92
S. Yanagisawa and I. Hamada
4.2 Theoretical Investigations on Electronic Properties
of Organic Molecular Materials
4.2.1 Structural Properties: Crystal Geometry and
Intermolecular Configuration
4.2.1.1 Crystal Structures Optimized with a Variant of Van der Waal
Density Functionals
Geometrical configurations in bulk systems of organic molecular crystals or heterojunctions such as organic-metal electrode and organic-organic interfaces are of
importance, in which the atomistic- or molecular-scale geometrical arrangements
significantly alter the electronic properties.
The structural properties of organic crystals such as crystal structures and
intermolecular geometrical configurations are one of the central issues dominating
the electronic properties such as charge transport [24, 25]. Subtle interplay among
the intermolecular van der Waals (vdW) forces, the exchange repulsion, and the
orbital hybridization leads to a rich variety of polymorphs with different electronic
properties. The stability of the polymorphs depends on temperature, and thus
attempts are going on to predict (meta-)stable polymorphs of the organic crystals
[26]. Nevertheless, first-principles theoretical investigations on electronic structure
of the most stable polymorph at 0 K may give us insights into the properties of the
materials at not only low temperature but also at room temperature.
Recently proposed vdW-inclusive methods have played roles in prediction of
stable crystal structure and molecular configurations, which allows us to discuss the
effects of the intermolecular interaction on electronic properties.
Here, we focus on the first-principles theoretical methods that have been successfully applied to organic semiconductor crystals (for a variety of the applications to
other molecular aggregates and complexes, see a recent comprehensive review such
as Ref. [27]).
The structure and the energetics of an organic crystal could be quite accurately
predicted with first-principles methods such as the adiabatic-connection fluctuationdissipation theorem within the random-phase approximation (RPA) [4, 28, 29], the
quantum Monte Carlo [30, 31], and the wave function-based quantum chemistry
[32, 33] methods. However, the computational cost involved at present prevents
application of the highly accurate methods to organic crystals that are of practical
or experimental interest. The vdW-inclusive density functional method, such as
the van der Waals density functional (vdW-DF) and its variants [10, 34–39], may
be a method of choice, because of its moderate computational cost [40] and its
reliability in prediction of structural properties of organic crystals. We employed the
revised vdW-DF2 (rev-vdW-DF2), with its reliable prediction of crystal geometries
of organic crystals such as rubrene [10] and metal phthalocyanine [41].
We first performed the structural optimizations and determined the stable
structures for naphthalene, anthracene, and tetracene crystals (Fig. 4.1) by using
S. Yanagisawa and I. Hamada
4.2 Theoretical Investigations on Electronic Properties
of Organic Molecular Materials
4.2.1 Structural Properties: Crystal Geometry and
Intermolecular Configuration
4.2.1.1 Crystal Structures Optimized with a Variant of Van der Waal
Density Functionals
Geometrical configurations in bulk systems of organic molecular crystals or heterojunctions such as organic-metal electrode and organic-organic interfaces are of
importance, in which the atomistic- or molecular-scale geometrical arrangements
significantly alter the electronic properties.
The structural properties of organic crystals such as crystal structures and
intermolecular geometrical configurations are one of the central issues dominating
the electronic properties such as charge transport [24, 25]. Subtle interplay among
the intermolecular van der Waals (vdW) forces, the exchange repulsion, and the
orbital hybridization leads to a rich variety of polymorphs with different electronic
properties. The stability of the polymorphs depends on temperature, and thus
attempts are going on to predict (meta-)stable polymorphs of the organic crystals
[26]. Nevertheless, first-principles theoretical investigations on electronic structure
of the most stable polymorph at 0 K may give us insights into the properties of the
materials at not only low temperature but also at room temperature.
Recently proposed vdW-inclusive methods have played roles in prediction of
stable crystal structure and molecular configurations, which allows us to discuss the
effects of the intermolecular interaction on electronic properties.
Here, we focus on the first-principles theoretical methods that have been successfully applied to organic semiconductor crystals (for a variety of the applications to
other molecular aggregates and complexes, see a recent comprehensive review such
as Ref. [27]).
The structure and the energetics of an organic crystal could be quite accurately
predicted with first-principles methods such as the adiabatic-connection fluctuationdissipation theorem within the random-phase approximation (RPA) [4, 28, 29], the
quantum Monte Carlo [30, 31], and the wave function-based quantum chemistry
[32, 33] methods. However, the computational cost involved at present prevents
application of the highly accurate methods to organic crystals that are of practical
or experimental interest. The vdW-inclusive density functional method, such as
the van der Waals density functional (vdW-DF) and its variants [10, 34–39], may
be a method of choice, because of its moderate computational cost [40] and its
reliability in prediction of structural properties of organic crystals. We employed the
revised vdW-DF2 (rev-vdW-DF2), with its reliable prediction of crystal geometries
of organic crystals such as rubrene [10] and metal phthalocyanine [41].
We first performed the structural optimizations and determined the stable
structures for naphthalene, anthracene, and tetracene crystals (Fig. 4.1) by using