4 Nanoscale First-Principles Electronic Structure Simulations of Materials. . .
91
predictable by the first-principles theoretical methods describing the quasiparticle
self-energy such as the GW approximation [12].
In this article, we review first-principles electronic structure calculations on
1. Structures of organic semiconductor crystals with the van der Waals (vdW)inclusive methods
2. Electronic properties of organic-metal interfaces: electronic properties such as
energy level alignment and the image potential states
3. Electronic charge (electron or hole) injection level depending on the molecular
orientation at the organic crystal surface
In the first topic, we focus on our first-principles study on prediction of the lattice
constants and the intermolecular configuration in oligoacene crystals, along with
analysis of the band structure and the intermolecular transfer integral [13]. Here, the
Wannier functions [14, 15] were derived, allowing us to estimate intermolecular
transfer integrals depending crucially on the cell volume or the intermolecular
configurations in the crystal. The Wannier functions in organic crystals could be
mapped on the crystal Hamiltonian such as the tight-binding Hamiltonian, thus
leading to multi-scale real-time simulations of charge carrier transport in organic
crystals [16]. In addition to the previous work on naphthalene, anthracene, and
tetracene [13], calculation of pentacene and hexacene crystals is demonstrated in
this article.
Secondly, the electronic properties induced at the organic-metal interface are
discussed. The energy level alignment at the organic-metal interface originating
from the rearrangement of the electronic clouds at the interface has attracted
considerable attention [17], because of its relevance to barrier for charge injection
into an organic layer. To elucidate the experimental measurements using techniques
such as photoemission spectroscopy proving the energy levels of the injected hole
or electron right at the interface, aid of a reliable theoretical method treating
charged excitations (quasiparticles) is necessary. Here, we focus on highly accurate
theoretical approaches beyond DFT within the LDA or GGA, such as the manybody perturbation theory within the GW approximation [12, 18]. In addition, we
demonstrate our recent theoretical investigation [19] to clarify the electronic nature
of the recently reported image potential state of graphite in the presence of an
organic overlayer [20].
In the third place, we discuss determination of charge injection levels in organic
semiconductor crystals or thin films, mainly based on the GW approximation.
The charge injection level relative to the vacuum level, i.e., ionization energy (IE)
or electron affinity (EA) of organic semiconductors, crucially affects the charged
carrier transport properties. It was practically difficult to predict IE and EA of
organic semiconductors using periodic slab models at the GW level of theory,
because of the computational cost to meet convergence criteria. Recently, there are
theoretical studies at the same level of theory handling the problem [21–23].
Finally, we demonstrate the future aspects of the theoretical methods, both in
terms of contribution to elucidation of the novel experimental measurements and
room for development of the theoretical methodologies.
91
predictable by the first-principles theoretical methods describing the quasiparticle
self-energy such as the GW approximation [12].
In this article, we review first-principles electronic structure calculations on
1. Structures of organic semiconductor crystals with the van der Waals (vdW)inclusive methods
2. Electronic properties of organic-metal interfaces: electronic properties such as
energy level alignment and the image potential states
3. Electronic charge (electron or hole) injection level depending on the molecular
orientation at the organic crystal surface
In the first topic, we focus on our first-principles study on prediction of the lattice
constants and the intermolecular configuration in oligoacene crystals, along with
analysis of the band structure and the intermolecular transfer integral [13]. Here, the
Wannier functions [14, 15] were derived, allowing us to estimate intermolecular
transfer integrals depending crucially on the cell volume or the intermolecular
configurations in the crystal. The Wannier functions in organic crystals could be
mapped on the crystal Hamiltonian such as the tight-binding Hamiltonian, thus
leading to multi-scale real-time simulations of charge carrier transport in organic
crystals [16]. In addition to the previous work on naphthalene, anthracene, and
tetracene [13], calculation of pentacene and hexacene crystals is demonstrated in
this article.
Secondly, the electronic properties induced at the organic-metal interface are
discussed. The energy level alignment at the organic-metal interface originating
from the rearrangement of the electronic clouds at the interface has attracted
considerable attention [17], because of its relevance to barrier for charge injection
into an organic layer. To elucidate the experimental measurements using techniques
such as photoemission spectroscopy proving the energy levels of the injected hole
or electron right at the interface, aid of a reliable theoretical method treating
charged excitations (quasiparticles) is necessary. Here, we focus on highly accurate
theoretical approaches beyond DFT within the LDA or GGA, such as the manybody perturbation theory within the GW approximation [12, 18]. In addition, we
demonstrate our recent theoretical investigation [19] to clarify the electronic nature
of the recently reported image potential state of graphite in the presence of an
organic overlayer [20].
In the third place, we discuss determination of charge injection levels in organic
semiconductor crystals or thin films, mainly based on the GW approximation.
The charge injection level relative to the vacuum level, i.e., ionization energy (IE)
or electron affinity (EA) of organic semiconductors, crucially affects the charged
carrier transport properties. It was practically difficult to predict IE and EA of
organic semiconductors using periodic slab models at the GW level of theory,
because of the computational cost to meet convergence criteria. Recently, there are
theoretical studies at the same level of theory handling the problem [21–23].
Finally, we demonstrate the future aspects of the theoretical methods, both in
terms of contribution to elucidation of the novel experimental measurements and
room for development of the theoretical methodologies.