Structure–property relationship on insertion of fluorine- versus nitrogen substituents in wide bandgap polymer donors for non-fullerene solar cells: an interesting case study

Abstract

In organic solar cell research, developing efficient and low-cost photovoltaic materials via insertion of fluorine (F) and nitrogen (N) substituents has proved as a highly successful strategy, thus raising the question of choosing between these substituents while designing new materials. In this work, two new low-cost polymer donors, P1-2F and P2-2N, based on an alternate chlorinated thienyl benzodithiophene donor and 2,5-difluorobenzene (2FBn) and pyrazine (Pz) as a unit as the acceptor core, respectively, were synthesized and compared in parallel to investigate the synergistic effects of the insertion of F and N substituents (functional group vs. atomic substitution) on the morphology and photovoltaic performance. Although both strategies effectively lower the frontier molecular orbital (FMO) energy levels because of the high electronegativity of these substituents, favorable positioning of the N atom in Pz led to further improved coplanarity, a lower bandgap of 2.07 eV, and enhanced crystallinity and molecular ordering with a shorter π–π spacing distance in P2-2N as revealed by density functional theory and X-ray diffraction results. Besides, combining with the 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (IT-4F) acceptor, P2-2N also maintained optimal nanoscale morphology, excellent charge transfer, and high and more balanced hole and electron mobilities, which subsequently resulted in a remarkable power conversion efficiency of 9.5% with a low energy loss (E_loss) of 0.61 eV and outperformed the corresponding F counterpart P1-2F (8.1% and E_loss of 0.65 eV). The in-depth study using various characterization tools suggested that the lower performance of P1-2F resulted from the inferior nanoscale morphology caused by poor mixing with IT-4F, which significantly reduced the charge carrier mobility and efficient charge transfer. Consequently, these results provide deeper insights and mechanisms for further designing efficient donors with either F or N substituents.