Heon Jin1,Henry Snaith1
University of Oxford1
Multi-junction tandem solar cells, utilizing complementary bandgaps, offer the potential to surpass the detailed balance limit for single-junction perovskite solar cells (PSCs). Through the tunability of perovskite bandgaps and recent advancements in mixed lead-tin (Pb:Sn) narrow bandgap PSCs, it is now possible to create highly efficient multi-junction solar cells solely using perovskites, achieving certified PCEs of up to 26.4% in all-perovskite tandem solar cells.<br/>However, there are two main challenges when using Pb:Sn perovskites. Firstly, methylammonium (MA) has been traditionally used in the most efficient Pb:Sn PSCs, but its thermal and chemical stability concerns prompt the search for MA-free alternatives. Second, conventional solution processing methods remain prevalent in lab-scale Pb:Sn PSCs fabrication. There were many attempts to thermal evaporate Pb:Sn as this method presents significant advantages, including high-quality thin film fabrication, precise thickness control, elimination of toxic solvents, large-scale compatibility, and reproducibility. Vacuum evaporation can also be applied to the fabrication of all-perovskite tandem solar cells without damaging underlying layers. Although vacuum deposition has proven successful in achieving high-efficiency and large-area PSCs for Pb-based perovskites, its application to Pb:Sn perovskites is less unexplored. Ball et al. and Igual-Munoz et al. are among the few to report Pb:Sn PSCs using vacuum evaporation.<br/>In this work, we demonstrate that through careful control of the environment and some fine-tuning, it is possible to deposit Pb:Sn perovskite films of high quality through thermal evaporation. We compare devices made using this method to those made from similar state-of-the-art solution processed methods. Our work demonstrates that it is possible to approach the performance of solution processed devices within a workable processing window using vacuum evaporation. Our champion devices reach PCEs of 17%, surpassing previously set records of 14% for thermally coevaporated Pb:Sn devices. This was done without requiring additional passivation or bulk additive, which are used in the previous reports.