Research progress on functionalized fullerenes in tin-based perovskite cells

In recent years, organic-inorganic halide perovskite solar cells (PSCs) have gained significant attention and experienced rapid advancements, thanks to their high light absorption coefficients, tunable bandgaps, low exciton binding energies, and solution processability. Within just over a decade, the power conversion efficiency (PCE) of PSCs has surged from 3.8% to 26.7%, positioning them as one of the leading technologies in the third generation of photovoltaics. However, the vast majority of high-efficiency PSCs currently rely on lead-based perovskite, raising concerns about the potential risks associated with lead leakage during production, transportation, storage, and usage, which could lead to biological hazards and environmental pollution. As a result, various low-toxicity or non-toxic lead-free perovskite materials have been developed in an attempt to replace the toxic lead-based compounds. Among these alternatives, tin-based perovskites (TPSCs) are considered promising. Tin, being in the same group as lead on the periodic table, shares similar atomic structures and ionic radii, enabling tin-based perovskites to exhibit many optoelectronic properties akin to lead-based perovskites, such as high carrier mobility and excellent light absorption coefficients. Additionally, tin-based perovskites possess a narrower bandgap, which approaches the theoretical optimal bandgap for single-junction solar cells (approximately 1.34 eV), potentially allowing TPSCs to achieve efficiencies exceeding 33%. However, the highest PCE of TPSCs to date is still only 15.7%, which is considerably lower than the theoretical efficiency. Fullerenes, which are spherical conjugated π systems made up of multiple five-membered and six-membered rings, have emerged as a potential solution. These molecules exhibit high electron mobility, efficient defect passivation properties, and the ability to introduce various functional groups. As a result, fullerene derivatives with specific functionalities have been extensively used in TPSCs to address several of the challenges these devices face. Due to their excellent conductivity, various functionalized fullerenes have been incorporated into tin-based perovskite precursor solutions. Studies show that these large spherical fullerene additives tend to concentrate at the grain boundaries of the perovskite, where they function as carrier transport media between grains. Additionally, fullerenes can passivate grain boundary defects by interacting with Sn²⁺ ions, preventing their oxidation. Some cross-linkable fullerene additives have been employed to delay the crystallization process, optimize lattice matching, and reduce mechanical stress within the thin film, which not only improves device efficiency but also makes them suitable for applications in wearable devices. Another key strategy to improve the performance of TPSCs involves optimizing the interface between the perovskite and electron transport layers (ETLs). The common-used solvents for fullerene interface modification do not damage the underlying tin-based perovskite, and these fullerenes can also enhance interactions with the tin-based perovskite by introducing suitable functional groups, thereby improving electron extraction and defect passivation, which effectively enhances device performance. Furthermore, the development of fullerene materials with higher lowest unoccupied molecular orbital (LUMO) levels and stronger coordinating interactions for direct use as ETLs can help resolve energy-level mismatches in TPSCs. Research has shown that a series of fullerene bisadducts, which possess suitable LUMO levels and enhanced passivation effects, have been successfully employed in TPSCs, resulting in substantial performance improvements. In conclusion, although there is still a noticeable PCE gap between TPSCs and lead-based perovskite solar cells (LPSCs), the promising theoretical efficiency and low toxicity of TPSCs highlight their commercial potential. Continued exploration of functionalized fullerene materials, along with ongoing advancements in device performance, will be key to unlocking the full capabilities of tin-based perovskites in photovoltaic energy generation.