High‐Performance Polymer Solar Cells Based on a Wide‐Bandgap Polymer Containing Pyrrolo[3,4‐f]benzotriazole‐5,7‐dione with a Power Conversion Efficiency of 8.63%

A novel donor–acceptor type conjugated polymer based on a building block of 4,8‐di(thien‐2‐yl)‐6‐octyl‐2‐octyl‐5H‐pyrrolo[3,4‐f]benzotriazole‐5,7(6H)‐dione (TZBI) as the acceptor unit and 4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)­benzo­[1,2‐b:4,5‐b′]dithiophene as the donor unit, named as PTZBIBDT, is developed and used as an electron‐donating material in bulk‐heterojunction polymer solar cells. The resulting copolymer exhibits a wide bandgap of 1.81 eV along with relatively deep highest occupied molecular orbital energy level of −5.34 eV. Based on the optimized processing conditions, including thermal annealing, and the use of a water/alcohol cathode interlayer, the single‐junction polymer solar cell based on PTZBIBDT:PC(71)BM ([6,6]‐phenyl‐C(71)‐butyric acid methyl ester) blend film affords a power conversion efficiency of 8.63% with an open‐circuit voltage of 0.87 V, a short circuit current of 13.50 mA cm(−2), and a fill factor of 73.95%, which is among the highest values reported for wide‐bandgap polymers‐based single‐junction organic solar cells. The morphology studies on the PTZBIBDT:PC(71)BM blend film indicate that a fibrillar network can be formed and the extent of phase separation can be mani­pulated by thermal annealing. These results indicate that the TZBI unit is a very promising building block for the synthesis of wide‐bandgap polymers for high‐performance single‐junction and tandem (or multijunction) organic solar cells.