2D-IR studies of annealing-induced changes to structural dynamics in organic semiconductor thin films

Polymer dynamics in blended films of poly(3-hexylthiophene-2,5-diyl) (P3HT) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine ruthenium(II)carbonyl (RuOEP) are examined with 2D-IR vibrational echo spectroscopy (2D-IR VES). Solvent vapor annealing generates three unique structural states in the films for electrical and spectroscopic characterization. The field-effect hole mobilities of the unannealed films are unaffected by the presence of the RuOEP guest species, and comparable mobility increases are observed during the early stages of the annealing process. The FTIR spectrum of the CO symmetric stretch exhibits dramatic changes over the course of annealing, indicating that substantial changes occur in the surrounding chemical environments in the P3HT film. During the early annealing steps, the 2D-IR VES measurements indicate a loss of dynamics on the time scale of 400 fs, concurrent with annealing-induced hole mobility improvements. We cast these results in light of several recent theoretical studies that predict that structural dynamics can have a profound influence on charge carrier mobilities.