Theoretical Studies On The Electronic Processes In Organic Optoelectronic Materials

Because of the unique properties, organic conjugated materials have been widely used in preparation of organic optoelectronic devices. The performances of devices are close related to the electronic processes of organics, such as charge transport, electronic excitation, exciton diffusion and excited state decays. Therefore, understanding those electronic processes is crucial to improving the the device efficiencies.We first focus on the excited state property of donor materials in OSCs. Poly(thienylene vinylene)(PTV) is a low band-gap polymer but shows poor performance on photovoltaics, and its pristine and alkyl substituted derivatives are nonluminescent. However a newly synthesized PTV derivative with carboxylate substitution presented one order of magnitude higher power conversion efficiency and was shown to be fluorescent. Such phenomenon indicates an intrinsically concomitant relationship between light-emitting and photovoltaic properties. Thus, using the quantum chemistry methods coupled with the correlation function formalism for optical spectra and excited state decays, we investigate a series of PTV derivatives targeting optimal optoelectronic performance. It is predicted that the carbonyl substituted PTV is a strongly fluorescent polymer with large spectral overlap between emission and absorption, and owning long exciton lifetime. Thus carbonyl PTV is expected to be a promising luminescent and photovoltaic polymer. Based on the correlation function methods, we further compute the molecular parameters for exciton diffusion in anthracene crystals,including radiative rate, nonradiative rate and energy transfer rate. With Monte Carlo model, we then simulate the intrinsic exciton diffusion process.Besides, we investigate the charge transport properties in both organic semiconductors and conducting polymers. Quantum nuclear tunneling effect is proposed to be significant for charge transport process. However there still is no direct experimental verification for such effect in organic materials. In order to manifest the nuclear tunneling effect, we propose a theoretical prediction on the isotope effect of mobility. Our calculation results from quantum nuclear tunneling model and semiclassical Marcus theory indicate that isotope substitution can reduce the charge mobility if nuclear-tunneling-assisted hopping dominates the transport mechanism. Then, to further reveal the intrinsic property of charge transport, we study a series of typical organic semiconductors with high mobility by Marcus theory, quantum nuclear tunneling model, time-dependent wavepacket diffusion(TDWPD) method as well as deformation potential theory. Our calculations show that both quantum nuclear tunneling model and TDWPD method can obtain reasaonble results, indicating the importance of nuclear tunneling effect. Finally, we pay attention to the controversy on the charge transport mechanism for donor-acceptor copolymers that whether charge transport occurs in a single chain or between chains. With quantum chemical methods, we calculate the absorption spectra of intrachain and interchain polarons for poly(DPP-DTT) film. Combined with charge modulation spectrum in experiment, it is expected that there exists both intrachain and interchain polarons in poly(DPP-DTT) film.