Unprecedented Efficiency Increase in a Ternary Polymer Solar Cell Exhibiting Polymer-Mediated Polymorphism of a Non-Fullerene Acceptor

Polymer solar cells, with the assistance of nonfullerene acceptors (NFAs) and ternary blend strategies, have exceeded 18% power conversion efficiency. However, most NFA-based ternary blends are constructed using the strategies developed for polymer–fullerene systems, and intrinsic properties of these NFAs have been overlooked when designing a ternary organic solar cell. Here, using a new NFA 2,2′-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl) bis(methaneylylidene)) bis(1H-indene-1,3(2H)-dione), referred to IDID, as the third component, we observed the appearance of a polymorph of IDID when it was introduced into a PTQ10: PC61BM binary blend and this ternary blend solar cell showed a significant improvement in efficiency from 3.38% to 6.04%. This relative increase (with respect to the best binary cell) is nearly 80% which is the highest among all the reported organic ternary blends to the best of our knowledge. Specifically, IDID was found to be nucleated by the host polymer donor PTQ10 under the assistance of the processing solvent to form a distinct polymorph, as proven by grazing incidence X-ray diffraction (GIXRD), differential scanning calorimetry (DSC), and supported by surface energy measurements. More interestingly, IDID, as a third component in the PTQ10: PC61BM system, was found to outperform the structurally similar NFA IDIC, which only boosted the efficiency from 3.38% to 3.55% in ternary polymer solar cells. This work highlights polymer-mediated polymorphism in NFAs as an important consideration in selection of components for and the optimization of ternary organic solar cells.