Amorphous electron donors with controllable morphology for non-fullerene polymer solar cells

Abstract

The aggregation and crystallinity of polymeric donors are significant for polymer solar cells (PSCs), which dominate the film forming properties, morphology, and micro-structures of the active layer, and consequently, influence the charge carrier generation and transport in the device. Regulation of the aggregation of polymers can be realized via the design of alkyl chains, and thus, herein, a cyclohexylmethyl side chain is employed and grafted on PBDD-T, which is one of the most efficient polymeric donors for non-fullerene PSCs, to construct two new polymers, PBDD-CH and PBDD-CH-S. Both polymers have a high number-average molecular weight (M_n) of 50.06 kDa and 58.50 kDa, respectively, but excellent solution processability. Their optical properties indicate that PBDD-CH has a weak aggregation characteristic even at room temperature, which is caused by the steric cyclohexylmethyl side chains. Therefore, the newly designed polymers exhibit amorphous aggregation behavior in their film states, which was characterized by grazing incidence X-ray diffraction analysis. PBDD-CH and PBDD-CH-S have a similar optical bandgap (E^opt_g) of 1.85 eV and low lying highest occupied molecular orbital (HOMO) energy levels of −5.52 and −5.47 eV, respectively. The polymer:ITIC morphologies and phase separation could be easily controlled using 1,8-diiodooctane (DIO) and thermal annealing post-treatment, which resulted in a smooth surface morphology and well-defined phase separation, as characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements. The best power conversion efficiency (PCE) for PBDD-CH:ITIC and PBDD-CH-S:ITIC was 8.21% and 9.63%, respectively, which are rarely reported for amorphous polymer:acceptor blends.