Abstract
Crystalline organic semiconductors (OSCs) are a zoo of various physical phenomena, from polymorphism to the (hypothesized as possible) room-temperature superconductivity. This, on one hand, makes them extremely flexible and favorable for molecular design, while on the other hand, obscures successful routes to attaining a number of practically important characteristics in them, including charge-carrier mobility, μ. In fact, there is no ultimate consensus on what actually limits μ in the most appealing OSCs with the so-called band-like mobilities above ~1 cm^2/Vs, making engineering of even higher-μ OSCs a challenging task. That said, the success of molecular design or screening of high-μ OSCs depends on the correct understanding of charge transport physics therein. In this tutorial, I will elucidate different suspected “killers” of efficient transport, mainly focusing on polaronic effects and the dynamic disorder. Both effects stem from the “softness” of molecular crystals formed by weakly-bound molecules, with their deformations or thermal relative motions resulting in a strong electron-phonon interaction . I will portray the corresponding competing theories, both of which predict a band-like mobility, however, from completely different physics. Along the way, I will review the fundamentals of such exciting phenomena as Anderson localization and ballistic quantum transport. Finally, I will give several examples of how one should and how one should not calculate the mobility in OSCs.