(318f) Band Gap Engineering In Donor-Acceptor Conjugated Copolymers

Here we report the principles of the change in the electronic properties (especially, HOMO/LUMO, band gap, and band width) by introducing different amounts of electron donating (D) or electron accepting (A) units to the backbone of a conducting polymer. We performed systematic studies on thiophene (T) based copolymers containing thienopyrazine (TP) or methoxythiophene (OT) at varying compostions using ab initio density functional theory (DFT). We found that

(1) The increase of acceptor (TP or OT) content in the mainchain leads to a smaller band gap of conjugated copolymers.

(2) For moderate D-A contrast system (e.g., T-OT), band gap varies linearly with composition at similar D-A ratio region; however, there is little composition effect at high D and A concentration regions.

(3) For strong D-A contrast system (e.g., T-TP), LUMO and band gap drop rapidly when a small amount of acceptor (TP) is added. The band gap become composition insensitive at high acceptor concentration regions.

(4) An extremely narrow bandwidth of conduction band would be observed for strong D-A contrast system especially at low concentration of π-electron acceptor.

Based on these results, we demonstrate an effective molecular design strategy by adjusting D-A composition toward desired electronic properties for donor-acceptor conjugated copolymers.