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

Abstract

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/cm(2), and a fill factor of 0.51 under air mass 1.5 global (AM 1.5 G) illumination conditions (100 mW/cm(2)). (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4136-4149