Understanding the Temperature Dependence of Hybrid Lead Halide Perovskite Band Gaps in Terms of the Effects of Thermal Expansion and Electron-Phonon Interaction
Adrián Francisco-López a, Bethan Charles b, Ma. Isabel Alonso a, Miquel Garriga a, Mariano Campoy-Quiles a, Mark T. Weller b c, Alejandro R. Goñi a d
a Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain
b Dept. of Chemistry & Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, UK
c Dept. of Chemistry, Cardiff University, Wales CF10 3AT, UK
d ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain
nanoGe Perovskite Conferences
Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO20)
Sevilla, Spain, 2020 February 23rd - 25th
Organizer: Hernán Míguez
Oral, Alejandro R. Goñi, presentation 031
Publication date: 25th November 2019

Hybrid lead halide perovskites exhibit an atypical temperature dependence of the fundamental gap for the phases stable at ambient conditions: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase [1], which speaks against the assumption of negligible thermal expansion effects. Here we show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling [2]. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts. As an example, recent results obtained for a series of FAxMA1-xPbI3 solid solutions, where FA stands for formamidinium [3], will be also presented. A striking result concerns the temperature dependence of the gap of a presumably tetragonal but disordered phase which is stable in a wide range of intermediate compositions and temperatures lower than ca. 250 K. This phase is found to exhibit a quadratic dependence of the band gap with temperature, which is again interpreted in terms of the combined effects of thermal expansion and electron-phonon interaction.

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