Temperature-independent vibrational dynamics in an organic photovoltaic material

Ultrafast orientational motion and spectral diffusion of the carbonyl stretch vibration of the functionalized fullerene, PCBM, blended with the conjugated polymer, CN-MEH-PPV, are examined with two-dimensional infrared and polarization-resolved IR pump probe spectroscopy. In previous contributions from our group, the carbonyl stretch frequency of PCBM has been used as a local vibrational reporter to measure the temperature dependence of the time scale for dissociation of charge transfer excitons in CN-MEH-PPV:PCBM polymer blends. It was found that the rate of charge separation is independent of temperature, indicating that charge separation occurs through an activationless pathway. This assignment was supported by the observation at room temperature that thermal fluctuations do not give rise to spectral diffusion of the carbonyl stretch vibration on the picosecond and longer time scale. In this contribution, we examine the temperature dependence of the carbonyl vibrational dynamics to determine whether thermal fluctuations might give rise to spectral diffusion at other temperatures. We find that the time scale for fast wobbling-in-cone orientational motion is independent of temperature on the subpicosecond time scale. Similarly, spectral diffusion is not observed on the picosecond and longer time scale at all temperatures examined confirming our earlier interpretation of the frequency shift dynamics exclusively in terms of charge separation. Interestingly, the half angle characterizing the wobbling-in-cone orientational motion does increase at higher temperature due to increased free-volume resulting from thermal expansion of the polymer blend.