Study of Optical Properties and Molecular Aggregation of Conjugated Low Band Gap Copolymers: PTB7 and PTB7-Th

A comparative study of the optical properties of two well-known copolymers for photovoltaic applications, the PTB7 and the PTB7-Th, is presented. This study contributes to the understanding of the photophysics and permits correlation of the morphological change to the optical properties. For this purpose, Variable Angle Spectroscopic Ellipsometry (VASE) measurements are performed to determine the optical transitions as well as the band gap of PTB7 and PTB7-Th thin films from the imaginary parts of the dielectric functions. Then, the molecular aggregations in chlorobenzene solutions are investigated by means of absorbance and photoluminescence measurements as a function of concentration and temperature. These solution measurements are compared to film measurements in order to isolate interchain and intrachain optical transitions.