Thiophene with Oligoethylene Oxide Side Chain Enables Random Terpolymer Acceptor to Achieve Efficient All‐Polymer Solar Cells

Abstract The development of high‐performance polymer acceptors is the key challenge for all‐polymer solar cells (All‐PSCs), especially related morphology control in the blend film. Herein, we introduce a thiophene with oligoethylene oxide side chain (TOE) as the third unit into a D‐A copolymer PYT by simple random polymerization. Rigid‐fused unit Y5 with large planar or quasi‐plane surfaces are partially substituted to obtain a series of novel random terpolymers PYT‐TOE(x) (x represents the molar ratio of TOE unit). The electron‐deficient unit Y5 partly replaced by electron‐rich unit TOE realizes the increase of LUMO energy levels. More importantly, a moderate amount loading of TOE not only modulate the polymer acceptor solubility and molecule arrangement, but also perfect the compatibility between the polymer donor and acceptor. Thus, optimized crystallinity and π‐π stacking of blend film are obtained, which is favorable for charge transfer and better film morphology. Eventually, accompanied by improved J SC , V OC and FF simultaneously, the device photoelectric conversion efficiency (PCE) increase from 11.75 % for PBDB‐T:PYT to 12.77 % for PBDB‐T:PYT‐TOE(10). These results indicate that the random ternary copolymerization of small molecule acceptor with functional third unit TOE is a simple but effective strategy to develop polymer acceptors for high‐efficiency All‐PSCs.