Exciton-Lattice Phonon Coupling in Organic Semiconductor Crystals beyond the Static Disorder

Investigations of the physical properties of pi-conjugated organic semiconductor single crystals represent the first step for understanding their role as active materials in devices and for determining what limits device performance. In this respect, the role of both static and dynamic disorder is pivotal, affecting exciton and charge-carrier mobility. Here, the effects of disorder on the optical response of a paradigmatic class of organic semiconductors, namely oligothiophenes, are addressed. The temperature dependence of the optical response in the spectral range related to the lowest-energy state is investigated by comparing the results of three different samples. The spectral features are identified and a peak never reported in the literature is discussed in the framework of the new classification of the exciton-phonon coupling in molecular aggregates (Spano et al., J. Chem. Phys. 2007, 127, 184703). The temperature dependence of the line shape of the low-energy tail of the lowest-energy peak is discussed in the framework of the so-called Urbach tail analysis for discriminating between the contribution of the static and dynamic disorder to the optical response of the crystals. The energy of the low-energy phonon modes coupled to the lowest-energy exciton is determined, and a measure of its strength is given.