Brauer Jan C, Tsokkou Demetra, Sanchez Sandy, Droseros Nikolaos, Roose Bart, Mosconi Edoardo, Hua Xiao, Stolterfoht Martin, Neher Dieter, Steiner Ullrich, De Angelis Filippo, Abate Antonio, Banerji Natalie
Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland.
Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland.
J Chem Phys. 2020 Mar 14;152(10):104703. doi: 10.1063/1.5133021.
Organic-inorganic perovskites are one of the most promising photovoltaic materials for the design of next generation solar cells. The lead-based perovskite prepared with methylammonium and iodide was the first in demonstrating high power conversion efficiency, and it remains one of the most used materials today. However, perovskites prepared by mixing several halides and several cations systematically yield higher efficiencies than "pure" methylammonium lead iodide (MAPbI) devices. In this work, we unravel the excited-state properties of a mixed-halide (iodide and bromide) and mixed-cation (methylammonium and formamidinium) perovskite. Combining time-resolved photoluminescence, transient absorption, and optical-pump-terahertz-probe experiments with density functional theory calculations, we show that the population of higher-lying excited states in the mixed material increases the lifetime of photogenerated charge carriers upon well above-bandgap excitation. We suggest that alloying different halides and different cations reduces the structural symmetry of the perovskite, which partly releases the selection rules to populate the higher-energy states upon light absorption. Our investigation thus shows that mixed halide perovskites should be considered as an electronically different material than MAPbI, paving the way toward further materials optimization and improved power conversion efficiency of perovskite solar cells.
有机-无机钙钛矿是设计下一代太阳能电池最有前景的光伏材料之一。用甲铵和碘化物制备的铅基钙钛矿是首个展现出高功率转换效率的材料，并且它至今仍是最常用的材料之一。然而，通过系统混合几种卤化物和几种阳离子制备的钙钛矿，其效率比“纯”甲铵碘化铅（MAPbI）器件更高。在这项工作中，我们揭示了一种混合卤化物（碘化物和溴化物）以及混合阳离子（甲铵和甲脒）钙钛矿的激发态特性。将时间分辨光致发光、瞬态吸收和光泵太赫兹探测实验与密度泛函理论计算相结合，我们表明，在高于带隙激发时，混合材料中高能激发态的占据增加了光生电荷载流子的寿命。我们认为，不同卤化物和不同阳离子的合金化降低了钙钛矿的结构对称性，这在一定程度上放宽了光吸收时占据高能态的选择规则。因此，我们的研究表明，混合卤化物钙钛矿应被视为一种在电子性质上与MAPbI不同的材料，为钙钛矿太阳能电池进一步的材料优化和提高功率转换效率铺平了道路。
