Ibaceta-Jaña Josefa, Muydinov Ruslan, Rosado Pamela, Mirhosseini Hossein, Chugh Manjusha, Nazarenko Olga, Dirin Dmitry N, Heinrich Dirk, Wagner Markus R, Kühne Thomas D, Szyszka Bernd, Kovalenko Maksym V, Hoffmann Axel
Technology for Thin-Film Devices, Technische Universitat Berlin, Einsteinufer 25, 10587 Berlin, Germany.
Institute of Solid State Physics, Technische Universitat Berlin, Hardenbergstr. 36, 10623 Berlin, Germany.
Phys Chem Chem Phys. 2020 Mar 11;22(10):5604-5614. doi: 10.1039/c9cp06568g.
Lead halide perovskite semiconductors providing record efficiencies of solar cells have usually mixed compositions doped in A- and X-sites to enhance the phase stability. The cubic form of formamidinium (FA) lead iodide reveals excellent opto-electronic properties but transforms at room temperature (RT) into a hexagonal structure which does not effectively absorb visible light. This metastable form and the mechanism of its stabilization by Cs+ and Br- incorporation are poorly characterized and insufficiently understood. We report here the vibrational properties of cubic FAPbI3 investigated by DFT calculations on phonon frequencies and intensities, and micro-Raman spectroscopy. The effects of Cs+ and Br- partial substitution are discussed. We support our results with the study of FAPbBr3 which expands the identification of vibrational modes to the previously unpublished low frequency region (<500 cm-1). Our results show that the incorporation of Cs+ and Br- leads to the coupling of the displacement of the A-site components and weakens the bonds between FA+ and the PbX6 octahedra. We suggest that the enhancement of α-FAPbI3 stability can be a product of the release of tensile stresses in the Pb-X bond, which is reflected in a red-shift of the low frequency region of the Raman spectrum (<200 cm-1).
具有创纪录太阳能电池效率的卤化铅钙钛矿半导体通常在A位和X位掺杂混合成分以提高相稳定性。甲脒（FA）碘化铅的立方晶型具有优异的光电性能，但在室温（RT）下会转变为六方结构，这种结构不能有效地吸收可见光。这种亚稳形式及其通过掺入Cs⁺和Br⁻实现稳定的机制尚未得到充分表征和理解。我们在此报告通过对声子频率和强度的密度泛函理论（DFT）计算以及显微拉曼光谱研究的立方FAPbI₃的振动特性。讨论了Cs⁺和Br⁻部分取代的影响。我们通过对FAPbBr₃的研究支持我们的结果，该研究将振动模式的识别扩展到了之前未发表的低频区域（<500 cm⁻¹）。我们的结果表明，掺入Cs⁺和Br⁻会导致A位成分位移的耦合，并削弱FA⁺与PbX₆八面体之间的键。我们认为α-FAPbI₃稳定性的增强可能是Pb-X键中拉应力释放的结果，这在拉曼光谱的低频区域（<200 cm⁻¹）的红移中得到体现。
