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 TiO2 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 TiO2 (me-TiO2) 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-TiO2 ETL and suppress the charge recombination effectively, improve the contact between me-TiO2 ETL and perovskite layer and increase the conductivity of me-TiO2 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 CH3NH3PbI3 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.