Tuning Hole-Collecting Monolayers with Hammett Constant-Engineered Benzoic Acid Derivatives for Inverted Perovskite Solar Cells

Although carbazole-based hole-collecting monolayers (HCMs) have been widely employed for achieving high-efficiency inverted perovskite solar cells (PSCs), enhancing their film uniformity remains a critical challenge. Here, we report the use of Hammett constant (σp)-engineered benzoic acid derivatives to improve the conformal coating of the tripodal triazatruxene-based 3PATAT-C3 HCM. Benzoic acid derivatives with positive σp values enhanced the power conversion efficiency (PCE), whereas those with negative σp values reduced device performance. Among the tested derivatives, 4-acetylbenzoic acid (4-AcBA), bearing an electron-withdrawing substituent that imparts a positive σp, promoted improved conformality and a favorable work function for efficient hole extraction. In contrast, 4-aminobenzoic acid (4-ABA), with an electron-donating substituent corresponding to a negative σp, led to poor conformality and an unfavorable work function. Furthermore, compared to 4-ABA, the incorporation of 4-AcBA strengthened the binding of both HCMs and the additive itself to the fluorine-doped tin oxide (FTO) surface. As a result, engineering the 3PATAT-C3 HCM with 4-AcBA increased the PCE from 23.55% to 24.43%, while also significantly enhancing operational stability compared with the untreated control device.