Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 19187
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Molecular orientation in small-molecule organic light-emitting diodes
Daisuke Yokoyama*
Received 19th July 2011, Accepted 8th September 2011
DOI: 10.1039/c1jm13417e
In research on small-molecule organic light-emitting diodes (OLEDs), the molecular orientation in
vacuum-deposited amorphous films has been disregarded for around 20 years, and its effects on device
performance have not been sufficiently discussed at the microscopic level. Only recently have theintermolecular interaction and subsequent horizontal molecular orientation in OLEDs been
investigated and reported. In this article, an overview of the recent studies on molecular orientation in
OLEDs is presented. The general properties of molecular orientation of OLED materials are shown,and its significant effects on the electrical and optical properties of devices are discussed to understanddevice physics and improve the future performance and reliability of OLEDs.

1. Introduction
Recently, research on organic electronics has significantly
developed, and it is now regarded as a promising field for the
future. We can now use some commercial organic light-emitting
diode (OLED) displays, and much wider applications are
expected.1 In the near future, it will inevitably be hoped thatorganic semiconductor devices will be widely used in our daily
lives as silicon devices. To reach that point, it is necessary to
Department of Organic Device Engineering, Graduate School of Scienceand Engineering and Research Center for Organic Electronics (ROEL),
Yamagata University, 4-3-16 Johnan, Yonezawa, Yamagata, 992-8510,
Japan. E-mail: d_yokoyama@yz.yamagata-u.ac.jp

Daisuke Yokoyama obtained his
BS and MS in Physical Chemistry from The University ofTokyo in 1998 and 2000,
respectively. After working in
Fuji Film Corporation asa research staff member for six
years, he joined Kyushu University, and conceived and designed
the research on molecularorientation in OLEDs. He
obtained his PhD degree inEngineering in 2009 and worked
Daisuke Yokoyama
as an assistant professor for one
year in Kyushu University. He iscurrently working at Yamagata
University as an assistant professor, and his main research interests
are the in-depth analysis of molecular orientation and ordering inorganic semiconductor solids and the application of their functionalities to organic optoelectronic devices.
This journal is ª The Royal Society of Chemistry 2011

achieve high, attractive, and stable performance for thesedevices.
If we reconsider why silicon devices have such high reliability
today, we find that it is because they are based on the sophisticated theory of semiconductor physics for the bulk state ofinorganic semiconductor materials.2 Organic semiconductors, on
the other hand, have significantly different properties because
they are composed of molecules bound by weak intermolecularinteractions. In particular, the films of small-molecule organic
semiconductors consist of units of small molecules, and eachmolecule has specific characteristics as a single molecule, such as
geometric and electronic structures, and absorption and emission
spectra. Thus, we usually deduce the properties of the solids oforganic semiconductor materials from the characteristics of thesingle molecules before fabricating the thin films or crystals. Thismeans that each molecule in small-molecule organic semiconductors is independent to some degree because electrons aremainly localized within the molecule, though its properties are
shifted or changed depending on intermolecular interactions. We
have efficiently used this independence to deduce, control, andimprove the properties of organic semiconductor films for thedevelopment of many new materials.
However, this independence of molecules in small-moleculeorganic semiconductors also leads to low charge mobility in mostof their films. Since the independence means that the interaction
between the molecular orbitals of adjacent molecules is not large,
the charge transport between them is generally not as efficient as
in inorganic semiconductors having a well-defined band structure.2 In particular, amorphous organic semiconductor films,
which are commonly used in small-molecule OLEDs,3 havemuch lower charge mobilities than those in other types of organic
semiconductors such as single crystals and polycrystals because
the molecular orientation and alignment is much more random.
Despite this disadvantage in charge transport, small-molecule
amorphous organic films are usually used in OLEDs mainly
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because of many advantages in fabrication of devices with a large
area and a high uniformity; that is, the amorphous films have theadvantages of (1) nanometre-scale surface smoothness, (2) easy
controllability of thickness, (3) no restriction in the choice ofunderlying layers, and (4) a simple high-purity fabrication
process. They make it possible to stack any organic semiconductor layers having different properties orderly in the
direction of thickness and achieve higher-order functionalities oforganic materials with high stability and durability even undera high electric field.
To improve the charge mobility in small-molecule amorphousorganic films with the above advantages, many kinds of amorphous semiconductor materials have been developed. These havecontributed very successfully to the continuous improvement of
the performance of OLEDs.4–7 However, when discussing theproperties of amorphous organic films and the performance of
OLEDs, we have focused mainly on the properties of films andcharacteristics of single molecules and have not sufficiently
considered how molecules are oriented and aligned in the bulk of
the films. Since Tang and VanSlyke3 began research on OLEDs
in 1987, it has been taken for granted for around 20 years thatmolecular orientation in small-molecule OLEDs is generally
random and isotropic. To date, this simple assumption of
random orientation has been available as a ‘‘0th approximation’’in OLED research, as shown in Fig. 1. In the early stage of
research on OLEDs, it has been more important to simplify theproperties of the films as isotropic and find out principal
phenomena than to discuss ‘‘trivial’’ details such as the molecularorientation.
Now, however, research on OLEDs has reached maturity. The
performance of the devices is now coming close to the theoreticallimit,8 and the technologies for fabrication, evaluation, andanalysis have also been developed to a much higher level than in
the early stage. At the present stage, we have to investigate andunderstand the device physics and the underlying chemistrybeyond the simple assumption to further improve the performance and reliability of OLEDs. Since it is the microscopic viewof chemistry that supports the device physics in organic semiconductors, the detailed investigation of molecular states such as
molecular orientation and ordering is very important for
explaining the relationship between the chemical characteristics
of single molecules and the physical properties of the films. This
will help build reliable fundamentals of organic devices.
The molecular orientation of OLED materials in vacuumdeposited amorphous films has been reported and discussed only
recently. In this article, an overview of the recent studies onmolecular orientation in OLEDs will be presented, and theimportance of considering the orientation will be demonstrated.
First, to understand the structures of molecules in amorphousfilms, molecular conformation of OLED materials will becovered briefly in Section 2. Then, Section 3 will discuss thegeneral properties of the molecular orientation of OLED materials and the control of the orientation by temperature. In
general, molecules in vacuum-deposited amorphous films arehorizontally oriented depending on the anisotropy of themolecular shape, and the orientation can be controlled by heating a substrate during deposition. Next, in Section 4, the significant effects of the molecular orientation on both the electricaland optical properties of devices will be demonstrated. The
horizontal molecular orientation has positive effects on chargetransport and light outcoupling. Furthermore, Section 5 willshow the active control of the molecular orientation by intermolecular interactions. Using weak intermolecular hydrogenbonds, it is possible to control the molecular orientation andmolecular stacking in vacuum-deposited films to improve theelectrical property of OLEDs. Finally, Section 6 will present thesummary and future outlook. Through the overview in thisarticle, we can find that an understanding of molecular statessuch as orientation and ordering is vital to clarifying the mechanism in OLEDs and improving their future performance.

2. Molecular conformation of materials

Fig. 1 ‘‘History’’ of the picture of molecular orientation in OLEDs.
Since the beginning of research in OLEDs, the molecular orientation in
vacuum-deposited amorphous films has been disregarded for around20 years (Section 1). The molecular orientation caused by the anisotropyof molecular shapes has been discussed only recently (Section 3).
Furthermore, the orientation control by intermolecular interactions wasreported more recently, leading to further improvement of the propertiesof films (Section 5).

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There seem to be two reasons why molecular orientation in
amorphous films has been disregarded for a long time in OLED
research, even though it is a matter of course in other kinds oforganic solids such as polymer films, polycrystalline films, andsingle crystals. One is the preconceived idea that the molecularorientation in ‘‘amorphous’’ films must be random and isotropic,
and the other is the difficulty in finding a proper method to detect
the molecular orientation in such films. Because amorphousmaterials used in OLEDs have many molecular conformation
structures, their films do not have a periodic structure witha long-range order that is usually seen in crystalline films. This
makes it difficult to investigate the orientation by X-ray
diffraction (XRD) measurement or other conventional methods.
On the other hand, the large number of the conformation
structures makes the performance of OLEDs highly stable. Since
the molecules used in OLEDs usually have multiple conformation structures with a local minimum steric energy, the structuresof the molecules in the films are not the same. This avoids crystallization in the films. The films keep an amorphous state witha very smooth surface without roughness or pinholes, which isThis journal is ª The Royal Society of Chemistry 2011

critical in OLEDs because we have to control holes and electrons
in the direction of small thickness under a high electric field.
Therefore, when we discuss the molecular structure in OLEDs,
we should always consider that the molecule has multiple
structures in the film. Calculation of molecular structures bymolecular mechanics9 is very informative for this. Although theaccuracy of the calculation is less than that of a quantum
chemical calculation such as a density functional theory (DFT)
calculation, it can find all possible conformation structures
without a large computational cost. For example, Fig. 2 shows
the conformation structures of 4,40 -bis(N-carbazole)-biphenyl(CBP) and 4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl(a-NPD) obtained by molecular mechanics calculations in freespace using an MMFF94s force field.10–13 Since two aromaticrings connected by a single bond can have two different dihedral
angles between them, there are different conformation structures.
When the structures symmetrically equivalent to each other are
excluded, the CBP molecule has three structures, and the a-NPD
molecule has 12 structures. In general, the number of conformation structures highly depends on the variation of the dihedral
angles and the symmetry in the molecule. We have to consider
this variation when we discuss in detail the properties of amorphous films based on the characteristics of single molecules.14
Then, in addition to the variation of the conformation structures,
we should also remember that the structures of molecules inamorphous films are slightly different from those in free space,though the analysis of the difference is currently quite difficult.

3. Molecular orientation depending on anisotropy ofmolecular shape
3.1 Variable angle spectroscopic ellipsometry
To analyze the molecular orientation in vacuum-deposited
amorphous films, we need a method to detect the anisotropy ofthis orientation even in a film without a long-range structuralorder. One of the best methods to detect the molecular orientation in amorphous organic films is variable angle spectroscopic
ellipsometry (VASE),15,16 which can nondestructively determine
the optical properties of thin films and the anisotropy in them.
Since the specialized details of this method can be readFig. 2 Variety of conformation structures of CBP and a-NPD obtainedby molecular mechanics calculation using an MMFF94s force field.

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elsewhere,17 the outline of the principle will be explained briefly
here to understand and interpret the experimental results shown
later.
Ellipsometry is an analytical method to model the opticalproperties of thin films through the use of the interference of
obliquely incident light, as shown in Fig. 3. A sample film isusually formed on a substrate having a smooth surface. The
incident light is linearly polarized, where the plane of thepolarization is usually tilted at 45 . This means that the s- andp-components of the electric field (Es and Ep in Fig. 3) of theincident light have the same amplitudes and phases. However,
because of the multiple interferences in the thin film, the amplitudes and phases of the s- and p-components in the reflected lightbecome different from those in the incident light. Generally, inlight having different amplitudes and phases for the s- andp-components, the plane of the polarization rotates as the lightpropagates (see the red arrows in Fig. 3); the light is ellipticallypolarized light. The difference in amplitudes between the s- andp-components in the reflected light is monitored as the ellipsometric parameter J, and the difference in the phases is monitored as the parameter D. The values of these parameters
significantly depend on the optical constants (refractive index n
and extinction coefficient k) and thickness of the film and also on
the wavelength and incident angle Q. The optical constants
generally have different values depending on the wavelength, andmany values of J and D at multiple wavelengths are usuallyobtained to make the analysis sufficiently reliable. From theexperimental values of J and D, it is possible to determine theoptical constants and thickness of the thin film. In the analysis, it
is also important to keep Kramers–Kronig consistency,17 which
is a required condition that refractive indices and extinction
coefficients should necessarily satisfy.
When the optical properties are complicated as in organic
semiconductor films, it is preferable to obtain many more valuesof J and D by changing the incident angle and use all the data to
get a unique solution in the analysis. This method of obtaining
spectra under variable angles is what we call VASE, which makes
it possible to analyze the complicated properties of thin films. In
particular, the VASE is sensitive to the optical anisotropy in films
because light propagating in an anisotropic film feels different
optical properties depending on the incident angle. The values ofFig. 3 Schematic of variable angle spectroscopic ellipsometry (VASE).
By analyzing the properties of the elliptically polarized light reflected bya sample, the optical constants and their anisotropies can be determined.
(Reprinted with permission from ref. 32; Copyright 2009, Elsevier.)

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J and D significantly depend on the anisotropy of the opticalconstants, and the optical anisotropies in films can be estimatedfrom this dependence. This means that we can determine theanisotropies in the refractive index n and the extinction coefficient k of the film. The VASE has been applied mainly to polymer
films,18–20 liquid crystals,21,22 and inorganic films23,24 to investigate the optical anisotropy in them. Only recently, it has started
to be applied to vacuum-deposited amorphous organic films.
For example, when a film has the same optical properties inboth two horizontal directions but a different property in thevertical direction, it means that the film has optical anisotropy,
where the optical constants (refractive index and extinction
coefficient) are different for horizontally and vertically polarized
light: ordinary refractive index and extinction coefficient (no andko) and extraordinary ones (ne and ke), respectively, as shown in
Fig. 4(a). This anisotropy in the optical constants of organic films
is related to the anisotropy of the molecular orientation in them.
To make this relationship clear, it is necessary to correlate theoptical properties of films with the electronic properties ofmolecules.
First, the refractive index of a film is directly related to themolecular polarizability and the number density of the moleculesin the film via the Lorentz–Lorenz equation,25 where the higher
the molecular polarizability or the number density is, the higher
the refractive index is. In amorphous films, the effect of themolecular polarizability is dominant because the densities of thefilms are not significantly different. The molecular polarizability
represents how easily the electrons in the molecule can be moved
by an external electric field. It has different values depending on
the direction of the external electric field and the direction of
motion of the electrons; more precisely, it has a tensor quantity.
In the case of linear-shaped organic molecules, for example, themolecular polarizability generally has a larger value in the
direction of the long molecular axis than in the other directions(see Fig. 4(a)), because the electrons in an organic molecule canbe easily moved within the molecule. Thus, the orientation ofmolecules with an anisotropic shape can be deduced from theanisotropy in the refractive index of the film.
Next, the extinction coefficient of a film is directly related tothe transition dipole moment of the molecule. The extinction
coefficient is represented as k ¼ (l/4p)a, where a is the absorption coefficient of the film and l is the wavelength of light. Thus,
the extinction coefficient represents light absorption and has
a larger value in the direction of the transition dipole moment of
the molecules. In the case of linear-shaped organic molecules, thetransition dipole moment of most of the molecules is along thelong molecular axis, though there are exceptions. Thus, we canlearn about the orientation of the molecular axis in the film byinvestigating the anisotropy in the extinction coefficient of thefilm and the direction of the transition dipole moment of themolecules.
In the analysis of vacuum-deposited amorphous organic films
by VASE, the in-plane rotation of the sample usually does notchange the result of the analysis even when the film has large
anisotropy. This means that the molecular orientation is random
in the plane even when the property of the molecular orientation
is different between the horizontal and vertical directions, as
shown in Fig. 4(b). Since amorphous materials have many
conformation structures as discussed in Section 2, the van der
Waals intermolecular interactions of amorphous materials is not
as strong as that of polycrystalline materials, leading to loose
binding of molecules in the amorphous films with no orderedstructure in plane. This molecular behavior in amorphousmaterials is quite different from that in polycrystalline materials,
which often shows a vertical orientation caused by strong
intermolecular interaction.26–28
The orientation of transition dipole moments in amorphousfilms can be quantified using an orientation order parameter S,29
which is defined as
S¼

3hcos2 qi  1
kemax  komax¼ max
2
ke þ 2komax(1)

Fig. 4 Relationship between the optical anisotropy in films and molecular orientation. (a) Optical anisotropy in vacuum-deposited amorphous organic
films and relationship between the optical properties of films and electronic properties of molecules. (b) Schematic of the horizontal orientation of linearshaped molecules in amorphous films. (c) Orientation order parameter for the films of linear-shaped molecules.

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where h/i indicates the ensemble average, q is the angle between
the axis of the transition dipole moment and the direction verticalto the substrate surface, and komax and kemax are the ordinary andextraordinary extinction coefficients at the peak of the band
attributed to the transition dipole moment, respectively. Whenthe transition dipole moment is completely along the longmolecular axis of a linear-shaped molecules, S ¼ 1 if the molecules are perfectly oriented vertically to the substrate surface, S ¼
0 if they are randomly oriented, and S ¼ 0.5 if they are perfectly oriented horizontally to the substrate surface, as shown in
Fig. 4(c). From the values of the ordinary and extraordinary
extinction coefficients obtained by the VASE analysis, we canestimate the orientation of the molecular axis quantitatively
using eqn (1). It should be mentioned that eqn (1) is derived
under the assumption that the band used for this estimation canbe attributed only to a single transition dipole moment of themolecule. If it cannot be attributed only to one, as in the case ofsymmetric planar molecules having two equivalent degeneratetransitions, the equation should be modified to include thecontributions of the multiple transitions.

3.2 Horizontal molecular orientation in amorphous films
The detection of the optical anisotropy and molecularorientation in a vacuum-deposited amorphous film of OLED
materials was first reported by Lin et al. in 2004.30 They applied
VASE to the vacuum-deposited films of two blue-emitting ter(9,9-diarylfluorene) materials and reported that the long molecular axis of these molecules was oriented parallel to the substratesurface. Although this report was very important and gave new
insight into molecular states in amorphous films and the OLED
mechanism, it did not get enough attention from researchers in
the field in OLEDs for several years. This is probably because it
focused only on the fluorene materials, and also because using

Fig. 5 Chemical structures of the OLED materials used to investigate
the difference in the anisotropy in the molecular orientation.

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VASE to study anisotropy was not popular in the field of organic
semiconductors at the time.
To show the importance of molecular orientation in OLEDs, it
was necessary to study the generality of the molecular orientationin vacuum-deposited films of various kinds of OLED materials.
Four years after the first detection, the generality of the molecular orientation was demonstrated by studies of the dependence
on molecular structures. In 2008 and 2009, we reported thehorizontal molecular orientation in vacuum-deposited amorphous films of various kinds of OLED materials, including
emitting, hole transport, and electron transport materials.31,32
Figure 5 shows some of the materials that we investigated. Somematerials have bulky or compact shapes, and others have linear,planar or long-and-winding shapes. The optical anisotropies intheir 100-nm films deposited at the rate of 0.2–0.3 nm s1 areshown in Fig. 6, where the wavelength dependences of therefractive indices (red) and extinction coefficients (blue) forhorizontally (solid) and vertically (broken) polarized light are
shown for each material. The values of the orientation orderparameter S obtained using eqn (1) are also shown. In thisanalysis, the ellipsometric parameters J and D were measured atseven variable incident angles in the range of 45–75 in steps of 5to make the analysis highly reliable.
First, in the case of bulky or compact materials such as 1–4,
which were developed in the early stage of OLED research,3,33–35
the optical properties are almost isotropic, and the values of theorientation order parameter are nearly zero. This means that theorientation of the transition dipole moment attributed to thehigh absorption is almost random in these films. The film ofmaterial 1 is almost optically isotropic because of the isotropic
molecular shape. Materials 2 and 3 have their large transition
dipole moments in the direction from one nitrogen atom to theother, and material 4 has its large transition dipole moment in the
direction parallel to the plane of the central triphenylaminegroup. The results of the VASE analysis show the randomorientation of these dipoles in the films. Both the extinction
coefficients and the refractive indices are almost isotropic, indicating that the molecular orientation is almost completely
random in three dimensions.
Next, the results of materials 5–836–39 show the significant
dependence of the orientation on molecular length. Thesemolecules all terminate with carbazole groups, and the molecularlengths are different in the order 5 < 6 < 7 < 8. They have largetransition dipole moments in the direction of the long molecularaxis. We can see the correlation between the molecular lengthand the optical anisotropy in the films; that is, the longer themolecular length is, the larger the optical anisotropy in the film
is. The fact that the ordinary refractive indices and extinction
coefficients are higher than the extraordinary ones shows that themolecules are horizontally oriented in the film, as shown in Fig. 4(b). We also see the clear correlation between the molecularlength and the orientation order parameter. These results clearly
demonstrate that the linear-shaped molecules are horizontallyoriented in the vacuum-deposited amorphous films depending on
the molecular length.
Then, from the results of materials 9–12, which are holetransport materials mainly composed of triphenylaminegroups,40 we see that not only linear-shaped molecules but also
the molecules having a planar or long-and-winding shape are
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Fig. 6 Anisotropies in the refractive indices (red) and the extinction coefficients (blue) of the films of the OLED materials in Fig. 5. The solid andbroken lines indicate the horizontal and vertical components of the optical constants, respectively. (Reprinted with permission from ref. 31; Copyright
2008, American Institute of Physics and ref. 32; Copyright 2009, Elsevier.)

Fig. 8 Schematic of horizontal molecular orientation on various kindsof underlying layers and even in an isotropic organic host matrix.

Fig. 7 General relationship between the anisotropies of molecular shapeand molecular orientation in vacuum-deposited amorphous organic
films.

also horizontally oriented in the films. As in materials 5–8, theorientation order parameter becomes larger as the molecularlength gets longer, though these molecules do not have a rigid
linear shape because of the variety of the conformation structures, as discussed in Section 2. Although the conformation
structures of these materials are quite complicated, the transition
dipole moment is along the p-conjugation system of the molecules. Thus, we can still obtain rough information about themolecular orientation.
The results of the VASE analysis of 1–12 clearly demonstrate
the generality of the molecular orientation in vacuum-deposited
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amorphous organic films; that is, the larger the anisotropy of themolecular shape is, the more significant the molecular orientation is, as shown in Fig. 7. This also means that we had overlooked the molecular orientation of various OLED materials
even though many materials having a significantly anisotropic
molecular shape have been developed in research on OLEDs. It is
worthwhile to investigate the molecular orientation of suchmaterials to understand the properties of the films.
It should be noted that the horizontal molecular orientation
occurs on any underlying films or substrates.32 For example,
molecules are similarly oriented on a Si substrate, an ITO electrode, a metal layer, and another organic layer as schematicallyshown in Fig. 8. This characteristic is specific only to vacuumdeposited amorphous films, and cannot be seen in other types oforganic layers such as vacuum-deposited polycrystalline films,
spin-coated films, or liquid crystals. Thus, we can use the horizontal molecular orientation in amorphous films without any
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Fig. 9 Dependence of the optical anisotropies in the films of 8–10 on deposition rate. The red and blue lines indicate the refractive indices and extinction
coefficients for a deposition rate of 2 nm s1, and the black and green lines indicate those for a deposition rate of 0.02 nm s1, respectively. The solid andbroken lines indicate the horizontal and vertical components of the optical constants, respectively.

restriction on the underlying layer or the thickness; that is, theorientation can be used effectively in any multilayer OLED
structure. Moreover, it is also worth noting that the horizontal
orientation of linear-shaped molecules occurs even in an
isotropic organic host matrix. This was qualitatively shown bymeasurement of the edge emissions from 6 wt%-8-doped CBPfilms with 15 different thicknesses,32 and a reliable quantitative
estimate of the orientation of dopant molecules was done41 by
combining angular dependence measurement of photoluminescence (PL) spectra and optical simulation.42–44 Since
emitting molecules in OLEDs are usually doped in host materials
to avoid a concentration quenching, the horizontal orientation ina doped matrix is important when we want to use the orientationof emitting molecules to improve OLED performance. Its effecton optical performance of OLEDs will be discussed in Section
4.2.
Furthermore, we need to know the dependence of the horizontal orientation on the deposition rate to make fabrication
processes highly reliable under different conditions. The dependence is shown in Fig. 9, where the optical constants at the higher
deposition rate (2 nm s1) and the lower one (0.02 nm s1) are
shown for materials 8–10. The anisotropies of the orientation
become slightly smaller with decrease of the deposition rate, but
the anisotropies do not change very much even when the deposition rate changes by two orders. This means that we canreproduce the films with similar orientations even if the deposition rate fluctuates to some degree.
Although the dependence on thickness is also important, it is
not significant. Since amorphous films do not have a long-range
structural order, the effect of the underlying layer does not reach
far into the bulk of the films. This is one reason why the orientation can occur on any underlying layer. This homogeneity oforientation in the direction of thickness was demonstrated by insitu ellipsometry,45 shown in the next section. However, there
remains a possibility that the molecules show different behavior
in their orientation just near an underlying layer. Since theorientation just at the interface cannot be investigated by VASE,
the difference in the molecular orientation such as in an initialmonolayer on the surface should be discussed carefully by othermethods.
From the above results and discussions, it is reasonable toassume that the molecular orientation in amorphous films iscaused simply by weak van der Waals interactions between
a substrate and molecules and between adjacent molecules.
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Fig. 10 Schematics of the orientation mechanism. (a) Vertical orientation of vacuum-deposited polycrystalline materials due to crystallization.
(b) Horizontal molecular orientation of vacuum-deposited amorphousmaterials. (c) Randomization of molecular orientation by deposition ona heated substrate.

Typical mechanisms of the molecular orientations in vacuumdeposited polycrystalline and amorphous films are schematicallyshown in Fig. 10(a) and (b), respectively.32 For example, in thecase of a polycrystalline film of a-sexithiophene (a-6T)26 on
silicon dioxide, molecules are horizontally oriented just after
the deposition of a monolayer to reduce the surface energy of thesubstrate. However, the molecules stand up vertically during thesubsequent deposition due to the strong interaction between
adjacent molecules and form polycrystalline islands while
keeping the high coverage of the substrate surface.46 This is why
molecules in vacuum-deposited polycrystalline films are often
vertically oriented.26–28 In amorphous films, on the other hand,since the molecules have a variety of conformation structures, thevan der Waals intermolecular interaction is not as strong as thatof polycrystalline materials. Thus, the horizontal molecularorientation is kept as a metastable glassy state in the films. This
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simple mechanism is highly possible because it is consistent withthe above experimental results: the simple relationship between
the orientation and molecular shapes, the orientation on any
underlying layer and in doped films, and the homogeneity oforientation in the direction of thickness.
3.3 Orientation control by deposition on heated substrate
To control the physical properties of vacuum-deposited amorphous organic films, the molecular orientation should becontrolled by an experimental condition during fabrication. One
method to control it is deposition on heated substrates.45,47 It is
well-known that the morphology and properties of vacuumdeposited amorphous films change and often degenerate when
the temperature of the films goes over the glass transition
temperature Tg.48,49 This also means that it is difficult to change
the physical properties of the films under Tg after the deposition
without a change in the morphology of the film. Although it wasreported that molecular orientation of fluorene materials in
amorphous films can be controlled by heating a substrate to overTg in an inert atmosphere after deposition,50 heating to over Tg in
general inevitably involves changes in the morphology, such as
crystallization, increase of surface roughness, or generation of
pinholes. Thus, it is preferable to control the properties ofamorphous films under Tg.
By heating a substrate during deposition, we found that themolecular orientation in amorphous films can be controlled evenunder Tg without crystallization, because molecules on thesurface during deposition can locally migrate more readily thanin the bulk after deposition.47,51 Figure 11 shows the opticalanisotropies in the 100-nm films of materials 8–10 deposited ona Si substrate at different temperatures. Although the molecules
are horizontally oriented when they are deposited at room
temperature, the orientation becomes more random withincrease of the substrate temperature under Tg during deposition,as schematically shown in Fig. 10(c). This result means that ‘‘theTg at the surface’’ of an organic film is lower than that in the bulk.
Moreover, in the case of 8, which has a lower Tg (116  C) thanthose of 9 and 10, the direction of the molecular orientation
changes to vertical at a substrate temperature of 110  C without
crystallization in the film. This result shows that we can control
the molecular orientation in vacuum-deposited amorphous films
from horizontal to vertical even at temperatures lower than Tg.
In addition, the controllability of the orientation by substratetemperature depends on the roughness of the substrate surface.47
The control by substrate temperature becomes inefficient whenincreasing the roughness because the roughness prevents themolecules on the surface from migrating locally.
A more detailed analysis of the control during deposition canbe performed by in situ ellipsometry. This technique has been
applied mainly to inorganic layers,52,53 and it had not been
applied to organic layers during vacuum deposition until
recently.45,54,55 An example of a measurement system is shown in
Fig. 12(a) and (b), where an ellipsometer with high response
speed is attached to a deposition chamber.45 The ellipsometricparameters J and D can be measured every several seconds
during deposition. Using this system, it is possible to track theorientation during deposition; for example, we can investigate
the dependence of the orientation on the film thickness.
Although the ellipsometric parameters are measured at a single
incident angle, measuring them many times every several seconds
during deposition increases the reliability of the results.

Fig. 11 Dependence of the optical anisotropies in the films of 8–10 on substrate temperature during deposition. The red and blue lines indicate therefractive indices and extinction coefficients, and the solid and broken lines indicate the horizontal and vertical components of the optical constants,
respectively. (Reprinted with permission from ref. 45; Copyright 2010, American Institute of Physics.)

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Fig. 12 In situ ellipsometry measurement during deposition. (a) Structure of the in situ ellipsometry system to measure ellipsometric parameters during deposition. (b) Photograph of an example of the system. (c)Schematic of the multilayer structure using a single material. (Reprinted
with permission from ref. 45; Copyright 2010, American Institute ofPhysics.)

Furthermore, this system is useful for analyzing the change ofthe orientation in detail. An interesting example is shown in
Fig. 12(c), where the randomly oriented film of a linear-shaped
molecule was deposited at a high substrate temperature, and thehorizontally oriented film was subsequently deposited on it atroom temperature using the same material after cooling it.45 By
in situ ellipsometry, we can nondestructively analyze sucha ‘‘multilayer’’ structure using a single amorphous material bymodeling separated layers of the same material with different
optical properties.

Molecular orientation in organic semiconductor films inevitably affects the charge transport characteristics in the films, suchas via charge-transfer integrals including intermolecular p–p
overlap in the Marcus theory57 and/or energetic and positional
disorders in B€
assler’s formalism.60 Actually, the effects ofmolecular orientation and alignment on electrical properties areoften discussed for other types of organic solids, such as single
crystals,63–65 polymer films,66,67 and liquid crystals.68,69 However,
the effects of the molecular orientation in amorphous films havegenerally not been considered for a long time because themolecular orientation itself has been ignored.
Now, the molecular orientation in vacuum-deposited amorphous organic films is generalized as shown in Section 3.2. Thus,
it is natural to think that the molecular orientation affects theelectrical properties even of amorphous films. Since the horizontal molecular orientation of charge transport materials causes
an increase of p–p overlap between adjacent molecules anda decrease of the positional disorder in the film,70 it improves the
charge transport characteristics of the films. There are roughly
two ways to demonstrate this effect in amorphous films.
One way is to compare the electrical properties of the films of
two similar materials where only the molecular orientations of
the molecules are different. To explain the correlation between
the molecular orientation and the electrical properties, it is

4. Effects of molecular orientation on device
performance
4.1 Electrical properties
The horizontal molecular orientation causes positive changes inboth the electrical and optical properties of OLEDs. First, we
focus on the effects of the horizontal molecular orientation of
charge transport materials on the electrical properties of OLEDs.
The electrical properties of amorphous films have been widely
studied to enhance the performance of OLEDs, and the fundamental mechanisms of carrier injection and transport have been
discussed using simple physical models that have also been used
in inorganic semiconductor physics.2,56 Furthermore, in the study
on organic semiconductor devices, the Marcus theory57–59 andB€assler’s formalism60–62 have often been used to discuss theelectrical properties of the films; the former is used to discuss the
charge transfer between two adjacent molecules in the microscopic view of chemistry, and the latter is used to discuss therandomness of the energy levels and intermolecular distances in
the macroscopic view of physics. However, when these theories
have been used to discuss the properties of vacuum-deposited
amorphous organic films, the molecular orientation or orderingin the films has not been sufficiently considered.
This journal is ª The Royal Society of Chemistry 2011

Fig. 13 Effect of the molecular orientation of oxadiazole electrontransport materials on electron mobility. (a) Anisotropies in the refractive
indices (red) and the extinction coefficients (blue) of the films of OXD7
and Bpy-OXD. The solid and broken lines indicate the horizontal andvertical components of the optical constants, respectively. (b) Schematicsof the enhancement of electron transport by horizontal orientation of
Bpy-OXD molecules. (Reprinted with permission from ref. 71; Copyright
2009, American Institute of Physics.)

J. Mater. Chem., 2011, 21, 19187–19202 | 19195

important to select two materials having similar single-moleculecharacteristics and focus on the difference in the molecularorientation independently of the characteristics of the single
molecules. A typical example is shown in Fig. 13.71 The twomaterials are electron transport materials of oxadiazole derivatives, 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene(OXD7)72 and 1,3-bis[2-(2,20 -bipyridin-6-yl)-1,3,4-oxadiazo-5-yl]
benzene (Bpy-OXD).73,74 The central part of an oxadiazole groupis common to both molecules. The conformation structures, thedistribution of the lowest unoccupied molecular orbitals(LUMOs), and their reorganization energies are also quite
similar. This means that the difference in the electronic structuresof the LUMOs is quite small between the two derivatives.
However, the electron mobility in the vacuum-deposited filmsof Bpy-OXD is 30 times higher than that of OXD7.71 Thisdifference in the electron mobilities cannot be explained by thedifference in the electronic structures of the LUMOs of the single
molecules. It was shown by ellipsometry that the reason is not thedifference in the characteristics of the individual molecules but
the difference in the molecular orientations in the films. The
orientations of these molecules in vacuum-deposited films arequite different because the terminal groups of the molecules aredifferent. Since the OXD7 molecule has the bulky tert-butyl
terminals, the OXD7 film is almost optically isotropic as shownin Fig. 13(a), meaning that the molecular orientation is random
in the film. On the other hand, since the 2,20 -bipyridyl terminalsof Bpy-OXD are completely planar due to the intramolecular C–
H/N interaction, the Bpy-OXD molecule has planar conformation structures. This leads to significantly large anisotropy ofmolecular orientation in the film. The large difference between
ordinary and extraordinary optical constants means that themolecular planes of Bpy-OXD molecules are significantly
oriented parallel to the substrate surface. The planar bipyridyl
terminals of Bpy-OXD do not significantly affect the electronic
structure of the LUMO but contribute largely to the horizontal
molecular orientation in the film. This horizontal orientation of
the Bpy-OXD molecular plane significantly enhances electron

Fig. 14 Effect of molecular orientation of the linear-shaped BSB-Cz
molecules (material 8) on hole and electron mobilities. The orientation
was controlled by the substrate temperature during deposition.
(Reprinted with permission from ref. 47; Copyright 2010, Wiley-VCH.)

19196 | J. Mater. Chem., 2011, 21, 19187–19202

transport as a consequence of the large intermolecular overlap of
the LUMOs, as shown in Fig. 13(b).
The other way to demonstrate the effect of molecular orientation on electrical properties is to compare the electrical properties of films of the same material where the orientation was
controlled during deposition by heating the substrate. This isa more direct investigation of the effects of molecular orientation
because the same material is used for the comparison. An
example is shown in Fig. 14,47 where the molecular orientationsof the linear-shaped BSB-Cz molecules (material 8 in Fig. 5) were
controlled during deposition by heating the ITO substrate, as
shown in Section 3.3. In this case, because the ITO surface wasrougher than the surface of the Si substrate, the change oforientation on the ITO substrate by heating is smaller than thaton the Si substrate at the same temperature (see Fig. 11). Thus,
the orientation on the ITO substrate becomes random by heating
the substrate at 110  C during deposition, whereas the molecules
are horizontally oriented at a substrate temperature of 25  C.
Figure 14 shows the difference in hole and electron mobilities
between the horizontally oriented and randomly oriented films.
Both the hole and electron mobilities in the horizontally oriented
film are higher than those in the randomly oriented film,
demonstrating the significant effect of molecular orientation on
the electrical properties of vacuum-deposited amorphous organic
films.
Since the horizontal molecular orientation occurs even ina multilayer OLED structure as discussed in Section 3.2, theorientation generally affects the electrical properties of OLEDs.
To further understand and improve the electrical properties of
OLEDs, we should always consider that the molecules of chargetransport materials are horizontally oriented in general, and thatthe orientation affects the electrical properties of the films.
4.2 Optical properties
The horizontal orientation of emitting molecules positively
affects the optical properties of OLEDs because the horizontal
orientation of the transition dipole moments enhances the lightoutcoupling efficiency of OLEDs. Since molecules emit lightmainly in the direction perpendicular to its transition dipole
moment, the horizontal molecular orientation is preferable to thevertical orientation for high light outcoupling efficiency of
OLEDs. This is schematically shown in Fig. 15(a), where theemitted light from vertically oriented molecules is almost
confined inside the device due to the total reflection at thetransparent electrode/glass or glass/air interface. The horizontal
molecular orientation leads to a decrease of the number ofvertically oriented molecules that cannot contribute to the lightoutcoupling, resulting in the enhancement of light outcoupling
efficiency of OLEDs without a significant change of the angular
distribution of the emission. This effect has already been discussed in studies of polymer OLEDs,75 where the molecularorientation of emitting polymers is a matter of course. It wasestimated that the light outcoupling efficiency of polymer
OLEDs with horizontally oriented emitters can be more than 1.5
times higher than that with randomly oriented emitters.75,76
Since the emitting molecules can be horizontally oriented evenin vacuum-deposited amorphous films, it is possible to enhance
the light outcoupling efficiency of small-molecule OLEDs using
This journal is ª The Royal Society of Chemistry 2011

This effect on the light outcoupling efficiency is critical todiscussing the upper limit of the performance of OLEDs. It also
provides us with valuable guideline for developing a new emitting
material for high OLED performance. Using the horizontal
orientation of emitting molecules, we can expect a light outcoupling efficiency of more than 30%, which is much higher thanthe value of around 20% that has been thought to be
typical.44,80,81 Since the EQE of OLEDs is currently reaching thetheoretical limit, while the maximum internal quantum efficiency
is almost unity,82,83 the enhancement of the light outcoupling
efficiency without any complicated outcoupling structure80,81,84–88
is especially important to improve the performance while keeping
the fabrication process simple. In addition, to achieve extremely
high EQEs of OLEDs, horizontal orientations of both fluorescent and phosphorescent emitters are very desirable. Most
recently, horizontally oriented phosphorescent emitters werediscussed and carefully demonstrated89 by the combining angulardependence measurement of PL and electroluminescence spectra
and optical simulation.90,91

5. Further active control by intermolecularinteractions
Fig. 15 Effect of the horizontal orientation of emitting molecules. (a)Schematic of the enhancement of outcoupling efficiency of OLEDs by
horizontal orientation of the transition dipole moment of emitting
molecules. Since the light is emitted mainly in the direction vertical to thetransition dipole moment, the horizontal orientation is preferable forhigh outcoupling efficiency. (b) Chemical structures of BDAVBi and
PEBA used for the comparison of horizontally and randomly oriented
emitters. The arrows roughly indicate the direction of the transition
dipole moment.

emitting materials having an anisotropic molecular shapecausing horizontal orientation. As was mentioned in Section 3.2,
the fact that the horizontal orientation can occur even in a doped
film is very important because the emitting molecules in OLEDs
are usually doped in the host materials to avoid concentrationquenching.
The effect of the horizontal orientation on the light outcoupling of small-molecule OLEDs was shown experimentally bycomparing two blue-emitting OLEDs with horizontally andrandomly oriented emitters.77 In this comparison, the horizontally oriented emitter was a linear-shaped molecule of 4,40 -bis[4(diphenylamino)styryl]biphenyl (BDAVBi),78 whose transition
dipole moment is highly oriented to the horizontal direction in
doped emitting layer having CBP host matrix in OLEDs,41 andthe randomly oriented emitter was a rather compact molecule of
4-(2,2-diphenylethenyl)-N,N-bis(4-methylphenyl)benzenamine(PEBA),79 as shown in Fig. 15(b). The PL quantum efficiencies
and PL spectra of their doped films were not very different, andthe electrical properties of the OLEDs using the doped films as
the emitting layer were also quite similar, but the external
quantum efficiencies (EQEs) of the OLEDs were significantly
different because of the difference in the molecular orientation of
the emitting molecules. The light outcoupling efficiency of the
OLEDs with the BDAVBi emitter was estimated to be 1.45 times
higher than those of the OLEDs with PEBA emitters, indicating
the significant effect of the horizontal orientation of the emitting
molecules on the light outcoupling efficiency of OLEDs.
This journal is ª The Royal Society of Chemistry 2011

Intermolecular interaction between organic molecules92 is one of
the most vital characteristics of organic materials whencompared with inorganic materials. In particular, the variety ofhydrogen bonds93,94 plays an essential role in controlling higherorder structures in both chemistry and biology and has also
contributed to maximizing the functionalities of organic materials, which cannot be achieved using inorganic materials.95
Thus, using intermolecular interactions more actively to control
molecular orientation and ordering96–98 seems to be a promisingnext step in improving device performance (see Fig. 1).
Recently, we showed that intermolecular C–H/N hydrogenbonds can be formed even in vacuum-deposited films having
a flat interface, and that the molecular stacking induced by thesebonds improves carrier mobility.14 In Fig. 16(a), four electrontransport materials named B0, B2–B4PyMPM are shown, whichwere used to investigate the effects of intermolecular hydrogenbonds in vacuum-deposited organic films. Some of them wereused as electron transport materials in extremely high-performance OLEDs.99
The analysis by VASE was performed to study the opticalanisotropies in the vacuum-deposited films of these materials,
and only the films of B3 and B4PyMPM were found to havesingularly large optical anisotropies as shown in Fig. 16(a).
Although the molecular shapes of these four materials are quite
similar, a singular difference in optical anisotropies was observed
depending on the position of the nitrogen atoms in the pyridine
rings. The much larger ordinary (horizontal) components of therefractive indices and extinction coefficients of the B3 andB4PyMPM films mean that the molecular planes of these molecules are nearly parallel to the substrate surface. Then, theabsorptions of the vertically incident light by the B3 andB4PyMPM films were higher than those by the B0 and
B2PyMPM films, though absorptions by solutions with the same
concentrations were almost the same for all four materials, as
shown in Fig. 16(b). This shows that the transition dipole
moments of the B3 and B4PyMPM molecules are horizontally
J. Mater. Chem., 2011, 21, 19187–19202 | 19197

Fig. 16 Singularly large anisotropies in B3 and B4PyMPM films by molecular stacking. (a) Refractive indices (red) and extinction coefficients (blue) of
B0PyMPM and B2–B4PyMPM films. The solid and broken lines indicate the horizontal and vertical components of the optical constants, respectively.
(b) Absorption spectra of 100-nm deposited thin films and solutions with a concentration of 105 M of B0PyMPM and B2–B4PyMPM. (c) Out-of-plane
XRD patterns of B0PyMPM, B2–B4PyMPM, and a-6T films. The patterns of B3 and B4PyMPM have a halo band, whose peak angle shows the
distribution center of the distance of the molecular stacking (red arrow). (Reprinted with permission from ref. 14; Copyright 2011, Wiley-VCH.)

oriented in the films. Furthermore, the out-of-plane XRD
patterns of the films suggest the formation of short-range orderin the direction of thickness in the B3 and B4PyMPM films.
Figure 16(c) shows the out-of-plane XRD patterns of the
B0PyMPM, B2–B4PyMPM, and a-6T films, where the a-6T film
was used as a reference to show the pattern of a polycrystalline
film having long-range order.26 The patterns of the B3 andB4PyMPM films have a broad halo peak around 2q ¼ 23 , which
 with short-range ordershows a periodic structure (3.8 A)
formed in the direction of thickness. This shows that the singularhorizontal orientation of the B3 and B4PyMPM molecules
causes the molecular stacking in the films.
The origin of the singular orientation in the B3 and B4PyMPM
films is the intermolecular C–H/N hydrogen bonds between
pyridine rings of adjacent molecules.14 Infrared (IR) absorption
spectra of the B2–B4PyMPM films, shown in Fig. 17(a), areinformative for investigating and discussing this origin. Because
the nitrogen atoms in the pyridine rings in B3 and B4PyMPM areat the outer side of the molecule, they can be connected byintermolecular C–H/N hydrogen bonds as shown in Fig. 17(b),causing a blue-shift of the IR absorption frequency of the C–Hstretching modes100–103 at the neighbor of the nitrogen atoms inthe spectra. These spectral shifts show that the intermolecular
C–H/N hydrogen bonds in the B3 and B4PyMPM films induce
molecular orientation and stacking. Similar intermolecular
C–H/N hydrogen bonds and subsequent two-dimensional
networks were also directly observed by scanning tunnelling
microscope measurement using similar molecules,104–108 though
the networks were a monolayer at the interface between a solution and a graphite surface.
The molecular stacking induced by the intermolecularhydrogen bonds enhances the electron transport in the films.
19198 | J. Mater. Chem., 2011, 21, 19187–19202

Fig. 17 Intermolecular C–H/N hydrogen bonds in films. (a) Spectral
blue-shift of IR absorption by the formation of intermolecular C–H/N
hydrogen bonds in the B3 and B4PyMPM films. (b) Examples of possible
interactions between B3PyMPM molecules and between B4PyMPM
molecules by intermolecular C–H/N hydrogen bonds. (Reprinted withpermission from ref. 14; Copyright 2011, Wiley-VCH.)

Figure 18(a) shows that the electron mobilities of these materials
significantly depend on the positions of the nitrogen atoms in thepyridine rings. The electron mobilities at an electric field of 6.4 
105 V cm1 are 1.6  106, 1.5  105, and 1.0  104 cm2 V1 s1for B2, B3, and B4PyMPM films, respectively, and the energetic
disorders s and the positional disorders S in these films are in theorder sB2 z sB3 > sB4 and SB2 > SB3 z SB4.109
Generally, the energetic disorder in a film increases with anincrease in the local field caused by the permanent dipole
This journal is ª The Royal Society of Chemistry 2011

Fig. 18 Effects of molecular orientation by intermolecular hydrogenbonds on electron mobility. (a) Dependences of electron mobilities of B2–
B4PyMPM films on electric field obtained by time-of-flight measurements at 298 K. (b) Schematic of the effects of molecular orientation andthe permanent dipole moment on the electrical properties of the B2–
B4PyMPM devices. The B3 and B4PyMPM molecules are bound by theintermolecular hydrogen bonds and form the stacking structure in thefilms. The arrows in the molecules indicate the large permanent dipole
moments of B2 and B3PyMPM. (Reprinted with permission from ref. 14;
Copyright 2011, Wiley-VCH.)

deposition, the molecular orientation can be controlled evenunder the glass transition temperature of the material. We havealso demonstrated that the horizontal orientation can improve
both the electrical and optical properties of OLEDs. The horizontal molecular orientation of charge transport materials
enhances the charge transport in the films, and the horizontal
molecular orientation of emitting molecules enhances the lightoutcoupling efficiency of OLEDs. Furthermore, we have shownthat the molecular orientation can be controlled more actively
using intermolecular C–H/N hydrogen bonds to improve theelectrical properties of films. These results will be helpful inunderstanding device physics and developing new materials forfurther improving the performance of OLEDs and other organic
devices.
Although the various properties of the molecular orientationin OLEDs have been found and studied, there remain many
challenging issues for the future. To construct reliable fundamentals of organic devices, it is very important to clarify thedetailed structure and ordering of the molecules in the films andtheir effect on the electrical and optical properties of devices. The
followings are the target issues to be solved next.
(1)

moments of the molecules, and the positional disorder increases
as the packing constraints between the molecules weaken.70
Thus, because the B4PyMPM molecules have smaller permanent
dipole moments than those of the B2 and B3PyMPM molecules,14 the energetic disorder in the B4PyMPM film is less thanthose in the B2 and B3PyMPM films. Then, because the B3 andB4PyMPM molecules are horizontally oriented in the films,
the positional disorders in these films are less than that in the
B2PyMPM film. These differences in the disorders lead to thesignificant difference in the electron mobilities m in the B2–
B4PyMPM films with the order mB2 < mB3 < mB4, as schematicallyshown in Fig. 18(b). This result shows the significant effects ofboth the molecular orientation and the permanent dipole
moment on the electrical characteristics of OLEDs. It also showsthat we can control the molecular stacking via the intermolecularhydrogen bonds even in vacuum-deposited organic films to
improve the performance of OLEDs. Furthermore, this result
provides us with a new strategy and much insight for developing
new materials for construction of an internal self-assembly
structure in vacuum-deposited organic films.

6. Summary and outlook
In this article, the studies on horizontal molecular orientation in
OLEDs have been briefly reviewed. Starting with the firstdetection of the molecular orientation of fluorene derivatives, themolecular orientation of various kinds of OLED materials havebeen investigated systematically. We have demonstrated thegeneral relationship between the anisotropy of molecular structures and the horizontal molecular orientation; the larger the
structural anisotropy is, the more significant the horizontal
molecular orientation becomes. For applications to OLEDs, it isalso noteworthy that the orientation occurs on any underlying
layer and even in doped films and is almost homogeneous in the
direction of thickness. By heating the substrate during
This journal is ª The Royal Society of Chemistry 2011

Orientation of functional groups in molecules

As shown in this article, the molecular orientation in OLEDs has
been investigated by VASE using UV and visible light, but we
can only know the anisotropies of the molecular axis roughly.
Although the anisotropies of the molecular polarizability andtransition dipole moment can be discussed from the results of
VASE in the UV and visible region as shown in Section 3.1, theorientation of each functional group in the molecules has notbeen the focus of sufficient discussion. When discussing the effectof the orientation of emitting molecules on the light outcouplingof OLEDs, we only have to know the orientation of the transition dipole moment of the emitting molecules. However, when
we want to elucidate the effect of the molecular orientation on
the electrical properties of the film at the microscopic level, we
need to study deeply the orientation not only of the molecularaxis but also of functional groups in the molecules. Such aninvestigation also leads to the elucidation of the difference in themolecular structures in free space and vacuum-deposited amorphous films. One of the methods to solve this issue is analyzing
the orientation of the vibrational modes of functional groups
using IR light.110–112 Since IR spectra are not sensitive to thedifference in conformation structures,14 we can discuss theorientation of functional groups without bothering about
the variety of the conformation structures. Now, we are investigating the orientation of functional groups of OLED materials
using IR-VASE,16,113–115 where an IR light source and an interferometer are attached to a VASE system.
(2)

Effect on charge injection at interfaces

It is reasonable to believe that the horizontal molecular orientation affects not only the charge transport in the bulk of films
but also the charge injection at an electrode/organic or organic/organic interface as schematically shown in Fig. 19(a), because
the orientation affects the overlap of the wave functions at theinterface.116 Although ellipsometry cannot detect the orientation
J. Mater. Chem., 2011, 21, 19187–19202 | 19199

(4) Effect of optical anisotropy on the optical properties ofdevicesFig. 19 Target issues to be solved in future. Schematics are shown as
guides for understanding of the issues discussed in Section 6. (a) Effect on
charge injection at interfaces. (b) Advantage of horizontal orientation in
OPVs. (c) Index ellipsoid in anisotropic films. (d) Further molecularalignment and ordering as in single crystals.

In this article, two kinds of effects on device performance werediscussed. One is the effect of the orientation of charge transportmaterials on the electrical properties of films, and the other is theeffect of the orientation of emitting materials on the opticalproperties of OLEDs. However, the effect of the orientation of
charge transport materials on the optical properties of OLEDs
has not been discussed sufficiently. To consider this effect, it
should be noted that large optical anisotropy of the chargetransport layer affects the light propagation in the layer.31 The
refractive index for light polarized parallel to a substrate surface[transverse electric (TE)-polarized light] is equal to no, but thatfor light with the polarization plane perpendicular to a substratesurface [transverse magnetic (TM)-polarized light] depends onthe direction of the wave vector. As shown in Fig. 19(c), this isrepresented by the equation of the index ellipsoid,121nTE ¼ no1
nTM ðfÞ

only at the interface, such as in a monolayer, we can expect
analysis by other methods to explain and clarify the effect atinterfaces. Since the effects of molecular orientation of polycrystalline materials at interfaces have been investigated,117,118 weshould extend their methods and discussions to the orientation of
amorphous materials at interfaces in OLEDs.
(3) Application to other organic semiconductor devices
Molecular orientation is commonly one of the critical viewpoints
in research on organic devices other than OLEDs, such asorganic thin-film photovoltaics (OPVs) or organic field-effecttransistors (OFETs). From the electrical viewpoint, the horizontal orientation is preferable in OLEDs and OPVs for chargetransport in the direction of thickness, and the vertical orientation is preferable in OFETs for lateral charge transport. From
the optical viewpoint, the horizontal orientation is also preferable for both OLEDs and OPVs. As in the case of OLEDs where
the light outcoupling is enhanced by the horizontal orientation,
the absorption of vertically incident light by OPVs is enhancedby the horizontal orientation of absorbing materials because thetransition dipole moments of absorbing molecules are parallel tothe electric field of the incident light as shown in Fig. 19(b). The
high absorption makes it possible to absorb sufficient light withsmall thickness, which can compensate for the short exciton
diffusion length and the low mobility of OPV materials. Some
amorphous OPV materials having a linear or planar shape wererecently reported,119,120 and they can be expected to be oriented
horizontally as OLED materials. Although the carrier mobilities
in amorphous films are generally lower than those in polycrystalline films which are usually used in OPVs, the horizontal
orientation in amorphous films is optically better than verticalorientation of polycrystalline materials.26–28 In addition, it ishighly possible that smooth interfaces of amorphous films
without any grain boundary improve fill factors of OPVs.119,120
The elucidation of the effects in OPVs is a promising theme inresearch on organic devices.
19200 | J. Mater. Chem., 2011, 21, 19187–19202

2¼

cos2 f sin2 f
þ
no 2
ne 2

(2)
(3)

where nTE and nTM are the refractive indices for TE- and TMpolarized light, respectively, and f is the angle between the
direction normal to the substrate plane and the wave vector of
the light. This effect requires modifications to the simulation oflight propagation in both OLEDs and OPVs. When the opticalanisotropy of a layer in a device is significant, we should include
this effect in the optical simulation to estimate precisely the lightoutcoupling efficiency of OLEDs and the light absorption efficiency of OPVs. It should also be noted that the large opticalanisotropy by the horizontal orientation leads to the significantly
high refractive index for the TE-polarized light, which is preferable for optimizing the interference effect in OLEDs and OPVs
using thin layers, as mentioned previously.14(5)Further alignment of molecules by simple fabrication

Although the horizontal molecular orientation affects the device
performance positively, the orientation in the plane is random asshown in Fig. 19(d). This means that the molecular orientation in
the films is not perfect. It is, of course, preferable to align themolecular axis only in one direction without defects in the film as
in single crystals of organic semiconductor materials. Actually,
the carrier mobility in single crystals is much higher than that of
vacuum-deposited amorphous films.63–65 It was also reported
that OLEDs fabricated using single crystals showed highperformance,122 though the size of the device is limited due to thedifficulty of fabricating a thin single crystal with a large area.
What is important for wide application in the future is to fabricate such a high-performance OLED with a large area bya simple process. The understanding and use of the intermolecular interaction seems essential for it. By deriving the maximum
potential of molecular interaction of organic semiconductormaterials, this challenge will be successfully overcome in future.
By solving the above issues, the performance of organic
devices will be further improved, and the fundamentals ofThis journal is ª The Royal Society of Chemistry 2011

organic devices will be much more sophisticated from the viewpoint of both chemistry and physics, as with silicon devices. We
believe that such fundamental studies in organic semiconductor
devices are critical for making them widely available in our daily
lives in the near future.

Acknowledgements
The author thanks all the coauthors of the publications cited inthis article. The author also gratefully acknowledges financial
support from the Promotion of Environmental Improvement
for Independence of Young Researchers Program of the
Ministry of Education, Culture, Sports, Science and Technology, Japan.

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