Vibronic Coupling In Organic Optoelectronic Materials: Its Role In Charge Transport And Luminescent Properties

Organic π-conjugated materials possessing great advantage such as low-cost,low-toxicity and easy-synthesis are of immense interest. They have been extensivelyinvestigated for optoelectronic and microelectronic applications, such as organiclight-emitting diodes (OLED), organic field-effect transistors (OFET), organicphotovoltaic (OPV) devices or various types of sensors. Therefore, in-depthunderstanding their structure-property relationships provides a fertile theoreticalground for the rational molecular design of organic optoelectronic materials.Vibronic coupling plays a crucial role in various optical and electrical processes.For organic charge-transport materials, electron-phonon (e-ph) couplings can beclassified as two parts, local and nonlocal one. Local and nonlocal e-ph couplingmainly tunes the molecular site-energy and intermolecular electronic coupling,respectively. During several decades, various types of physical and chemical modelshave been developed to investigate the roles of e-ph couplings in charge-transportproperties for organic electronics. For example, Holstein’s small polaron model andMarcus’ electron transfer theory which mainly consider the case of local e-phcoupling have extensively been employed to describe charge-transport properties inorganic semiconductors. Since the dependence of carrier mobility on temperature inultrapure organic crystals, that is,μ∝T n, much more attention has been paid tonon-local e-ph coupling by theoretical physicists and chemists. Holstein-Peierlspolaron model and dynamic disorder theory can reproduce the temperaturedependence of carrier mobility well, but the effects of nonlocal coupling on electronwave-function and spectra properties are known very little. Therefore, based ontrimer’s Peierls-polaron model, the problem was studied. Our results havedemonstrated that when nonlocal coupling is very strong or electronic coupling isvery small, electron wave-function is completely delocalized on every site, which is contrary to that in Holstein-polaron case. Simultaneously, nonlocal coupling not onlycan make energy splitting (or bandwidth) broaden, but also reduce it. Throughutilizing well-known theoretical models, charge-transport properties in pentacenederivatives with pyrazine ring as excellent n-type organic semiconductors were alsoinvestigated. Our theoretical results have demonstrated that due to the introduction ofheteroatom N, intermolecular interactions in their organic crystals are strengthenedand thus the fluctuation of transfer integral is suppressed. Furthermore, neither bandmechanism nor hopping mechanism can not be employed to describe theircharge-transport properties.In the framework of time dependent second-order perturbation theory, the rateexpression of the nonradiative transition including spin-orbit and non-adiabaticcouplings was derived, and was employed to understand reverse intersystem crossingfrom the lowest triplet (T1) to lowest singlet (S1) excited states by thermal activationin metal-free organic emitters. Our result demonstrated that except S1-T1energy gaps,non-adiabatic effect between low-lying excited states should play a key role in the T1�'S1up-conversion for metal-free organic emitters.