Elimination of Charge Transfer Energy Loss by Introducing a Small-Molecule Secondary Donor into Fullerene-Based Polymer Solar Cells

Abstract

The fullerene acceptor, which has historically been used in bulk-heterojunction (BHJ) organic solar cells, is being replaced by non-fullerene acceptors due to its problem of high energy loss (Eloss). To reduce the energy loss in BHJ organic solar cells as a means to improve open-circuit voltage (Voc), several approaches have been applied to diminish recombination loss, notably enhancements to film morphology and decreasing of trap states by improving the crystallinity of active components. However, the adjustment of trap density by means of morphology control only has an inevitable limit. In this work, we have investigated the use of a ternary configuration as an alternative way to mitigate the energy loss in fullerene-based BHJ solar cells. A ternary system based on the wide-band-gap polymer PBDTTPD-HT, the small molecule DRCN5T, and PC71BM showed cascading charge transfer from PBDTTPD-HT through DRCN5T to PC71BM, an indirect electron transfer that avoided the deep charge transfer state between PBDTTPD-HT and PC71BM. DRCN5T:PC71BM mixture has a high ECT level close to the singlet state energy of DRCN5T, leading to low energy loss between DRCN5T and PC71BM and thereby greatly reducing the probability of first-order recombination. Consequently, the incorporation of a small amount of DRCN5T as a secondary donor into the PBDTTPD-HT:PC71BM system enhanced overall PCE, an effect mostly attributed to enhancement of Voc by means of eliminating charge transfer energy losses.