Improving the stability of organometallic halide perovskite photoactive layers and solar cells via film passivation with iodide salts

Abstract

Methylammonium lead halide perovskite materials are receiving great attention as promising candidates for industrial applications, but their long-term stability when used in solar devices remains problematic. In particular, the perovskite layers based on CH3NH3PbI3 (methylammonium lead iodide, MAPI) experience severe degradation caused by oxygen and light. An understanding of this process, and how to enhance stablility, is distinctly important in the evolution of devices utilising perovskite. The systematic study of MAPI-based solar cells is investigated in first results chapter. Factors such as compact layer thickness, mesoporous layer thickness, TiCl4 surface treatment and Spiro-OMeTAD oxidation are studied. Furthermore, the issues of how photo- and oxygen-induced degradation, and thus the superoxide formation, will affect the efficiency and stability of the perovskite devices are addressed. The second results chapter builds on the knowledge that iodide vacancies in MAPI crystals are the most-preferred sites for superoxide formation. First, a film passivation technique was devised using methylammonium iodide (MAI) coating. The treatment could effectively reduce the suface defects and improve device stability by inhibiting superoxide generation. Second, the relation between superoxide formation and device stability as a function of bromide composition for Br-I mixed halide solar cells is examined. Use of an alternative passivating agent, namely phenylethylammonium iodide (PEAI) shows that it is highly effective on enhancing film and device stability under light and oxygen without reducing performance. The results confirm that this iodide salt could help to reduce problematic vacancies and therefore lower superoxide yields and improve device stability. Extending the investigation further, in final results chapter, a novel Iodide Salt Assisted Anti-solvent Treatment (ISAAT) for preparing MAPI layer is developed. The ISAAT strategy could effectively enhance the morphology and crystallinity of the perovskite films. With ISAAT treatment, devices are more stable and efficient than without treatment. More importantly, this ISAAT method shows more reproducibility than direct PEAI treatment, thereby elucidating an effective methodology of future design and optimisation of perovskite solar cells with better efficiency, long-term durability and reproducibility.

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