The lead–halide-based hybrid organic–inorganic perovskite material, methylammonium lead iodide (MAPbI₃), has recently garnered increased attention among researchers worldwide as a promising photovoltaic material. As of 2015, perovskite photovoltaic devices have achieved laboratory efficiencies in excess of 20%. Many challenges regarding the stability and toxicity of the MAPbI₃ material, however, remain at the forefront of current research. Due to the toxicity of lead and a purely fundamental interest in dicationic organic groups in place of monocationic MA⁺, less toxic dicationic bismuth–halide materials are the object of my research. The bismuth–iodide networks of the materials under investigation were hypothesized to form chains that extend in one dimension, whereas in MAPbI₃ the lead–iodide network forms a lattice in three dimensions. If a small dicationic organic group is incorporated into the structure, however, it was hypothesized that the bismuth–iodide chains are in close enough proximity to each other to provide for long-range charge transport in three dimensions, similar to that observed in MAPbI₃.

A library of dicationic bismuth–halide materials has been investigated to determine optimal dicationic groups based on film quality, optical band gap, and current density versus potential (J–E) characteristics. XRD data of dicationic bismuth–halide thin films have suggested larger average crystal grain size than MAPbI₃ thin films and SEM images of dicationic bismuth–halide thin films have indicated greater surface coverage than MAPbI₃ thin films. UV–Vis absorption spectra of dicationic bismuth–halide thin films have revealed onsets of light absorption in the range of 565 nm to 620 nm, which correspond to optical band gaps between 2.2 and 2.0 eV. J–E measurements have indicated that dicationic bismuth–halide photovoltaic devices exhibit a photovoltage of almost 400 mV (Fig. 1). Thermal stability tests suggest that dicationic bismuth–halide thin films are significantly more robust than MAPbI₃ thin films. Large single crystals have been obtained for a dicationic bismuth–halide material, and single-crystal XRD data have indicated that the material formed binuclear clusters of Bi₂I₁₀^4-, with two dicationic organic groups and two H₂O molecules associated with each binuclear cluster. To my knowledge, this represents the first report of a crystal structure of this material. Efficient photovoltaic devices featuring a dicationic bismuth–halide material may offer a more environmentally friendly, stable alternative to MAPbI₃ photovoltaic devices that currently dominate emerging photovoltaic technology research.