Publication date: 9th October 2020
Unraveling the structure-property relationships in hybrid lead halide perovskites is instrumental for the fundamental understanding not only of their exceptional optoelectronic performance but also their in-operando stability. Surprisingly, hybrid halide perovskites exhibit a great defect tolerance in their performance, despite their mechanical softness and the consequent low energetic barriers for point defect formation and ion migration. In a recent publication [1], we have constructed the complete phase diagram of organic-cation solid-solutions of lead iodide perovskites (FAxMA1-xPbI3, where MA stands for methylammonium and FA for formamidinium) with compositions x ranging from 0 to 1 in steps of 0.1 and in the temperature range from 10 to 365 K by combining Raman scattering and photoluminescence (PL) measurements. In this talk, I will present an interesting byproduct of this work, which concerns the study of the evolution of shallow-defect signatures observed in the PL spectra at low temperatures. The strong free-exciton PL of the perovskites, apart from the discontinuous changes in the energy of its maximum at the different structural transitions, also exhibits a strong decrease in linewidth at low temperatures. This allows for the observation of the emission related to radiative recombination of bound exciton complexes (BECs) associated with shallow defects (donor and/or acceptor). I will report a tentative assignment of all PL features to the different shallow-defects typically present in hybrid perovskites, attained with the aid of state of the art ab-initio calculations [2]. The defect-related signatures exhibit a clear trend regarding the composition of the mixed crystals, indicating that the material becomes less prone to defect formation with increasing FA content.
The Spanish Ministerio de Ciencia e Innovación is gratefully acknowledged for its support through grants No. SEV-2015-0496 (FUNMAT) and CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program and through grants MAT2015-70850-P (HIBRI2) and PGC2018-095411-B-100 (RAINBOW). We also thank the Catalan agency AGAUR for grant 2017-SGR-00488. AFL acknowledges a FPI fellowship (BES-2016-076913) from the Spanish Ministerio co-financed by the European Social Fund and the PhD programme in Materials Science from Universitat Autònoma de Barcelona in which he was enrolled. BC thanks the EPSRC for PhD studentship funding via the CSCT CDT (EP/G03768X/1).
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