Multilayer Formation, Interfacial Binding, and Stability of Self-Assembled Molecules in Perovskite Solar Cells

Carbazole-based self-assembled monolayers (SAMs) have played a key role in advancing the efficiency and stability of inverted perovskite solar cells (PSCs). However, weakly bound SAM molecules can be removed by polar solvents used in perovskite deposition. The loss of SAM molecules, especially those bound to the transparent conductive oxide substrate, disrupts interfacial energetics and accelerates PSC degradation. Quantifying initial SAM coverage and tracking its loss during fabrication are therefore critical yet experimentally challenging. Here, we develop a computational-experimental approach combining density functional theory with multimodal surface characterization, including X-ray photoelectron spectroscopy and cyclic voltammetry, to selectively remove and quantify SAM molecules in distinct adsorption modes, enabling reconstruction of their initial structure and evolution during processing. We reveal that SAMs, although commonly treated as single molecular layers, comprise multiple layers. Furthermore, SAMs undergo major restructuring upon exposure to N,N-dimethylformamide (DMF), a common perovskite precursor solvent, which removes all upper layers and nearly half of the first-layer molecules. To mitigate these losses, we implemented a redeposition strategy introducing new SAM molecules onto the DMF-washed SAM. Redeposition resulted in 13-21% more molecules retained after a second DMF wash compared to the DMF-washed SAM without redeposition. Devices with redeposited SAMs retain 90% of the initial efficiency for 480 h under 85 °C and 50% relative humidity─a 5-fold improvement in operational stability compared to unwashed samples. More broadly, this work establishes a first-layer-sensitive, quantitative method to track SAM evolution during fabrication and offers a simple, generalizable route to durable interfaces in PSCs.