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

Abstract

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 C60/C70. 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 C60/C70 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.

Photoinduced Self-Gating of Perovskite Photovoltaic Cells in Ionic Liquid

“…By simulating an ionically gated perovskite solar cell using the DD model in our previous work, 32 we demonstrated that penetrated ions inside CNTs can raise the electrode Fermi level and convert a Schottky contact into an ohmic one through the reduction of the potential barrier at the ETL/ CNT interface. Additionally, at higher gate voltages, positive ions can drift in high concentrations inside the ETL, thereby doping it.…”

“…In this study, we present an architecture similar to one recently described in our previous work . We then fabricate and characterize the internal p–i–n structure within the perovskite photoactive layer and the n-ETL/n-CNT interface controlled by a horizontal CNT ionic gate CE placed next to the PSC pixel.…”

“…29−31 In our previous work, we successfully implemented ILG in perovskite photovoltaic devices using an external gate-source. 32 In this article, we present a novel architecture scheme that utilizes the ionic gating effect in PSC devices without requiring a gate voltage from an external source. Specifically, the voltage applied to the gate in our devices is generated by the device itself as V oc and is applied as a self-gating voltage to the CNT gate counter-electrode (CE) upon illumination of the PSC.…”

“…Ionic liquid gating (ILG) is a commonly used technique in transistors as it forms a double layer, resulting in a significant increase in capacitance. − Similarly, ILG has been applied in organic photovoltaic (OPV) devices with CNTs as an electrode. , In this approach, gating voltage is applied from an external source to dope the CNT electrode with ions from the ionic liquid. While CNTs have been used as both anode and cathode in perovskite photovoltaics, − they also offer several advantages over traditional metal electrodes, including high transparency, low cost, and hydrophobicity, as well as superior stability. − In our previous work, we successfully implemented ILG in perovskite photovoltaic devices using an external gate-source …”

Temperature Dependence of Photoinduced Phase Segregation in Bromide-Rich Mixed Halide Perovskites

“…Since 2009, hybrid perovskites have proven to be an excellent material for optoelectronics, including solar cells (SCs), − light-emitting diodes (LEDs), − and various other device types. − Additionally, the perovskite bandgap can be continuously tuned by incorporating different halides into its structure. By introducing chloride, bromide, and iodide in different proportions to the perovskite composition, the bandgap can be adjusted within the range of 1.5–3.2 eV .…”