compare finite-difference, unweighted least-squares and thermally weighted least-squares approaches. We find that the thermally weighted least-squares method reduces sensitivity to the choice of sampling density. Second, we employ a Kane quasi-linear dispersion to quantify the extent of non-parabolicity, and compare results from different electronic structure theories to consider the effect of spin-orbit coupling and electron exchange. Finally, we focus on the halide perovskite CH$_3$NH$_3$PbI$_3$ as a model system to assess the impact of non-parabolicity on calculated electron transport and optical properties at high carrier concentrations. We find that at a concentration of 10$^{20}$ cm$^-3$ the optical effective mass increases by a factor of two relative to the low carrier-concentration value, and the polaron mobility decreases by a factor of three. Our work suggests that similar adjustments should be made to the predicted optical and transport properties of other semiconductors with significant band non-parabolicity.