Strongly Enhanced Photovoltaic Performance and Defect Physics of Air‐Stable Bismuth Oxyiodide (BiOI)

Abstract

Bismuth‐based compounds have recently gained increasing attention as potentially nontoxic and defect‐tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All‐inorganic solar cells (ITO|NiOx|BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short‐circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.

Research Update: Bismuth-based perovskite-inspired photovoltaic materials

“…3(d)]. 39 This indicates that BiOI is tolerant to all anti-site and vacancy defects investigated since deep transition levels are not expected to be present. However, for Cs 2 AgBiBr 6 , low formation-energy transition levels have been found [ Fig.…”

“…6(d)]. 39 Films were more compact than those fabricated by SILAR, and a photoluminescence lifetime of 2.7 ns was measured. These BiOI thin films were also shown to be stable in air for the duration of the 197-day experiment, when the relative humidity varied from 45% to 67%.…”

“…BiOI is another example where the device performance has been shown to be limited due to sub-optimal band alignment, in which downward band bending of BiOI occurs at the interface with the NiO x hole transport layer. 39 In a study by Greul et al 42 on Cs 2 AgBiBr 6 double perovskite devices, the large band offset between the spiro-OMeTAD (5.13 eV) and Cs 2 AgBiBr 6 (6.04 eV) has been highlighted as one of the main barriers for limited V OC and inefficient charge extraction. It is therefore important to find new hole transport materials for bismuth-based compounds.…”

“…These BiOI thin films were also shown to be stable in air for the duration of the 197-day experiment, when the relative humidity varied from 45% to 67%. 39 There are few reports of double perovskite thin films, owing to difficulties in dissolving the precursors in a common organic solvent. The first report is from Greul et al, 42 who fabricated Cs 2 BiAgBr 6 by a one-step spin casting method.…”

“…37 Two contemporary investigations of BiI 3 devices based on solution-processed thin films reported open-circuit voltages >1.3 V below the bandgap with low shunt resistances, and pinholes in the thin films may contribute to this shunting. 38,79 As well as single crystals, thin films of BiOI, with an indirect bandgap ∼1.9 eV, 35,39 have been synthesized for photovoltaic and photocatalytic applications by methods such as successive ionic layer adsorption and reaction [SILAR, Fig. 6(b)] 80 and the solvothermal method.…”