First-principles simulations of charge transfer in optoelectronics

Abstract

There are growing experimental and theoretical studies about charge transfer process in optoelectronic devices. Until recently, little is known about how to investigate this process in real space with energy-resolved details on realistic molecular systems at atomic level. In this thesis, all first-principles simulations are performed using the time-dependent density functional tight-binding (TD-DFTB) implementation in Lodestar. To study charge transfer at the atomic level, TD-DFTB method for open systems (OS) is used, in which the Hamiltonian is partitioned into blocks corresponding to the left electrode, device and right electrode. Such open-system simulations provide a realistic description for the transport of charge and energy within the system of interest and to its environment. To investigate the contribution of nuclear motion to the charge transfer process, Ehrenfest molecular dynamics are adopted, using random initial velocities for the nuclei according to a Maxwell distribution at room temperature. This method is applied to a donor–acceptor blend system composed of thiophene polymer and fullerene. The simulation results reveal coherent oscillations of the charge density between neighboring donor sites, persisting for about 200 fs and promoting charge transport within the polymer stacks. At the donor–acceptor interface, vibronic wave packets are launched, propagating coherently over distances of more than 3 nm into the acceptor region. This supports earlier experimental observations of long-range ballistic charge carrier motion in organic photovoltaic systems and highlights the importance of vibronic coupling engineering as a concept for tailoring the functionality of hybrid organic devices. TD-DFTB method is also applied to bilayer graphene quantum dots. Under the influence of external electric field, the induced density and dipole responses are decomposed to first-order and higher order terms with respect to the strength of field. Charge transfer between two layers is studied in real space with different intensities of external field. Significant charge transfer is observed when the external field is strong enough and the process is mainly induced by the second-order response. This study on nonlinear optics could assist in the design of optoelectronic devices with high optical sensitivity.