The electron-acoustic phonon scattering for charge transport in organic semiconductors has been studied by first-principles density functional theory and the Boltzmann transport equation with relaxation time approximation. Within the framework of deformation-potential theory, the electron-longitudinal acoustic phonon scattering probability and the corresponding relaxation time have been obtained for oligoacene single crystals (naphthalene, anthracene, tetracene and pentacene). Previously, the electron-optic phonon scattering mechanism has been investigated through Holstein-Peierls model coupled with DFT calculations for naphthalene. Numerical results indicate that the acoustic phonon scattering intensity is about 3 times as large as that for the optic phonon and the obtained mobility is in much better agreement with the result of the experiment done for ultrapure single crystals. It is thus concluded that for closely packed molecular crystal where the electron is partly delocalized, acoustic phonon scattering mechanism prevails in the charge transport. Moreover, it is found that the intrinsic electron mobility is even larger than hole mobility. A frontier orbital overlap analysis can well rationalize such behavior.
The non-Condon effect plays an important role in the process of electron transfer (ET). Several theoretical models have been proposed to investigate its effect on ET rates. In this paper,we overview a theoretical method for the calculations of the non-Condon ET rate constants proposed by us,and its applications to organic semiconductors. First,full quantum expressions of the non-Condon ET rates are presented with the electronic couplings having exponential,Gaussian and linear dependences in terms of the nuclear coordinates,respectively. The proposed formulas have closed forms in time domain and they thus can be easily applied in multi-mode systems. Then,the driving force dependences of the ET rates involving the non-Condon effect are calculated with the use of full quantum mechanical formulas. It is found that these dependences show very different prop-erties from the Marcus one. As an example of applications,the approaches are used to investigate the non-Condon effect on the mobility of the organic semiconductor dithiophene-tetrathiafulvalene (DT-TTF). The results manifest that the non-Condon ef-fect enhances ET rates compared with the Condon approximation,and static fluctuations of electronic coupling dominate the ET rate in the DT-TTF,which has been confirmed by the molecular dynamics simulation.
