Abstract
The understanding of the excited-state properties of electron donors, acceptors and their interfaces in organic optoelectronic devices is a fundamental issue for their performance optimization. In order to obtain a balanced description of the different excitation types for electron-donor-acceptor systems, including the singlet charge transfer (CT), local excitations, and triplet excited states, several ab initio and density functional theory (DFT) methods for excited-state calculations were evaluated based upon the selected model system of benzene-tetracyanoethylene (B-TCNE) complexes. On the basis of benchmark calculations of the equation-of-motion coupled-cluster with single and double excitations method, the arithmetic mean of the absolute errors and standard errors of the electronic excitation energies for the different computational methods suggest that the M11 functional in DFT is superior to the other tested DFT functionals, and time-dependent DFT (TDDFT) with the Tamm–Dancoff approximation improves the accuracy of the calculated excitation energies relative to that of the full TDDFT. The performance of the M11 functional underlines the importance of kinetic energy density, spin-density gradient, and range separation in the development of novel DFT functionals. According to the TDDFT results, the performances of the different TDDFT methods on the CT properties of the B-TCNE complexes were also analyzed.
Highlights
In terms of device architecture and materials [1], promising organic devices usually contain electron donor and acceptor materials constructing an active layer [2,3]
In order to clearly present the differences, the exchange-correlation energy functionals are listed in Table 1, where the superscripts and subscripts in the formulas stand for method and exchange-correlation energy terms, respectively
In order to quantitatively evaluate the performance of the different computational methods for the excitations of the donor-acceptor B-TCNE complexes, we defined the arithmetic mean of the absolute errors (AMAE) and standard errors (SE) of the vertical electronic excitation energies for the B-TCNE
Summary
In terms of device architecture and materials [1], promising organic devices (organic photovoltaics, organic light-emitting diodes, organic photon detectors, etc.) usually contain electron donor and acceptor materials constructing an active layer [2,3]. The conventional hybrid functionals, such as B3LYP [31,32,33] and PBE0 [34], usually underestimate the CT excitation energy due to their incorrect long-range asymptotic behavior of exchange-correlation potential [30] To remedy this deficiency, various functionals, including meta-GGA-based functionals and long-range corrected range-separated hybrid (LRCRSH) functionals, have been developed. The important issue that should be noted is that if we adopt the LRCRSH functionals with the optimally tuned ω for different systems, the calculated results will be rooted in different exchange potentials in DFT. This reduces the comparability of computational studies for different systems, which is important in the design of novel materials. Based on the benchmark calculations, we assessed several wavefunction-based methods and the TDDFT methods (both the full TDDFT and the TDA-TDDFT) with different functionals for the excitations of the benzene-tetracyanoethylene (B-TCNE) complexes, which was selected as the reference donor-acceptor model system
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