Annealing free tin oxide electron transport layers for flexible perovskite solar cells

Electron transport layer (ETL) is a key component in the perovskite solar cells (PSCs). Low temperature, even room temperature deposition technique for ETLs is especially intriguing for flexible solar cell fabrication on polymer substrates. Here, we proposed a facile amine gas fuming strategy to prepare SnO2 ETLs at room temperature without thermal annealing. It is found that the SnO2 films treated with methylamine or other amine gases are smoother and more compact than the SnO2 films with thermal annealing. A base-catalyzed densification mechanism is proposed that the SnO2 colloidal is converted into compact film through particle connections and attachments facilitated by basic amine gas treatments. XRD, FTIR and TEM were used to verify the densification mechanism. With methylamine treated SnO2 ETLs, the rigid and flexible PSCs achieved high PCEs of 20.36% and 19.80%, respectively. The flexible PSCs with the annealing free SnO2 ETLs showed excellent flexibility with 94.5% of the initial PCE remaining after 3000 bending cycles. The amine fuming method provides a new and facile way for annealing free deposition of charge transport layers for flexible solar cells and optoelectronics.