Correlating the structures and photovoltaic properties in phase-separated blends of conjugated donor polymers and acceptors

The power conversion efficiency of polymer solar cells strongly depends on the microscale morphology of the interpenetrating network structures between the polymer donor and acceptor materials. Therefore, it is essential to understand the relationship between photovoltaic properties and phase-separated structures in the blend active layer. Here, we discuss the relationship between charge generation and collection and phase-separated structures, which was analyzed by a ternary phase diagram for polymer solar cells based on blends of a thiophene-based conjugated polymer donors and the following different acceptors: a fullerene derivative, a nonfullerene acceptor, and a conjugated polymer acceptor. By considering the ternary phase diagram based on the Flory–Huggins interaction parameters, we discuss the binodal point and acceptor volume fraction in the mixed phase in each material combination. Furthermore, we suggest strategies for improving the efficiency of polymer solar cells according to the molecular weight of acceptor materials. These findings will provide a guideline for developing highly efficient polymer solar cells. Ternary phase diagrams of the solvent/donor/acceptor system were studied to discuss the relationship between the photovoltaic properties of polymer solar cells and the phase-separated structures. The downshifted intersection points between the solvent drying pathway and binodal lines suggest the smaller domain sizes, and hence the higher exciton diffusion efficiency. The appropriate acceptor volume fraction close to the percolation threshold in mixed-phase shows high charge collection efficiency due to an effective charge transport and a suppressed bimolecular recombination.