The Role of Self‐Assembled Monolayers in the Performance‐Stability Trade‐Off in Organic Solar Cells

In recent years, self‐assembled monolayers (SAMs) have been proven to work efficiently as hole‐selective materials in both organic solar cells (OSCs) and perovskite solar cells. Although competitive performances are reported with these materials, a mechanistic understanding on device stability remains elusive. This study reveals that while various SAM molecules can increase the indium tin oxide (ITO) work function versus vacuum, they may not consistently result in monolayers that ensure simultaneous improvement in performance and operational stability of devices. Energetically, achieving alignment between the work function of the SAM‐modified electrode and the ionization energy (IE) of the donor is shown to be crucial for a low hole injection barrier, irrespective of the SAM's IE. Light‐induced degradation in the widely used SAM, (2‐(9H‐carbazol‐9‐yl)ethyl) phosphonic acid (2PACz), is identified through diverse aging tests and comprehensive chemical and electronic characterizations. This degradation involves SAM molecule decomposition and chemical reactions with the photoactive layer, contributing further to device degradation. Addressing these challenges, sputtered nickel oxide/SAM bilayers are proposed as hole‐selective contact with tailored interface energetics for both efficient and photostable OSCs, offering a promising alternative to commonly used hygroscopic PEDOT:PSS in OSCs. This study reveals that spin‐coated films of SAM molecules (2PACz and analogs) do not conform to the common assumption of forming monolayers. Compared to PEDOT:PSS, SAM‐based devices show significantly reduced photostability. Light‐induced degradation of 2PACz and chemical reactions with photoactive layer materials are the molecular origins of device degradation. NiOx/Br‐2PACz bilayer exhibits simultaneous enhancement of device performance and photostability.