The Role of Oxygen Insertion on Performance of A-D-A-Type Non-Fullerene Acceptors (NFA) in Organic Solar Cells

Charge transfer (CT) in bulk heterojunction (BHJ) organic solar cells (OSCs) can occur through two pathways: photo-induced electron transfer from the donor (D) to the acceptor (A), controlled by the electron affinity (EA) offset, and photo-induced hole transfer from the acceptor to the donor, governed by the ionization energy (IE) offset. Here, we report the synthesis of a novel non-fullerene acceptor (NFA) coded TPTI-BT, whose properties were first predicted by computational chemistry before the material was synthesized: i.) high ionization energy, creating sufficient IE offsets with many typical donor materials, and ii.) large acceptor quadrupole moment (QM), facilitating interfacial charge transfer and separation. However, despite these allegedly favorable properties for high device performance, TPTI-BT exhibited surprisingly moderate device performance, in particular when compared with the structurally-related and efficient acceptor O-IDTBCN. In fact, the chemical structure of TPTI-BT is very similar to that of O-IDTBCN, it contains only two additional oxygen atoms in the donor core of the A-D-A-type acceptor backbone structure. Interestingly, this causes TPTI-BT to exhibit overall lower device performance. We present a comparative study of these two NFAs (TPTI-BT vs. O-IDTBCN) and elucidate the origin of the lower performance of the TPTI-BT acceptor caused by the modification of the chemical structure upon insertion of two additional oxygen atoms.