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

•Polybenzodithiophene derivative containing alkoxyphenyl side chain.•Organic photovoltaic cells.•enhanced π–π QUOTE QUOTE stacking.

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] and [5] thiadiazole (BT), or 4,7-bis(5-bromothiophen-2-yl)benzo[c] [1], [2] and [5] thiadiazole (DTBT) units as electron acceptor units. Both polymers exhibited a low-lying highest occupied molecular orbital (HOMO, −5.43 for PhBDT-BT, −5.66 eV 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 1.69 eV, 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 0.86 V 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 0.73 V, a short-circuit current density (Jsc) value of 7.06 mA/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 4.7×10−2 cm2 V−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.

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