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.