Modified fullerenes as acceptors in bulk heterojunction organic solar cells - a theoretical study

In the present study, electronic structure calculations were used to provide strategies for designing poly(3-hexylthiophene) (P3HT)-fullerene-derivative-based donor-acceptor materials for use in high-efficiency bulk heterojunction organic solar cells (BHJ OSCs). The work systematically analyses the impact of electron-donating and -withdrawing substituents on the opto-electronic properties of the fullerene structures. Parameters relating to the absorption spectra, orbital distributions, and energy ordering of the frontier molecular orbitals (FMO), the interactions between P3HT and the fullerene derivatives, and charge transfer across the interface were investigated. We found that substitution with the electron-withdrawing group NO₂ enhances the electronic coupling between the fullerene and P3HT; however, it reduces the open-circuit voltage (V_OC) of the OSC through lowering the LUMO energy level. Furthermore, the results show that substitution with an electron-withdrawing group (NO₂) and electron-donating group (OCH₃) can improve the power conversion efficiency (PCE) of the OSC, since this slightly improves the photon absorption abilities and charge transfer coupling at the interface without overly compromising V_OC relative to PC₆₁BM. Our study shows that alkyl chain modification in the PC₆₁BM acceptor is a promising strategy for improving the performances of OSCs.