The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Zimu Wei,Jos Thieme,Jacky Even,Tom J Savenije,Claudine Katan,Valentina M Caselli,Dengyang Guo

doi:10.1038/s41467-019-13136-y

Abstract

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Highlights

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored
MHPs, which is explained for CH3NH3PbI3 by a combination of multiple bandgap transitions derived from a symmetry-based empirical tight-binding model: a primary bandgap of 1.58 eV and a secondary bandgap above 2.25 eV owing to the spin-orbit coupling induced band splitting
To probe 2PA in the CH3NH3PbI3 film, the sample was measured by the timeresolved microwave conductivity (TRMC) technique[25,26] for a wide range of photon energies varying from 0.775 to 1.55 eV

Summary

Introduction

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. We recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. Over the past few years, metal halide perovskites (MHPs) have attracted immense attention due to their extraordinary performance as the light absorbing layer in solar cells[1,2] This exciting family of semiconductors exhibits large carrier mobilities[3,4], long charge carrier lifetimes[5,6], and a linear absorption coefficient over 105 cm−1 above the bandgap[1]. We have recorded the 2PA spectrum of methylammonium lead iodide perovskite (CH3NH3PbI3) polycrystalline thin films using the time-resolved microwave conductivity (TRMC) technique, as well as CH3NH3PbBr3 thin films and single crystals, and CsPbI3 thin films. We investigated for CH3NH3PbI3 the impact of the tetragonal-to-orthorhombic phase transition on the 2PA coefficient, β, by changing the temperature

Methods

Results

Conclusion