Crosslinkable Conjugated Polymers for Improvement in the Stability of Flexible Polymer Solar Cells

The polymer solar cells (PSCs) have been extensively researched in recent years, and have the advantage that they can be applied to flexible devices. Although PSCs with bulk-heterojunction (BHJ) systems are composed of an active layer with binary blends of polymer donors and polymer acceptors and display high performance, the device stability is still challengeable. Even in well-mixed polymer blend films, long-term high-temperature conditions can induce phase separation in the films. Therefore, the shelf-life and operational stability of the PSCs in ambient and high-temperature environments should be carefully considered for the practical application of all-PSCs. Among the several strategies reported for improving the high-temperature stability of all-PSCs, the crosslinking reaction has emerged as an effective method for improving the stability and functions of the polymer films. On the other hand, single-component polymers via conjugated donor-acceptor block copolymers (CDABPs) have been proposed for use in many strategies to overcome the drawback of BHJ. These polymers can suppress phase separation and exhibit inert internal morphologies compared to the polymer blends; however, their thermal stability remains an issue to be addressed.<br/> In this study, we demonstrated a novel crosslinkable polymer system: extrinsic single component system(PBDBT-BV₂₀ and N2200-TV₁₀) and intrinsic single component system(P(OXBTT-co-NDI2T)). First, random terpolymers were successfully synthesized by introducing thiophene-based monomers bearing vinyl functional groups in the side-chain. The physical properties of the blend films of PBDBT-BV₂₀ and N2200-TV₁₀ before and after thermal crosslinking were extensively investigated and compared to those of the homogeneous individual polymer films. In addition, the PSCs with the crosslinked blend film exhibited an excellent shelf-life of over 1200 h and a thermally stable power conversion efficiency(PCE). Furthermore, the crosslinked blend film exhibited excellent mechanical stability under bending stress in flexible PSCs using plastic substrates. After all, we synthesized a new CDABP (P(OXBTT-<i>co</i>-NDI2T)) bearing crosslinking groups to minimize phase separation and secure thermal stability of the internal morphology. The P(OXBTT-<i>co</i>-NDI2T) film exhibited a relatively high PCE and excellent thermal stability compared to the all-PSC device based on the P(OXBTT):P(NDI2T) blend film. The results of these studies indicate the promising potential of PSCs with crosslinkable conjugated polymers for future PSCs that require a long shelf-life, thermal and mechanical stability, and high performance.