Synthesis of Thiophene-Based π-Conjugated Polymers Containing Oxadiazole or Thiadiazole Moieties and Their Application to Organic Photovoltaics

We report the synthesis, properties, and photovoltaic applications of new pi-conjugated polymers having thiophene, 3,4-dihexylthiophene, and 1,3,4-oxadiazole (OXD) or 1,3,4-thiadiazole (TD) units in the main chain, denoted as PI and P2. They were synthesized by the Stifle coupling reaction of 2,5-bis(trimethylstannyl)thiophene and the corresponding monomers of 2,5-bis(5'-bromo-3',4'-dihexylthien-2'-yl)-1,3,4-oxadiazole or 2,5 -bis (5'-bromo-3',4'-dihexylthien-2'-yl) -1,3,4-thiadiazole, respectively. The experimental results indicated that the introduction of an electron accepting moiety of OXD or TD lowered the highest occupied molecular orbital (HOMO) energy levels, resulting in the higher the open circuit voltage (V-oc) values of polymer solar cells (PSCs). Indeed, the PSCs of 131 and P2 showed high V-oc, values in the range 0.80-0.90 V. The highest field-effect transistor (PET) Mobilities of PI and P2 with the OXD and TD moieties, respectively, were 1.41 X 10(-3) and 8.81 X 10(-2) cm(2) V-1 s(-1). The higher mobility of P2 was related to its orderly nanofibrillar structure, as evidenced from the TEM images. Moreover, the higher absorption coefficient and smaller band gap of P2 provided a more efficient light harvesting ability. The power conversion efficiency (PCE) of the PSC based on P2:PCBM = 1:1 (w/w) reached 3.04% with a short-circuit current density (J(sc)) value of 6.60 mA/cm(2), a V-oc value of 0.80 V, and a fill factor (FF) value of 0.576 during the illumination of AM 1.5, 100 mW/cm(2). In comparison, the parent PDHBT without the electron-accepting moiety exhibited an inferior device performance (PET mobility = 2.10 x 10(-4) cm(2) V-1 s(-1) and PCE = 1.91%). The experimental results demonstrated that incorporating the electron-acceptor moiety into the polythiophene backbone could enhance the device performance due to the low-lying HOMO levels, compact packing structure, and high charge carrier mobility. This is the first report for the achievement of PCE > 3% using PSCs based on polythiophenes having TD units in the main chain.