Enhanced Photostability and Reduced Burn‐In Losses in Organic Solar Cells via a 2PACz‐Modified PEDOT:PSS Hole Transport Layer

ABSTRACT Over the past decade, organic solar cells have witnessed remarkable improvements in power conversion efficiency, driven by advances in material design and device engineering. However, their limited operational stability remains a key obstacle to commercialization. The hole transport layers (HTLs), PEDOT:PSS, and more recently, self‐assembled molecules (SAMs), such as 2‐(9H‐carbazol‐9‐yl)ethyl phosphonic acid (2PACz), have shown promising performance. Nevertheless, using these HTLs individually is not ideal for the long‐term stability of organic solar cells. In this study, we investigated the combination of these two well‐established HTLs in a hole transport bilayer in PM6:Y6 OSCs. The PEDOT:PSS/2PACz bilayer is prepared using a simple two‐step spin‐coating process. This approach notably improves device reproducibility and, more importantly, reduces initial burn‐in photodegradation, maintaining stable PCEs for over 650 h (27 days) of continuous illumination. In comparison, solar cells employing PEDOT:PSS or 2PACz as single HTLs retain only 80% of their initial PCE (t 80 ) after two and one day of illumination, respectively. Moreover, transient photovoltage measurements reveal that the enhanced photostability of the devices based on the bilayer HTL originates from lower interfacial disorder induced during the photo‐aging process.