Solvent engineering for triple cationic ITO-based mesoscopic tin perovskite solar cells

Besides negligible toxicity, tin-based perovskite solar cells (TPSCs) hold bright prospects in light of broad light harvesting beyond ∼850 nm rendering theoretical efficiencies higher than lead-based counterparts. However, tin perovskites which are as thin as ∼200 nm (cf., ∼500 nm or beyond for lead analogs) and populate tin defects (i.e., Sn⁴⁺ defects) especially on a top surface do not suffice to absorb all the incoming light and to convert it into electricity. Herein, we explore triple mesoscopic TPSCs and develop suitable tin perovskites via cation and solvent engineering. The predetermined body frame of TiO₂ (ETM) / Al₂O₃ (insulator) / ITO (HTM and electrode) mesoporous scaffolds enables thickness control of tin perovskites with ease over micrometers (∼2 µm herein), where ETM stands for the electron transport material and HTM stands for the hole transport material. In mesoscopic TPSCs, ITO is partly buried in tin perovskites so that the Sn⁴⁺ defects-enriched surface is least influential to charge transfer. By leveraging a Cs cation and a N,N′-dimethylpropyleneurea solvent, demanding quality of tin perovskites for efficient conversion of light-to-electricity is satisfied such as relaxed and passivated crystals and Sn⁴⁺-lean defects. The resulting perovskites are well-suited in performance and stability to the mesoscopic TPSCs. Importantly, they are highly reproducible and attain high quantum efficiency of >90% over a broad range of light absorption from 400 to 700 nm. This remarkable quantum efficiency translates to photocurrent higher than >27 mA cm⁻² (cf., theoretical limit: ∼31 mA cm⁻²). We herein enlighten the future of TPSCs by proposing a route for maximal light harvesting and its conversion.