Effect of annealing on P3HT:PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy

We employ sub-picosecond TA spectroscopy on operating P3HT:PCBM devices to probe the effect of annealing on charge transfer dynamics and nanoscale morphology. Our measurement configuration allows us to remove the effect of high excitation densities that would otherwise dominate. Charge transfer in pristine P3HT:PCBM devices proceeds on a sub-picosecond time scale, implying molecular level intermixing and explaining the more localized character of excitons and charges. In annealed devices, annealing results in diffusion-limited charge generation with a half-life time of approximately 3 ps, complete only after 30 ps. This is the result of exclusion of PCBM molecules and ordering of P3HT domains and is correlated with improved photovoltaic efficiency. We are able to use the spectra and dynamics of optical excitations themselves to interpret blend morphologies on the appropriate time- and length scales of photoinduced charge generation.