Highly Efficient and Stable Organic Solar Cells Enabled by a Commercialized Simple Thieno[3,2‐b]thiophene Additive

Abstract Delicately manipulating nanomorphology is recognized as a vital and effective approach to enhancing the performance and stability of organic solar cells (OSCs). However, the complete removal of solvent additives with high boiling points is typically necessary to maintain the operational stability of the device. In this study, two commercially available organic intermediates, namely thieno[3,2‐b]thiophene (TT) and 3,6‐dibromothieno[3,2‐b]thiophene (TTB) are introduced, as solid additives in OSCs. The theoretical simulations and experimental results indicate that TT and TTB may exhibit stronger intermolecular interactions with the acceptor Y6 and donor PM6, respectively. This suggests that the solid additives (SAs) can selectively intercalate between Y6 and PM6 molecules, thereby improving the packing order and crystallinity. As a result, the TT‐treated PM6:Y6 system exhibits a favorable morphology, improved charge carrier mobility, and minimal charge recombination loss. These characteristics contribute to an impressive efficiency of 17.75%. Furthermore, the system demonstrates exceptional thermal stability ( T 80 > 2800 h at 65 °C) and outstanding photostability. The universal applicability of TT treatment is confirmed in OSCs employing D18:L8‐BO, achieving a significantly higher PCE of 18.3%. These findings underscore the importance of using appropriate solid additives to optimize the blend morphology of OSCs, thereby improving photovoltaic performance and thermal stability.