Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy.

Fabio G Santomauro,Jakob Grilj,Mario Gutierrez Tovar,Dominik Kinschel,Valerie Samson,Grigory Smolentsev,Maksym V Kovalenko,Giacomo Rossi,Daniel Grolimund,Thomas Rossi,Georgian Nedelcu,André Al Haddad,Gloria Capano,Sergii Yakunin,Dario S Ferreira,Lars Mewes,Majed Chergui,Maarten Nachtegaal,James Budarz

doi:10.1063/1.4971999

Abstract

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge, and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.

Highlights

Perovskites are a class of crystalline materials with the formula ABX3, in which A and B are cations and X represents an anion
Our results show that while for both systems, the hole is localized at Br atoms in the valence band (VB), electrons remain delocalized in the conduction band (CB) and Cs atoms exhibit no response to photoexcitation
We have investigated the fate of the charge carriers in photoexcited inorganic pure (Br) and mixed halide (Br-Cl) perovskites nanoparticles in solution using picosecond X-ray absorption spectroscopy

Summary

Introduction

Perovskites are a class of crystalline materials with the formula ABX3, in which A and B are cations and X represents an anion. Lead-halide organic-inorganic perovskites (in which A is a monovalent organic cation) have recently emerged as highly promising optoelectronic materials for photovoltaics, photodetection, light-emitting diodes, and laser devices.. Solar cells with lead-halide organic-inorganic perovskites (e.g., CH3NH3PbI3), prepared by lowtemperature solution-based methods, have recently achieved outstanding performances with photo-conversion yields near 22%.9,10. This high conversion efficiency, together with the low processing temperature (150 C), make them ideal candidates for the development of low-cost solar cells. A)On leave from Dipartimento di Fisica e Astronomia, Alma Mater Studiorum - Universita di Bologna Viale Berti Pichat. B)On leave from Departamento de Quımica Fısica, Facultad de Ciencias Ambientales y Bioquımica, y INAMOL, Campus.

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Discussion

Conclusion