Synthesis and photovoltaic properties of an alternating polymer based on benzo[1,2-b:4,5-b′]dithiophene and fluorine substituted 4,7-dithiophene-2-yl-2,1,3-benzothiadiazole

• A fluorine substituted polymer (PBDTFC10DBT) was synthesized.
• The PBDTFC10DBT polymer shows a low-lying HOMO energy level as −5.42 eV.
• A PCE of 1.19% was achieved for the polymer solar cell blending with PC61BM.

An alternating polymer (PBDTFC10DBT) with benzo[1,2-b:4,5-b′]dithiophene (BDT) as electron-rich unit and 4,7-bis(4-decylthiophen-2-yl)-5-fluorine-2,1,3-benzothiadiazole (FC10DBT) as electron-withdrawing unit was synthesized and characterized. PBDTFC10DBT showed similar absorption property with that of the counterpart polymer without fluorine atom (PBDTC10DBT). However, the low-lying highest occupied molecular orbit (HOMO) energy level of PBDTFC10DBT was −5.42 eV, about 0.22 eV deeper than that of PBDTC10DBT. In order to study the photovoltaic properties of the materials, polymer solar cells (PSCs) were fabricated with PBDTFC10DBT as donor blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor. The power conversion efficiency (PCE) of PSC was 1.19% with a high open circuit voltage (Voc) of 0.62 V for an optimized PBDTFC10DBT:PC61BM ratio of 1:4, in comparison with that of PBDTC10DBT-based device (PCE of 0.99% with Voc of 0.53 V). This study indicated that fluorine substituted 4,7-dithiophene-2-yl-2,1,3-benzothia-diazole based copolymers would be promising material for the application in polymer solar cells.

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