Parallel evaluation of the BiI3, BiOI, and Ag3BiI6 layered photoabsorbers

The bismuth-based (oxy)iodides BiI3, BiOI, and AgxBiIx+3 share similar layered crystal structures, optimal band gaps for top absorbers in tandem solar cells, and moderate synthesis temperatures. Similarly to halide perovskite absorbers, they contain a heavy cation with a lone pair of electrons (Bi3+), which has been proposed as an important feature enabling defect tolerance in perovskites. The aim of this work is to grow and characterize BiI3, BiOI, and Ag3BiI6 absorbers and solar cells using a consistent synthesis and analysis routine. In this way, the individual strengths and weaknesses of the three absorbers, as well as their common challenges, can be outlined. The proposed synthesis method based on (oxy)iodization of metallic precursor films results in similar room-temperature photoluminescence features in all three materials, possibly indicating a similar degree of defect tolerance. At the device level, the open circuit voltage of BiI3 solar cells and the fill factor of BiOI solar cells are improved compared to their respective state of the art. To improve short circuit currents, control of growth orientation should be a priority in view of the anisotropic properties of these compounds. P-type bulk doping and selection of hole transport layers with deep valence bands are also key areas for future work. Beyond photovoltaics, the very low (<1.1) dark diode ideality factor in BiI3 devices and the existence of both electronic and ionic conduction in Ag3BiI6 may open up applications in other areas of optoelectronics.