Time-Dependent Density-Functional Approaches for Exciton Dynamics

点击翻译按钮获取中文摘要

TECHNICAL SUMMARYThis award supports theoretical and computational research and education to study optical excitation processes in extended systems, with a particular emphasis on excitonic effects in bulk semiconductors and organic chains. The PI will use time-dependent density-functional theory. Exchange-correlation functionals with a long spatial range are required to capture excitonic effects, and only a few of these are available. The primary goal is to develop and test exchange-correlation functionals which produce excitonic binding in the frequency-dependent linear-response domain and in the nonlinear real-time domain.The PI will use time-dependent density-functional theory in the linear-response domain to calculate excitonic binding energies in bulk semiconductors and insulators. A two-band model, demonstrated to produce excitonic binding for various simple exchange-correlation kernels, will be extended to include additional bands. Various long-range exchange-correlation kernels will be implemented and tested, and singlet-triplet exciton splittings will be calculated using a spin-dependent formalism.The PI will apply real-time time-dependent density-functional theory to simulate short-time exciton dynamics in organic chain molecules. The exchange-correlation functionals required for excitonic binding will be carried over from the frequency-dependent linear-response regime into the real-time domain. A new computational tool to visualize exciton dynamics, the time-dependent transition density matrix, will be developed. Real-time simulations will be carried out for simple polymer chains to study how localized excitations spread out along the chains and connect to neighboring units.An accurate time-dependent density-functional theory description of excitons will be relevant and applicable for a wide range of materials, from bulk inorganic semiconductors to polymers and organic heterojunctions. The latter systems will be explored in real-time calculations, to test the time-dependent transition density matrix. Developing these methodologies may have impact on organic optoelectronics and photovoltaics.An undergraduate condensed-matter physics course developed under prior NSF support will be broadened in scope so as to address a wider audience. A new graduate course in theoretical materials science will be developed with the goal to introduce students to a variety of topics in materials theory and simulation, including hands-on computational exercises.NON-TECHNICAL SUMMARYThis award supports theoretical and computational research and education to develop new theoretical and computational methods to describe the optical properties of materials, specifically semiconductors with particular emphasis on semiconductor materials made of long chain-like molecules called polymers. An accurate and computationally efficient approach to simulate the optical properties of these materials will provide important assistance for understanding and designing novel solar cell and optoelectronic devices.The PI will further develop and use a computationally efficient method to describe the fundamental processes that take place during the interaction of semiconductor materials with light. The method known as time-dependent density functional theory has successfully described the response of electrons in molecules to time varying electric fields. This theory will be used to simulate and visualize in real time the basic steps involving the interaction of the electrons in polymer semiconductors with light. An undergraduate condensed-matter physics course developed under prior NSF support will be broadened in scope so as to address a wider audience. A new graduate course in theoretical materials science will be developed with the goal to introduce students to a variety of topics in materials theory and simulation, including hands-on computational exercises.