Exploring the Origin of High Thermal Stability of the Performance of Pseudo-Quaternary All-Polymer Solar Cells

As all-polymer solar cells (all-PSCs) have achieved impressive power conversion efficiencies (PCEs), extending their lifetime under long-term operation is also increasingly important. To address this issue, in this study, a new pseudo-quaternary blend composed of conjugated block copolymer donors and acceptors, PM6-b-TT:b-PYT, is introduced as the active layer for all-PSCs. Compared to the all-PSC based on the traditional binary blend, PM6:BTTP-T, those based on pseudo-quaternary active layer exhibited significantly improved thermal stability after thermal annealing under harsh conditions of 150 °C in an ambient atmosphere. More importantly, to elucidate the morphological stability of the pseudo-quaternary active layer, visible evidence of the thin film's surface and internal structure is carefully investigated by multiple advanced techniques. After extended thermal stress at 150 °C, the binary bulk heterojunction (BHJ) films exhibit excessive polymer chain aggregation, phase separation of the polymers, and increased surface roughness, forming bulk charge traps and increasing the exciton recombination. Meanwhile, the pseudo-quaternary BHJ films maintain the crystallinity and nanostructure of the active layer, improving the stability of the all-PSCs. Overall, this study provides a detailed understanding of the long-term stability of high-efficiency all-PSCs, offering key insights into the polymer section and proposing promising polymer structures for the long-term stability of all-PSCs.