Theoretical Insights Into The Excited State And Carrier Transport Processes From Nonadiabatic Dynamics

The excited state and charge transport dynamics are fundamentalchemicalphysicalprocessesin organic electronics. Understanding these processes is critical to thedesign of new functional materials which are crucial to the performance of organicelectronic devices like organic light emitting diodes(OLEDs), organic field effecttransistors(OFETs) and organic photovoltaics (OPVs). The integration of fastdeveloping electronic structure theory with quantum dynamicsprovides a verypromising tool to clarify these processes. In this thesis, the mixed quantum/classicalnonadiabatic dynamics including Ehrenfest and surface hopping, have beendevelopedbased on density functional tight binding (DFTB) theory which is aparameterized electronic structure theory. During the dynamics, the equations of motionfor electron and nuclei are coupled and evolved with feedback effects from each other.The surface hopping dynamics whichhave been implemented with Tully’s fewestswitchesalgorithm can run excited stated dynamics up to picosecond; while for theEhrenfest (mean field) dynamics, the equations of motion for electron have beenevolved under the fixed subspace spanned by the active molecular orbitals during eachnuclear time step. The linear interpolation of Hamiltonian method has been applied todeal with energy crossing or near degeneration problems in large systemsup to2×103atoms. With the application of the nonadiabatic dynamics, we focus on two problems:1. The surface hopping dynamics have been applied to explore the excited statedynamics for the molecule with exotic Aggregation Induced Emission (AIE)phenomena. The diphenyldibenzofulvene (DPDBF) molecules which present two forms:ring open and ring closed, have been taken as examples. The former fluoresces weaklyin solution, but it becomes strongly emissive in solid phase, exhibiting the AIEphenomena. The latter presents the normal aggregation quenching phenomena, asexpected from common sense. From the analysis on the nonadiabatic dynamics, it isfound that the low frequency twisting motion in the ring open DPDBF couples stronglywith the electronic excitation and dissipates the energy efficiently. While in the closedform, such motion is blocked by a chemical bond. This leads to the nonradiative decay rate for open form (1.4ps) becomes much faster than the closed form (24.5ps). It isexpected that in solid state, the low frequency motion in the open form will be hinderedand the energy dissipation path by nonradiative decay is slowed, presenting remarkableaggregation enhanced emission phenomena.2. We have applied the nonadiabatic Ehrenfest dynamics to investigate the carriermotion in the donor acceptor type polymer for photovoltaics. The ladder-typepoly(p-phenylenes)(LPPP) and poly-(diketopyrrolo-pyrrole (DPP)) series have beentake as example. The carriermobilities are evaluated via the diffusion process. It wasfound that the diffusion abilities are determined by the magnitude of transfer integralsand localization length for frontier orbital, which is caused by the self-trapping effects(polaron) arising from the double bond stretching and twisting motions.From the two applications, the developed mixed quantum/classical nonadiabaticdynamics can be useful in exploring the excited state dynamics, the underlying chargetransport behavior and improving the structure design of materials in organicelectronics.