4 Nanoscale First-Principles Electronic Structure Simulations of Materials. . .
121
4.3 Conclusions and Outlook
In this review, we have outlined recent theoretical works on basic electronic
properties of organic semiconductors relevant to organic electronic devices with
first-principles electronic structure methods. Firstly, we have demonstrated the
performances of recently proposed van der Waals (vdW)-inclusive methods in prediction of crystal structures of the oligoacene crystals. It was found that a variant of
the vdW density functional (vdW-DF) [10], with its reasonable computational cost
being comparative to that of DFT-LDA or DFT-GGA, predicted the lattice constants
in good agreement with the experimental values obtained with the diffraction data
measured at low temperatures [13]. For the optimized crystal geometries, the manybody perturbation theory within the GW approximation [12, 18] predicted the
fundamental band gap and the density of states in agreement with experiments.
More importantly, different polarization energies were obtained for the different
cell volumes measured at different temperatures, corresponding to the different
molecular packing densities and thus the different screenings. Furthermore, based
on the maximally localized Wannier functions (MLWF) [14, 15], the impact of
the molecular configurations in the unit cell on the transfer integrals between
molecules at the neighboring sites was discussed. The result demonstrates the
subtle interplay between the intermolecular configurations such as distance and
angle, and their displacement, and, therefore, the importance of predicting accurate
geometry of organic semiconductor crystals is underscored [13]. Prediction of the
crystal geometry of organic semiconductors and calculation of the MLWF, and
thus the resulting intermolecular transfer integrals, may lead to multi-scale realtime simulation of the carrier transport in organic crystals [16]. With the aid
of the present-day machine learning and artificial intelligence, a theoretical tool
for automatic and efficient high-throughput screening of organic semiconductor
materials might be realized in the near future.
Secondly, we discussed theoretical treatment of electronic phenomena in organicmetal interfaces. As an example of theoretical study on the interface energy
level alignment, we summarized the works based on the GW approximation.
GW approximation allows rigorous theoretical treatment of the charged excitation
(quasiparticle) at the interface. Actually, there have been works successfully
applying the same methodology to the energy level alignment at the physisorbed
or hybridized interface. In consideration of the molecular gap renormalization at
physisorbed interfaces dominated by the image potential, an approximated approach
of adding the image potential effect 1/4z to the fundamental gap of an isolated
molecule is found to be valid (DFT+) [112, 116, 117]. However, the large
computational cost which formally scales as N 4
P W [168], where N P W is the number
of plane-wave basis set, the poor convergence of the quasiparticle energies including
the work function of the metal, and the effect of self-consistency required [114, 115]
critically prohibit one from applying the methodology to organic-metal interfaces
in general. In terms of the effect of self-consistency, another formalism within the
framework of DFT such as the optimally tuned screened range-separated hybrid
(OT-SRSH) can be a method of choice [118–120].
121
4.3 Conclusions and Outlook
In this review, we have outlined recent theoretical works on basic electronic
properties of organic semiconductors relevant to organic electronic devices with
first-principles electronic structure methods. Firstly, we have demonstrated the
performances of recently proposed van der Waals (vdW)-inclusive methods in prediction of crystal structures of the oligoacene crystals. It was found that a variant of
the vdW density functional (vdW-DF) [10], with its reasonable computational cost
being comparative to that of DFT-LDA or DFT-GGA, predicted the lattice constants
in good agreement with the experimental values obtained with the diffraction data
measured at low temperatures [13]. For the optimized crystal geometries, the manybody perturbation theory within the GW approximation [12, 18] predicted the
fundamental band gap and the density of states in agreement with experiments.
More importantly, different polarization energies were obtained for the different
cell volumes measured at different temperatures, corresponding to the different
molecular packing densities and thus the different screenings. Furthermore, based
on the maximally localized Wannier functions (MLWF) [14, 15], the impact of
the molecular configurations in the unit cell on the transfer integrals between
molecules at the neighboring sites was discussed. The result demonstrates the
subtle interplay between the intermolecular configurations such as distance and
angle, and their displacement, and, therefore, the importance of predicting accurate
geometry of organic semiconductor crystals is underscored [13]. Prediction of the
crystal geometry of organic semiconductors and calculation of the MLWF, and
thus the resulting intermolecular transfer integrals, may lead to multi-scale realtime simulation of the carrier transport in organic crystals [16]. With the aid
of the present-day machine learning and artificial intelligence, a theoretical tool
for automatic and efficient high-throughput screening of organic semiconductor
materials might be realized in the near future.
Secondly, we discussed theoretical treatment of electronic phenomena in organicmetal interfaces. As an example of theoretical study on the interface energy
level alignment, we summarized the works based on the GW approximation.
GW approximation allows rigorous theoretical treatment of the charged excitation
(quasiparticle) at the interface. Actually, there have been works successfully
applying the same methodology to the energy level alignment at the physisorbed
or hybridized interface. In consideration of the molecular gap renormalization at
physisorbed interfaces dominated by the image potential, an approximated approach
of adding the image potential effect 1/4z to the fundamental gap of an isolated
molecule is found to be valid (DFT+) [112, 116, 117]. However, the large
computational cost which formally scales as N 4
P W [168], where N P W is the number
of plane-wave basis set, the poor convergence of the quasiparticle energies including
the work function of the metal, and the effect of self-consistency required [114, 115]
critically prohibit one from applying the methodology to organic-metal interfaces
in general. In terms of the effect of self-consistency, another formalism within the
framework of DFT such as the optimally tuned screened range-separated hybrid
(OT-SRSH) can be a method of choice [118–120].