Extending lead-free hybrid photovoltaic materials to new structures: thiazolium, aminothiazolium and imidazolium iodobismuthates

We report on the synthesis, crystal structures, optoelectronic properties and solar cell device studies of three novel organic-inorganic iodobismuthates - [C 3 H 4 NS] 3 [Bi 2 I 9 ] ( [TH] 3 [Bi 2 I 9 ] ), [C 3 H 4 N 2 ] 3 [Bi 2 I 9 ] ( [IM] 3 [Bi 2 I 9 ] ) and [C 3 H 5 N 2 S][BiI 4 ] ( [AT][BiI 4 ] ) as lead-free light harvesters. [TH] 3 [Bi 2 I 9 ] and [IM] 3 [Bi 2 I 9 ] show zero-dimensional structures, whereas a one-dimensional edge-sharing chain structure of BiI 6 -octahedra was observed in [AT][BiI 4 ] , with interchain short I I contacts also giving rise to the possibility of three-dimensional charge transport ability. Accordingly, greater energy dispersion in the band structure of [AT][BiI 4 ] can be observed, and less contribution from the organic moities at the conduction band minimum in [AT][BiI 4 ] than [TH] 3 [Bi 2 I 9 ] have been confirmed by density functional theory calculations. Moreover, bandgap values are redshifted from 2.08 eV for [TH] 3 [Bi 2 I 9 ] and 2.00 eV for [IM] 3 [Bi 2 I 9 ] to 1.78 eV for [AT][BiI 4 ] , determined by UV-Vis reflectance spectroscopy. Power conversion efficiency of 0.47% has been achieved by using ( [AT][BiI 4 ] ) as the light absorber in a hole-conductor-free, fully printable solar cell, with relatively good reproducibility. We also note the observation of a capacitance effect for the first time in a photovoltaic device with bismuth-based solar absorber, which may be related to the mesoporous carbon counter-electrode. We report on the synthesis, crystal structures, optoelectronic properties and solar cell device studies of three novel hybrid bismuth-halide materials as lead-free light harvesters.