Phase-separation Mechanism and Morphological Control in All-polymer Solar Cells

Compared to the polymer/fullerene system, all-polymer solar cells, based on conjugated polymers as both donor and acceptor, have many potential advantages such as achieving more efficient light absorption and high open-circuit voltage, as well as easily solution processing and large-area fabrication. Strongly promoted by developments of materials and device structure, the power conversion efficiency (PCE) has been reached 9%. However, conjugated polymers have more rigid molecules compared to the flexible polymers and thus will form chain entanglement and p-p interaction with each other, leading to a more complex phase separation process in the conjugated polymer system. Besides, the strong molecular interaction between donor and acceptor polymers may generate a long-range phase domain in the blend films, which will inhibit the excitons to diffuse to the donor/acceptor (D/A) phase interface. In addition, the difference of thermodynamics steady state between the donor and the acceptor polymers may lead to the formation of different molecular orientation, which will impede the exciton dissociation. To solve these problems, by tuning the thermodynamic and dynamics factors, including molecular rigidity and blend ratio, the phase-separated structure of the conjugated polymer blend system was adjusted and the phase separation mechanism was identified, based on which the phase diagram of the conjugated polymer blend was depicted. By controlling phase separation structure, the interpenetrating networks were obtained, facilitating the charge transfer and collection. Besides, the domain size and film crystallinity were adjusted by reducing the solvent-polymer interaction parameter and polymer-polymer interaction parameters. Due to the decreased domain size, the efficiency of the exciton diffusion was enhanced. In addition, the solution state or molecular diffusion rate was adjusted to adjust the molecular orientation. By increasing the aggregation of the polymers in solution and introducing the epitaxial crystallization, the molecular orientation could change from edge-on to face-on. The identical molecular orientation for the donor and the acceptor improved the exciton dissociation efficiency and the device performance.