Bismuth Complex Controlled Morphology Evolution and CuSCN-Induced Transport Improvement Enable Efficient BiI(3) Solar Cells

Bismuth triiodide (BiI(3)) is a particularly promising absorber material for inorganic thin-film solar cells due to its merits of nontoxicity and low cost. However, one key factor that limits the efficiency of BiI(3) solar cells is the film morphology, which is strongly correlated with the trap states of the BiI(3) film. Herein, we report a coordination engineering strategy by using Lewis base dimethyl sulfoxide (DMSO) to induce the formation of a stable BiI(3)(DMSO)(2) complex for controlling the morphology of BiI(3) films. Density functional theory calculations further provide a theoretical framework for understanding the interaction of the BiI(3)(DMSO)(2) complex with BiI(3). The obtained BiI(3)(DMSO)(2) complex could assist the fabrication of highly uniform and pinhole-free films with preferred crystallographic orientation. This high-quality film enables reduced trap densities, a suppressed charge recombination, and improved carrier mobility. In addition, the use of copper(I) thiocyanate (CuSCN) as a hole transport layer improves the charge transport, enabling the realization of solar cells with a record power conversion efficiency of 1.80% and a champion fill factor of 51.5%. Our work deepens the insights into controlling the morphology of BiI(3) thin films through the coordination engineering strategy and paves the way toward further improving the photovoltaic performances of BiI(3) solar cells.