Carbazole-Phenylbenzotriazole Copolymers as Absorber Material in Organic Solar Cells

An alternating copolymer comprising a 2,7-functionalized carbazole donor and a 2-phenyl-2H-benzotriazole acceptor with an octyldodecyloxy substituent was synthesized. The polymer was blended with [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM) and incorporated as absorber layer into solution processed organic solar cells. By adding the processing additive 1,8-diiodooctane (DIO) to the host solvent 1,2-dichlorobenzene (DCB), the solar cell fill factor increased to remarkable 70% and the power conversion efficiency approached 4.6%. Low energy scanning transmission electron microscopy (low-keV STEM) investigations indicated a finer bulk morphology of the active layer upon deposition from DCB:DIO. Further, the low-energy shoulder of the absorption spectrum was enhanced, indicating stronger polymer aggregation. According to external quantum efficiency measurements, the enhanced absorption also promoted better photon harvesting. Grazing incidence X-ray diffraction experiments revealed face-on polymer aggregates being beneficial for the vertical hole transport.