Mg$_{2}$Si和Ca$_{2}$Si半导体在光伏应用中的研究：基于密度泛函理论和Bethe-Salpeter方程的计算
Mg$_{2}$Si and Ca$_{2}$Si semiconductors for photovoltaic applications: Calculations based on density-functional theory and the Bethe-Salpeter equation
AI总结：
本研究通过密度泛函理论和BSE方法评估Mg2Si和Ca2Si的电子光学性质及光伏性能，发现Ca2Si适合单结薄膜光伏，Mg2Si适合多结底电池。
摘要：
我们利用基于密度泛函理论和Bethe-Salpeter方程（BSE）的方法，对Mg$_{2}$Si和Ca$_{2}$Si的电子和光学性质以及光伏（PV）性能参数进行了全面评估。当使用PBE、PBEsol和mBJ泛函时，Mg$_{2}$Si（Ca$_{2}$Si）的带隙在0.25-0.6（0.57-0.96）eV范围内。在独立粒子近似（IPA）下，介电函数的实部和虚部在2.6（1.0）eV处达到最大值50（16.3），在约3.24（3.4）eV处达到最大值61（16.2）。在BSE框架内，这些值分别变为在2.5（0.86）eV处的59（17）和在2.68（3.1）eV处的65（16.6）。研究发现，激子效应对于理解Mg$_{2}$Si的实验光学光谱至关重要，但该效应在Ca$_{2}$Si中相对较弱。本研究强调了不同理论近似水平对于高精度获取硅化物光学光谱数据的重要性。最后，我们通过光谱限制最大效率（SLME）计算评估了光伏效率。在辐射复合的基础上，我们还通过Shockley-Read-Hall（SRH）机制纳入了缺陷陷阱态的非辐射载流子复合以评估效率。在所研究的缺陷中，间隙Mg（Si）被确定为Mg$_{2}$Si（Ca$_{2}$Si）中最稳定的缺陷，并提供2微秒（11.3毫秒）的SRH寿命。使用BSE吸收光谱估算的最大SLME为1.3%（31.2%），由于SRH复合而降至1.2%（28.5%）。本研究表明，Ca$_{2}$Si（Mg$_{2}$Si）是单结（多结电池中的底电池）薄膜光伏器件的潜在候选材料。
英文摘要：
We conduct a comprehensive assessment of the electronic and optical properties, as well as photovoltaic (PV) performance parameters for Mg$_{2}$Si and Ca$_{2}$Si using density-functional theory and Bethe-Salpeter equation (BSE) based methods. The band-gap for Mg$_{2}$Si (Ca$_{2}$Si) is found to be in the range of 0.25-0.6 (0.57-0.96) eV when PBE, PBEsol and mBJ functionals are used. In the independent-particle approximation (IPA), the real and imaginary parts of dielectric function show maximum values of 50 (16.3) at 2.6 (1.0) eV and 61 (16.2) at $\sim$3.24 (3.4) eV, respectively. Within BSE, these respective values change to 59 (17) at 2.5 (0.86) eV and 65 (16.6) at 2.68 (3.1) eV. The excitonic effect is found to be crucial in understanding the experimental optical spectra of Mg$_{2}$Si. However, this effect is relatively weaker in Ca$_{2}$Si. Present study highlights the importance of different levels of theoretical approximations for obtaining the optical spectroscopy data of silicides with a high level of accuracy. Finally, we have evaluated PV efficiency by using spectroscopic limited maximum efficiency (SLME) calculation. On the top of radiative recombination, we have also incorporated non-radiative carrier recombination at a defect trap state via Shockley-Read-Hall (SRH) mechanism to evaluate the efficiency. Among the studied defects, the interstitial Mg (Si) is identified as the most stable in Mg$_{2}$Si (Ca$_{2}$Si) and this provides SRH lifetime of 2 $μs$ (11.3 $ms$). The estimated maximum SLME using BSE absorption spectrum is 1.3 (31.2)\%, which decreases to 1.2 (28.5)\% due to SRH recombination. The present study suggests that Ca$_{2}$Si (Mg$_{2}$Si) is a potential candidate for single-junction (bottom cell in multi-junction) thin-film PV devices.
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