Mechanism of improving the performance of perovskite solar cells through alkali metal bis(trifluoromethanesulfonyl)imide modifying mesoporous titania electron transport layer

Alkali metal bis(trifluoromethanesulfonyl)imide (TFSI) modification of TiO 2 electron transport layer (ETL) has been widely adopted to improve the performance of perovskite solar cells (PSCs), however, only a handful of investigations have proposed some corresponding mechanisms, and more in-depth investigations are urgent. In this investigation, Li-TFSI and K-TFSI were adopted to modify mesoporous TiO 2 (me-TiO 2 ) ETL of PSCs, electrochemical impedance spectroscopy , photoluminescence spectrum, space-charge limited current, and X-ray photoelectron spectroscopy measurements were performed to investigate the modification mechanisms. Alkali metal-TFSI modification could passivate the oxygen vacancies in me-TiO 2 ETL and suppress the charge recombination effectively, improve the contact between me-TiO 2 ETL and perovskite layer and increase the conductivity of me-TiO 2 ETL which lead to rapid charge extraction and transfer at ETL/perovskite interface and balance the electron flux and hole flux in devices. Moreover, sulfates induced by alkali metal-TFSI act as nucleation sites in the crystallization process of perovskite which could increase the grain size and improve the uniformity of CH 3 NH 3 PbI 3 films obviously, and it is a positive factor for suppressing charge recombination and improving the cell performance. As a result, the power conversion efficiencies of the champion devices for Li-TFSI and K-TFSI modified PSCs have reached 20.02% and 19.64%, respectively.