Side chain engineering of indacenodithieno[3,2-b]thiophene (IDTT)-based wide bandgap polymers for non-fullerene organic photovoltaics

Abstract

Indacenodithieno[3,2-b]thiophene (IDTT), as one of the widely utilized electron-donating (D) building blocks, has achieved significant success in the design of small molecule nonfullerene acceptors (NFAs). However, IDTT-based polymer donors exhibited inferior performance when paired with fullerene or NFAs, mainly due to weak crystalinity, high HOMO (highest occupied molecular orbital) energy levels and large energy loss (E_loss). To obtain high-efficiency IDTT-based polymers, here we designed three polymers PIDTT-Th, PIDTT-Ph and PIDTT-PhF via side chain engineering, where hexylthiophene (Th), hexylphenyl (Ph) and meta-fluorinated hexylphenyl (PhF) are attached onto the IDTT unit, respectively. The molecular packing and morphology of active layer can be effectively modulated by replacing thiophene side chain with phenyl side chain, as a result of which the charge generation and transport properties can be improved. The power conversion efficiency (PCE) increased from 9.40% for PIDTT-Th:Y6 to 10.72% for PIDTT-Ph:Y6. Further introducing fluorine on phenyl can further decrease the HOMO energy level, improve the miscibility of polymer with Y6, and finally lead to an excellent PCE of 12.5% with an improved V_OC of 0.81 V. The non-radiative energy losses of PIDTT-Th:Y6, PIDTT-Ph:Y6 and PIDTT-PhF:Y6 decrease gradually with values of 0.340, 0.334 and 0.242 eV, respectively. The PCE of 12.5% is the highest value of IDTT-based polymers, which proves that IDTT has the potential to construct high-efficiency polymers by delicate side chain modification.