Solvent-molecule-mediated manipulation of crystalline grains for efficient planar binary lead and tin triiodide perovskite solar cells

Binary lead and tin perovskites offer the benefits of narrower band gaps for broader adsorption of solar spectrum and better charge transport for higher photocurrent density. Here, we report the growth of large, smooth crystalline grains of bianry lead and tin triiodide perovskite films via a two-step solution process with thermal plus solvent vapor-assisted thermal annealing. The crystalline Sn_xPb_1-xI₂ films formed in the first step served as the templates for the formation of crystalline CH₃NH₃Sn_xPb_1-xI₃ films during the second step interdiffusion of methylammonium iodide (MAI). Followed by dimethylsulfoxide (DMSO) vapor-assisted thermal annealing, small, faceted perovskite grains grew into large, smooth grains via the possible mechanism involving bond breaking and reforming mediated by DMSO solvent molecules. The absorption onset was extended to 950 and 1010 nm for the CH₃NH₃Sn_xPb_1-xI₃ perovskites with x = 0.1 and 0.25, respectively. The highest PCE of 10.25% was achieved from the planar perovskite solar cell with the CH₃NH₃Sn_0.1Pb_0.9I₃ layer prepared via the thermal plus DMSO vapor-assisted thermal annealing. This research provides a way to control and manipulate film morphology, grain size, and especially the distribution of metal cations in binary metal perovskite layers, which opens an avenue to grow perovskite materials with desired properties to enhance device performance.