Influence of One Specific Carbon–Carbon Bond on the Quality, Stability, and Photovoltaic Performance of Hybrid Organic–Inorganic Bismuth Iodide Materials

Hybrid organic–inorganic halide perovskites are promising materials for thin-film solar cells. However, the toxicity and instability of best-in-class lead–halide perovskite materials make them nonideal. To combat these issues, we replaced lead with bismuth and explored the sensitivity of these new lead-free materials to the valency and bonding of their cationic organic groups. Specifically, we synthesized and characterized the materials properties and photophysical properties of hexane-1,6-diammonium bismuth pentaiodide ((HDA2+)­BiI5) and compared them to an analogue containing a more volatile organic group with half the number of carbon and nitrogen atoms in the form of n-propylammonium ((PA+)x­BiI3+x, where 1 x 2+)­BiI5 and (PA+)x­BiI3+x were resolved by single-crystal X-ray diffraction. (HDA2+)­BiI5 was shown to be pure-phase and have a one-dimensional structure, whereas (PA+)x­BiI3+x was shown to be a mix of one-dimensional and zero-dimensional phases. Structures of the materials were confirmed by synchrotron X-ray diffraction of powders. Both (HDA2+)­BiI5 and (PA+)x­BiI3+x exhibit steady-state photoluminescence at room temperature. Density functional theory calculations of (HDA2+)­BiI5 predict electronic absorption features and a ∼2 eV bandgap that are consistent with those observed experimentally. Structure–property relationships of the materials were examined, and moisture tolerance and film quality were found to be superior for dication-containing (HDA2+)­BiI5 in relation to monocation-containing (PA+)x­BiI3+x. We hypothesize that these trends are in part due to a molecular bridging effect enabled by the presence of the dicationic hexanediammonium groups in (HDA2+)­BiI5. Solar cells fabricated using (HDA2+)­BiI5 as the photoactive layer exhibited photovoltaic action while those containing (PA+)x­BiI3+x did not, suggesting that organic dicationic groups are beneficial to light-absorber morphology and ultimately solar-cell performance.