Forming enlarged grain and fixed boundary via a two-step surface modification to achieve stable inverted perovskite solar cells

Having a stable interface between perovskite and electron transport layer at p -i -n hybrid halide perovskite solar cells (PSCs), has been considered to be crucial to improve the performance of device. Here, a two-step sulfurcontaining molecules surface treatment procedure (TST) was utilized, which involves sequentially coating 2-thiazolamide hydrochloride (SFACl) and methylamine sulfate (MA2SO4) onto the surface of perovskite film to achieve solid interface. Consequently, SFACl induced grains enlarged following a dissolution-crystallization model and formed 2D/3D heterojunction (n = 1); MA2SO4 and residual PbI2 reacted to form PbSO4, which priorly appeared at the grain boundaries. Owing to the interaction between sulfur-containing molecules and perovskite/PbI2, TST film showed improved photoluminescence intensity and prolonged lifetimes. Importantly, TST solar cell (Target 2) achieved a champion efficiency of 21.94 % for CsFA-based device (23.19 % for CsFAMAbased device, certified 23.02 %) compared with that of 20.05 % (Control). The improved device performance was primarily attributed to the larger grain size and defects passivation via multifunctional sulfur-containing molecules. Operational stability results shown that Target 2 device remained 78 % of the initial efficiency while Control remained 57 % of that under continuous illumination after 700 hrs in N2 at room temperature, which can be ascribed to the stable interface inhibiting ion migration. This study offers a comprehensive understanding of sulfur-containing molecules surface modification in PSCs.