Enhancing Charge Collection of Tin-Based Perovskite Solar Cells by Optimizing the Buried Interface with a Multifunctional Self-Assembled Monolayer

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a widely used hole transport material in inverted tin-based perovskite solar cells (Sn-PSCs). However, the efficiency and stability of these Sn-PSCs that utilize PEDOT:PSS are unsatisfactory, partly due to concerns about their mismatched work functions, hydrophobicity, and chemical interactions. Here, we introduce a self-assembled monolayer (SAM), (2-(7<i>H</i>-dibenzo[c,g]carbazol-7-yl)ethyl) phosphonic acid (2PADCB) as a multifunctional buffer molecule at the buried PEDOT:PSS/Sn perovskite interface. The phosphate group in the 2PADCB molecule reacts with the sulfur atom on the thiophene ring in PEDOT:PSS. This reaction process effectively anchors the SAM molecule firmly to the surface of PEDOT:PSS. Additionally, it reduces the binding sites between PEDOT and PSS, alleviating the acidification of the PEDOT:PSS surface and the poor conductivity caused by excessive PSS. Furthermore, the presence of two additional benzene rings in the 2PADCB molecule terminal group increases the electron density around Sn²⁺, thereby inhibiting its oxidation. Additionally, the hydrophobic characteristics of the 2PADCB molecule mitigate moisture infiltration from PEDOT:PSS, thereby protecting the degradation of Sn perovskite. Consequently, the Sn-PSCs based on the PEDOT:PSS/2PADCB film achieve a champion efficiency of 14.7%, higher than that of their pristine counterpart (12.5%). Moreover, the 2PADCB molecule improves the stability of the device by maintaining 90% of its initial efficiency after 160 h under 1 Sun illumination. Such enhancement in efficiency and stability is mainly attributed to the improved interface quality with the 2PADCB molecule, leading to better carrier transport and suppressed charge recombination at the buried PEDOT:PSS/Sn perovskite interface. Our work suggests that introducing the 2PADCB molecule at the PEDOT:PSS/perovskite interface is a promising method for efficient and stable Sn-PSCs.