Journal of

Materials Chemistry C
PAPER

Cite this: J. Mater. Chem. C, 2015,
3, 11178

Advantages and disadvantages of vacuum-depositedand spin-coated amorphous organic semiconductor
films for organic light-emitting diodes†
Maki Shibata,a Yoshiya Sakaib and Daisuke Yokoyama*abc
To realize low-cost fabrication processes for high-performance organic light-emitting diode (OLED) display
and lighting, it has recently become important to understand the properties and structures of solutionprocessed amorphous films. In particular, to choose an appropriate process to produce OLEDs to meetthe demands of a realistic situation, it is necessary to know the general advantages and disadvantages of
vacuum- and solution-processed films quantitatively. However, the diﬀerences between the films formedby these processes are not yet suﬃciently clear. Here we systematically compare vacuum-deposited andspin-coated amorphous organic semiconductor films used for OLEDs mainly by ellipsometry, anddemonstrate the critical diﬀerences in film density, transition temperature, and molecular orientation. We
found that the film density, transition temperature, and degree of horizontal molecular orientation ofsmall-molecule spin-coated films are inherently lower than those of the corresponding vacuum-deposited

Received 27th June 2015,
Accepted 24th September 2015
DOI: 10.1039/c5tc01911g

ones. In addition, we show that the transition temperature and molecular orientation of small-molecule
spin-coated films of glassy materials are identical to those of ‘‘deteriorated’’ vacuum-deposited films thathave experienced a transition induced by heating. Our comprehensive comparisons lead to guidelines forselecting suitable processes and materials for production and clarify the future challenges to be addressedwww.rsc.org/MaterialsC

to facilitate the development of high-performance solution-processed OLEDs.

1. Introduction
Research on organic light-emitting diodes (OLEDs), which arepromising optoelectronic devices for display and lighting, hasgrown continuously with the development of amorphousorganic semiconductor materials. Since the first report of
OLEDs using thin films,1 amorphous organic films have beenused in most OLEDs mainly because of their smooth surface,which ensures the high uniformity and stability of devices
under a high electric field. As the performance of OLEDs has
been improved to a high level to allow wide commercialization,it has become important to obtain a much deeper understandingof the higher-order structures in the amorphous films to controla

Department of Organic Device Engineering, Yamagata University, 4-3-16 Jonan,
Yonezawa, Yamagata 992-8510, Japan. E-mail: d_yokoyama@yz.yamagata-u.ac.jp;
Fax: +81-238-26-3890; Tel: +81-238-26-3890
bDepartment of Polymer Science and Engineering, Yamagata University,
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japanc
Research Center for Organic Electronics (ROEL), Yamagata University,
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan† Electronic supplementary information (ESI) available: UV-vis absorption, XRR,
ellipsometry, XRD, and surface morphology results for sample films, along withexperimental conditions for sample fabrication. See DOI: 10.1039/c5tc01911g

11178 | J. Mater. Chem. C, 2015, 3, 11178--11191

and further improve OLED performance,2 even though therandomness in amorphous films makes this task diﬃcult.
Amorphous films for OLEDs can be roughly categorized into
two groups depending on their fabrication process: vacuumprocessed films, and solution-processed films. In addition,
solution-processed films can be further divided into two groups
with respect to their molecular size: small-molecule solutionprocessed films, and polymer solution-processed films. Most
commercialized OLEDs are currently based on vacuum-processedfilms, because they enable us to easily fabricate ideal multilayer
structures with very high purity. Meanwhile, the use of solutionprocessed films has recently drawn increasing attention to realize
low-cost fabrication of large-area printable OLEDs with highperformance.3 The films in the above three categories haveadvantages and disadvantages when compared to each other.
Thus, when we choose an appropriate process and materials to
produce OLED displays or lighting to meet the demands of arealistic situation, it is very important to quantitatively understandthe general advantages and disadvantages of these films.
However, the diﬀerences between small-molecule vacuum- andsolution-processed films, and also between small-molecule andpolymer solution-processed films, have not been systematically
discussed from the viewpoint of the higher-order structures ofthe molecules in the amorphous films. Although the diﬀerences

This journal is © The Royal Society of Chemistry 2015

Paperbetween small-molecule vacuum- and solution-processed filmsof some OLED materials have been reported,4–9 they were mainly
for a single material or device, and fundamental discussion of
the differences based on comprehensive investigation has notbeen carried out sufficiently. In addition, in some cases, wrong
conclusions have been drawn, probably because of poor accuracyof analysis or misinterpretation of the experimental results.
To consider the diﬀerences between small-molecule vacuum- andsolution-processed films, the recent studies on vacuum-processedfilms reported by Ediger and co-workers are very informative.
They reported a relatively high density and specific stability of
vacuum-deposited glasses of some organic materials comparedwith those of the corresponding ordinary glasses commonly
formed from supercooled liquids.10–14 Ishii and co-workers also
investigated the specific behaviour of vacuum-deposited organic
glasses and discussed their densities.15,16 The reports by thesegroups show the distinguished nature of vacuum-processedfilms, which inspired us to investigate the critical differences
between vacuum- and solution-processed films.
In this study, we systematically investigate the diﬀerences
between the physical properties of small-molecule vacuumdeposited and spin-coated films used for OLEDs by ellipsometry,ultraviolet-visible (UV-vis) absorption and X-ray reflectivity (XRR)
measurements. In particular, we focus on the diﬀerences in thethree factors of the films: (i) film density (with absolute values),(ii) transition temperature, and (iii) molecular orientation. As
shown in Fig. 1, these factors are important to discuss thecharacteristics of devices, because (i) the film density aﬀects
the overlap of molecular orbitals, which can aﬀect charge transport and energy transfer between two adjacent molecules;(ii) the transition temperature is directly related to the thermal
durability of films and devices;17 and (iii) the molecular orientationstrongly aﬀects both the electrical and optical characteristics ofdevices.2 We found that the film density, transition temperature,and degree of horizontal molecular orientation of small-molecule
spin-coated films are inherently lower than those of vacuumdeposited films. We also show that the molecular orientationand transition temperature of small-molecule spin-coated filmsof glassy materials are identical to those of the ‘‘deteriorated’’vacuum-deposited films that have experienced a transition
induced by heating. In addition, we compare the transition

Fig. 1 Schematic diagram outlining the importance of the film density,
molecular orientation, and transition temperature of amorphous organic
semiconductor films in OLEDs.

This journal is © The Royal Society of Chemistry 2015

Journal of Materials Chemistry C

temperatures and molecular orientations of small-moleculeand polymer spin-coated films with similar molecular units
and confirm that the transition temperature and degree ofhorizontal molecular orientation of the former are lower thanthose of the latter. Our results reveal some general disadvantagesof the higher-order structures in small-molecule spin-coatedfilms at present and lead to guidelines for choosing a process
or materials for device production. Our systematic comparisonsand related discussions also clarify the future challenges to beaddressed to facilitate the development of high-performancesolution-processed OLEDs.

2. Results and discussion
2.1

Film density

We used three methods to accurately determine the absolutedensities of vacuum-deposited and spin-coated films. The firstwas UV-vis absorption measurements of solutions of dissolved
films.18,19 From the absorbance of a solution of a dissolved filmwith a definite film volume, we determined the absolute density
averaged in the overall film by comparison with the absorbance ofreference solutions. The second method was XRR measurement.20
From the critical angle for an organic film on a substrate obtained
from its XRR pattern, we determined the absolute density at thefilm surface (to a depth of a few nanometers).21,22 These two
methods can determine the absolute density of films, but are notsuitable for all films; they both need a highly uniform film with alarge area, and are time-consuming when there are many samples.
In UV-vis absorption measurements, it is necessary to prepare
sample films with high uniformity over a large area to precisely
define the film volume. In addition, the dissolved solution andreference solutions need to be prepared very carefully to minimize
concentration errors. XRR measurements also require uniform
sample films with a large area. Because the critical angle for organic
films in XRR patterns is generally very small, uniform large films
are needed to suﬃciently shine the X-rays with such a small grazing
incident angle onto the sample film. Thus, these two methods were
too diﬃcult to apply to the numerous sample films (4100) in thisstudy, especially to the spin-coated films, which are not as uniform
as the vacuum-deposited ones.
The third method is ellipsometry, which enables us toanalyze films quickly, and only needs a small uniform area ofa few millimeters square. However, ellipsometry can only
determine relative densities. From the refractive index of a filmin the transparent spectral region, relative densities can be
determined using the Lorentz–Lorenz equation (see Section 4.4
for details). By combining the relative densities of all films of amaterial determined by ellipsometry with the absolute densities
of a few films of the material determined by UV-vis absorption
and XRR measurements, we can determine the absolute densitiesof all films of the material.
To compare the densities of vacuum-deposited and spin-coatedfilms, we first used four common OLED materials shown on theleft side of Fig. 2: 4,4 0 -bis[N-( p-tolyl)-N-phenylamino]biphenyl(TPD), 4,4 0 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl (a-NPD),

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Fig. 2Paper

Amorphous OLED materials used in this study.

4,40 -bis(N-carbazole)biphenyl (CBP), and tris(8-hydroxyquinoline)aluminum (Alq3). We selected these materials for the analysis ofdensity, because their birefringences in the transparent spectral
region are small23,24 enough to simplify the XRR and ellipsometry
analyses using isotropic optical models. Fig. 3(a) shows the averagedensities of the vacuum-deposited TPD films obtained from UV-vis
absorption measurements of solutions of dissolved films withthicknesses of B50, 100, 150, and 200 nm (see Fig. S1(a), ESI†for more details). The average density of vacuum-deposited TPD

Fig. 3 (a) Densities averaged in the overall thickness of vacuumdeposited TPD films determined by absorption measurements of solutionsof the dissolved films. (b) XRR pattern of a vacuum-deposited TPD film witha thickness of 330 nm on a Si(100) substrate. The theoretical curve for adensity of 1.18 g cm3 is also shown.

Table 1

films with a thickness of 450 nm was 1.14  0.02 g cm3.
Meanwhile, the density at the surface of a vacuum-deposited TPD
film with a thickness of 330 nm on a Si(100) substrate wasdetermined by XRR measurements (Fig. 3(b)). Fitting analysis using
theoretical curves indicated that the density at the surface was
1.18  0.04 g cm3, which is slightly higher than the averagedensity, though the diﬀerence is within the error. Table 1
summarizes the average and surface densities of vacuumdeposited films of each material (detailed data are provided inFig. S1(a)–(d) and S2, ESI†). For all of the four materials, thesurface densities of vacuum-deposited films (values in parentheses)
are slightly higher than their average densities. We think that
this is related to the time for surface diffusion of molecules.
Molecules buried in the bulk of a film do not have much time forsurface diffusion enough to find a sufficiently stable state as
those on the surface because of the deposition of successive
molecules overlaying onto them. However, we can assume that
this high-density region at the film surface is not thick because
the average densities for the overall films do not decrease
apparently with increasing film thickness. Thus, hereafter wewill mainly discuss the average densities determined from UV-vis
absorption measurements of the solutions of dissolved films.
These values can be directly associated with the relative densities
determined by ellipsometry measurements, which are opticallyaveraged in overall films.
Compared to the densities of the vacuum-deposited films,
those of the corresponding spin-coated films were lower for all
four materials. Fig. 4 shows the thicknesses and densities ofspin-coated TPD films fabricated on Si(100) substrates using
chloroform as the solvent with diﬀerent spin speeds andsolution concentrations. The absolute values of the densitiesof the spin-coated films were relatively determined from thoseof the vacuum-deposited films and the diﬀerences between therelative densities of vacuum-deposited and spin-coated films.
In general, the slower the spin speed is and also the higher thesolution concentration is, the thicker the spin-coated film
becomes. The densities of the spin-coated films converged toa value of 1.12  0.02 g cm3, which is lower than that of thevacuum-deposited films, when the film thickness exceeded
18 nm. The densities of very thin (o10 nm) films wereseemingly much lower than this value because such thin films
are not uniform on Si substrates, so the values are a mixture of
the densities of the material and air. We also performed thesame experiments for the other three materials, resulting in

Absolute densities and packing coeﬃcient Kp of single crystals, vacuum-deposited films, and spin-coated films of the materials

Materials

TPD

a-NPD

CBP

Alq3

a-ONPD

2-TNATA

TPT1

TSBF

BDAVBi

DensityaSingle crystalb
Vacuum-depositedc
Spin-coated

1.21
1.14 (1.18)
1.12—
1.17 (1.19)
1.14

1.31
1.17 (1.21)
1.13

1.42
1.30 (1.34)
1.28

—
1.16
1.15

—
1.17
1.15

—
1.15
1.13

—
1.16
1.14

—
1.18
1.17

Kp

Single crystalb
Vacuum-deposited
Spin-coated

0.68
0.66
0.65

—
0.67
0.65

0.70
0.65
0.63

0.69
0.66
0.65

—
0.69
0.68

—
0.66
0.65

—
0.65
0.64

—
0.64
0.63

—
0.68
0.68

a
The unit of all densities is g cm3. b The values were obtained using the crystallographic data in ref. 25–27. c The values in parentheses are
densities at the film surface determined by XRR measurements.

11180 | J. Mater. Chem. C, 2015, 3, 11178--11191

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Fig. 4 (a) Thicknesses and (b) film densities of spin-coated TPD films fabricated using chloroform as the solvent with diﬀerent spin speeds and solution
concentrations.

qualitatively similar trends (see Fig. S3(a)–(c), ESI†). The absolutedensities of the spin-coated films fabricated using chloroform
are also summarized in Table 1. The densities of the spin-coatedfilms of all these materials were lower than those of the vacuumdeposited films.
To investigate the dependence of the film density on thesolvent, we further determined the densities of spin-coated TPD
films fabricated using other many kinds of solvents having
diﬀerent boiling points: 1,4-dioxane (101 1C), toluene (111 1C),butylacetate (126 1C), cyclopentanone (131 1C), chlorobenzene(132 1C) and o-dichlorobenzene (180 1C), which all have boilingpoints that are higher than that of chloroform (61 1C). Because
the boiling point of a solvent aﬀects how quickly the solvent isvaporized during the spin-coating process, we considered thepossibility that the solvent might also aﬀect the density offilms. However, as shown in Fig. 5, the densities of the films
fabricated using these solvents were almost the same as thoseof the films fabricated using chloroform when the film thickness was 430 nm (see Fig. S4, ESI† for more details). Thissuggests that the dependence of the densities of spin-coatedfilms on the solvent is small, and that spin-coated films
generally have lower densities than vacuum-deposited ones.
In addition to the densities of the films, Table 1 also shows theFig. 5 Densities of spin-coated TPD films fabricated using diﬀerent solvents,
spin speeds, and solution concentrations. The densities of vacuum-depositedfilms with diﬀerent thicknesses are also shown for comparison.

This journal is © The Royal Society of Chemistry 2015

densities of single crystals of three materials, which were
calculated from the crystallographic data.25–27 From these results,
we conclude that the densities of the solids of small-molecule
amorphous OLED materials have the order of single crystal 4
vacuum-deposited film 4 spin-coated film.
We also estimated the packing coeﬃcient Kp,28 which represents how densely molecules are packed in each solid, using
the densities obtained above and the following equation:
Kp ¼

VvdW rNA VvdW
;
¼
Vm
M

(1)

where VvdW is van der Waals volume of a molecule,29 Vm ismolecular volume (the volume per molecule), r is density of thesolid, NA is the Avogadro constant, and M is molecular weight.
The value of 1  Kp indicates the void in the solid. Here thevalues of VvdW in vacuum-deposited and spin-coated films were
estimated using the optimized molecular structures obtainedby density functional theory (DFT) B3LYP/6-31G(d) calculations.30 The results are summarized in Table 1, showing thatthe packing coeﬃcients of vacuum-deposited and spin-coatedamorphous films are not highly dependent on materials.
The absorption spectra of solid films on fused silica substrates
also show definite diﬀerences between vacuum-deposited andspin-coated films. Fig. 6 shows the UV-vis absorption spectra of
vacuum-deposited and spin-coated TPD films on fused silica
substrates. To accurately and fairly compare the spectra, wefabricated sample films with approximately the same thicknessof 53  4 nm. We used the similar thickness to unify theinterference eﬀect on spectra in absorption measurements
using a spectrophotometer, which can substantially change
the spectral shape (see Fig. S5, ESI†). The absorption spectraof the four spin-coated films fabricated using diﬀerent foursolvents were the same, but diﬀered from that of the vacuumdeposited film. The diﬀerence between the absorption spectraof the vacuum-deposited and spin-coated films would originate
from slight diﬀerences in molecular orientation and the overlap of electronic orbitals of adjacent molecules. A similar
diﬀerence was also observed for a-NPD (see Fig. S6, ESI†).
The fact that the absorption spectra of the spin-coated films
are independent of the solvent suggests that they are macroscopically identical.

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Fig. 6 Normalized absorption spectra of spin-coated TPD films fabricated using four diﬀerent solvents and a vacuum-deposited TPD film. Allfilms had approximately the same thickness of 53  4 nm: 49.6 nm(chloroform), 51.1 nm (toluene), 53.8 nm (chlorobenzene), 56.3 nm(o-dichlorobenzene), and 52.3 nm (vacuum-deposited).

In addition, we determined the densities of vacuum-depositedand spin-coated films of 4-[N-(1-naphthyl)-N-phenylamino]-40 -[N-(1naphthyl)-N-(4-octylphenyl)amino] biphenyl (a-ONPD, see Fig. 2) toinvestigate the eﬀect of the alkyl side chain attached to a-NPD forhigh solubility. As shown in Table 1 (see also Fig. S1(e), ESI† formore details), the diﬀerence between the densities of vacuumdeposited and spin-coated a-ONPD films is smaller than that for
a-NPD films, though the vacuum-deposited films still have aslightly higher density than that of the spin-coated ones. This isprobably because the void is filled by the thin octyl chains.
However, this result does not mean that spin-coated films of amaterial with an alkyl chain have better properties than those
without it. If we exclude the volume of the octyl group from VvdW
in eqn (1), the Kp values of the vacuum-deposited and spincoated a-ONPD films are 0.56 and 0.55, respectively. Because thealkyl groups are nonconductive, these are the substantive Kp
values of a-ONPD as a semiconducting material.
2.2

Transition temperature

To understand the diﬀerence in the density of films fabricated by
diﬀerent methods, we investigated the temperature dependenceof film density. Using an ellipsometer with a temperature-control
stage, we performed in situ ellipsometry measurements andtraced the change of the densities of vacuum-deposited andspin-coated films during heating–cooling cycles.
Fig. 7 shows the variations in the density of vacuum-depositedand spin-coated TPD films with a thickness of B70 nm withtemperature. We heated each sample at a rate of 3 1C min1 up
to 100 1C, which is much higher than the glass transition
temperature of 60 1C of the TPD powder,17 kept the sample at
100 1C for 30 min, and then naturally cooled it to 30 1C. We
repeated this cycle twice for each sample to clarify what occurs in
the first cycle. When the vacuum-deposited film was first heated(0–15 min in the bottom axis), the density gradually decreased
because of the thermal expansion of the film. Then, at atemperature of 72 1C (around 19 min), the density abruptly
decreased. This is caused by the transition of the film induced

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Fig. 7 Changes of the densities of vacuum-deposited (red) and spin-coated(blue) TPD films with a thickness of B70 nm during heating–cooling cycles.
The temperature during the cycles is also shown (green). The gray dottedhorizontal line shows the identical densities of the transition-experienced
vacuum-deposited film and the spin-coated film at room temperature.

by heating. After the temperature was kept at 100 1C (30–60 min),the density gradually increased during the cooling process(60–115 min) because of the thermal shrinkage of the film. In
the second cycle, the film repeated the thermal expansion andshrinkage of the first cycle, except that the abrupt decrease ofdensity was not observed in the heating process. We note herethat the film density did not completely recover after cooling in
the first cycle; that is, there was a significant difference between
the film densities before and after the first cycle, whereas thedensities before and after the second cycle were the same.
Although similar changes in the density of vacuum-depositedorganic films have been reported,11,13 here we emphasize thequantitative difference between the absolute densities before
and after the first cycle and the identity before and after thesecond cycle.
The density of spin-coated TPD films showed markedly
diﬀerent behaviours during heating–cooling cycles, which has
not been reported before. In the first heating process, an abrupt
decrease in density was not observed, though the slope of thedecrease did change at 55 1C (12 min), which is the glasstransition of the spin-coated film. The important facts that
we should note are that (1) the density of the spin-coated film
completely recovered after the first and second cycles, though
the original density of the film was lower than that of thevacuum-deposited one, (2) the densities of the spin-coatedfilms before and after each cycle are quantitatively identicalto that of the ‘‘transition-experienced’’ vacuum-deposited filmat room temperature following its first heating cycle, and (3) thetransition temperature of the spin-coated film in the firstheating cycle (55 1C) is much lower than that of the vacuumdeposited film (72 1C). Qualitatively similar results were also
observed using other materials (see Fig. S7, ESI†). In addition,
we confirmed that the density of a spin-coated film after
heating did not come close to that of a vacuum-depositedfilm, even when the film was cooled very slowly over 4 h (seeFig. S8(a), ESI†).
To clearly demonstrate the diﬀerences in the transition
temperature and thermal stability of TPD films fabricated by
diﬀerent methods, we show the thickness change (d  d0)/d0 ofthe vacuum-deposited film in Fig. 8(a), where d is the thickness

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Table 2 Transition temperatures of the powder, vacuum-deposited films,
and spin-coated films of various OLED materialsa

Materialb

PowderVacuum-deposited film
Spin-coated film

TPD

a-NPD

2-TNATA

TPT1

TSBF

60
72
55

96
108
86

110
124
101

144
151
137

231
227
217

a
The unit of all temperatures is 1C. b The glass transition temperaturesof all powders are cited from ref. 17 and 31–34.

Fig. 8 Thickness changes of (a) vacuum-deposited and (b) spin-coated
TPD films during the first and second heating processes. The slopes are thethermal expansion coeﬃcients, and the cross points of the slope lines(black lines) show the transition temperatures.

of the film during the first and second heating processes, andd0 is the thickness before each heating process. The slopes
correspond to the thermal expansion coeﬃcients of the films.
In the first heating, the thickness increased linearly up toaround 72 1C, and then it suddenly jumped in a discontinuous
manner, where a drastic change from a stable solid to a supercooled liquid occurred in the film.13 This temperature is higher
than the glass transition temperature of the TPD powder(60 1C), meaning that the molecular aggregation state in thevacuum-deposited films is more stable than that in the bulk of
the powder. However, once the film experienced the transitionin the first cycle, the thickness showed diﬀerent behavior in thesecond cycle; the thickness changed continuously, and thethermal expansion coeﬃcient changed at 56 1C. This is theglass transition temperature of the vacuum-deposited film that
had already experienced the transition once in the first heating
cycle, and it is lower than the glass transition temperature of
the TPD powder. In contrast, the thickness of the spin-coated
TPD film changed continuously in a similar manner in both thefirst and second heating cycles, as shown in Fig. 8(b). The
thermal expansion coeﬃcient changed at 55 1C in both the firstand second heating cycles, which is nearly the same as the glasstransition temperature of the transition-experienced vacuumdeposited film in its second heating cycle. These results clearly show
the diﬀerences in the transition temperatures of the vacuumdeposited and spin-coated films, and demonstrate that the thermal
stability of the spin-coated film is lower than that of the vacuumdeposited one. We emphasize that the thicknesses of the spin-coatedfilm in the first and second heating cycles showed a very similar
trend to that of the transition-experienced vacuum-deposited film in
its second heating cycle. This result suggests the macroscopic
identity of the spin-coated and transition-experienced vacuumdeposited films. The vacuum-deposited and spin-coated films
of other materials also qualitatively showed the same results(see Fig. S9, ESI†). Table 2 summarizes the diﬀerences in thetransition temperatures of powders, vacuum-deposited films,
and spin-coated films of various OLED materials.

This journal is © The Royal Society of Chemistry 2015

The UV-vis absorption spectra of the solid films also support
the macroscopic identity of the spin-coated and transitionexperienced vacuum-deposited films. In Fig. 9, the absorption
spectra of three TPD films with approximately the same thicknessof 48  2 nm are shown without normalization; the spectra are ofan as-prepared vacuum-deposited film, a transition-experienced
vacuum-deposited film, and a spin-coated film. We can clearly
see the diﬀerence between the spectra of the vacuum-depositedand spin-coated films, as already discussed, and the identity of thespectra of the spin-coated film and the transition-experienced
vacuum-deposited one. Similar results were also observed using
other materials (see Fig. S10, ESI†).
The lower transition temperatures of the spin-coated films thanthose of the vacuum-deposited ones mean that their thermal
stabilities are lower, showing a disadvantage of small-molecule
spin-coated films compared to vacuum-deposited ones. Furthermore, the identical densities, transition temperatures, and absorption spectra of the transition-experienced vacuum-deposited films
and spin-coated ones indicate that the small-molecule spin-coatedfilms are macroscopically equivalent to the ‘‘deteriorated’’ vacuumdeposited films that have experienced a transition during a heating
cycle. Although it seems that the change in the density of the spincoated films during the heating–cooling cycles is reversible, it does
not mean that they possess high thermal stability. The change inspin-coated films seems to be reversible only when the film is asingle layer; in a multilayer structure, the irreversible mixing ofmaterials at interfaces cannot be avoided at temperatures exceeding the transition temperature.35 Such interfacial mixing at hightemperature is currently being investigated by ellipsometry and will
be reported in a future study.

Fig. 9 Absorption spectra without normalization of the as-prepared andtransition-experienced vacuum-deposited TPD films, and a spin-coated
TPD film. The original thicknesses of the vacuum-deposited and spincoated films were approximately the same (47.0 and 49.1 nm, respectively).

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2.3

Molecular orientation

Another important diﬀerence between vacuum-deposited andspin-coated films is their molecular orientation. Recently, we
have found that horizontal molecular orientation generally
occurs depending on the molecular shape even in smallmolecule vacuum-deposited OLED films and have demonstrated its considerable eﬀects on both the electrical andoptical characteristics of films and devices.2 Thus, clarificationof the diﬀerence in molecular orientation in films formed by
vacuum deposition and spin coating is very important to
explain the diﬀerences in device performance. To investigate
the general diﬀerences between the molecular orientations in
vacuum-deposited and spin-coated films of small-molecule
OLED materials, we selected four materials shown in Fig. 2:
4,4 0 ,400 -tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA),
00
N4,N4 0 -(biphenyl-4,4 0 -diyl)bis(N4,N4 ,N4 -triphenylbiphenyl-4,4 0 diamine) (TPT1), 2,7-bis(9,9-spirobifluoren-2-yl)-9,9-spirobifluorene(TSBF), 4,40 -bis[4-(diphenylamino)styryl]biphenyl (BDAVBi), which
are materials known to prefer horizontal molecular orientationand that exhibit large birefringence.23,33,36 The absolute densitiesof their vacuum-deposited and spin-coated films determined by
UV-vis absorption measurements of solutions and ellipsometry
analysis are summarized in Table 1 (also see Fig. S1(f)–(i), ESI† formore details). Similar to the cases described above, the densitiesof the vacuum-deposited films were higher than those of the spincoated ones. We also confirmed that the densities of spin-coatedfilms of 2-TNATA and TPT1 were constant when the film thickness
was 430 nm (see Fig. S3(d) and (e), ESI†).
Fig. 10 shows the changes in the density of vacuumdeposited and spin-coated 2-TNATA films during heating–coolingcycles, which were determined by in situ ellipsometry using ananisotropic optical model. The changes in the density werequalitatively the same as those of TPD and a-NPD films. The
density of the vacuum-deposited film abruptly decreased during
the first heating cycle, and it did not completely recover evenafter cooling. In contrast, the densities of the spin-coated films
before and after a heating cycle were the same, and identical tothe density of the transition-experienced vacuum-deposited film.

Fig. 10 Changes of the densities of vacuum-deposited (red) and spincoated (blue) 2-TNATA films with a thickness of B100 nm during heating–cooling cycles. The temperature during the cycles is also shown (green).
The gray dotted horizontal line shows the identical density of thetransition-experienced vacuum-deposited film and the spin-coated filmat room temperature.

11184 | J. Mater. Chem. C, 2015, 3, 11178--11191

Paper

Furthermore, the transition temperature of the vacuum-depositedfilms in the first heating cycle was higher than that of the spincoated film. We also observed similar behaviour for TPT1 and TSBF
(see Table 2 and Fig. S7 and S9, ESI†).
The analysis of the ellipsometry data clearly revealed thediﬀerence between the molecular orientations of vacuumdeposited and spin-coated 2-TNATA films with a thickness of
B100 nm, as shown in Fig. 11(a) and (b), respectively. For thevacuum-deposited film, the refractive index and the extinction
coeﬃcient in the horizontal direction (no and ko, respectively)
were much larger than those in the vertical direction (ne and ke,
respectively), meaning that the molecules are significantly
oriented in the horizontal direction.2 In contrast, the opticalconstants of the spin-coated film were isotropic, meaning thatmolecules in the spin-coated film are randomly oriented.
Interestingly, both the spin-coated and transition-experienced
vacuum-deposited films exhibited random molecular orientations, as shown in Fig. 11(b) and (c), respectively. The molecularorientation in the spin-coated film is random as that in thevacuum-deposited film that had experienced a transition
induced by the heating process. Although the change of theorientation from horizontal to random induced by heating
vacuum-deposited films has long been known,37 the analogyof spin-coated and transition-experienced films observed here
has never been discussed. The absorption spectra of solid films
on fused silica substrates in Fig. 12 further confirm theidentical random orientation of the spin-coated film and thetransition-experienced vacuum-deposited one, because a large
diﬀerence in orientation is reflected in the absorbance of theabsorption peaks.38,39 These trends of anisotropy and absorption spectra were also qualitatively common to TPT1, TSBF and
BDAVBi (see Fig. S10 and S11, ESI†).
In addition, we found that the anisotropy of spin-coated
2-TNATA films is not strongly aﬀected by the experimental
conditions of spin speed and solution concentration, as shown

Fig. 11 Anisotropic optical constants of the (a) as-prepared vacuumdeposited film, (b) spin-coated film, and (c) transition-experienced
vacuum-deposited film of 2-TNATA with a thickness of B100 nm. The
red solid and broken lines show the refractive indices in the horizontal andvertical directions (no and ne), respectively, and the blue solid and brokenlines show the extinction coeﬃcients in the horizontal and vertical directions (ko and ke), respectively.

This journal is © The Royal Society of Chemistry 2015

PaperFig. 12 Absorption spectra without normalization of the as-prepared andtransition-experienced vacuum-deposited 2-TNATA films, and a spincoated 2-TNATA film. The original thicknesses of the as-preparedvacuum-deposited and spin-coated films were approximately the same
(46.5 and 47.8 nm, respectively).

in Fig. 13. Because ellipsometry analysis becomes relatively
insensitive to anisotropy in thin films with a thickness ofo100 nm, here we used a simple optical model for the analysisof the transparent spectral region: the Cauchy model with theadditional anisotropic parameter of birefringence Dn, which isa constant representing the diﬀerence between no and ne.
Because Dn is directly related to the anisotropy of the molecularpolarizability in a film,2 the more significant the horizontal
orientation of 2-TNATA molecules is, the larger the Dn becomes.
For all experimental conditions investigated (spin speed of 1000–
7000 rpm and solution concentration of 10–20 mg ml1), thebirefringences of the spin-coated films were not more than
0.025, which is much lower than that of the vacuum-deposited
film (0.107). This result shows that the molecular orientation in
the spin-coated films is nearly random over a wide range of spincoating conditions, and the degree of the horizontal molecularorientation in the spin-coated films is much lower than that in
the vacuum-deposited ones. This conclusion is also strongly
supported by the absorption spectra of the as-prepared andtransition-experienced spin-coated films shown in Fig. 14. In
contrast to the considerable decrease of absorbance of a vacuumdeposited 2-TNATA film by a transition, the absorbance of all
spin-coated 2-TNATA films fabricated with a solution concentration of 15 mg ml1 and spin speeds of 1000–7000 rpm did not
significantly change even after they experienced a transition

Journal of Materials Chemistry C

Fig. 14 Absorption spectra without normalization of the as-prepared andtransition-experienced spin-coated 2-TNATA films fabricated with asolution concentration of 15 mg ml1 and diﬀerent spin speeds. Absorption spectra of the as-prepared and transition-experienced vacuumdeposited 2-TNATA film with a thickness of B100 nm are also shown.

induced by heating. Assuming that molecular orientations inall of these films become random via the transition, this resultmeans that molecular orientation of spin-coated films is originally almost random independently of the spin speed. Similar
results were also found for spin-coated TPT1 films (see Fig. S12
and S13, ESI†).
Fig. 15 shows how the molecular orientation in a vacuumdeposited film changed during the heating–cooling cycles,
which was determined from the in situ analysis of the anisotropy
in the extinction coefficients of a vacuum-deposited 2-TNATA
film with a thickness of B100 nm. The change of the anisotropyof the extinction coefficients Dk = ko  ke at 332 nm (thewavelength of the highest peak) is shown. Once the original
vacuum-deposited film with a large Dk of 0.23 experienced atransition at 124 1C during its first heating cycle, the anisotropy
became very small and did not recover during the first andsecond cooling cycles. We also confirmed that the molecularorientation of 2-TNATA did not recover even when we cooled thefilm very slowly over 4 h (see Fig. S8(b), ESI†). In addition, we
further investigated the anisotropy and absorption spectra of
2-TNATA films fabricated using three other solvents having
different boiling points (Fig. S14 and S15, ESI†). There was no
significant dependence of molecular orientation and absorption on solvent under the experimental conditions in this study.

Fig. 13 (a) Thicknesses and (b) birefringences of spin-coated 2-TNATA films fabricated using chloroform as the solvent with diﬀerent spin speeds andsolution concentrations. The birefringence of the vacuum-deposited 2-TNATA film is also shown as a red dotted line.

This journal is © The Royal Society of Chemistry 2015

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Journal of Materials Chemistry C

Paper

Fig. 15 Change of the anisotropy of extinction coeﬃcients Dk = ko  ke at
332 nm of vacuum-deposited 2-TNATA films with a thickness of B100 nm
during heating–cooling cycles. The temperature during the cycles is also
shown (green).

Our results show the irreversibility of the density and molecularorientation of vacuum-deposited films during the first heating
cycle, and the macroscopic identity of transition-experienced
films and spin-coated films of OLED materials having large
anisotropy of molecular shape.
The nearly random orientation of small-molecule spin-coatedfilms is a disadvantage when compared to vacuum-depositedfilms because horizontal molecular orientation has positiveeﬀects on both the electrical and optical characteristics of films
and devices. Our results show that it is not easy to achieve a highdegree of horizontal orientation by a simple spin-coating processusing common amorphous materials. Therefore, it is diﬃcult toenhance charge transport in small-molecule spin-coated films or
the outcoupling eﬃciency of OLEDs containing them by actively
utilizing the horizontal molecular orientation of charge transport materials or emitting materials, respectively.2 However, weshould also focus on the advantages of small-molecule spincoated films: low cost and ease of fabricating large-area devices.
If we can solve the diﬃculty in realizing the high degree of thehorizontal orientation in small-molecule solution-processedfilms, the OLED devices using them will have many advantages.

2.4 Origin of the diﬀerences between small-molecule
vacuum-deposited and spin-coated films
To overcome the disadvantages of small-molecule spin-coatedfilms in the future, it is important to understand the origin of
their diﬀerences from vacuum-deposited films. The distinct
similarity of spin-coated and transition-experienced films
demonstrated above provides a clue to it.
In vacuum-deposited films, each molecule on the surface ishighly mobile without being strongly restricted by other molecules in its diﬀusion. The molecules on the surface have a timein the order of seconds to sample diﬀerent ways of molecular
packing and find a stable higher-order structure before being
overlaid by successive molecules. (If we roughly assume that theeﬀective diameter of a small molecule is B1 nm, the deposition
rate of 2 Å s1 means that each molecule has B5 s for surface
diﬀusion.) This surface diﬀusion of highly mobile molecules
contributes to the high density and stability of vacuum-deposited
amorphous organic films.12

11186 | J. Mater. Chem. C, 2015, 3, 11178--11191

Fig. 16 Thickness changes of vacuum-deposited 2-TNATA films fabricatedat diﬀerent deposition rates of 0.2, 2, and 20 Å s1 during a heating–coolingcycle. To compare the densities of the as-prepared films, the thicknesses at
40 1C after cooling d0 0 are used as the standards for normalization.

To demonstrate this eﬀect of surface diﬀusion, we investigatedthe diﬀerences in density and thermal stability of vacuumdeposited 2-TNATA films when we changed the deposition rate.
Fig. 16 shows the thickness changes (d  d0 0 )/d0 0 of 2-TNATA
films fabricated at diﬀerent deposition rates of 0.2, 2, and20 Å s1 when they were heated at a rate of 3 1C min1 up to
150 1C and then naturally cooled down, where d0 0 is the
thickness of the film at 40 1C after cooling. Here, we assumed
that the densities of these three films become the same after
the transition by heating, and used the thicknesses of d0 0 as thestandards for normalization. This assumption is reasonablebecause the state of supercooled liquids after the transition
does not keep the higher-order structure before the transitionat all. The film fabricated at 0.2 Å s1 showed the maximum
diﬀerence between the thickness changes before and after theheating–cooling cycle, demonstrating that the as-prepared film
fabricated at a low deposition rate of 0.2 Å s1 has a higher
density than the other as-prepared ones. Furthermore, Fig. 16
shows that the film fabricated at 0.2 Å s1 has a higher
transition temperature than the others, meaning its higher thermal
stability. Similar trends were also observed for vacuum-deposited
TPD and TPT1 films (see Fig. S16, ESI†). These results directly
demonstrate that the long time for surface diffusion contributes
to high densities and high thermal stabilities of vacuumdeposited films.
In addition, we also found another important fact to understandthe mechanism of the molecular orientation; the horizontal
molecular orientation in vacuum-deposited films occurs justbecause horizontally oriented molecules on the surface are fixedby successively overlaying molecules, NOT because the state ofhorizontal orientation is more thermodynamically stable thanthat of random and vertical orientations in the bulk of the film.
Fig. 17 shows the optical anisotropies of the vacuum-deposited
2-TNATA films fabricated at diﬀerent deposition rates of 0.2, 2,
and 20 Å s1. Interestingly, the degree of the horizontal orientation is highest in the film fabricated at a high deposition rateof 20 Å s1, whose density and thermal stability are lowest
among the three films. A similar trend was also observed forThis journal is © The Royal Society of Chemistry 2015

PaperFig. 17 Anisotropic optical constants of vacuum-deposited 2-TNATA films
with a thickness of B100 nm fabricated at deposition rates of (a) 0.2, (b) 2,
and (c) 20 Å s1. The red solid and broken lines show the refractive indices in
the horizontal and vertical directions (no and ne), respectively, and the blue
solid and broken lines show the extinction coeﬃcients in the horizontal andvertical directions (ko and ke), respectively.

vacuum-deposited TPD films, where the film fabricated at
20 Å s1 shows horizontal orientation though the one fabricatedat 2 Å s1 shows nearly random orientation (see Fig. S17, ESI†).
These results show that the horizontal molecular orientation ofthese materials is not directly related to the high density or
thermal stability of the vacuum-deposited films. Molecules
deposited on the surface first adopt horizontal orientation justto minimize the surface energy, but if they can have a longer
time for surface diﬀusion without being overlaid by successively
deposited molecules, the molecular orientation changes to random(and further vertical), which is more thermodynamically stable,by the collaborative structural relaxation with other diﬀusing
molecules on the surface.2,38,40 This mechanism is consistent
with the fact that the molecular orientation in amorphous
vacuum-deposited films can be controlled from horizontal torandom and further vertical by heating the substrate during
deposition.38,40 Thus, the horizontally oriented states are causedby the restriction of the structural relaxation in amorphoussolids, and they are metastable states having a local minimum
energy with kinetic stability in the energy landscape.41
In contrast to vacuum-deposited films, spin-coated andtransition-experienced films are formed by a mechanism distinctly
diﬀerent from that of vacuum-deposited ones; many small molecules in the films are simultaneously condensed and solidified.
During these processes, molecular motion for structural relaxationis strongly limited by the surrounding molecules. The higher-order
structures of spin-coated and transition-experienced films arereadily trapped at a state having a local minimum energy in theenergy landscape that is less stable than that of vacuumdeposited ones. As a result, films have relatively larger voids,
resulting in a lower thermal stability.
At the same time, we should also note that the times for thechange from a liquid to a solid are diﬀerent for spin-coated andtransition-experienced films. In the spin-coating process, mostof the solvent is vaporized in a short time, at most a few tens ofseconds. Conversely, the cooling process of films after transition is very slow; the longest time for cooling in this study wasThis journal is © The Royal Society of Chemistry 2015

Journal of Materials Chemistry C

4 h (see Fig. S8, ESI†). Although these time scales are quite
diﬀerent, the films are macroscopically identical as discussed
above. This shows that the time for the formation of the solid
films is not the primary factor that causes the diﬀerences
between vacuum-deposited and spin-coated films, at least when
we fabricate films in a realistic time for production (withinseveral hours). This conclusion is consistent with the estimation thatit takes more than thousands of years to achieve ordinary glasses
with a density as high as that of vacuum-deposited amorphousfilms.11
Because simply lengthening the time for film formation
cannot compensate the diﬀerences, we have to find anotherway to realize small-molecule solution-processed films having
high thermal stability and a high degree of horizontal molecularorientation. From the above discussion, we can see that one
possible strategy is to develop new materials that can adopt a
stable higher-order structure even when their molecular motion
is strongly restricted by surrounding molecules. We think thatthe active use of intermolecular interactions39,42 is a possible way
to achieve this in the future, even though it has not yet been
explicitly discussed in studies on small-molecule solutionprocessed OLEDs.
2.5

Polymer film

Although small-molecule spin-coated films currently have thedisadvantages of their smaller anisotropy of molecular orientation and lower thermal stability than vacuum-deposited ones, it
has been well known for a long time that spin-coated polymer
films often tend to form horizontal molecular orientation.43–46
Furthermore, in general, the larger the molecular weight of apolymer or an oligomer is, the higher its glass transition
temperature is.17,47 Thus, we can expect that the use of spincoated polymers enables us to avoid the disadvantages of thesmall anisotropy and lower thermal stability of small-molecule
spin-coated films. To demonstrate this, we fabricated a spincoated film of poly[N,N 0 -bis(4-butylphenyl)-N,N 0 -bis(phenyl)benzidine] (poly-TPD), whose chemical structure is shown in
Fig. 2, and analyzed the optical anisotropy and transition
temperature of the resulting film. Because the unit structureof poly-TPD is very similar to those of TPD and TPT1, we canroughly regard poly-TPD as a molecule consisting of a large
number of TPD units, and TPT1 as a molecule consisting of two
TPD units. To investigate the effect of the number of units on
the film properties, we compared the optical anisotropies andtransition temperatures of spin-coated films of TPD, TPT1 andpoly-TPD with a thickness of B100 nm, which were fabricatedusing chlorobenzene as the solvent. The optical anisotropies ofthe films are shown in Fig. 18, revealing that both TPD and
TPT1 molecules are almost randomly oriented in the spincoated films, whereas the chain axis of poly-TPD molecules
are horizontally oriented. In Fig. 18(c), the optical anisotropy on
the long-wavelength side of the absorption band of poly-TPD
can be assigned to the anisotropy of the transition dipole
moment along the long axis of the TPD units.38 The results ofin situ ellipsometry analysis during heating processes also
confirmed that the transition temperature of the spin-coatedJ. Mater. Chem. C, 2015, 3, 11178--11191 | 11187

Journal of Materials Chemistry C

Fig. 18 Anisotropic optical constants of spin-coated films of (a) TPD, (b) TPT1,
and (c) poly-TPD with a thickness of B100 nm. The red solid and broken lines
show the refractive indices in the horizontal and vertical directions (no and ne),respectively, and the blue solid and broken lines show the extinction coeﬃcients in the horizontal and vertical directions (ko and ke), respectively.

Fig. 19 Thickness changes of spin-coated films of TPD (blue), TPT1 (green),
and poly-TPD (red) during heating. The slopes are the thermal expansion
coeﬃcients, and the arrows show the glass transition temperatures.

poly-TPD film (180 1C) is much higher than those of the spin-coated
TPD and TPT1 films (56 and 136 1C, respectively), as shown in
Fig. 19.
The fact that polymer films have larger anisotropy of molecularorientation and higher thermal stability than small-molecule
spin-coated films highlights the advantages of polymer films.
However, polymer films also have disadvantages, including
higher impurity and lower solubility. We should take into
account both the advantages and disadvantages of materials
when choosing a spin-coating system for OLED fabrication.

Paper

molecular orientation of small-molecule spin-coated films ofglassy materials are identical to those of the ‘‘deteriorated’’vacuum-deposited films that had experienced a transition
induced by heating. We also confirmed the higher thermal
stability and larger anisotropy of molecular orientation of apolymer spin-coated film compared to those of the corresponding
small-molecule spin-coated films.
The relative advantages and disadvantages of each type offilm at present are summarized as follows.
(i) Small-molecule vacuum-processed films:– Advantages: ease of stacking an ideal multilayer structure,high thermal stability, high material purity, and high degree ofhorizontal molecular orientation– Disadvantages: high cost of fabrication processes anddifficulty in fabricating large-area devices(ii) Small-molecule solution-processed films:– Advantages: low cost of fabrication processes and ease of
fabricating large-area devices– Disadvantages: low thermal stability of the film, small
anisotropy of molecular orientation, and difficulty in fabricating
an ideal multilayer structure(iii) Polymer solution-processed films:– Advantages: low cost of fabrication processes, ease of
fabricating large-area devices, high thermal stability, and highdegree of horizontal molecular orientation– Disadvantages: low material purity, low solubility, anddifficulty in fabricating an ideal multilayer structure
Our comprehensive comparisons lead to guidelines to helpchoose an appropriate process and materials to produce OLED
displays or lighting according to the requirement of a specificsituation.
Although we have mainly demonstrated the disadvantagesof small-molecule spin-coated films in this study, we would
finally like to emphasize that our results do not exclude thepossibility of overcoming them. One of the purposes of thisstudy is to clarify the future challenges that need to be solved torealize high-performance solution-processed OLEDs using small
molecules, which is important for researchers who are trying tosolve the problems and improve device performance. We believethat it will be possible to overcome some of the above disadvantagesby actively utilizing intermolecular interactions, which have notyet been considered explicitly in relation to solution-processed
OLED films.

4. Experimental
3. Conclusions
We systematically compared vacuum-deposited and spin-coatedfilms of the small-molecule OLED materials by ellipsometry,UV-vis absorption, and XRR measurements, and found that thefilm density, transition temperature, and degree of horizontal
molecular orientation of small-molecule spin-coated films are
inherently lower than those of the corresponding vacuumdeposited films. In addition, the transition temperature and11188 | J. Mater. Chem. C, 2015, 3, 11178--11191

4.1

Sample fabrication

TPD, a-NPD, a-ONPD, 2-TNATA, TPT1, TSBF, BDAVBi, and poly-TPD
were purchased from Luminescence Technology Corporation.
(a-NPD and a-ONPD are sold as ‘‘NPB’’ and ‘‘ONPB’’, respectively.)
CBP and Alq3 were purchased from e-Ray Optoelectronics
Technology. All of the small-molecule materials were of sublimed
grade. Si(100) substrates without a thermally oxidized layer wereused for the UV-vis absorption measurements of the dissolved
solutions of the films, XRR measurements, and ellipsometry.

This journal is © The Royal Society of Chemistry 2015

PaperFused silica substrates were used only for the UV-vis measurementsof solid films. All vacuum-deposited films were fabricated at adeposition rate of 2 Å s1, unless noted otherwise, under a vacuumof o1  103 Pa. Spin-coated films were fabricated under a wide
range of experimental conditions, including diﬀerent solvents,
solution concentrations, and rotation speeds, depending on the
purpose of each analysis. Chloroform was used as the solvent unless
noted otherwise. The spin-coating time was 50 s. After spin-coating,all of the spin-coated films except the TPD ones were baked at 80 1Cfor 30 min under a nitrogen atmosphere. TPD films were baked at
50 1C for 30 min because they have a low transition temperature.
When comparing the absorption spectra of the vacuumdeposited and spin-coated films without normalization, thespin-coated films were first fabricated and analyzed, and thenthe vacuum-deposited films were fabricated so that the thickness of the vacuum-deposited films were the same as that of thespin-coated ones. When preparing the transition-experienced
films, the as-prepared films were annealed on a hot plate undera nitrogen atmosphere at a temperature much higher than theglass transition temperature of the material and then naturallycooled to room temperature.
All of the experimental conditions used to fabricate films
and the resulting thickness of each sample are summarized inTable S1 (ESI†).
4.2

UV-vis absorption measurements of dissolved films

The main procedures used to obtain film densities from UV-vis
absorption measurements of dissolved films have been
described elsewhere.18,19 In our case, films were deposited on
Si(100) substrates using a metal mask to precisely define thearea of the films (20  20 mm2 with an error of 4 mm2). Thisguaranteed a small error of the area of 1%. From this area andthe thickness obtained by ellipsometry analysis, film volumes were
determined. The films were dissolved in chloroform or toluene,
and the volumes of the solutions were carefully adjusted to
10  0.03 ml. Reference solutions of each material with definite
concentrations were also prepared to make calibration lines.
UV-vis absorption spectra were recorded using a spectrophotometer (UV2450, Shimadzu). From the peak absorbance,
the concentrations of each dissolved film were determinedusing the calibration lines, and the densities of the films were
estimated. The detailed results of this analysis are presented inFig. S1 (ESI†).
4.3

XRR measurements

The films for XRR measurements were deposited on Si(100)substrates. Because the densities of organic materials areusually lower than those of inorganic substrates such as a Si
substrate, the critical angles for the organic films appear at asmaller angle than that for the Si substrate.21,22 In this case, thecritical angle for an organic film in the XRR patterns often
becomes blurred, especially when the organic film is thin. Thus,
to make the critical angle for organic films clearly apparent,
films having a large thickness of B300 nm were fabricated.
Although it is possible to analyze the fringing patterns in thelarger-angle region of XRR patterns of thin films, this was not

This journal is © The Royal Society of Chemistry 2015

Journal of Materials Chemistry C

done here because the analysis of the fringing patterns oforganic films is easily aﬀected not only by density but alsosurface roughness, which makes it diﬃcult to obtain a uniquesolution of clear density results.
The XRR patterns of the samples were measured using
an X-ray diﬀractometer (SmartLab, Rigaku) under Cu Ka(l = 1.54178 Å) radiation at 45 kV and 200 mA. The beam
divergence angle was 0.041, and the widths of the divergence,scattering and receiving slits were 0.1, 0.3 and 0.4 mm, respectively.
The incident angles were scanned from 01 to 0.51 in steps of 0.0021
at a scan rate of 0.21 min1. Considering the diﬀerences between
the sample area and the footprint area of the X-ray with very small
grazing incident angles, a correction of the measured patterns was
performed. The fitting analysis was performed using GlobalFit
software (Rigaku). We estimated the error of the absolute densities
at the sample surface as 0.04 g cm1 or less. The detailed results
are presented in Fig. S2 (ESI†).
4.4

Ellipsometry analysis

All sample films for ellipsometry analysis were fabricated on
Si(100) substrates, whose definite optical constants minimize
the error of the results. This choice of substrate is very
important because the errors of the optical constants of theunderlying substrate aﬀect the results of the analysis of theoverlaying organic film and often lead to an incorrect result,especially when analyzing the optical anisotropy of a very thinfilm. (For example, the use of a glass or fused silica substrate to
analyze the anisotropy of very thin films makes the reliability of
the results very low, because their transparency causes
unwanted back-side reflection and their optical constants
depend on the fabrication process.)
For the analysis of density and anisotropy, variable angle
spectroscopic ellipsometry (VASE) measurements were performed
using a fast spectroscopic ellipsometer (M-2000U, J. A. Woollam) atseven angles of incident light from 451 to 751 in steps of 51. At eachangle, experimental ellipsometric parameters C and D were
simultaneously obtained in steps of 1.6 nm throughout the spectral
region from 245 to 1000 nm.
For the in situ measurement during the heating–coolingcycles, we used a temperature-control stage (J. A. Woollam),which was attached to the ellipsometer. The stage was coveredby a metallic box with optical windows to avoid exposure to air,and it was kept under a nitrogen atmosphere during the cycles.
The angle of incident light was fixed at 701, and the ellipsometry
parameters were monitored in time steps of B6 s and spectral
steps of 1.6 nm from 245 to 1000 nm. The sample films were
heated at a constant rate of 3 1C min1 up to a temperaturemuch higher than the transition temperature (the temperatures
used for each sample are listed in Table S1, ESI†). After the hightemperature was maintained for 30 min, the sample films were
naturally cooled to B30 1C. This heating–cooling cycle wasrepeated twice.
The Cauchy model was used to analyze the relative densities ofthe films of TPD, a-NPD, CBP, and Alq3, because their birefringencesin the long-wavelength transparent spectral region are notlarge. For these films, we analyzed the transparent spectral

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Paper

region of 600–1000 nm, which is apart from the absorption bandsof the four materials. After the fitting analysis, we estimated therelative densities of the films using the Lorentz–Lorenz equation,48
n2  1 4p NA a¼
r / r;
n2 þ 23 M(2)where n is the refractive index, and a is molecular polarizability.
We used the refractive indices at 600 nm for the estimations.
Because it is difficult to know the precise values for themolecular polarizabilities of each material, we can only estimate
the relative densities of the films of materials based on the
proportional relation in eqn (2).
In the analysis of the anisotropy of TPD, 2-TNATA, TPT1,
TSBF, BDAVBi, and poly-TPD films with a thickness of B100 nm(Fig. 11, 17 and 18), we used the general oscillator model, thedetails of which are described in ref. 24. When estimating therelative densities of these anisotropic films, we calculated theisotropic refractive index niso using the following equation48 andreplaced n in eqn (2) with niso,sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2no 2 þ ne 2:
(3)
niso ¼
3
In the in situ analysis of these anisotropic films during theheating–cooling cycles (Fig. 10, 15, and 16), we fixed the positions (peak energies) and widths of the oscillator bands in thegeneral oscillator model based on the results of the VASE
analysis of the vacuum-deposited films before heating, because
it is necessary to decrease the number of fitting parameters to
obtain reliable unique results for an in situ analysis performed
with only a single angle of incident light. In the in situ analysis,
the temperature dependence of the optical constants of the Si
substrate was also considered.
When analyzing thin spin-coated films with a thickness ofo100 nm using the anisotropic optical model, we should notethat the VASE analysis becomes relatively insensitive to anisotropy
as the films become thinner. This insensitivity often leads to an
unreliable solution of the general oscillator model. In particular,
the reliability of the anisotropy of extinction coeﬃcients ko and ke
becomes low with decreasing thickness. Thus, when we investigated the dependence of optical anisotropy on the experimental
conditions of spin speed and solution concentration (Fig. 13), we
analyzed the transparent spectral region only (600–1000 nm) andused the Cauchy model with an additional anisotropic parameter of birefringence Dn, which is a constant representing thedifference between no and ne. This optical model with only four
fitting parameters (two Cauchy parameters, Dn, and thickness) is
sufficiently simple to guarantee the uniqueness of solutions evenwhen we analyze thin films with a thickness of several tens ofnanometers.

Acknowledgements
We acknowledge Prof. Mark D. Ediger for fruitful discussion.
We also thank Prof. Shizuo Tokito and Dr. Masashi Mamada for
their help with preliminary experiments. J. A. Woollam Japan11190 | J. Mater. Chem. C, 2015, 3, 11178--11191

Co. supplied the heating–cooling stage and its controller. This
work was financially supported in part by KAKENHI (Grant
Number 25708038) from the Japan Society for the Promotion of
Science (JSPS). The DFT calculations were performed using
computers in the Research Center for Computational Science,
Okazaki, Japan.

Notes and references
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