Lead-free pseudo-three-dimensional organic-inorganic iodobismuthates for photovoltaic applications

Two organic-inorganic iodobismuthates, C5H6NBiI4 ([py][BiI4]) and C6H8NBiI4 ([mepy][BiI4]), have been prepared with their structures revealed by single-crystal X-ray diffraction. One-dimensional BiI4- anionic chains built by edge-sharing BiI6 octahedra were found in both materials; short I center dot center dot center dot I, I center dot center dot center dot C contacts and hydrogen bonding give rise to three-dimensional intermolecular interactions. Both compounds are semiconductors, with band gaps of around 2.0 eV, and the contribution from the organic moieties to the conduction band minimum has been derived by density functional theory. Solid-state optical and electrochemical studies performed on powders and thin films were carried out, and their stabilities under an ambient environment have been demonstrated. Their use as the absorber layer in printable mesoscopic solar cells without any hole transport material has led to efficiencies up to 0.9%, showing a promising new approach towards the development of lead-free third-generation photovoltaic materials.