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Temperature-dependent studies of exciton binding energy and phase-transition suppression in (Cs,FA,MA)Pb(I,Br)₃ perovskites

Lookup NU author(s): Dr Pablo Docampo

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

Abstract

© 2019 Author(s). Multiple-cation mixed-halide (Cs,FA,MA)Pb(I,Br) 3 perovskites containing cesium, formamidinium (FA), and methylammonium (MA) possess excellent properties for a wide range of optoelectronic applications such as thin-film photovoltaics or lasers. We investigate the role of excitons and the exciton binding energy E B , relevant for the effectiveness of charge separation in solar cells, as well as the temperature-dependent bandgap energy E g which is used as an indicator for crystal phase transitions. Generalized Elliott fits of absorption spectra offer the possibility to determine both E B and E g . However, since excitonic effects are non-negligible even at room temperature, a careful and detailed analysis of the spectra is crucial for a correct interpretation. Therefore, an additional evaluation based on a so-called f-sum rule is applied to achieve an improved reliability of the results at higher temperatures. The obtained E B values of 20-24 meV for Cs-containing mixed perovskite compounds are below the ones of 24-32 meV and 36-41 meV for pure methylammonium lead iodide (MAPbI 3 ) and bromide (MAPbBr 3 ), respectively, and, thus, facilitate charge-carrier separation in photovoltaic applications. Furthermore, temperature-dependent (T = 5-300 K) studies of E g in (Cs,FA,MA)Pb(I,Br) 3 indicate a suppressed crystal phase transition by the absence of any phase-transition related signatures such as the well-known jump of about 100 meV in MAPbI 3 . We verify these results using temperature-dependent electroreflectance spectroscopy, which is a very reliable technique for the direct and non-destructive determination of optical resonances of the absorber layer in complete solar cells. Additionally, we confirm the suppression of the phase transition in Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 by temperature-dependent X-ray diffraction.

Publication metadata

Author(s): Ruf F, Ayguler MF, Giesbrecht N, Rendenbach B, Magin A, Docampo P, Kalt H, Hetterich M

Publication type: Article

Publication status: Published

Journal: APL Materials

Year: 2019

Volume: 7

Issue: 3

Online publication date: 26/03/2019

Acceptance date: 01/03/2019

Date deposited: 23/05/2019

ISSN (electronic): 2166-532X

Publisher: American Institute of Physics Inc.

URL: https://doi.org/10.1063/1.5083792

DOI: 10.1063/1.5083792

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