Abstract
Spectral and transport properties of small molecule single-crystal organic semiconductors have been theoretically analyzed focusing on oligoacenes, in particular on the series from naphthalene to rubrene and pentacene aiming to show that the inclusion of different electron-phonon couplings is of paramount importance to interpret accurately the properties of prototype organic semiconductors. While, in the case of rubrene, the coupling between charge carriers and low frequency inter-molecular modes is sufficient for a satisfactory description of spectral and transport properties, the inclusion of electron coupling to both low frequency inter-molecular and high frequency intra-molecular vibrational modes is needed to account for the temperature dependence of transport properties in smaller oligoacenes.
Highlights
In recent years, the interest in plastic electronics has grown considerably
We show that the coupling to the organic semiconductor bulk phonon modes affects the behavior of mobility below room temperature, enhancing the coherent contribution, but it is ineffective on the incoherent small polaron contribution dominated by the interface coupling at high temperatures
The intrinsic reduction of the bare band due to local modes provides a simple and direct explanation of the difference in the bandwidth evidenced in the series of oligoacenes from naphthalene to pentacene
Summary
The interest in plastic electronics has grown considerably. The realization of devices, such as organic field-effect transistors (OFETs), represents a key step in this field. Single-crystal OFETs made of ultra-pure small molecule semiconductors are characterized by mobilities up to one order of magnitude larger those typical of thin film transistors [1]. Transport measurements from 100 K to room temperature in single crystal semiconductors, such as rubrene, show a behavior of the charge carrier mobility μ that can be defined as band-like (μ ∝ T −γ , with the exponent, γ, close to two), similar to that observed in crystalline inorganic semiconductors [2]. The order of magnitude of mobility is much smaller than that of pure inorganic semiconductors, and the mean free path for the carriers has been theoretically estimated to be comparable with the molecular separation at room temperature [3]
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