Achieving high-efficiency tin-based and wide-bandgap lead-based perovskite solar cells through thermally evaporable fullerene diadditions

Professor Chengbo Tian's research group at Huaqiao University recently reported a methylene-functionalized fullerene diadduct electron transport material (CH₂)₂C₆₀ without isomers. They applied this material to tin-based perovskite and wide-bandgap perovskite solar cells via vapor deposition, effectively improving the efficiency and stability of the devices. This work focused on the energy level mismatch between tin-based and wide-bandgap perovskites and commonly used fullerene electron transport materials, as well as the poor charge transport performance of disubstituted fullerenes with large-sized groups. They designed and synthesized a disubstituted fullerene molecule (CH₂)₂C₆₀ with a small-sized methylene group . Due to its energy level fit, high electron mobility, and excellent thermal stability, the efficiencies of tin-based and wide-bandgap perovskite solar cells based on (CH₂)₂C₆₀ reached 15.35% and 20.81%, respectively. The article was published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Tin-based perovskites are considered candidate materials for high-performance photovoltaics due to their low toxicity and ideal band gap, with their theoretical single-junction efficiency approaching the Shockley-Queisser limit. However, despite continuous breakthroughs in efficiency through composition and structural engineering, their actual performance remains far below theoretical potential. One key bottleneck is the energy level mismatch with commonly used fullerene electron transport materials (such as C60 and PCBM). This problem also exists in 1.77 eV wide-bandgap lead-based perovskite solar cells. In contrast, disubstituted fullerenes are considered ideal electron transport materials for tin-based and wide-bandgap lead-based perovskite solar cells due to their suitable LUMO energy levels. However, these materials often exist as isomer mixtures and contain bulky substituents such as carbonyl or aromatic rings, leading to disordered molecular packing, low electron mobility, and difficulty in preparing dense thin films via thermal evaporation. Therefore, developing disubstituted fullerene molecules with both high thermal stability and high electron mobility is crucial for improving the performance of these solar cells.

Highlights of this article:

The research team designed and synthesized a disubstituted fullerene molecule (CH₂)₂C₆₀ with methylene as the functional group and used it as an electron transport layer in tin-based perovskite solar cells and wide-bandgap perovskite solar cells. Thanks to its excellent thermal stability, suitable energy level, and high electron mobility, (CH₂)₂C₆₀ effectively improves the open-circuit voltage loss problem in tin-based perovskite solar cells and wide-bandgap perovskite solar cells, thus achieving highly efficient and stable devices.

Single-crystal X-ray diffraction revealed that the small-sized methylene groups not only did not affect the close packing of fullerene molecules, but also led to a certain degree of shortening of the centroid distance of the fullerene carbon cages compared to C60 due to the CH··π interactions between molecules _. This smaller centroid distance helps the film to transport charge more efficiently; therefore, the conductivity and electron mobility of the (CH₂)₂C₆₀ film are both improved compared to C60 and CH₂C₆₀.

Furthermore, the dimethylene functional group of ((CH₂)₂C₆₀ reduces its π-conjugation, resulting in a better energy level match between it and tin-based perovskites compared to C₆₀ and CH₂C₆₀. PL and TRPLcharacterization results show that, due to the better energy level match, the perovskite film coated with (CH₂)₂C₆₀ exhibits a higher PL peak intensity and the shortest fluorescence lifetime, decreasing from 2.27 ns for the original perovskite film to 1.41 ns. Statistical results from surface potential analysis also show that this film has the lowest surface potential, indicating that the improved energy level alignment facilitates charge extraction.

Thanks to improved energy level matching and excellent charge transport performance, the tin-based perovskite solar cell based on (CH₂)₂C₆₀ achieved a power conversion efficiency of 15.35%, maintaining over 95% of its initial performance after 3000 hours of storage. Furthermore, using (CH₂)₂C₆₀ in a wide-bandgap perovskite solar cell also yielded an efficiency of 20.81%. This device also exhibited good stability, retaining over 90% of its initial efficiency after 1000 hours of aging under ISOS-L-1 conditions.

Summary and Outlook:

In summary, this study designed and synthesized a methylene disubstituted fullerene electron transport material (CH₂)₂C₆₀ without isomers. Benefiting from the characteristics of the methylene group, the (CH₂)₂C₆₀ molecule not only possesses suitable LUMO energy levels and thermal stability, but its corresponding thin film also exhibits excellent charge transport performance, thereby effectively improving the performance of tin-based and wide-bandgap lead-based perovskite solar cells. This strategy effectively solves the problem of achieving both high LUMO energy levels and high electron mobility in previous fullerene materials.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.