Amorphous Electron Donors with Controllable Morphology for Non-fullerene Polymer Solar cells
The aggregation and crystallinity of the polymeric donor is of great significance for polymer solar cells (PSCs), which will dominate the film forming property, morphology, micro-structures of the active layer, and in turn, influence the charge carrier generation and transport in the device. The regulation of the polymer’s aggregation can be realized by the design of alkyl chains, here, cyclohexylmethyl side chain is employed and grafted on PBDD-T, which is one of the most efficiency polymeric donors for non-fullerene PSCs, to construct two new polymers PBDD-CH and PBDD-CH-S. Both of the polymers have high number-average molecular weight (Mn) of 50.06 kDa and 58.50 kDa, respectively, but with excellent solution process-ability. The optical properties indicate PBDD-CH has a weak aggregation characteristic even at room temperature, caused by the steric cyclohexylmethyl side chains. Therefore, the newly designed polymers exhibit amorphous aggregation behaviors in their film states characterized by grazing incidence X−ray diffraction analysis. PBDD-CH and PBDD-CH-S have similar optical bandgap (Egopt) 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 can be easily controlled by using 1,8-diiodooctane (DIO) and thermal annealing post-treatment, which show smooth surface morphology and well-defined phase separation characterized by atomic force microscope (AFM) and transmission electron microscopy (TEM) measurements. The best power conversion efficiency (PCE) for PBDD-CH:ITIC and PBDD-CH-S:ITIC is 8.21% and 9.63%, respectively, which are rarely reported for the amorphous polymer:acceptor blends.