Dual nature of the excited state in organic–inorganic lead halide perovskites

Abstract

The rapid increase in efficiency of methylammonium lead halide perovskite solar cells necessitates further investigation into the nature of perovskite absorption features and optical properties. Films obtained from the deposition of solutions containing lead halides and the CH₃NH₃⁺ organic cation is known to yield the CH₃NH₃PbI₃ perovskite structure upon annealing. In examining the precursor solution used in the processing of CH₃NH₃PbI₃ solar cells, we find that Pb²⁺ readily forms plumbate complexes in the presence of excess iodide ions and exhibits characteristic absorption bands at 370 (PbI₃⁻) and 425 nm (PbI₄²⁻). Through comparative spectral analysis of the absorption features of charge transfer complexes in the solution phase and the final solid-state perovskite films, we are able to fully classify the absorption features in the excited state of CH₃NH₃PbI₃ across the transient absorption spectrum recorded following laser pulse excitation. In particular, we attribute the broad photoinduced absorption to a charge-transfer excited state, and show correlation between the photoinduced absorption and 480 nm bleach signals. These observations lead us to propose a band structure composed of two distinct transitions that is consistent with the various spectral features and kinetic behavior of the CH₃NH₃PbI₃ excited state. Characterization of this unique dual excited state nature provides further insight into the optoelectronic behavior of hybrid lead halide perovskite films and thus aids in elucidating their exceptional photovoltaic properties.