Homogenizing Self-Assembled Monolayers via Alkali Modulation: Toward Efficient and Reproducible Organic Nonfullerene Solar Cells.

Self-assembled monolayers (SAMs), such as 2PACz, have emerged as effective hole transport layers (HTLs) in organic nonfullerene solar cells, enabling enhanced power conversion efficiency (PCE) to over 20%. However, they have been recognized to suffer from incomplete interfacial coverage and thus poor reproducibility. To address these issues, herein, we report a simple yet efficient alkali modulation strategy to achieve homogeneous interfacial coverage and contact through an acid-base reaction between potassium hydroxide (KOH) and the phosphonic acid group in SAM solution. Specifically, the resulting electrostatic repulsion effectively suppresses aggregation of SAM molecules in solution, which further promotes their homogeneous adsorption at the interface. Consequently, the KOH-modulated 2PACz effectively enhances both PCE and reproducibility for various representative donor:acceptor systems, with a champion efficiency of 20.11% achieved in D18:L8-BO-based cells. Moreover, the long-term stability of both SAM solutions and relevant devices is improved. We believe these findings provide an efficient interfacial engineering concept for optimizing SAM morphology to advance both efficiency and reproducibility of organic solar cells.