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Perovskite solar cells (PSCs) have drawn unprecedented attention due to their skyrocketed power conversion, however the reported fill factors (FFs) still lag behind commercialized solar cells, and there lacks comprehensive...

Low-temperature solution-processed TiO 2 nanocrystals (LT-TiO 2 ) have been extensively applied as electron transport layer (ETL) of perovskite solar cells (PSCs). However, the low electron mobility, high density of electronic trap states, and considerable photocatalytic activity of TiO 2 result in undesirable charge recombination at the ETL/perovskite interface and notorious instability of PSCs under ultraviolet (UV) light. Herein, LT-TiO 2 nanocrystals are in situ fluorinated via a simple nonhydrolytic method, affording formation of Ti─F bonds, and consequently increase electron mobility, decrease density of electronic trap states, and inhibit photocatalytic activity. Upon applying fluorinated TiO 2 nanocrystals (F-TiO 2 ) as ETL, regular-structure planar heterojunction PSC (PHJ-PSC) achieves a champion power conversion efficiency (PCE) of 22.68%, which is among the highest PCEs for PHJ-PSCs based on LT-TiO 2 ETLs. Flexible PHJ-PSC devices based on F-TiO 2 ETL exhibit the best PCE of 18.26%, which is the highest value for TiO 2 -based flexible devices. The bonded F atoms on the surface of TiO 2 promote the formation of Pb─F bonds and hydrogen bonds between F − and FA/MA organic cations, reinforcing interface binding of perovskite layer with TiO 2 ETL. This contributes to effective passivation of the surface trap states of perovskite film, resulting in enhancements of device efficiency and stability especially under UV light.

Ionic defects at the surfaces of organolead halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells (PSCs). Herein, sodium p-toluenesulfonate (STS) is applied during the surface modification of perovskite layer for the first time, leading to the efficient surface passivation of the perovskite film and consequently significant enhancements in both efficiency and stability of mixed-cation PSC devices. Upon incorporating STS atop the perovskite layer, the power conversion efficiency of the Cs 0.05 MA 0.12 FA 0.83 PbI 2.55 Br 0.45 (abbreviated as CsMAFA) mesoporous-structure mixed-cation PSC devices improves from 18.70% to 20.05% with reduced hysteresis. The sulfonate (-SO 3 À ) anion of STS coordinates with the Pb 2þ of CsMAFA perovskite, and the Na þ cation of STS electrostatically interacts with the anions (I À /Br À ) of CsMAFA perovskite, resulting in the surface passivation of the CsMAFA perovskite film with reduced electron and hole trap state densities. In addition, STS modification induces an upshift of the valence band of perovskite, facilitating hole extraction from the perovskite layer to the hole transport layer with suppressed interfacial charge recombination. Moreover, such a trap state passivation of perovskite film leads to improvement of the ambient stability of PSC devices.