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Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells

April 5, 2021

(Published in Nature)
Perovskite solar cells (PSCs) are based on halide perovskites which are of general ABX3 structure: where A is a monovalent cation such as caesium, methylammonium, or formamidinium, B is a divalent cation lead, tin, or germanium, and X is an anion iodide, chloride, or bromide. Since their emergence from dye-sensitized solar cells (DSSC) in 2009, PSCs have been at the forefront of research on cheaper solar cell technologies.

They are usually manufactured by mixing and layering various materials together on a transparent conducting substrate. Over the last decade, countless research has been published on improving the efficiency and stability of PSCs. To this effect, a large number of chemical compositions have been investigated. However, the cubic phase of formamidinium lead iodide (FAPbI3) has emerged as the most promising material for stable and efficient PSCs.

In this study, collaborative research efforts between several groups at the EPFL, Switzerland led by Professor Michael Grätzel and UNIST, Korea led by Professor Jin Young Kim, have developed an innovative chemical synthesis technology to maximize the solar to the power conversion efficiency of FAPbI3 based PSCs. The resulting solar cells achieved a power conversion certified record efficiency of 25.21%, thereby surpassing other solar cell technologies of CdTe, CIGS, and the current market-leader, polycrystalline silicon. One of the major problems in these materials is the formation of halide vacancies (defects) which are mainly formed during their fabrication procedure. PSCs suffer from these defects which cause low efficiency and poor stability. In the new method, perovskite films are synthesized by using anion engineering that uses formate (HCOO-) anions to suppress the formation of notorious halide defects and improve the morphology of thin films.

The group of Ursula Röthlisberger has used molecular simulations to figure out the unique mechanistic role of HCOO- anions. First principles molecular dynamics simulations were performed to first understand the effect of HCOO- anions during the crystallization process. We have found that HCOO- strongly coordinates with Pb2+ cations that help in slowing down the crystallization process resulting in larger stacked grains of thin-films. Secondly, we performed carefully designed first principles calculations to understand the passivating effects of HCOO-. We compared the relative binding affinities of different anions to I- vacancy sites on the surface of perovskite crystal and found that HCOO- is one of the best candidates for passivating halide vacancies among similar pseudo halides alternatives.

Reference: Jeong, J., Kim, M., Seo, J., Lu, H., Ahlawat, P., Mishra, A., Yang, Y., Hope, M.A., Eickemeyer, F.T., Kim, M., Yoon, Y.J., Choi, I.W., Darwich, B.P., Choi, S.J., Jo, Y., Lee, J.H., Walker, B., Zakeeruddin, S.M., Emsley, L., Röthlisberger, U., Hagfeldt, A., Kim, D.S., Grätzel, M., and Kim, J.Y. (2021). Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells. Nature. (10.1038/s41586-021-03406-5).

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