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

太阳是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。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良好的太阳能转换效率，并且可以利用富含地球的元素以较低的成本、基于溶液的制备方法来制备。然而，这些钙钛矿材料含有有毒的铅，而且它们在潮湿的情况下也会降解，这阻碍了它们的商业使用。为了解决这些限制，该项目将开发基于铋元素的新型太阳能类钙钛矿型太阳能材料，铋元素含量丰富，毒性低。这类尚未探索的材料是通过理论和计算技术确定的，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用于光伏应用的新材料的性能。实验结果与理论分析相结合，将反馈到材料设计标准，以帮助寻找有前途的新材料进行继续研究。与该项目相关的教育活动包括与少数群体服务组织协调提供的实验室实习，动手学习模块，以及继续开发一门为期一学期的太阳能光伏课程。具有钙钛矿型能带结构的铋(铋)化合物，包括三元卤化铋和硫代硫代铋，在三个方面有望成为太阳能光伏应用的吸收材料，在三个方面超越传统的钙钛矿材料。首先，它们的电子结构和强大的自旋-轨道耦合能够以一种类似于甲基铵铅碘钙钛矿材料的方式容忍固有缺陷，但不含铅。其次，铋离子具有较大的本征有效电荷，以提供高介电常数并屏蔽带电缺陷。第三，无铅甲基碘化铋铵材料在水蒸气存在下，由于保护层的优先形成，可以保持相稳定。鉴于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型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闪锌矿来研究对称性对良好输运性质的重要性，偏硫闪石是为数不多的几类没有层状的铋基材料之一。第二个目标是开发模型，通过时间分辨光致发光测量来确定材料中的复合过程和复合速率。第三个目标是发展生长高纯度和杂质含量与少数载流子寿命相关的新的铋化合物的策略。密度泛函理论将被用来确定电子结构、介电常数和载流子有效质量。实验研究将建立有效吸收太阳能的材料设计标准，重点确定晶内结构缺陷、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影响。薄膜的扩散长度将通过时间分辨光致发光测量以及用于深度分辨寿命测量的单光子和双光子光谱来获得，该深度分辨寿命测量消除了表面复合的影响。

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.