Nanocrystal Precursors to Doped Cesium Metal Halide Perovskite Photovoltaics

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，目前的有机-无机钙钛矿型光伏器件在空气中存在水分时会退化，而有机成分是这种缺乏稳定性的原因之一。该项目的目标是开发完全由无机纳米晶体组成的新型钙钛矿型太阳能电池材料。这种方法的创新之处在于，通过将含有铯和氯化物元素的不同类型的纳米晶体融合在一起，钙钛矿在水分中的稳定性将大大提高。作为与该项目相关的教育活动的一部分，太阳能光伏概念将被整合到宾夕法尼亚州费城材料日的实践演示中，作为加强STEM教学中州教育标准的努力的一部分。这项研究的总体目标是利用胶体纳米晶体的化学诱导烧结来生产用于太阳能光伏器件的掺杂的多晶铯金属卤化物钙钛矿薄膜。铯碘铅和铯锡碘钙钛矿相材料与有机/无机混合甲基铵碘铅钙钛矿太阳能光伏材料具有许多共同的特性。然而，相对于有机金属卤化物钙钛矿材料，全无机铯钙钛矿材料的挥发性和化学活性都较低，但在大气湿度存在下仍然不稳定。据推测，与氯化物掺杂将稳定钙钛矿形式的碘化铯铅。为了实现氯化物掺杂，采用异相反应烧结了氯化铯铅和碘化铯纳米晶混合固体薄膜。用这种方法可以制备出非平衡的混合纳米晶薄膜。这种基于纳米晶体的方法还可以利用原位依赖于温度的吸收光谱来跟踪合成反应的进展和产物，以在微观水平上检测混合和相分离，从而能够阐明动力学控制过程与热力学控制过程。本文提出的分析方法为从力学上理解两种不同卤化物钙钛矿相的本征混溶提供了一个机会，以提高混晶钙钛矿薄膜的整体相稳定性。

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, present organic-inorganic perovskite photovoltaic devices degrade in the presence of moisture in air, and the organic component is one reason for this lack of stability. The goal of this project is develop new class of perovskite solar cell materials that are made entirely of inorganic, nanometer sized crystals. The innovative aspect of this approach is that by fusing together different types of nanocrystals which contain the elements cesium and chloride, the stability of the perovskite in moisture will be much improved. As part of the educational activities associated with this project, solar photovoltaic concepts will be integrated into hands-on demos at Philly Materials Day in Philadelphia, PA, as part of an effort to reinforce state education standards in STEM teaching. The overall goal of this research is to employ chemically-induced sintering of colloidal nanocrystals to produce doped, polycrystalline thin films of cesium metal halide perovskites for use in solar photovoltaic devices. Cesium lead iodide and cesium tin iodide perovskite phase materials share many attributes with the hybrid organic/inorganic methylammonium lead iodide perovskite materials for solar photovoltaics. However, relative to organic metal halide perovskite materials, the all-inorganic cesium perovskites are both less volatile and less chemically reactive, but are still not stable in the presence of atmospheric moisture. It is hypothesized that doping with chloride will stabilize the cesium lead iodide in the perovskite form. To carry out the chloride doping, a hetergenous-phase reaction is used to sinter solid films of mixed cesium lead chloride and cesium lead iodide nanocrystals. By this approach, non-equilibrium, mixed nanocrystal thin films can be synthesized. This nanocrystal-based approach also makes it possible to follow the progress and products of the synthesis reactions with in-situ, temperature-dependent absorption spectroscopy to detect both mixing and phase-separation at microscopic levels, allowing for elucidation of kinetically vs. thermodynamically controlling processes. The analytical approaches proposed here present an opportunity to mechanistically understand the intrinsic miscibility of two dissimilar halide perovskite phases to promote the overall phase stability of mixed crystal perovskite thin films.