Synthesis, characterization and photovoltaic properties of two-dimensional conjugated polybenzodithiophene derivatives appending diketopyrrolopyrrole units as side chain

Two side-chain donor-acceptor (D-A) narrow-band-gap two-dimensional (2D) conjugated polymers of POBDTDPPs and PTBDTDPPs with benzodithiophene (BDT) as D unit and appending diketopyrrolopyrrole (DPP) as A unit were designed and synthesized. The optical, thermal, electrochemical and photovoltaic properties have been investigated. It was found that both polymers exhibit better solution processability and a deeper HOMO energy level relative to its corresponding main-chain conjugated polymers (PBDT-DPP). Moreover, the PTBDTDPPs with an additional di(alkylthiophene)- substituted BDT unit shows a deeper HOMO energy level than the POBDTDPPs with a dialkoxy- substituted BDT unit. Using PTBDTDPPs as an electron donor and [6,6]-phenyl-C71-butyric acid methyl ester as an electron acceptor, the resulting polymer solar cells (PSCs) exhibited meliorative photovoltaic properties with a power conversion efficiency (PCE) of 2.92% and open circuit voltage (Voc) of 0.94 V. The PCE and Voc levels are 1.52–2.92 and 1.14–1.18 times higher than those in the PBDT-DPP- and POBDTDPPs-based PSCs, respectively. This work demonstrates a good example for tuning energy level and photovoltaic properties of the polymers by the application of 2D conjugated structure.