非铅钙钛矿太阳能电池材料A2InBiX6的电子结构与光伏性能理论研究

The rapid developments of perovskite solar cells based on APbX3 have recently made halide perovskites become great promising candidates as photovoltaic absorbers, while the toxicity due to lead and their intrinsic material instability are the two challenges still standing in their way to large-scale commercial applications. Over the past few years, some lead-free halide perovskites have been formed by using single nontoxic cations to replace lead and almost have shown poor photovoltaic performance, which urgently calls for researchers to find other material design strategy of replacing lead. Recently, halide double perovskites A2InBiX6, where using two heterovalent cations (In+ and Bi3+) to replace Pb2+, have been proposed as very promising lead-free halide perovskite photovoltaic absorbers. Therefore in this project, we will deeply investigate the properties of lead-free A2InBiX6 halide double perovskites based on the Density Functional Theory calculations. In order to screen out potential photovoltaic absorber in A2InBiX6, their band structures, thermal and dynamics stabilities, carrier effective masses, exciton binding energies and defect tolerances will be studied. For those candidate materials with optimal performance, we will calculate the effect of doping or pressure on the electronic structure and structure stability, which could help us to put forward the performance optimization scheme and possible synthesis of stable A2InBiX6. We hope this project can help to make a better understanding of optimization strategies of halide perovskite absorbers, and also provide a theoretical guidance for next experimental attempts to synthesize stable, environmentally-friendly and high-efficient halide perovskite photovoltaic absorber.

APbX3型铅基钙钛矿太阳能电池的迅猛发展使卤化钙钛矿成为极具潜力的光伏材料，然而含铅带来的毒性问题和结构不稳定性是其商业化面临的两个重大挑战。过去几年利用单一无毒元素替换铅得到的系列类钙钛矿材料的光伏性能并不理想，亟需研究者找到替换铅元素的其他材料设计方案。利用两种异价金属元素（In+和Bi3+）共同替换Pb2+而衍生出的A2InBiX6双钙钛矿体系被认为是一种很有潜力的非铅钙钛矿光伏材料。本项目拟通过第一性原理计算，对A2InBiX6体系材料的带隙特征、热力和动力学稳定性、载流子有效质量、激子束缚能及缺陷容忍度等性质进行系统研究，澄清体系中电子结构与光伏性能之间的关系；对于其中性能最优的材料，关注掺杂或加压对其电子结构及稳定性的影响，提出可能的材料制备和光伏性能优化的方案。本项目有助于更好地理解钙钛矿材料光伏性能的优化调控机制，并为实验合成稳定高效且环境友好的钙钛矿光伏材料提供参考。

通过两种异价元金属共同替换铅元素，并且在相应衍生的双钙钛矿体系中寻找潜在的钙钛矿太阳能电池吸光材料是本项目的创新之处。本项目基于第一性原理计算，主要从材料的晶格结构、热力学稳定性、电子结构以及光电性质四个方面进行研究，预测了一类有潜力的非铅钙钛矿太阳能电池的吸光材料，即双钙钛矿(CH3NH3)2InBiX6 (X=I,Br,Cl)。我们主要从两个方面深入研究了其热力学稳定性，一方面分析了其分解能，即比较双钙钛矿能量和可能的分解产物的能量关系，所考虑的可能的分解方式的分解能都大于0，表明双钙钛矿是热力学稳定的。另一方面，我们进行了分子动力学模拟，进一步证实了双钙钛矿在室温下可以稳定存在。另外，我们发现当施加低程度压强（2Gpa~4Gpa）时，分解能有增加的趋势，说明其热力学稳定性在提高，因此，我们提出施加低强度的压强有助于双钙钛矿(CH3NH3)2InBiX6 (X=I,Br,Cl)的材料稳定。随后，我们继续仔细研究了其电子结构和光电性质，发现双钙钛矿(CH3NH3)2InBiX6 (X=I,Br,Cl)具有合适的直接带隙，载流子的有效质量小以及三维电子特性，这些特性又有利于其获得强的光吸收，以上电子和光电性质都使得双钙钛矿(CH3NH3)2InBiX6 (X=I,Br,Cl)有潜力成为太阳能电池的光吸收材料。以前关于双钙钛矿材料的研究主要集中在晶体合成和基本物理性质上，例如相变、磁性、电子结构等，因此本项目中关注其在光伏领域的应用不仅可以加深对该体系材料的认识，也可以为实验制备无毒且高效的太阳能电池吸光材料提供理论参考。