Versatile Acetyl‐Functionalized Phenothiazine Self‐Assembled Monolayers for Efficient Perovskite and Organic Solar Cells

The development of versatile, stable, and cost‐effective hole‐transport layers (HTLs) is essential for advancing next‐generation photovoltaic technologies. Herein, we report a series of phenothiazine‐based self‐assembled monolayers (SAMs) as HTLs for perovskite and organic solar cells. By systematically tuning the substituents, including unsubstituted phenothiazine (PTZ), dibromo‐PTZ (PTZ‐diBr), and diacetyl‐PTZ (PTZ‐diACE), we achieve control over interfacial energetics and surface properties. The dual coordination sites of carbonyl groups and sulfur in PTZ‐diACE provide Lewis basic sites that coordinate undercoordinated Pb 2 + ions, facilitating passivation at the buried HTL/perovskite interface. Additionally, polar acetyl groups induce interfacial dipoles that reinforce the built‐in electric field, promoting efficient hole extraction. Among the series, PTZ‐diACE exhibits superior film formation, a deeper HOMO level, and stronger interfacial interactions, resulting in enhanced charge extraction and suppressed nonradiative recombination. Consequently, inverted perovskite solar cells employing PTZ‐diACE achieve a power‐conversion efficiency (PCE) of 25.5% with excellent operational stability, retaining 93% of its initial efficiency after 500 h under the ISOS‐D‐2I protocol. These advantages extend to organic solar cells, where PTZ‐diACE delivers a PCE of 20.4% in D18:L8BO devices and demonstrates compatibility across multiple active layers. This study demonstrates phenothiazine‐based SAMs as HTLs and provides design guidelines for efficient, stable interfacial materials.