Highly Efficient All‐Polymer Solar Cells Enabled by Random Ternary Copolymer Acceptors with High Tolerance on Molar Ratios

Finding effective molecular design strategies and fine tuning the molar ratios of donor/acceptor (D/A) random copolymers to optimize the blend microstructure of the photoactive layer is one of the main long‐standing challenges in developing and fabricating highly efficient all‐polymer solar cells (all‐PSCs). Herein, a random ternary copolymerization strategy to develop four random copolymer acceptors PYE x ( x = 10, 20, 30, 40) is used by polymerizing a fused‐ring A–D–A‐type acceptor unit modified from Y5 with a thiophene‐connecting unit and a controlled amount of an ester‐substituted thiophene (EST) unit. Compared with PYT (PYE0) of only Y5‐like units and thiophene units, the ternary copolymers PYE x show slightly down‐shifted lowest unoccupied molecular orbital (LUMO) energy levels, reduced absorption coefficients, and decreased electron mobilities. However, it is also demonstrated that this design approach rationally modifies the molecular aggregations of polymer acceptors, effectively fine tuning the blend morphology and physical mechanisms, and enhances the device performance of the PYE x ‐based all‐PSCs. Among them, blends of PYE20 with donor polymer PBDB‐T combine 13.6% power conversion efficiency (PCE). Of particular note is that all of the PYE x ‐based devices exhibit the best PCEs of over 13%, indicating the high tolerance on molar ratios.