Chlorinated Dithiazole‐Based Polymer Donor Enables Over 18% Efficiency for Organic Photovoltaics

Abstract Forming an optimal donor:acceptor blend morphology with a low trap state density is highly desired to reach a perfect compromise between charge generation and charge recombination for organic photovoltaics (OPVs), wherein polymer donor plays a key role. Here, two wide bandgap polymer donors of PN‐HD and PN‐BO are reported, featuring the same low‐cost chlorinated dithiazole as the electron‐withdrawing unit but varied alkyl chains on the π‐bridged thiophene ring. It is found that shorter alkyl chain endows PN‐BO with merits of better miscibility with the acceptor and longer crystallization time, leading to more favorable phase separation and lower trap density of states (tDOS) in the resulting active layer. Notably, tDOS as low as 3.51 × 10 15 cm −3 eV −1 in PN‐BO:BTP‐eC9‐based device has enabled an exceptionally high photocurrent of 28.59 mA cm −2 , thereby a high efficiency of 18.01%, far beyond those (24.64 mA cm −2 , 15.51%) of PN‐HD:BTP‐eC9‐based one with a higher tDOS value. The work reveals the relationship between the flexible alkyl chain of polymer donor and trap state density, providing a new route to efficient OPVs.