Collaborative Research: SusChEM: Air-stable, high-lifetime bismuth compounds as solar absorbers with perovskite-like band structures

太阳代表着地球上最丰富的潜在可持续能源。用于发电的太阳能电池需要吸收太阳能量并将其光子转化为电子的材料，这一过程被称为光伏发电。最近，基于无机-有机卤化物钙钛矿材料的材料已经取得了接近硅太阳能电池的有前途的太阳能转换效率，并且可以使用低成本的基于溶液的制造方法从地球丰富的元素中制造出来。然而，这些钙钛矿材料含有铅，这是有毒的，而且它们在有水分的情况下也会降解，这阻碍了它们的商业用途。为了解决这些限制，该项目将开发基于元素铋的新型类似太阳能钙钛矿的太阳能材料，这种元素既丰富又低毒。这种未开发的材料类别是通过理论和计算技术确定的，该研究将利用这些理论见解来制造和测试这种潜在的令人兴奋的光伏应用新型材料的性能。实验结果，结合理论分析，将反馈到材料设计标准，以帮助确定有前途的新材料，继续研究。与此项目相关的教育活动包括与少数民族服务组织合作提供的实验室实习、动手学习模块，以及持续开发一学期的太阳能光伏课程。具有钙钛矿类带结构的铋（Bi）化合物，包括三元卤化铋和胆卤铋，是太阳能光伏应用的有前途的吸收材料，在三个方面超越了传统的钙钛矿材料。首先，它们的电子结构和强大的自旋轨道耦合能够以类似于碘化铅甲基铵钙钛矿材料的方式容忍内在缺陷，但不含铅。其次，铋阳离子具有较大的玻恩有效电荷，可提供高介电常数和屏蔽带电缺陷。第三，无铅碘化铋甲基铵材料在水蒸气存在的情况下可以保持相稳定，因为它优先形成保护氧化层。考虑到这些相对于铅基有机金属卤化物钙钛矿材料的潜在优势，本研究的总体目标是通过理论和实验研究，对含铋化合物作为类钙钛矿太阳能光伏材料的光伏性能有一个基本的了解。为此，本研究有三个目标。第一个目标是通过研究碱金属偏硫铋铋矿来研究对称性对有利输运性质的重要性，偏硫铋矿是少数几种不分层的铋基材料之一。第二个目标是开发模型，以确定从时间分辨光致发光测量材料的重组过程和重组率。第三个目标是发展策略，以生长新的双化合物具有更高的纯度和相关的杂质含量与少数载流子寿命。密度泛函理论将用于确定电子结构、介电常数和载流子有效质量。实验研究将建立有效太阳能吸收的材料设计标准，重点是确定颗粒内结构缺陷、分子阳离子和晶体对称性对传输扩散长度、光学性质和整体器件性能的作用。薄膜的扩散长度将通过时间分辨光致发光测量以及单光子和双光子光谱的深度分辨寿命测量来获得，从而解耦表面复合的影响。

The sun represents the most abundant potential source of sustainable energy on earth. Solar cells for producing electricity require materials that absorb the sun's energy and convert its photons to electrons, a process called photovoltaics. Recently, materials based on inorganic-organic halide perovskite materials have achieved promising solar energy power conversion efficiency approaching that of silicon solar cells, and can be made from earth-abundant elements using lower-cost, solution based fabrication methods. However, these perovskite materials contain lead, which is toxic, and they also degrade in the presence of moisture, which prevents their commercial use. To address these limitations, this project will develop new solar perovskite-like solar materials based on the element bismuth, which is both abundant and has low toxicity. This unexplored class of materials was identified through theoretical and computational techniques, and the research will use these theoretical insights to make and test the performance of this potentially exciting new class of materials for photovoltaic applications. The experimental findings, in combination with theoretical analysis, will feed back to the materials design criteria to help identify promising new materials for continued study. The educational activities associated with this project include laboratory internships offered in coordination with minority-serving organizations, hands-on learning modules, and the continued development of a semester-long solar photovoltaics course.Bismuth (Bi) compounds with perovskite-like band structures, including ternary bismuth halides and bismuth chalcohalides, are promising absorption materials for solar photovoltaic applications that go beyond conventional perovskite materials in three ways. First, their electronic structure and strong spin-orbit coupling can enable tolerance to intrinsic defects in a way that is similar to methylammonium lead iodide perovskite materials, but do not contain lead. Second, the bismuth cation has a large Born effective charge to provide high dielectric constants and screening of charged defects. And third, lead-free methylammonium bismuth iodide materials can be phase stable in the presence of water vapor due to the preferential formation of protective oxide layers. Given these potential advantages over lead-based organic metal halide perovskite materials, the overall goal of this research is to gain a fundamental understanding of the photovoltaic performance of bismuth-containing compounds as perovskite-like solar PV materials through theoretical and experimental investigation. Towards this end, the research has three objectives. The first objective is to investigate the importance of symmetry for favorable transport properties by investigating the alkali metal metathiobismuthites, one of the few classes of Bi-based materials that are not layered. The second objective is to develop models to determine the recombination processes and recombination rate in materials from time-resolved photoluminescence measurements. The third objective is to develop strategies for growing the new Bi-compounds with higher purity and correlate impurity content with minority carrier lifetime. Density functional theory will be used to determine the electronic structure, dielectric constant, and charge carrier effective masses. Experimental studies will establish the materials design criteria for efficient solar absorption, focusing on determining the role of intra-granular structural defects, molecular cations, and crystal symmetry on transport diffusion length, optical properties, and overall device performance. The diffusion length of the thin films will be obtained through time-resolved photoluminescence measurements as well as single photon and two-photon spectroscopy for depth-resolved lifetime measurements that decouple the effects of surface recombination.