Phonons in organic semiconductors are crucial for determining their charge transport properties. Current theoretical models treat these phonons within the harmonic approximation. These models fail many times to predict important electronic properties such as the charge-carrier mobility. Since organic solids have weak intermolecular interactions, the anharmonic components of their low-frequency vibrations are expected to be significant. We hypothesize that the reason for the failure to predict the electronic properties is the neglection of these anharmonic components.
I performed temperature-dependent low-frequency Raman measurements of oligo-acenes to quantify their anharmonicity. Results show a much stronger change in the peak position of the low-frequency modes with temperature compared to inorganic semiconductors. Analysis of these peak shifts shows strong anharmonic behavior - contributed from both thermal expansion and phonon-phonon interactions - likely to have a significant impact on electron-phonon interactions. Comparison of a series of oligo-acenes shows an inverse correlation between anharmonicity and intermolecular electronic coupling. I will also show photoluminescence and reflectance measurements to corroborate the connection between vibrational anharmonicity and the electronic properties of organic semiconductors.