New quinoxaline derivatives as accepting units in donor-acceptor type low-band gap polymers for organic photovoltaic cells

A series of new donor–acceptor-type low-band-gap semiconducting polymers were synthesized as electron donors for organic photovoltaic cells. The polymers comprised quinoxaline derivatives as the acceptors and a benzodithiophene (BDT) derivative as the donors. 5,8-Dibromoquinoxaline (Qx), 8,11-dibromobenzo[a]phenazine (BPz), 10,13-dibromodibenzo[a,c]phenazine (DBPz), and 8,11-dibromo-5-(9H-carbazol-9-yl)benzo[a]phenazine) (CBPz) were synthesized and polymerized with 2,6-bis(trimethyltin)−4,8-diethylhexyloxybenzo-[1,2-b;3,4-b]dithiophene (BDT) through Stille cross-coupling to produce four types of fully conjugated semiconducting polymers: PBDT-Qx, PBDT-BPz, PBDT-DBPz, and PBDT-CBPz, respectively. Intramolecular charge transfer between the electron donating and accepting units in the polymeric backbone induced a broad absorption from 300 to 800 nm. The optical band gap energies of the polymers were measured from their absorption onsets to be 1.54–1.80 eV depending on the polymer structure. Solution-processed field-effect transistors were fabricated to measure the hole mobilities of the polymers, and bulk hetero-junction photovoltaic devices were fabricated using the synthesized polymers as electron donors and fullerene derivatives as electron acceptors. One of these devices showed a high power conversion efficiency of 3.87% with an open-circuit voltage of 0.78 V, a short-circuit current of 9.68 mA/cm2, and a fill factor of 0.51 under air mass 1.5 global (AM 1.5 G) illumination conditions (100 mW/cm2).