Synthesis and photovoltaic properties of low-bandgap polymers based on N-arylcarbazole

Low-bandgap poly(2,7-carbazole) derivatives with variable N-substituent of ethyl (PEtCzBT), phenyl (PPhCzBT) and 4-diphenylaminophenyl (PTPACzBT) on the carbazoles, were synthesized through Suzuki coupling reaction. The polymers show excellent solubility in organic solvents (readily soluble in chloroform, THF and toluene etc.), good thermal stability (5% weight loss temperature of more than 417 °C), and electrochemical properties (reversible redox process with narrow bandgap), and deep HOMO energy levels (∼5.1 eV), allowing them promising candidates in the solar cell fabrication. Bulk-heterojunction solar cells with these polymers as electron donor and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptor exhibit high Voc (0.91–0.95 V) and good power conversion efficiency (PCE) of 1.69% for PEtCzTB, 2.01% for PPhCzTB, and 2.42% for PTPACzTB.