Ultrasound-spray deposition of multi-walled carbon nanotubes on NiO nanoparticles-embedded perovskite layers for high-performance carbon-based perovskite solar cells

Carbon-based perovskite solar cells (C-PSCs) are prized for their simple device structure, high cost-efficiency and good stability. However, the absolute efficiency of C-PSCs is still low due to inappropriate C/perovskite interfaces. Herein, we report a simple ultrasound spray method to deposit pure multi-walled carbon nanotubes (MWCNT) films. When directly deposited on perovskite films, MWCNT could form a seamless contact at the perovskite/carbon interface, thus boosting the power-conversion efficiency (PCE) of C-PSCs to 14.07% as compared to drop cast MWCNT electrodes. In addition, when the perovskite surface was embedded with a sub-monolayer of nickel oxide nanoparticles (NiO NPs) prior to the MWCNT deposition, we achieved a champion efficiency as high as 15.80%, which is among the highest C-PSCs efficiencies reported to date. Detailed characterizations revealed that by planting NiO NPs into the surface region of CH₃NH₃PbI₃ crystals, the hole extraction efficiency was effectively enhanced and interfacial recombination was reduced. With silicon dioxide (SiO₂ NPs) as a control, the NiO NPs layer was found to favorably bend the energy levels at the interface for selective hole extraction, thereby enhancing the overall photovoltaic performance. The combination of ultrasound spray and nanoparticle embedment technologies opens the way to cost-efficient and scalable production of C-PSCs.