Structure and properties of small molecule-polymer blend semiconductors for organic thin film transistors

Article abstract of DOI:10.1021/ja804013n

A comprehensive structural and electrical characterization of solution-processed blend films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconductor and poly(α-methylstyrene) (PαMS) insulator was performed to understand and optimize the blend semiconductor films, which are very attractive as the active layer in solution-processed organic thin-film transistors (OTFTs). Our study, based on careful measurements of specular neutron reflectivity and grazing-incidence X-ray diffraction, showed that the blends with a low molecular-mass PαMS exhibited a strong segregation of TIPS-pentacene only at the air interface, but surprisingly the blends with a high molecular-mass PαMS showed a strong segregation of TIPS-pentacene at both air and bottom substrate interfaces with high crystallinity and desired orientation. This finding led to the preparation of a TIPS-pentacene/PαMS blend active layer with superior performance characteristics (field-effect mobility, on/off ratio, and threshold voltage) over those of neat TIPS-pentacene, as well as the solution-processability of technologically attractive bottom-gate/bottom-contact OTFT devices. Copyright

Full text of DOI:10.1021/ja804013n

Published on Web 08/23/2008
Structure and Properties of Small Molecule-Polymer Blend Semiconductors
for Organic Thin Film Transistors
Jihoon Kang,† Nayool Shin,† Do Young Jang,† Vivek M. Prabhu,*,‡ and Do Y. Yoon*,†
Department of Chemistry, Seoul National UniVersity, Seoul, 151-747, Korea, and Polymers DiVision, National
Institute of Standards & Technology, Gaithersburg, Maryland 20899
Received June 4, 2008; E-mail: dyyoon@snu.ac.kr; vprabhu@nist.gov
Organic semiconductors are generally regarded to be the most
critical element of the new generation of optoelectronic devices
such as printable flexible electronics and photovoltaic devices.1,2
Therefore, organic small molecule and polymer semiconductors3-5
have been actively investigated to obtain the desired electrical
properties. However, their field-effect mobility, solution process-
ability and long-term performance stability must still be substantially
improved6 in order to match conventional amorphous silicon
semiconductors whose field-effect mobilities reach as high as ca.
1 cm2/Vs. Recently, small molecule organic semiconductors have
achieved field-effect mobilities over 1 cm2/Vs due to the intelligent
Figure 1. Schematic drawings of (a) top-gate/bottom-contact and (b)
bottom-gate/bottom-contact OTFTs, and chemical structures of (c) TIPS-
pentacene and (d) poly(R-methylstyrene) (PRΜS).
design and synthesis of organic molecules with novel molecular
packing structure and high crystallinity.4,7-9 However, the solution
processing of small molecules, necessary for printable electronic
devices, encounters serious issues including the nonuniform film
thickness and coating problems such as dewetting. In comparison,
polymer semiconductors are well suited for the solution-processing
because of their excellent film-forming characteristics. But semi-
conducting polymer films generally show a limited charge carrier
mobility (<0.3 cm2/Vs) and poor performance stability, most likely
due to the low crystallinity and the resultant high permeability of
moisture and oxidizing species.
A possible solution to this problem was proposed by Brown et
al., who applied solution-processed blend films of a semiconducting
small molecule and an insulating polymer as the active layer.10
This approach of combining the advantages of small molecules and
polymers led to a stable field-effect mobility over 0.3 cm2/Vs. It
was postulated that the semiconducting small molecules were phase-
segregated to the air surface of the thin blend film, since this
excellent mobility was reported only for the organic thin film
transistor (OTFT) devices with a top-gated structure (Figure 1a).
However, no concrete experimental evidence has been presented
to support this proposal. Moreover, such a top-gate/bottom-contact
OTFT structure presents serious problems for fabricating techno-
logically important devices due to the potential damage to the active
layer during the subsequent process steps including the gate
electrode patterning and wiring. Instead, a bottom-gate/bottom-
contact structure as shown in Figure 1b is strongly favored. For
fabricating such structures it is critical to have the segregation of
the semiconducting small molecules at the interface with the
dielectric substrate in the solution-processed thin blend active layer
of ca. 100 nm thickness.
characteristics in thin blend films, so that we could optimize the
conditions for the segregation of small molecule semiconductors
at the bottom interface with the solid substrate. Third, we
investigated the crystalline morphology of small molecule semi-
conductors in thin blend films in order to determine whether they
have the desired molecular packing and orientation. Finally, we
characterized the electrical properties of the optimized blend active
layer in bottom-gate/bottom-contact OTFT devices, and demon-
strated that the optimized blend active layer exhibited superior
functional properties (field-effect mobility, on/off ratio, and thresh-
old voltage) over those of neat small molecule system.
For our experiments, we employed blend films of 6,13-bis(tri-
isopropylsilylethynyl)pentacene (TIPS-pentacene) (see Figure 1c)
and poly(R-methylstyrene) (PRMS) (see Figure 1d). TIPS-pentacene
is a soluble semiconductor known to show a high mobility of 0.17
cm2/Vs when solution processed upon SiO2 dielectric,9 and PRMS
insulator was shown to work quite well with a variety of small
molecule semiconductors.10
To obtain the most critical structural information underlying the
observed electrical properties of the thin blend active layer, we have
carried out neutron reflectivity (NR) experiments which allow the
measurements of TIPS-pentacene depth profiles with nanometer-
scale resolution. Since this technique relies on the contrast of
neutron scattering length density arising primarily from the variation
in the hydrogen and deuterium concentration, the NR sensitivity
was provided by employing a deuterium-labeled TIPS-pentacene
(d-TIPS-pentacene), synthesized as described in the Supporting
Information. Therefore, the reflectivity arising from the segregation
of d-TIPS-pentacene in the blend film with PRMS could be
measured with a sufficiently high precision for accurately determin-
ing the d-TIPS-pentacene depth profile.
Measurements were performed on the NG 7 reflectometer at the
Center for Neutron Research, National Institute of Standards and
Technology (NIST). The specular reflectivity (R) curves, corrected
for background, were measured as a function of wave vector (Q)
normal to the film (Q ) 4πλ-1 sin θ, where λ is the fixed incident
neutron wavelength of 4.75 Å and θ is the angle of reflection) with
Therefore, the scope of this work is 4-fold. First, we delineated
the structural origin of the enhanced performance of the small
molecule-polymer blend active layer as reported by Brown et al.10
Second, we explored a key material parameter, molecular mass of
the insulating polymer, which strongly influences the segregation
† Seoul National University.
‡ National Institute of Standards & Technology.
9
10.1021/ja804013n CCC: $40.75
2008 American Chemical Society
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C O M M U N I C A T I O N S
Figure 2. Neutron reflectivity profiles and fitted concentration profiles (in the insets) of d-TIPS-pentacene in the 1:1 ratio (by mass) blend films of d-TIPS-
pentacene and PRMS for (a) the blend with PRMS of Mr,n ≈ 1300 g ·mol-1 and (b) the blend with PRMS of Mr,n ≈ 575 000 g ·mol-1 in the as-cast and the
annealed (20 min at 100 °C) films. The fitted reflectivity curves from the modeled density profiles are shown by black solid lines.
the resolution ∆Q/Q ) 0.04. The reflectivity data were then fit to
the results calculated from the modeled depth profiles using the
Parratt algorithm, in units of scattering length density (F).
pentacene to segregate preferentially to the air interface. This
segregation feature, as found from the NR experiments, is com-
pletely consistent with the excellent field-effect mobility of this
blend active layer only in the top-gated OTFT structure, as reported
by Brown et al.10
Figure 2a shows the specular NR data for the blend films (1:1
ratio by mass) of d-TIPS-pentacene with PRMS of relative number
average molecular mass Mr,n ≈ 1300 g ·mol-1, spin-cast on thick
silicon substrate, for the initial as-cast and vaccuum-dried film and
the film subsequently annealed at 100 °C for 20 min in nitrogen
atmosphere, respectively. This low molecular-mass PRMS blend
system is identical to the system reported by Brown et al.,10 and
thus was chosen to understand the excellent electrical properties
they reported. In general, the well-defined oscillating fringes arise
from the interference of the reflected neutrons from the air and the
substrate interfaces, such that the period of the fringes is inversely
proportional to the film thickness. The reflectivity data in Figure
2a indicates that the blend films with the low molecular-mass PRMS
exhibit distinctly different features between the as-cast film and
the film annealed at 100 °C, as noticed by the appearance of an
enveloping longer wavelength profile upon the short wavelength
features in the annealed film data. From the fits to the NR data the
scattering length density profile F(z) was interpreted as d-TIPS-
pentacene volume fraction as a function of distance from the
substrate using the relation: φd-TIPS × Fd-TIPS + (1 - φd-TIPS) × FPRMS
) Fblend, where φd-TIPS corresponds to the volume fraction of d-TIPS-
pentacene. Reference pure component films provide the values of
As a means of changing the segregation characteristics, we
changed the molecular mass of the insulating polymer (PRMS) to
a very high value of Mr,n ≈ 575 000 g ·mol-1. Figure 2b shows
the corresponding NR data and the depth profiles of d-TIPS-
pentacene in the 1:1 ratio (by mass) blend. For this blend with such
a high molecular-mass PRMS, a strong interface segregation of
TIPS-pentacene occurred at both interfaces in the initial spin-cast
film. That is, the phase-segregated structure formed a nearly pure
TIPS-pentacene layer not only at the air surface, with ca. 134 Å
thickness, but also at the silicon substrate interface with ca. 117 Å
thickness. And the annealing step at 100 °C did not cause any
substantial change in the segregation profile other than a slight
increase in the concentration of TIPS-pentacene at both interfaces,
probably due to the high glass transition temperature (Tg ≈ 140
°C) of this high molecular-mass sample well above the annealing
temperature. There appears to be a very slight decrease in the overall
PRMS fraction and a small decrease in the total film thickness
possibly due to the evaporation of residual solvent.
Most surprising and significant is the observation that in the blend
with the high molecular-mass PRMS the segregated nearly pure
TIPS-pentacene of ca. 117 Å thickness was obtained at the bottom
silicon substrate interface, which was not seen for the blend system
with the low molecular-mass PRMS (see Figure 2a). As discussed
above, this feature is critically important for fabricating bottom-
gate/bottom-contact OTFT structures, provided that the TIPS-
pentacene molecules in the segregated thin films form a highly
crystalline structure with the π-π stacked molecular layers oriented
parallel to the film surface.9,11
Fd-TIPS and FPRMS as 4.37 × 10-6 Å-2 and 1.12 × 10-6 Å-2
,
respectively. Insets in Figure 2 show the depth profiles of d-TIPS-
pentacene for the as-cast film and the film subsequently annealed
at 100 °C. In the as-cast film, the blend film with a low molecular-
mass PRMS (Mr,n ≈ 1300 g ·mol-1) has a mostly uniform
distribution of d-TIPS-pentacene with a concentration of 51% by
volume throughout the film thickness, except in the two narrow
interfacial regions: 11% excess at the blend/silicon substrate and
20% excess at the blend/air interface. Upon annealing this blend
film at 100 °C under nitrogen atmosphere, a large-scale restructuring
occurred which resulted in a strong segregation of d-TIPS-pentacene
at the film/air interface. This segregated film structure consists of
nearly pure TIPS-pentacene layer of ca. 177 Å thickness at the air
surface, PRMS-rich middle layer of ca. 326 Å thickness, and a
slightly TIPS-pentacene enriched layer of ca. 70 Å at the silicon
substrate interface. Since the glass transition temperature (Tg) of
the PRMS with Mr,n ≈ 1300 g ·mol-1 is observed around 74 °C,
the annealing step at 100 °C allowed sufficient mobility for TIPS-
Figure 3 shows the grazing-incidence X-ray diffraction (GIXD)
patterns, recorded at Pohang Accelerator Laboratory, Korea (PAL)
for the thin blend films of TIPS-pentacene and PRMS with Mr,n
≈
575 000 g ·mol-1, identical to those employed for the neutron
reflectivity experiments. Both the as-cast film and the film
subsequently annealed at 90 °C show highly crystalline morphology
of TIPS-pentacene molecules, with the layer spacing of 16.6 Å,
which is in close agreement with 16.8 Å reported by Anthony et
al.12 Moreover, the molecular layers are highly oriented parallel to
the film surface (i.e., the normal to the (00l) planes oriented along
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C O M M U N I C A T I O N S
expected.13,14 (See the Supporting Information for more details and
comparison with previous works.)
The charge transport in OTFT devices is critically dependent
on the thin semiconducting layer less than a few nanometer
thickness adjacent to the gate dielectric surface.15,16 Therefore, the
excellent electrical properties of the OTFT devices prepared with
the blend active layer arises from the nearly pure TIPS-pentacene
layer segregated and crystallized at the bottom substrate interface,
as indicated by the results of Figure 2b and Figure 3. Moreover,
significant improvements in key performance characteristics (field-
effect mobility, on/off ratio, and threshold voltage) of the OTFT
devices are most likely due to the important changes in the chemical
and physical structure at the semiconductor/dielectric interface, as
compared with neat TIPS-pentacene system. This is the topic that
is poorly understood at the moment and hence needs much further
study, in order to realize successful technological applications of
organic semiconductors.
Figure 3. GIXD patterns of spin-cast films of TIPS-pentacene/PRMS (Mr,n
≈ 575 000 g ·mol-1) blend, observed for the as-cast and the 90 °C-annealed
films. The scale of the scattering vector is denoted in the film plane by qxy.
The (00l) diffraction spots are indicated by the arrows in the figure.
Acknowledgment. The authors acknowledge the helpful dis-
cussions with Drs. Dean DeLongchamp and Eric Lin of NIST, and
the financial support by the grants from the Korea Science and
Engineering Foundation (F01-2006-000-10200-0) and the Informa-
tion Display R&D Center (F0004031-2007-23), one of the 21st
Century Frontier R&D Program of Korean Government. This
research was also supported by the Chemistry and Molecular
Engineering Program of the Brain Korea 21 Project and by Samsung
Electronics Co., LTD. GIXD facility at PAL is funded by Korean
government and operated by POSTECH. The NIST Center for
Neutron Research is funded by the U.S. Department of Commerce.
Figure 4. (a) Transfer characteristic and (b) output characteristic of bottom-
gate/bottom-contact OTFT with TIPS-pentacene/PRMS (Mr,n ≈ 575 000
g·mol-1) blend film. The field-effect mobility calculated in the saturation
regime is 0.54 cm2/Vs.
the film thickness (z) direction). As discussed by Anthony and his
co-workers,8,9,11,12 such an orientation is essential to obtaining a
high field-effect mobility of TIPS-pentacene active layer in OTFT
devices. Therefore, we found that the phase-segregated TIPS-
pentacene layers in the blend films with the high molecular-mass
PRMS exhibited a highly crystalline structure with the desired
crystal orientation on silicon solid substrate.
Supporting Information Available: Description of the employed
materials and experimental methods (NR, GIXD, OTFT device fabrica-
tion). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
Figure 4 shows the performance characteristics of an excellent
bottom-gate/bottom-contact OTFT device prepared with the TIPS-
pentacene blend (1:1 ratio by mass) with the high molecular-mass
PRMS of Mr,n ≈ 575 000 g ·mol-1. The OTFT devices were
fabricated with thermally grown SiO2 dielectric and the surface-
modified (with pentafluorobenzenethiol) Au electrodes. The active
layer of neat TIPS-pentacene or the blend with PRMS was deposited
by drop-casting, relevant to the ink-jet printing process, from 0.5%
(by mass) solution of TIPS-pentacene or the blend solution in
toluene, followed by solvent evaporation at 90 °C for 20 min. Neat
TIPS-pentacene devices exhibited an average field-effect mobility
of 0.05 cm2/Vs with on/off ratio of 105 and threshold voltage |Vth|
≈ 4 V. But they showed a serious problem of large fluctuations of
device performance due to the poor film-forming characteristics
and variation in crystal-growth behavior in the channel area (length
L ) 100 µm and width W ) 1000 µm). In comparison, the OTFT
devices fabricated with the blend with PRMS of Mr,n ≈ 575 000
g·mol-1 showed a much higher average mobility of 0.3 cm2/Vs
and on/off ratio of 5 × 105, together with a reduced threshold
voltage |Vth| < 2 V (see Figure 4). Moreover, the deposited film
quality was nearly uniform and satisfactory in all the fabricated
OTFTs, and therefore the device performance variation decreased
significantly. As an additional benefit, since the semiconducting
layer was encapsulated in situ by the insulating polymer layer (see
Figure 2b), an improved stability of the active layer in OTFTs is
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