Sn-Based Perovskite Solar Cells: Additive Engineering and Surface Modification to Improve Stability

Perovskite solar cells (PSCs) have attracted much attention because of the fast progress of their power conversion efficiency (PCE), from 3.8% to 25.7% in the past 14 years [1]. However, the toxicity of Pb effectively hinders the large-scale commercial production of PSCs. Therefore, researchers have turned their attention to Sn-based perovskite due to their similar or even superior optoelectronic properties, such as optimum band gap, higher charge carrier mobility, and low exciton binding energy [2–3]. However, the stability of the Sn-based PSCs is low compared to Pb-based PSCs, which limits the practical use of the solar cells. One of the reasons for this low stability is the rapid crystallization, which results in inhomogeneous films with a lot of pin holes. Another reason is the easy oxidation of Sn²⁺ to Sn⁴⁺, which leads to high nonradiative recombination of carriers, low efficiency, and low stability. According to recent studies, Sn²⁺ oxidation is more dominant on the perovskite surface [2].<br/>To enhance the quality of perovskite film, additive engineering is a well-known process for Sn-based PSCs to slow down the crystallization rate [3]. Recently, polydentate additives have shown effectiveness in modulating the growth of Sn-based perovskite films [4]. However, the stability of perovskite film is dependent on crystallinity and pin-hole-free large-grain compact films. The highly oriented crystal offers smooth charge transfer, whereas the larger grain reduces the grain boundaries, which are regarded as the recombination centers for the charges. In this work, we used a bifunctional additive in Sn precursor solution that acted as a modulator to dictate the growth of perovskite film and were able to fabricate highly crystalline and stable uniform perovskite film.<br/>The additive-added FASnI₃ film showed large and compact grains without any pinholes. This is because the bifunctional additive had a strong interaction with SnI₂ through hydrogen bonding, which delayed the FASnI₃ crystal growth. At the same time, the bifunctional groups helped to create strong adherence between perovskite and PEODOT:PSS and dictated the growth of perovskite crystals in a preferred orientation. The XRD pattern of the modified FASnI3 film remains unchanged for up to 3h when exposed to air, which indicates that the addition of this bifunctional additive to the precursor solution significantly enhanced the environmental stability of the FASnI3 film.<br/>With the addition of a bifunctional additive, the PCE jumped from 10.0% to 13.2%. It is important to state that the addition of the additive significantly improved the open circuit voltage (V_OC) from 0.85 V to 0.95 V as well as the short circuit current (J_SC) from 16.5 mA/cm² to 18.2 mA/cm². The enhancement of V_OC may be due to the suppression of leakage current with the addition of additives in the precursor solution, as we confirmed from our dark current measurement. This modification also reduced the charge recombination center, such as Sn⁴⁺ content in the film, as we observed from the XPS result. The PCE of the modified PSC retained its 90% initial PCE even after 100h. As a result, we achieved a certified (AIST, Japan) PCE of over 12% with superior device stability for Sn-based PSCs.<br/><br/><b>References</b><br/>NREL Best Research-Cell Efficiency Chart. [https://www.nrel.gov/pv/cellefficiency.html]<br/>Zhang, J., Yu, C., Wang, L., Li, Y., Ren, Y., & Shum, K. Scientific Reports, 2014, 4, 1-6.<br/>Kayesh, M. E., Matsuishi, K., Kaneko, R., Kazaoui, S., Lee, J. J., Noda, T., Islam, A. ACS Energy Letters, 2018, 4, 278-284.<br/>Hu, S., Otsuka, K., Murdey, R., Nakamura, T., Truong, M. A., Yamada, T., Wakamiya, A. Energy & Environmental Science, 2022,15, 2096-2107.