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 <
3). The full crystallographic structures of (HDA2+)­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.