Self-assembled monolayers (SAMs) are widely employed as effective hole-selective layers (HSLs) in inverted perovskite solar cells (PSCs). However, most SAM molecules are amphiphilic in nature and tend to form micelles in the commonly used alcoholic processing solvents. This introduces an extra energetic barrier to disassemble the micelles during the binding of SAM molecules on the substrate surface, limiting the formation of a compact SAM. To alleviate this problem for achieving optimal SAM growth, a co-solvent strategy to disassemble the micelles of carbazole-based SAM molecules in the processing solution is developed. This effectively increases the critical micelle concentration to be above the processing concentration and enhances the reactivity of the phosphonic acid anchoring group to allow densely packed SAMs to be formed on indium tin oxide. Consequently, the PSCs derived from using MeO-2PACz, 2PACz, and CbzNaph SAM HSLs show universally improved performance, with the CbzNaph SAM-derived device achieving a champion efficiency of 24.98% and improved stability.
Keywords: amphiphilic molecules; carbazole; hole-selective layer; perovskite solar cells; self-assembled monolayer.
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