Manipulating Backbone Structure to Enhance Low Band Gap Polymer Photovoltaic Performance

A pair of polymers, PBDTBT and PBDTDTBT, was synthesized for application in polymer solar cells (PSCs). Although these two polymers have similar absorption bands and molecular energy levels, PBDTDTBT exhibits much better photovoltaic performance in polymer solar cell (PSC) devices with power conversion efficiency (PCE) of 7.4%. To understand the differences between PBDTDTBT and PBDTBT, we have investigated the correlations of the molecular structure, morphology, dynamics and efficiency of these two polymers. A theoretical investigation using density functional theory (DFT) and time-dependent DFT (TDDFT) has been employed to investigate the electron density and electron delocalization extent of the unimers. TEM data showed that PBDTDTBT phase separates from PC71BM, while PBDTBT suffers from having a proper morphology on different processing conditions. Grazing incidence wide angle X-ray diffraction (GIWAXD) was used to probe the crystal structure of the polymers in thin film. A polymorph crystal structure was observed for PBDTBT. Grazing incidence small angle X-ray scattering (GISAXS) was used to probe the size scale of phase separation, with an optimized 25 nm feature size observed for PBDTDTBT/PC71BM blends, which agrees well with TEM results. Femtosecond transient absorption (TA) spectroscopy was used to probe the dynamics of the fundamental processes in organic photovoltaic (OPV) materials, such as charge separation and recombination. The enhanced absorption coefficient, good charge separation, optimal phase separation and higher charge mobility all contribute to the high PCE of the PBDTDTBT/PC71BM devices.