Fullerene-Anchored Core-Shell ZnO Nanoparticles for Efficient and Stable Dual-Sensitized Perovskite Solar Cells

Among all the solutions to improve long-term stability of perovskite solar cells, p-i-n heterojunction with all-metal-oxide charge transporting layers show their attractive features. However, these devices require an efficient electron-transporting layer (ETL) fabricated on top of the perovskites in which the commonly used oxide nanocrystals are limited by their imperfect surface. Here, a novel surface-passivation strategy was adopted by anchoring ZnO nanoparticles with fullerene nano-shells (Fa-ZnO) to mitigate trap states and passivate surface hydroxyl groups. We evidence the Fa-ZnO can be easily processed on top of perovskite as a high-quality ETL that improves electron extraction efficiency and suppresses ion diffusion/moisture penetration. In addition, the presence of fullerene shells allows Fa-ZnO nanoparticles to penetrate into perovskite precursors before crystallization, resulting in n-type sensitized configuration. With mesoscopic NiO_x as hole-collecting contact, the p-n dual sensitization configuration has enabled all-metal-oxide devices to achieve state-of-the-art efficiencies of 21.1% with greatly improved performance longevity. With the consideration of long-term stability for commercialization, research on perovskite solar cells is clearly moving toward the perovskite composition tuning and the development of charge transporting layers. Compared with commonly used organic materials, inorganic oxides offer greater versatility and stability. However, the great barrier for all-metal-oxide devices is to achieve a high-quality “capping layer” on top of perovskites. Here, we adopted a surface modification by grafting fullerenes onto ZnO nanoparticles (Fa-ZnO) through the binding of catechol on ZnO. We found that Fa-ZnO meets the strict requirements of an effective ETL, including high conductivity, low electron traps, and ions/moisture diffusion barrier. Furthermore, the fullerene shells allow the penetration of Fa-ZnO into the perovskite film to form a heterojunction upper layer. As a result, the PSCs with dual sensitization architecture exhibit a high PCE over 21% and show high tolerance to environmental stresses. The perovskite solar cells with all-inorganic selective contacts have demonstrated impressive long-term stability and triggered great interests; however, their device performances are still lagging behind their organic counterparts. Here a new design of fullerene-anchored ZnO nanoparticles with low surface defects meets the strict requirements of electron-transporting layer on top of perovskite. Delicate control over surface of oxide nanocrystals also allows the formation of graded heterojunction, and thereby enables the fabrication of p-n dual-sensitized solar cell with high efficiency and stability.