Inter-Domain Charge Transfer as a Rationale for Superior Photovoltaic Performances of Mixed Halide Lead Perovskites

TL;DR

This study reveals that mixed halide lead perovskites exhibit inter-domain charge transfer that enhances photovoltaic performance by facilitating charge separation, challenging the traditional view that heterogeneity reduces efficiency.

Contribution

It provides direct spectroscopic evidence of charge transfer excitons across halide domains, explaining the superior performance of mixed halide perovskites in solar cells.

Findings

Charge transfer excitons form across halide domain boundaries.

Charge transfer facilitates hole separation along bromide content gradients.

Mixed halide perovskites outperform pure compositions in photovoltaic efficiency.

Abstract

Organic-inorganic lead halide perovskites containing a mixture of iodide and bromide anions consistently perform better in donor-acceptor heterojunction solar cells than the standard methylammonium lead triiodide material. This observation is counterintuitive, as it is generally expected for photovoltaic materials that heterogeneities and compositional disorder cause reduced carrier diffusion length and conversion efficiency. Here, we combine ultrafast photoinduced electroabsorption and broadband fluorescence up-conversion spectroscopy measurements to scrutinize the carrier dynamics in mixed-cations, mixed-halide lead perovskite thin films. Our results evidence the formation of charge transfer excitons (CTE) across the boundaries of domains of various halide compositions. A global analysis of photoinduced transient Stark signals shows that CTE evolve gradually from Br-rich to I-rich domains over tens to hundreds of picoseconds. Rather than constituting recombination centres, boundaries between domains of various halide compositions appear then to favour charge carrier separation by driving photogenerated holes along channels of decreasing bromide content.