The covalent connection of donor (D) and acceptor (A) materials as conjugated block copolymers (CBCs) has attracted increasing attention in recent years. When used individually as the photoactive material in organic photovoltaics (OPVs), CBCs can exhibit several advantages, such as excellent mechanical properties and superior long-term stability, as compared to the typical bulk-heterojunction (BHJ) blends. However, relevant studies to date mainly focus on clarifying the device performance difference between CBCs and the blend systems based on the commensurate D and A segments. To the best of our knowledge, the influence of the D-A segment ratios for CBCs on the device performance has not been investigated yet.

In this context, we herein synthesize a series of CBCs based on an n-type PNDI2T and a p-type PBDB-T segments with three different D-A ratios (P1-P3) and introduce these CBCs separately as the single-component photoactive materials in OPVs to scrutinize their photovoltaic performance (Figure 1, left). Compared to P2 and P3, the P1 device with a higher content of PNDI2T exhibits superior exciton dissociation and charge transfer behaviors, leading to the highest power conversion efficiency (PCE). This is due to the more dominant face-on orientation of P1 than the others. On the other hand, the n-type block is revealed to play a more critical role in the inter/intra-chain charge transfer than the p-type block. Finally, we show that the P1 device also possesses the lowest energy loss as a result of the suppressed non-radiative energy loss. This result provides the first discussion on the impact of the segment ratio for a CBC on the resultant chain stacking behavior that would pave the way for the future development of single-component OPVs.

Besides being employed as single-component photoactive materials, CBCs can also serve as compatibilizers for modifying the morphology of BHJ. Typically, BHJ in high-performance OPVs based on non-fullerene small molecular acceptors (NFAs) suffer from insufficient long-term stability due to the higher diffusion coefficient of NFAs compared with polymer donors. Furthermore, the NFAs in the BHJ blends tend to aggregate, forming a larger pure domain region, which can significantly impact the charge dissociation/transfer efficiency and the BHJ morphology stability. To address this issue, we develop PM6-b-PYIT, which possesses an analogous structure of PM6 and Y-series NFAs, and we introduce it as compatibilizers into BHJ blends to optimize their morphology (Figure 1b, right). As a result, adding 0.5 wt% of PM6-PYIT into PM6:L8-BO system can obtain over 18% of PCE, making it the best performance for the inverted OPV device we are aware of. Moreover, adding PM6-PYIT optimizes the morphology of BHJ and thus enhances charge transport properties. Compared with PM6 and Y-series NFA materials, the analogous structure of PM6-PYIT could fix initial morphology through van der Waals force, leading to superior long-term stability and mechanical properties.