Computational Evaluation of Optoelectronic Properties for Organic/Carbon Materials

Abstract

CONSPECTUS: Organic optoelectronic materials are used in a variety of devices, including light-emitting diodes, field-effect transistors, photovoltaics, thermoelectrics, spintronics, and chemico- and biosensors. The processes that determine the intrinsic optoelectronic properties occur either in the photoexcited states or within the electron-pumped charged species, and computations that predict these optical and electrical properties would help researchers design new materials. In this Account, we describe recent advances in related density functional theory (DFT) methods and present case studies that examine the efficiency of light emission, carrier mobility, and thermoelectric figures of merit by calculation of the electron-vibration couplings. First we present a unified vibrational correlation function formalism to evaluate the excited-state radiative decay rate constant kr, the nonradiative decay rate constant knr, the intersystem crossing rate constant kISC, and the optical spectra. The molecular parameters that appear in the formalism, such as the electronic excited-state energy, vibrational modes, and vibronic couplings, require extensive DFT calculations. We used experiments for anthracene at both low and ambient temperatures to benchmark the calculated photophysical parameters. In the framework of Fermi's golden rule, we incorporated the non-adiabatic coupling and the spin-orbit coupling to evaluate the phosphorescence efficiency and emission spectrum. Both of these are in good agreement with experimental results for anthracene and iridium compounds. Band electron scattering and relaxation processes within Boltzmann theory can describe charge transport in two-dimensional carbon materials and closely packed organic solids. For simplicity, we considered only the acoustic phonon scattering as modeled by the deformation potential approximation coupled with extensive DFT calculations for band structures. We then related the carrier mobility to the band-edge shift associated with the lattice dilation of longitudinal waves. The calculated relaxation time was in good agreement with experimental data for the graphene sheet, which supports the methodology. We then found that the intrinsic electron mobility for a 6,6,12-graphyne sheet can be even larger than that of graphene. We extended this approach to investigate the thermoelectric transport of electrons in metal phthalocyanines, which showed reasonable Seebeck coefficients when compared with experiments. For the thermal lattice transport, we employed nonequilibrium molecular dynamics simulations. Combining both electron transport and lattice thermal conductivity, we can evaluate the thermoelectric figure of merit.