Small molecules based on 2,7-carbazole for efficient solution-processed organic solar cells

A series of small molecules (DTCz-TBT, DTCz-2TBT and DTCz-3TBT) with 2,7-carbazole as a center core donor, benzothiadiazole as acceptor units and oligothiophenes as end-capping units were synthesized and characterized. Their thermal, optical, electrochemical and photovoltaic properties were examined. With increasing the number of the terminal thiophene units and conjugation length, these molecules exhibit red-shifted absorption (529–594 nm), up-shifted HOMO levels (−5.27 to −5.08 eV), and reduced optical band gaps (2.04 to 1.80 eV). Solution processed organic solar cells (OSCs) based on DTCz-TBT/PC71BM exhibit a power conversion efficiency of 2.26% with a high open-circuit voltage up to 1.02 V, while the OSCs based on DTCz-2TBT/PC71BM and DTCz-3TBT/PC71BM exhibit power conversion efficiencies of 3.44% and 3.90% without any post-treatment.