Photoelectron extraction in BiOI: an atomistic perspective

Abstract

With the increasing demand for indoor photovoltaics (IPV) to power autonomous and low-power electronic devices, lead-free perovskite-inspired materials (PIMs) have gained significant attention. Among the many lead-free alternatives, bismuth oxyiodide (BiOI) displays an electronic structure similar to that of high-performance lead-halide perovskites, but without the same toxicity limitations. However, its photoconversion efficiency (PCE) under indoor conditions is limited to 4.0–4.4%. A leading cause for such low efficiency is the contact between flake-like BiOI crystallites and electron and hole transport layers (ETL and HTL respectively). In this work, we thoroughly investigated an experimentally motivated (110) BiOI/ETL interface by means of density functional theory (DFT) calculations, to uncover the structural, mechanical and electronic characteristics of this heterostructure, identifying the atomistic origins of the sub-optimal performance of BiOI in photovoltaic applications. We focused on zinc oxide (ZnO) as an ETL, a material that attracted interest for its low annealing temperature (T_ann = 100–300 °C) and higher electron mobility (µ = 5–30 cm² V⁻¹ s⁻¹), compared to the prototypical TiO₂ ETL (T_ann ∼500 °C; µ = 0.5–8 cm² V⁻¹ s⁻¹). Our calculations reveal that a suitable orientation between the surfaces exists that induces limited strain on BiOI, potentially allowing the formation of an ideal heterostructure. Nevertheless, severe reconstruction occurs at the interface between BiOI and ZnO due to undercoordination of the I and O atoms of the terminal layer of the two solids. This reconstruction is observed to introduce states deep within the band gap. A trace of electron energy from the bulk of BiOI to the bulk of ZnO reveals a minimum at the BiOI/ZnO interface, which can hinder extraction and consequently reduce the PCE. Thus, we report for the first time the atomistic origins of limited PCE of BiOI/ZnO based photovoltaic devices and offer design principles to engineer more efficient interfaces.