Excited-State Photophysics in a Low Band Gap Polymer with High Photovoltaic Efficiency

We report an experimental investigation of a newly synthesized polymer, PNDT-DTP7T, with a 1.55 eV band gap tuned for optimal photovoltaic performance. Several time-resolved optical spectroscopies are used to examine fundamental photophysics in pure polymer films. Clear signatures of a distribution in photoexcitation sizes are found in steady state absorption and fluorescence measurements conducted from 100 to 300 K. Electronic structure calculations are combined with an empirical model to establish that, on average, the excitations span less than two repeat units because of strong interactions with the surrounding environment. These strong vibronic interactions are attributed to the charge transfer nature of the excitation, which involves a significant change in the spatial charge distribution. Femtosecond transient grating (TG) measurements find energy transfer processes occurring with roughly 1 and 10 ps time constants. This distinction in time scales most likely has a geometric origin (e.g., processes involving stacked and nonstacked neighbors). Fluorescence quenching and TG experiments show that the addition of PCBM to blends composed of both PNDT-DTPyT and PCBM correlates with an increase in the charge separation efficiency following photoexcitation. This increase in charge separation efficiency is caused by a reduction in the average distance that photoexcitations in the polymer must traverse before injecting electrons into PCBM. Photovoltaic power conversion efficiencies measured for the films are consistent with this interpretation, where a maximum efficiency of 4.1% is found for the 1:1 blend of polymer and PCBM. Overall, the present study covers several fundamental processes taking place in photovoltaic devices; from the primary events surrounding the creation of photoexcitations in the polymer to their deactivation at the PCBM interface.