Measuring the barrier to charge transfer state dissociation in organic photovoltaic materials with ultrafast vibrational spectroscopy

We have developed a technique based on ultrafast vibrational spectroscopy to measure the barrier to charge transfer state dissociation, or alternatively labeled charge separation, in organic photovoltaic materials. Visible pump--infrared probe spectroscopy is used to generate excitons within organic photovoltaic blend films and the photochemical sequence of events that follows is monitored with an infrared probe pulse tuned to a vibrational mode specific to the electron acceptor. A gradient of vibrational frequencies exists in blend films such that the motion of electrons away from sites where electron transfer occurs can be measured. This gradient of vibrational frequencies arises from a gradient of solvent environments present in the blend film. Using two-dimensional infrared spectroscopy and other IR third-order techniques, we characterize the dynamics of the vibrational mode in the ground-state electronic potential and demonstrate that these dynamics do not interfere with the interpretation of the frequency shift observed in the visible pump--infrared probe experiment in terms of electron motion.;We have used this technique to measure the barrier to charge separation at low excitation densities in thin blend films consisting of device-relevant organic photovoltaic materials. For example, we have measured the barrier to charge separation in organic photovoltaic blend films consisting of the pi-conjugated polymer RR-P3HT as electron donor blended with either the functionalized fullerene PCBM or soluble perylene diimide derivative BTBP as electron acceptor. We observe barrierless charge separation in the blend with PCBM while we find charge separation is activated in the blend with BTBP. In combination with X-ray scattering measurements performed by our collaborators, we argue that fullerenes are capable of barrierless charge separation in the presence of structural disorder while perylenes may require structural order to effect barrierless charge separation. Lastly, we find that separated charge carriers can be generated without a donor/acceptor heterojunction in films consisting exclusively of PCBM via the dissociation of charge transfer excitons.