Fluorinated fullerene interlayers for tin halide perovskite solar cells with enhanced operational air stability and minimized voltage losses

Abstract

Tin halide perovskite solar cells (Sn-PSCs) are a promising alternative to their lead-based analogues due to their favorable optoelectronic properties. However, their performance is still limited by two main factors: on one hand, the tendency of Sn²⁺ to oxidize to Sn⁴⁺, which leads to high non-radiative recombination; and on the other hand, the offset between the perovskite conduction band and the LUMO level of the standard C₆₀ fullerene, which hinders efficient charge transport and limits overall device performance. While the former has received significant attention, the latter has been less explored, despite being a major bottleneck for Sn-PSC development. In this work, we designed and synthesized two novel fullerene derivatives, namely JM10 and JM12, functionalized with different fluorine-rich moieties and incorporated them as interlayers between the perovskite and the C₆₀ electron transport layer (ETL). These derivatives demonstrated suitable LUMO levels and effectively alleviated the energy level misalignment at the perovskite/ETL interface, minimizing the undesirable charge recombination and enhancing the charge extraction processes. As a result, the best-performing Sn-PSCs incorporating JM12 showed a nearly 20% increase in power conversion efficiency with respect to the reference devices. Moreover, the fluorinated groups increased the hydrophobicity of the interlayers, significantly improving the operational stability of the unencapsulated devices under harsh ambient conditions (60% RH and 25 °C). As a result, devices containing JM10 and JM12 retained up to 80% of their initial performance after more than 15 and 18 hours, respectively. These results highlight the potential of fullerene chemistry's versatility to mitigate carrier losses at the interfaces while simultaneously improving both the performance and stability of Sn-PSCs, paving the way for future advancements in their design and development.