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

Metallic cathodes are one of reasons for instability in perovskite 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 C-60/C-70. In this work, we show that gating of such top CNT cathode in ionic liquid by changing gate voltage (V (g)) tunes the Fermi level of CNT by electrical double layer charging and causes lowering of the barrier at C-60/C-70 electron transport layer (ETL). Moreover, at higher V (g) the ions further propagate into fullerene ETL by electrochemical n-doping, which increases dramatically performance by further raising mostly two parameters: short-circuit current and fill factor, resulting in solar cell efficiency growth from 3% to 11%. Surprisingly, open-circuit voltage (V (oc)) is not sensitive to V (g) in the perovskite solar cells, on the contrary to strongly enhanced V (oc) in ionically gated organic solar cells, where it is the main effect for ionic gating. This insensitivity of V (oc) to lowering of the work function of CNT electrode is a clear indication that V (oc) in the perovskite solar cell is determined by inner p-i-n junction formation in the perovskite layer itself.