Carbon Nanotube Bridging Method for Hole Transport Layer-Free Paintable Carbon-Based Perovskite Solar Cells

The incredible stability of carbon-based perovskite solar cells (C-PSCs) has aroused enormous interest. However, for the paintable C-PSCs, the fill factor (FF) and power conversion efficiency (PCE) remain low, which is because of the insufficient contact at the interface between the perovskite and the electrode and the low conductivity of the electrode. In this work, a carbon nanotube (CNT) bridging method is introduced into the devices by adding single-walled CNTs (SWCNTs) in both perovskite and carbon layers to form a high-quality perovskite/carbon interface. The CNT bridges penetrating into both the CH₃NH₃PbI₃ layer and the carbon cathode not only facilitate charge extraction and transport between the two layers but also promote the electrical conductivity of the carbon electrode. The hole transport layer-free C-PSC with a structure of fluorine-doped tin oxide/compact TiO₂/mesoporous TiO₂/CH₃NH₃PbI₃-SWCNT/SWCNT-C gained a remarkable PCE of 15.73% with an FF of 0.72, accompanied by an outstanding stability of 90 days in the dark under high-humidity [65 ± 5% relative humidity (RH), 25 ± 5 °C] and high-temperature (75 ± 5 °C, 25 ± 5% RH) conditions. The low-cost fabrication process makes highly stable and efficient C-PSCs promising candidates for future applications.