Small-molecule/fullerene acceptor alloy: A powerful tool to enhance device efficiency and thermal stability of ternary polymer solar cells

Along with the rapidly increased power conversion efficiency (PCE) of polymer solar cells (PSCs), the stability of PSCs becomes one of crucial issues for the commercial application of this technology. Ternary PSCs exhibit a great potential in improving device efficiency and stability. However, the main cause behind the improvement in the efficiency and stability has been lack of in-depth investigation. Moreover, the basic working mechanism of ternary solar cells is still ambiguous and urgently necessary to further understand. Here, a small-molecule acceptor (SMA) IDT-OT was developed and introduced into the binary system containing polymer donor PBDB-T (Donor) and fullerene acceptor (FA) PC71BM to construct Donor/FA/SMA ternary PSCs. Results reveal that the incorporation of IDT-OT into the PBDB-T/PC71BM binary system can improve the PCE up to 9.09% and enhance the thermal stability of the ternary PSCs relative to all the binary systems. In-depth investigations indicate that the enhanced device performance and stability is closely related with an intermixed small-molecule/fullerene acceptor alloy formed by IDT-OT and PC71BM in which the structural nature of the two acceptors is remarkably different. This distinctive alloy system is verified via investigating the dependence of LUMO energy level and open-circuit voltage on the weight ratio of IDT-OT in the acceptor blend, miscibility of two acceptors by morphological characterization and water contact angle measurement. This small-molecule/fullerene acceptor alloy phase is beneficial for improving charge dissociation, collection and transport as well as carrier mobility, and suppressing bimolecular recombination, thus leading to the increase of Jsc and FF of the ternary PSCs. Interestingly, the photoluminescence and donor-free device performance studies demonstrate that there exists not only efficient charge transfer but also the Forster resonance energy transfer between PC71BM and IDT-OT, which has rarely been found in the previously reported ternary PSCs. Moreover, we suppose the stable active layer morphology benefitted from the formation of this small-molecule/fullerene acceptor alloy as the main reason behind the observed improvement in the thermal stability of the ternary PSCs. This work suggests introducing a SMA into FA to form small-molecule/fullerene acceptor alloy could be a powerful tool to realize high-performance and stable PSCs.