90
S. Yanagisawa and I. Hamada
methodologies, being enhanced by rapid progress in computational resource and
algorithm, might lead to in silico material simulation or design.
Keywords Organic semiconductors · Intermolecular van der Waals interaction ·
van der Waals density functional · GW approximation · Organic-metal
interfaces · Image potential states
4.1 Introduction
Semiconductors comprised of organic molecules have attracted considerable attention as candidates for next-generation flexible electronics materials. It is believed
that organic semiconductor electronics will be competitive against inorganic semiconductors for applications to mechanically flexible, large-area, and low-cost
electronics [1]. Some organic-based electronic devices such as organic lightemitting diodes (OLED) are on the market, while other electronics materials such
as organic field-effect transistors (OFET) and organic photovoltaics (OPV) are at
the stage of a fundamental research. However, there is much room for investigation
on the electronic properties of the organic materials, for instance, carrier transport
property. Poor knowledge of the transport property in organic solids is contrast to
the situation in inorganic semiconductors [2]. More understanding of their basic
electronic properties is urgent, which may accelerate development of the nextgeneration flexible electronics.
Here, we review recent theoretical works with the first-principles electronic structure calculation, i.e., (i) those on the intermolecular van der Waals (vdW) interaction
dominating the structural and electronic properties of the organic semiconductor
materials and (ii) the effect of the many-body electrons (or the quasiparticle effect)
upon the injected charge in the bulk of the organic semiconductors or at the organicmetal interface.
We demonstrate the roles of the first-principles electronic structure calculations
which could clarify a relation between the electronic structure and the geometrical
configurations. As an example, the impact of the atomic-scale geometric structures
on the electronic states is discussed. It was not possible to predict the crystal
geometry or the molecular configuration in the unit cell of the organic semiconductor crystals with the density functional theory (DFT) within the local density
(LDA) or generalized gradient approximation (GGA), because of the failure of the
methodology to describe the intermolecular vdW interaction, which is nonlocal
and long-ranged in nature. However, there are more and more methodologies
being developed capable of describing the vdW interaction between the constituent
organic molecules [3–11].
The quasiparticle energy is essentially related to the barrier for charge injection,
i.e., the electronic energy upon charged excitation, and also the interaction between
the electron-hole pair upon optical excitation, dominating the optical gap. Fundamental gap and band dispersion of organic molecular solids are becoming accurately
S. Yanagisawa and I. Hamada
methodologies, being enhanced by rapid progress in computational resource and
algorithm, might lead to in silico material simulation or design.
Keywords Organic semiconductors · Intermolecular van der Waals interaction ·
van der Waals density functional · GW approximation · Organic-metal
interfaces · Image potential states
4.1 Introduction
Semiconductors comprised of organic molecules have attracted considerable attention as candidates for next-generation flexible electronics materials. It is believed
that organic semiconductor electronics will be competitive against inorganic semiconductors for applications to mechanically flexible, large-area, and low-cost
electronics [1]. Some organic-based electronic devices such as organic lightemitting diodes (OLED) are on the market, while other electronics materials such
as organic field-effect transistors (OFET) and organic photovoltaics (OPV) are at
the stage of a fundamental research. However, there is much room for investigation
on the electronic properties of the organic materials, for instance, carrier transport
property. Poor knowledge of the transport property in organic solids is contrast to
the situation in inorganic semiconductors [2]. More understanding of their basic
electronic properties is urgent, which may accelerate development of the nextgeneration flexible electronics.
Here, we review recent theoretical works with the first-principles electronic structure calculation, i.e., (i) those on the intermolecular van der Waals (vdW) interaction
dominating the structural and electronic properties of the organic semiconductor
materials and (ii) the effect of the many-body electrons (or the quasiparticle effect)
upon the injected charge in the bulk of the organic semiconductors or at the organicmetal interface.
We demonstrate the roles of the first-principles electronic structure calculations
which could clarify a relation between the electronic structure and the geometrical
configurations. As an example, the impact of the atomic-scale geometric structures
on the electronic states is discussed. It was not possible to predict the crystal
geometry or the molecular configuration in the unit cell of the organic semiconductor crystals with the density functional theory (DFT) within the local density
(LDA) or generalized gradient approximation (GGA), because of the failure of the
methodology to describe the intermolecular vdW interaction, which is nonlocal
and long-ranged in nature. However, there are more and more methodologies
being developed capable of describing the vdW interaction between the constituent
organic molecules [3–11].
The quasiparticle energy is essentially related to the barrier for charge injection,
i.e., the electronic energy upon charged excitation, and also the interaction between
the electron-hole pair upon optical excitation, dominating the optical gap. Fundamental gap and band dispersion of organic molecular solids are becoming accurately