Even though the performance of tin perovskite solar cell (TPSC) has been reported to attain PCE 14.8%,¹ the hole-transport layer was based on a hydrophilic polymer, PEDOT:PSS, and this is the case for most TPSC reported in the literature. PEDOT:PSS is hygroscopic and this could easily degrade the device performance due to moisture penetration. Other HTM has not been reported in the literature until recently. This is because the reaction of SnI₂ with FAI to form FASnI₃ is a much more rapid process than for its lead analogue, control of crystal growth and nucleation rates are important factors to be considered. The traditional one-step method did not work for other hydrophobic HTM because of the poor hydrophilic nature of the film that enables retarded nucleation comparable to the crystal growth. We therefore developed a two-step method² with the procedure to deposit SnI₂ first for its rapid and smooth nucleation and to retard the crystal growth in the second step using an appropriate solvent and additive. This is a very important development for tin PSC because of the versality of the two-step method which can be applied for all the charge-transport materials developed elsewhere. For example, it is possible to fabricate a HTM-free tin-based PSC using the concept of a self-assembled monolayer (SAM) to modify the ITO surface with tin perovskite layer deposition in a two-step approach.³ We also deposited a smooth and uniform tin-perovskite layer on a hydrophobic conducting polymer, (bis (4-phenyl) (2,4,6-trimethylphenylamine) (PTAA), on modification of the PTAA surface with an organic ammonium salt, phenylethylammonium iodide (PEAI), according to a two-step approach.⁴ Our approach is also applicable to other prospective HTL materials to match the energy levels between perovskite and HTL so as to enhance further the performance of the device in the future.

References:

[1] B. B. Yu, Z. Chen, Y. Zhu, Y. Wang, B. Han, G. Chen, X. Zhang, Z. Du and Z. He, Adv. Mater. 2021, 33, 2102055.

[2] S. Shahbazi, M.-Y. Li, A. Fathi and E. W.-G. Diau, ACS Energy Lett. 2020, 5, 2508-2511.

[3] D. Song, S. Narra, M.-Y. Li, J.-S. Lin and E. W.-G. Diau, ACS Energy Lett. 2021, 6, 4179-4186.

[4] C.-H. Kuan, G.-S. Luo, S. Narra, S. Maity, H. Hiramatsu, Y.-W. Tsai, J.-M. Lin, C.-H. Hou, J.-J. Shyue and E. W.-G. Diau, Chem. Eng. J. 2022, 450, 138037.