Perovskite materials are one of the most promising candidates for the realization of cheaper, more efficient solar cells but some details of their photovoltaic properties remain unclear. Specifically, this includes the importance of excitonic effects on their operation. Researchers report in Applied Physics Letters that the most commonly used perovskite compound, methylammonium lead iodide, shows a fundamental optical transition that is discretely excitonic in nature, rather than arising from unbound continuum states.

By performing simultaneous temperature-dependent electroabsorption (EA) and standard absorption spectrum measurements on methylammonium lead iodide solar cells, at temperatures from 10 Kelvin up to room temperature, the authors were able to directly compare line shapes from both sets of spectra. The comparison shows almost identical energy positions, indicating an excitonic optical transition due to bound electron-hole pairs. This is seen across the entire observed temperature range.

The authors found a best fit with a first-derivative line shape consistent with excitonic phenomena. Applying an Elliott fit, which describes linear absorption in solids, to the absorption spectra also provided the values of the exciton binding energy in the orthorhombic (26 meV) and tetragonal phase (19 meV).

The experimenters note that despite the bound electron-hole characteristics of this clear excitonic signature, high power-conversion efficiencies are quite achievable in these perovskite devices. The present work indicates that the exciton binding energy is low enough at room temperature to permit clear and distinct charge separation, and the Sommerfeld enhancement linked to the excitonic effects leads to an improved absorption.

Source: “Excitonic nature of optical transitions in electroabsorption spectra of perovskite solar cells,” by Fabian Ruf, Alice Magin, Moritz Schultes, Erik Ahlswede, Heinz Kalt, and Michael Hetterich, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5017943.