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

Abstract

We report on the synthesis, crystal structures, optoelectronic properties and solar cell device studies of three novel organic–inorganic iodobismuthates – [C₃H₄NS]₃[Bi₂I₉] ([TH]₃[Bi₂I₉]), [C₃H₄N₂]₃[Bi₂I₉] ([IM]₃[Bi₂I₉]) and [C₃H₅N₂S][BiI₄] ([AT][BiI₄]) as lead-free light harvesters. [TH]₃[Bi₂I₉] and [IM]₃[Bi₂I₉] show zero-dimensional structures, whereas a one-dimensional edge-sharing chain structure of BiI₆-octahedra was observed in [AT][BiI₄], 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][BiI₄] can be observed, and less contribution from the organic moities at the conduction band minimum in [AT][BiI₄] than [TH]₃[Bi₂I₉] have been confirmed by density functional theory calculations. Moreover, bandgap values are redshifted from 2.08 eV for [TH]₃[Bi₂I₉] and 2.00 eV for [IM]₃[Bi₂I₉] to 1.78 eV for [AT][BiI₄], determined by UV-Vis reflectance spectroscopy. Power conversion efficiency of 0.47% has been achieved by using ([AT][BiI₄]) 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.