Optimal Interface Engineering through SAM Energy Alignment & Functionalization

Open circuit voltage losses (V_OC) in the perovskite top cell remain a significant challenge for the commercialization of perovskite tandem solar cells. We show through simulations that common hole transport layers like PTAA and Me-4PACz are energetically mismatched with the perovskite active layer which lowers the quasi-Fermi level splitting and limits V_OC, particularly for wide bandgap compositions relevant to tandems (E_g ≥ 1.7 eV). We identify the ideal energy offset between the hole extraction and the wide bandgap perovskite layers and modulate the anode work function to this ideal using novel halide self-assembled monolayers (SAMs) as interface modifiers. Mixing these SAMs allows for selective tunability of both wettability and work function. Using a two-step SAM-perovskite deposition, we have not found a clear correlation between improved alignment of the SAM-modified ITO-perovskite interface and improved V_OC in solar cell devices. However, we found that mixing the SAMs into the perovskite precursor solution using a one-step deposition method improves their solar cell performance. This suggests that interfacial chemistry, including SAM functionalization and SAM interactions with perovskite in solution, may supersede energy alignment as the primary factor currently limiting V_OC. Understanding their respective roles will enable the development of efficient wide gap perovskite devices for Si-tandem applications.