Band-Like Transport and Cation Off-Centring in Ag/Bi-Based Solar Absorbers

TL;DR

This study reveals that AgBiS2, a promising lead-free solar absorber, exhibits band-like transport due to cation off-centring and close packing, despite local distortions and cation disorder, suggesting new pathways for improving photovoltaic efficiency.

Contribution

It demonstrates that AgBiS2 can support band-like transport in both ordered and disordered phases, challenging previous assumptions about carrier localization in this material.

Findings

Band-like transport occurs in both ordered and disordered AgBiS2 phases.

Cation off-centring and close packing are key to high electronic dimensionality.

Extrinsic factors influence carrier localization in nanocrystals.

Abstract

Ag(I)-Bi(III)-based semiconductors have gained substantial attention as nontoxic, stable alternatives to lead-halide perovskites for optoelectronics, but are widely limited by carrier localization, which severely restricts diffusion lengths. The most efficient Ag/Bi solar absorber is AgBiS2, but diffusion lengths in nanocrystal films are <50 nm. Carrier localization in this rock-salt (Fm-3m) system is believed to arise from cation disorder, and so we herein investigate the layered cation-ordered analogue. Through beyond-DFT simulations combined with neutron and X-ray powder diffraction, we reveal that off-centring of Ag+ and Bi3+ cations is energetically-favoured in this cation-ordered phase. Despite local distortions in the AgS6 and BiS6 octahedra, band-like transport takes place, which, surprisingly, also occurs in the cation-disordered rock-salt phase when these materials are made as bulk powders. The cubic-phase powders have the same degree of cation disorder as the nanocrystals that have carrier localization, which suggests that extrinsic factors play a determining role. We ascribe the intrinsic band-like transport of both phases of AgBiS2 to its close packing, ensuring high electronic dimensionality. These insights offer pathways for designing solar absorbers avoiding carrier localization limitations, and call for future efforts to enhance the efficiency of AgBiS2 photovoltaics to focus on large-grained thin films, or improved nanocrystal surface passivation.