3. Polaron Mechanism

The intrinsic charge transport in organic semiconductors is an electron–phonon interacting process. Due to the “soft” nature of organic materials, the existence of an electron can cause significant deformation of local nuclear vibrations, which moves together with the electron itself, and thus the effective diffusing quasiparticle is composed of the electron and its accompanying phonons. This is the basic idea of the polaron mechanism. In principle, it is a more general description for charge transport since it does not presume that the charge is localized within one molecule as in the hopping mechanism described in Chap.​ 2. In this chapter, we adopt the general Holstein-Peierls Hamiltonian coupled with first-principles calculations to investigate the fundamental aspects concerning charge transport. All kinds of electron–phonon couplings, including both local and nonlocal parts for inter- and intra-molecular vibrations, have been taken into considerations. Detailed studies are performed to study their contributions to the total electron–phonon coupling strength and the temperature dependence of mobility, especially the band-hopping crossover feature. We also investigate the pressure- and temperature-dependent crystal structure effects on the charge transport properties.