Transport Properties Of The Carrier In Organic Molecular Crystals

Organic molecular crystals are new type of semiconductor, which have an importantapplication prospect in the field of organic field-effect transistors, organic light-emittingdiodes, and organic solar cells. The mobility of the organic molecular crystals is higher thanthat of the conjugated polymers because of the ordered structures. However, the mechanismof transport of organic molecular crystals is still unclear and is a open question. Therefore, theresearch of carrier transport mechanism has very important significance.The electron-phonon coupling is the strength of the interactions between electron andlattice, which is an important feature of physical quantities in organic materials. Under theeffect of intermolecular electron-phonon coupling, the molecular thermal motion results in thechange of the transfer integral between the molecules and the destruction of the translationalsymmetry of the system. The reorganization energy in organic materials is the change of themolecular structure when the organic molecule is in the pros and cons of electronic processes,which affect the transport properties of organic molecular crystals. Considering the influenceof molecular thermal motion on the transfer integral and reorganization energy, we introducethe intermolecular electron-phonon coupling and intramolecular electron-phonon couplingrespectively, and calculate the transport properties in organic molecular crystals with thesemi-classical molecular dynamic method.The calculation results show that: with the gradual increasing of the intermolecularelectron-phonon coupling, the carrier’s mobility first increases and then decreases in organicmolecular crystals. With the change of the electron-phonon coupling, the mobility has theextreme point, and because of the temperature-dependent disorder, the extreme pointcorresponding to the intermolecular electron-phonon coupling strength changes withincreasing temperature. With the variation of the intramolecular electron-phonon coupling, themobility is also first increases and then decreases. Electron-phonon coupling in the systemplays the role of two aspects: on the one hand, the electron-phonon coupling enhancement isin favor of exchanging energy between electron and lattice, so the electronic thermallyassisted transition process increases, and improves the carrier’s mobility; on the other hand, the electron-phonon coupling enhancement leads to the increase of the disorder. Theintermolecular electron-phonon coupling results in the thermal disorder of the system, and theintramolecular electron-phonon coupling results in the disorder of the potential energy whichis the Anderson localization. Neither are conducive to the transport of electrons in organicmolecular crystals, and they make the carrier mobility decreased. This shows energy exchangeplays a major role in the weak coupling region, so the carrier’s mobility increases with theenhancement of electron-phonon coupling; in the strong coupling region, the disorderaccounts for the main position, so that the mobility of charge carrier in organic molecularcrystals decreases with the electron-phonon coupling enhanced.With the introduction of the intramolecular electron-phonon coupling in the model, themobility values change, but their trends with the intermolecular electro-phonon couplingdon’t change. Because the double impact of thermally assisted transition process and thermaldisorder, the carrier’s mobility of organic molecular crystals still first increases and thendecreases with the increasing of the intermolecular electron-phonon coupling.In summary, the intramolecular electron-phonon coupling and intermolecularelectron-phonon coupling affect the energy exchange and the disorder in organic molecularcrystals, thus they have important influence on the transfer characteristics of the chargecarriers in organic molecular crystals, and the results are important for understanding thetransport mechanism of organic molecular crystals.