Study of the doping effect on imperfect morphology at photovoltaic heterojunctions in bilayer organic solar cells

A twisted backbone, steric side-chains, and weak intermolecular interactions inevitably produce morphological defects in organic films, which lead to electronic defects in all kinds of devices, such as organic solar cells (OSCs). Tuning the semiconductor morphology or straightforward electronic doping have been developed for the performance enhancement of OSCs. However, it has not been examined if electronic doping offers more scope for OSC improvement than morphology modification, and whether electronic doping can compensate for imperfect morphology-induced defects. Herein, we prepare polymer films with distinct morphologies in terms of molecular orientation and crystallinity. Maintaining the pre-controlled morphologies in planar heterojunction (PHJ) devices, we find that doping is more effective in the original poor-performing device with less crystalline face-on orientation. Density function theory (DFT) calculations show the roles of dopant-polymer donor electrostatic interactions on the dopant arrangement geometry and energy landscape shift, in which the former is considered the dominant cause for efficient doping in the face-on orientation. This work emphasizes the importance of the design of matched charge-quadrupole interaction between the dopant and donor-acceptor (D-A) copolymer to achieve high doping efficiency. It also provides answers to the above question that doping might be able to alleviate the imperfect morphology-induced performance loss in OSCs.