Balance between Energy Transfer and Exciton Separation in Ternary Organic Solar Cells with Two Conjugated Polymer Donors

The ternary strategy as a straightforward way for organic solar cells to improve the device performance attracts many interests in the field. The ternary strategy usually focuses on processing the third light-absorbing material having a complementary absorption to the binary system. However, studying the third component with similar absorption spectra as those of the binary counterpart is equally essential to understand the in-depth mechanism of the performance improvement from the third component. In this work, we filled this blank and derived a type of ternary device consisting of two conjugated polymer donor materials of PTB7-Th and PffBT4T-2OD and the nonfullerene acceptor material IEICO-4F. The average power conversion efficiency value of the optimized ternary device reached 12.1%, which is around 16% higher than that of its PTB7-Th:IEICO-4F binary counterpart. For the third component of PffBT4T-2OD containing a similar absorption spectrum as that of PTB7-Th, it was found that the J(sc) increase contributes to the primary performance enhancement. Further investigations indicate that the L-sc increase in the optimized ternary device mainly came from the improved light absorption ability, current extraction process, charge transport process, and suppressed nonradiative recombination. Moreover, there is a balance found between the exciton separation process and the energy transfer process when optimizing ternary blend ratios. The optimized ternary device is suspected to reach this balance point and thus exhibits the enhancement in device performance. Morphology investigation reveals that the addition of POT4T-2OD can tune the morphology and increase the crystallinity in the active layer. The optimized ternary blend shows a well-mixed donor and acceptor morphology with small domain size and slightly increased crystallization, which further explains its best device performance.