Ionic Liquid Gating in Perovskite Solar Cells with Fullerene/Carbon Nanotube Collectors

Metallic cathodes are one of reasons for instability in perovskite (PS) solar cells due to reaction with halogens I-/Br-, and it is desirable to have stable carbon-based cathodes, particularly carbon nanotubes (CNTs), and transport layers such as C60/C70. In this work, we show that gating of such top CNT cathode in ionic liquid (IL) by changing bias voltage Vg tunes the Fermi level of CNT by electrical double layer (EDL) charging and causes lowering of the barrier at C60/C70 electron transport layer (ETL). Moreover, at higher gating Vg the ions further propagate into fullerene ETL by electrochemical n-doping, which increases dramatically performance by further raising mostly two parameters: short-circuit current (Isc) and fill factor (FF), resulting in solar cell efficiency growth from 3% to 11%. N-type doping of ETL is quite different for C70 as compared to C60: it stronger enhances charge collection by C70 at low Vg independently of ETL thickness. Surprisingly, open-circuit voltage Voc is not sensitive to external Vg gating in the perovskite solar cells, on the contrary to strongly enhanced Voc in ionically gated organic solar cells, where it is the main effect for ionic gating. This insensitivity of Voc to lowering of the work function of Vg gated CNT electrode is a clear indication that Voc in the perovskite solar cell is determined by inner p-i-n junction formation in the device itself, via accumulation of its intrinsic mobile ionic species of halogens (I-, Br-) and cations (MA+, Cs+) and their vacancies. To support our experimental results, we show by modeling using an equivalent diode circuit model that short circuit current increment in the case of the architecture with C60 ETL layer can be interpreted by a current sink that could be related to the suppression of trap states at the perovskite/ETL interface by IL. This article is protected by copyright. All rights reserved.