New polybenzo[1,2-b:4,5-b′]dithiophene derivative with an alkoxyphenyl side chain: Applications in organic photovoltaic cells and organic semiconductors

Abstract

We have used Stille coupling polymerization to synthesize new crystalline deep HOMO level polymers, PhBDT-BT and PhBDT-DTBT, which consist of 4,8-bis(2-octyldodecyloxyphenyl)-benzo-[1,2-b:4,5-b′]dithiophene (PhBDT) as an effective electron donor unit and 4,7-dibromobenzo [c] [1,2,5] thiadiazole (BT), or 4,7-bis(5-bromothiophen-2-yl)benzo[c] [1,2,5] thiadiazole (DTBT) units as electron acceptor units. Both polymers exhibited a low-lying highest occupied molecular orbital (HOMO, −5.43 for PhBDT-BT, −5.66eV for PhBDT-DTBT) to obtain a high open circuit voltage (Voc). The band gaps of PhBDT-BT and PhBDT-DTBT were tuned to 1.72 and 1.69eV, respectively. As a result, bulk heterojunction photovoltaic devices derived from these polymers and fullerenes provided open-circuit voltages (Voc) as high as 0.73 for PhBDT-BT/PC70BM (1:2) and 0.86V for PhBDT-DTBT/PC70BM (1:2). In particular, photovoltaic devices fabricated from the PhBDT-DTBT/PC70BM (1:2) blend system exhibited an excellent photovoltaic (PV) performance with a Voc value of 0.73V, a short-circuit current density (Jsc) value of 7.06mA/cm2, a fill factor (FF) value of 0.67, and a promising power conversion efficiency (PCE) of 3.5%. In addition, organic thin film transistor (OTFT) devices using PhBDT-DTBT as the semiconductor also showed excellent performance with a value of hole mobility of 4.7×10−2cm2V−1s−1. In the PhBDT-DTBT, the thiophene bridge between the PhBDT and benzthiadiazole provides enough space for the intedigitaion of the bulky alkoxyphenyl side chains, increasing the planarity with enhanced π–π stacking.