Indenothiophene-Based Wide Bandgap Copolymer for Polymer Fullerene Solar Cells with 9.01% Efficiency and 1.0 V Open Circuit Voltage

Despite the frequently renewed record high power conversion efficiencies (PCEs) in recent years, achieving single-junction polymer solar cells (PSCs) with both high PCE and large open circuit voltage (V _OC) remains a challenge because there is a trade-off between high short circuit current density (J _SC) and large V _OC values. Here, the design and synthesis of a novel donor–acceptor (D–A) copolymer (PIT2FBT) based on an asymmetric-indenothiophene donor and a difluorinated benzo[c][1,2,5]thiadiazole acceptor is demonstrated. PIT2FBT exhibits a wide optical bandgap of 1.82 eV with a large ionization potential of 5.54 eV. Through systematic morphology control and device engineering, the best performance solar cell based on PIT2FBT shows a high PCE of 9.01% in combination with a large V _OC of 1.0 V. To the best of our knowledge, the PCE is the highest for single-junction polymer fullerene solar cells with V _OCs up to 1.0 V. At the same time, the influences of solvent additive on the active layer morphology together with the resulting device performance are investigated in-depth. These findings and detailed studies provide an important guidance to tune and optimize the morphology and photovoltaic properties of indenothiophene-based copolymers, which are promising candidates for short-wavelength absorbing materials in tandem PSCs.