Monochlorination enabled an elevated solar cell performance in the region-asymmetric benzothiadiazole-based medium bandgap conjugated copolymers

Abstract

Constructing the asymmetric polymeric semiconductors was proved to be an effective strategy of key photoactive layer materials present in the high performing organic solar cells (OSCs). In this investigation, two region-asymmetric medium bandgap (MBG) type conjugated copolymers, namely, PFBDT-TBT and PFBDT-TClBT, in which electron-rich 4,8-bis(4-fluoro-5-(2-butyl-octyl)thien-2-yl)benzo[1,2-b:4,5-b′]dithiophene (FBDT), electron-deficient benzothiadiazole (BT)/5-chloro-benzothiadiazole (ClBT), in conjunction with 3-octylthiophene, were sequentially designated as donors, acceptors, and π-conjugated bridging moieties, were developed, with the primary objective of investigating the functional role of monochlorination modifications along the polymer main chain. The monochlorinated PFBDT-TClBT possessed the decreased thermal-stability but better photostability. And monochlorination occasioned the slightly blue-shifted absorption and unchanged optical bandgap, the weakened aggregation phenomena occurring both in the solution state and the thin-film states, deepened EHOMO, induced grater molecular geometry twisting, and markedly increased natural dipole moment. Influenced by this, chlorine-free PFBDT-TBT:Y6-based device delivered the PCE of 7.77%, with the VOC of 0.80 V, the JSC of 18.67 mA cm‒2 and the FF of 51.9%. In contrast, monochlorination facilitated the device based on PFBDT-TClBT acquired a 0.07 V elevated VOC of 0.87 V, a 5.78% heightened JSC of 19.75 mA cm‒2, and an 7.32% increased FF of 55.7%, collectively contributing to 11.52% increased PCE as high as 9.53%. This increase in PCE after monochlorination was mainly due to the deepened EHOMO, the better exciton dissociation, the suppressed bimolecular recombination and the more balanced μe/µh ratio originating from the desired predominant face-on molecular orientation and reduced aggregation, the better miscibility and smoother surface morphology. This finding demonstrated that monochlorination was an effective and promising designing approach to enlarging the PV performance by regulating molecular structure, energy level, packing and microstructural morphology.