Elucidating the Role of a Tetrafluoroborate‐Based Ionic Liquid at the n‐Type Oxide/Perovskite Interface

• Published in: Advanced energy materials Vol. 10; no. 4
• Main Authors: Noel, Nakita K., Habisreutinger, Severin N., Wenger, Bernard, Lin, Yen‐Hung, Zhang, Fengyu, Patel, Jay B., Kahn, Antoine, Johnston, Michael B., Snaith, Henry J.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.01.2020; Wiley
• Subjects: Conduction bands; Crystallization; Energy conversion efficiency; fermi level; Interfaces; Ionic liquids; Ions; Optoelectronics; perovskite solar cell; Perovskites; Photoelectrons; Photoluminescence; Photovoltaic cells; reduced defect density; Solar cells; Spectrum analysis; Substrates; time‐resolved microwave conductivity; Tin dioxide; Work functions
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201903231

Halide perovskites are currently one of the most heavily researched emerging photovoltaic materials. Despite achieving remarkable power conversion efficiencies, perovskite solar cells have not yet achieved their full potential, with the interfaces between the perovskite and the charge‐selective layers being where most recombination losses occur. In this study, a fluorinated ionic liquid (IL) is employed to modify the perovskite/SnO2 interface. Using Kelvin probe and photoelectron spectroscopy measurements, it is shown that depositing the perovskite onto an IL‐treated substrate results in the crystallization of a perovskite film which has a more n‐type character, evidenced by a decrease of the work function and a shift of the Fermi level toward the conduction band. Photoluminescence spectroscopy and time‐resolved microwave conductivity are used to investigate the optoelectronic properties of the perovskite grown on neat and IL‐modified surfaces and it is found that the modified substrate yields a perovskite film which exhibits an order of magnitude lower trap density than the control. When incorporated into solar cells, this interface modification results in a reduction in the current–voltage hysteresis and an improvement in device performance, with the best performing devices achieving steady‐state PCEs exceeding 20%. Crystallizing perovskites on an ionic liquid‐modified SnO2 substrate causes a shift of the perovskite Fermi level toward the conduction band and decreases the density of trap states in the perovskite. This results in a reduction of nonradiative recombination losses and, consequently, improved solar cell efficiencies.