Organic thin film transistors

Abstract

Since John Bardeen, William Shockley, and Walter Brattain invented the world's first transistor in 1947, inorganic field-effect transistors (FETs) have dominated the mainstream microelectronics industry. They are the fundamental building blocks for basic analytical circuits, such as amplifiers, as well as the key elements for digital combinational logic circuits, such as adders, shifters, inverters, and arithmetic logic units, and are used to build sequential logic circuits, such as flip-flops. Moreover, transistors are essential to the modern memory devices, integrated circuits, and microprocessors used in personal computers and laptops.

Full text

Organic thin film field-effect transistors (OTFTs) are
particularly interesting as their fabrication processes
are much less complex compared with conventional Si
technology, which involves high-temperature and
high-vacuum deposition processes and sophisticated
photolithographic patterning methods. In general,
low-temperature deposition and solution processing
can replace the more complicated processes involved
in conventional Si technology. In addition, the
mechanical flexibility of organic materials makes
them naturally compatible with plastic substrates for
lightweight and foldable products. Since the report of
the first organic field-effect transistor in 1986
1
, there
has been great progress in both the materials’
performance and development of new fabrication
techniques. OTFTs have already been demonstrated in
applications such as electronic paper
2-4
, sensors
5,6
,
and memory devices including radio frequency
identification cards (RFIDs)
7,8
. Although OTFTs are
not meant to replace conventional inorganic TFTs –
because of the upper limit of their switching speed –
they have great potential for a wide variety of
applications, especially for new products that rely on
their unique characteristics, such as electronic
newspapers, inexpensive smart tags for inventory
control, and large-area flexible displays.
In this article, we will describe the basic materials
requirements and fabrication methods for building these
by Colin Reese, Mark Roberts, Mang-mang Ling, and Zhenan Bao*
Organic thin
film transistors
Department of Chemical Engineering,
Stanford University,
381 North South Mall,
Stanford, CA 94305-5025 USA
*E-mail: zbao@chemeng.stanford.edu
September 200420
ISSN:1369 7021 © Elsevier Ltd 2004
Since John Bardeen, William Shockley, and Walter
Brattain invented the world’s first transistor in 1947,
inorganic field-effect transistors (FETs) have
dominated the mainstream microelectronics industry.
They are the fundamental building blocks for basic
analytical circuits, such as amplifiers, as well as the
key elements for digital combinational logic circuits,
such as adders, shifters, inverters, and arithmetic logic
units, and are used to build sequential logic circuits,
such as flip-flops. Moreover, transistors are essential
to the modern memory devices, integrated circuits,
and microprocessors used in personal computers and
laptops.

REVIEW FEATURE
devices, and outline the related technical issues and
challenges. Promising applications of OTFTs will also be
discussed.
Operation and materials
An OTFT is analogous to its inorganic counterpart in basic
design and function. It is a three-terminal device, in which a
voltage applied to a gate electrode controls current flow
between a source and drain electrode under an imposed bias.
A basic schematic is shown in Fig. 1, where
V
g
and
V
ds
are
the applied gate and source-drain voltages, respectively. The
control of source-drain current in FETs via a third terminal
has resulted in their widespread use as switches. Their utility
in this capacity is gauged by several key measures of their
performance. The mobility,
µ
, describes how easily charge
carriers can move within the active layer under the influence
of an electric field and is, therefore, directly related to the
switching speed of the device. This parameter can be
extracted from current-voltage measurements, and would
ideally be as large as possible. Typical values range from
0.1-1 cm
2
/V.s for amorphous-Si (a-Si) devices, with the best
organic materials achieving mobilities of 1-10 cm
2
/V.s
9,10
.
The on/off ratio, defined as the ratio of the current in the ‘on’
and ‘off’ states, is indicative of the switching performance of
OTFTs. A low off current is desired to eliminate leakage while
in the inactive state. Ratios as high as 10
6
– suitable for most
applications – can be reached by current-generation
OTFTs
11,12
.
In a traditional inorganic device, the active semiconductor
layer is generally comprised of lightly doped Si, or
combinations of Group III-V elements, such as GaAs. In these
materials, the applied gate voltage causes an accumulation of
minority charge carriers at the dielectric interface,
e.g. electrons in a
p
-type material, termed an ‘inversion
layer’. In this very shallow channel, carriers injected from the
source and drain electrodes may pass, resulting in current
flow. In an organic transistor, the active layer is comprised of
a thin film of highly conjugated small molecules or polymers
(Fig. 2), such as
p
-channel
13-15
pentacene
9,16
(
1
),
α-sexithiophene (α-6T)
17
(
2
), and poly-3-hexylthiophene
(P3HT)
18-21
(
6
), or
n
-channel
22-25
benzobisimidazobenzophenanthroline (BBL)
26,27
(
10
) and
perfluorinated copper-phthalocyanine (F
16
-CuPc)
28
(
12
). In
stark contrast to inorganic materials, organics pass current by
majority carriers, and an inversion regime does not exist. This
fundamental difference is related to the nature of charge
transport in each of these semiconductors. In well-ordered
inorganics, e.g. single-crystal Si, the delocalization of
electrons over equivalent sites leads to a band-type mode of
transport, with charge carriers moving through a continuum
of energy levels in the solid. In less-ordered organic materials,
the proposed mechanism is hopping between discrete,
localized states of individual molecules. The presence of
impurities or inconsistencies in structure may result in ‘traps’
that alter the relative energy levels, and inhibit the flow of
charge carriers. The complexities of current flow in organic
materials – which are still poorly understood – have added
another dimension to the development of the devices that
incorporate them. In addition to the challenges presented by
fabrication, particular attention must be paid to the design of
materials that will meet the performance demands of the
OTFT in its parent applications. This aspect of development
Fig. 1 Basic schematic of a field-effect transistor.
September 2004
21
Organic layer
Dielectric layer
Source
V
g
V
ds
Drain
Gate

S S
NN
Cu
N
N
N
N
N
N
F
F
F
F
F
F
F
F
O
O
NNRR
S S
N-type
(8) PTCDI-R
22
(9) NTCDI-R
23
NC
CN
n
(11) TCNNQ
24
(10) BBL
26,27
F
F
F
F
CN
CN
n
(12) F
16
-CuPc
28
C
6
F
13
(13) DFH-nT
25
C
6
F
13
O
N
NN
N
O
O
O
O
O
NR
NR
O
O
F
F
F
F
has received much attention, and will continue to play an
important role as the technologies progress.
The limitations of current organic technologies are clearly
posed by the performance and processability of the active
layer component. In addition to meeting benchmarks for
performance criteria, such as mobility and on/off ratio, active
layer materials should ideally be easy to process, mitigating
potential fabrication challenges, and have long-term stability
for device longevity. This has proven a difficult balance. The
organics possessing the best electronic characteristics to
date, small molecules such as pentacene
16,29
(
1
) and α-6T
17
(
2
), are insoluble and therefore difficult to process. Efforts to
solubilize these materials have included the incorporation of
side chains, such as the addition of alkyl groups to
polythiophene polymers (
6
,
7
). The size
30
, type
31
, and
regioregularity
20,21
of these groups have been explored
extensively, with the goal of electronic property optimization.
From these studies, additional insight has also been gained
regarding the relationships between morphological
characteristics and charge transport. The nature of
substituents, chain length, and processing conditions all
affect the packing structure in the films, which is reflected in
the electronic properties. For example, differences in
regioregularity in P3HT (
6
) samples have been shown to
cause distinctly different orientations relative to the
substrate
32
. The two films, shown schematically in Fig. 3,
have mobilities that differ by two orders of magnitude. In this
manner, careful substituent selection has been used to tune
organic semiconductors for the best all-around
characteristics
33
. Molecular weight (M
w
), as well as the
solvent used for film deposition, has also been shown to have
a significant effect on ordering and properties
15,34
. Atomic
force microscopy of regioregular P3HT polymer films (Fig. 4)
illustrates the dramatic differences in morphology between
films of 3.2 kD and 31.1 kD M
w
, whose mobilities again differ
by more than two orders of magnitude
34
. Clearly, many
REVIEW FEATURE
September 200422
S
S
S S
R
R
S
NN
Cu
N
N
N
N
N
N
S
S
S S
S
S
S
P-type
(1) Pentacene
9,16
(2) α-Sexithiophene
17
(3) Copper phthalocyanine
13
-
15
nn
n
(4) Rubrene
10
(5) Polythiophene (6) Regioregular
poly(3-hexylthiophene)
18
-
21
(7) Poly(3,3"'-dialkyl-quaterthiophene)
33
C
6
H
13
Fig. 2 Prominent (a) p- and (b) n-type organic semiconductor materials.
(a) (b)

REVIEW FEATURE
factors are at play when active materials do not exhibit the
order of single crystals, and the various effects have yet to be
deconvoluted. Recent reports of mobilities up to 35 cm
2
/V.s
for high-purity pentacene single crystals
29
, however, give
hope that further investigation will continue to pay off for
organic semiconductors.
Device design and fabrication
As previously mentioned, the OTFT inherits its design
architecture from its inorganic counterpart, namely, the
metal-oxide-semiconductor field-effect transistor (MOSFET).
It is composed of three main components: source, drain, and
gate electrodes; a dielectric layer; and the active
semiconductor layer. Within the basic MOSFET design, there
are two types of device configuration: top contact and
bottom contact. The former involves building source and
drain electrodes onto a preformed semiconductor layer,
whereas the latter is constructed by depositing the organic
over the contacts. The structures are illustrated
schematically in Fig. 5. Top contact devices have been
reported to have superior performance for certain organic
semiconductors compared with their bottom contact
counterparts. It has been suggested that this is a result of
reduced contact resistance between the electrode and the
organic layer because of an increase in the area for charge
injection
35
. Each of these devices has particular advantages
and disadvantages in the fabrication process, which will be
discussed below.
Deposition
of thin film organic semiconductor layers is
primarily performed through vapor or solution phase
processes. Vacuum deposition is used for small molecules and
oligomers. It is somewhat costly because of expensive
equipment and low deposition throughput, but produces films
with high field-effect mobility and on/off ratios. Examples of
organic semiconductor films that have been deposited in this
manner are oligothiophene and oligofluorene
derivatives
12,36-38
, metallophthalocyanines
15,39
, and
acenes
40-44
(pentacene and tetracene). OTFT device
performance can be improved by controlling the deposition
rate and temperature, which affect the morphology of the
semiconductor. Modification of the interface between the
substrate and the organic layer and post-deposition
treatments, such as annealing, also improve molecular
ordering.
Fig. 3 Packing orientation of P3HT films relative to substrate with (a) 81% and (b) 95%
regioregularity, as determined by X-ray diffraction
32
. The films differ in mobility (parallel
to substrate) by a factor >100, illustrating the effect of morphology on performance.
September 2004
23
Fig. 4 Atomic force microscopy images of regioregular P3HT films with M
w
of (a) 3.2 kD
and (b) 31.1 kD
34
. The large molecular weight polymer forms a more ordered, crystalline
film, yet has a mobility that is two orders of magnitude lower than the 3.2 kD film.
(Reprinted with permission from
34
. © 2003 Wiley-VCH.)
Fig. 5 Top and bottom contact OTFT architectures.
Source Drain
Organic semiconductor
Dielectric layer
Gate
Gate
Substrate
Substrate
Bottom contact
Top contact
Source Drain
Organic
semiconductor
Dielectric layer

For solution-soluble organic semiconductors, two forms of
deposition are available: deposition of a soluble precursor
from a solution and subsequent conversion to the final
film
45-49
, and direct deposition from solution. The motivation
to use soluble precursors is that most conjugated oligomers
and polymers are insoluble in common solvents unless side
chain substitutions are incorporated into the molecular
structures. The addition of side chains can interfere with
molecular packing or increase the π-π stacking distance
between molecules, decreasing mobility, but when used
properly can be incorporated to promote better molecular
packing, such as in the case of regioregular P3HT.
Determining the processing temperature can be challenging,
however, as the conversion temperature from precursor to
semiconductor may be too high for compatibility with low-
cost plastic substrates. Furthermore, the precursor conversion
requires an additional processing step.
Spin-coating and solution casting are two popular ways of
direct solution deposition, which is often used for polymers
such as regioregular P3HT
21,32-33
or various soluble
oligomers
50-52
. Post-processing treatments, such as thermal
annealing, improve molecular ordering and grain sizes of the
thin film and frequently result in better device performance.
It is often difficult, however, to purify the polymers and
achieve good molecular ordering over large-area substrates.
Another major concern for solution processing methods is the
effect of the solvent on underlying organic features, requiring
chemically compatible materials. For this reason, dry
processing methods are being developed
53
. Generally, it is
important to consider the organic solution concentration and
solubility, solvent evaporation rate, and substrate surface.
Because of their effect on the quality of the resulting
semiconductor film, these processing parameters should be
carefully controlled. High mobilities have been reported with
several oligomers by optimizing the deposition
conditions
50-52
.
Dielectric films are fabricated in a similar manner to the
semiconductor layer. Examples of vacuum-deposited
dielectrics include silicon dioxide
41
and parylene
54
. An
example of a solution-processed dielectric layer is
poly-4-vinylphenol (PVP)
55
, which is deposited by spin-
coating and then cross-linked at 200°C.
Patterning
is a crucial part of the fabrication of OTFTs. The
organic semiconductor must be confined to the channel
region to eliminate parasitic leakage and reduce cross-talk in
order to achieve better device performance
56
. The drain,
source, and gate electrodes need to be patterned with the
required feature size depending on the application. Typically,
the smaller the distance between the drain and source
electrode (channel length), the higher the current output and
the faster the transistor switching speed. The following
sections will discuss some typical patterning methods used
for OTFT fabrication. The most desirable methods involve
direct printing of the active materials, in which the patterning
and deposition are carried out in one single step. Such
methods provide the possibility of processing over a large
area, increasing production throughput and therefore
reducing the cost per device
30
.
Optical lithography
is a well-developed conventional
technique for the patterning of mesoscopic features and
components for microelectronic and photonic devices
57
. In
this process, geometric shapes from a mask are transferred to
a substrate (e.g. Si wafer), enabling patterning of the active
materials and electrodes. Both metal and conducting polymer
electrodes can be fabricated using standard photolithography
followed by lift-off
12,58
. Conducting polymer electrodes have
also been patterned by light exposure to change their
conductivity without having to remove the polymer in the
channel region
3,49
. Although optical lithography can achieve
100 nm resolution, it is a relatively expensive process. This
method is also less suitable for the patterning of organic
semiconductors because the exposure of organic
semiconductors to solvents and etchant tends to cause
degradation in device performance.
Screen printing
involves squeezing a specially prepared ink
through a screen mask onto a substrate surface to form the
desired pattern. This method is capable of printing all the
active components in OTFTs
59
, but has limited feature size
resolution (75 µm or larger). Components of OTFTs can also
be deposited using ink-jet printing, which is similar to the
operation of a conventional ink-jet printer, but uses specially
formulated inks
60,61
. Resolution on the order of 25 µm can be
achieved without surface modifications, while hydrophobic
dewetting patterns
62
can be used to obtain resolutions
approaching 200 nm. This hybrid method, in which ink-jet
printing and traditional photolithography are combined to
achieve the resolution necessary for device applications, is
being pursued by Plastic Logic
63
. An additive dry printing
method
53
for depositing conducting polymers has been
developed using a thermal (laser) imaging technique for the
REVIEW FEATURE
September 200424

REVIEW FEATURE
ablative transfer of a patterned layer onto a flexible receiver
layer with resolution down to 5 µm. This can be used to
process successive layers without the use of a solvent that
could degrade the underlying organic layers.
Soft lithography
encompasses a wide variety of patterning
techniques in which a master structure is fabricated in a
material, such as Si, by conventional lithographic processes,
and then used to make elastomeric replicas in a material such
as poly(dimethylsiloxane) (PDMS). In microcontact printing,
elastomeric stamps are used for molecular transfer to
produce a contact-induced chemical modification of a
surface
64-66
. The chemical modification can produce
hydrophobic and hydrophilic patterns allowing for selective
solution phase deposition of the organic semiconductor.
Alternatively, an alkanethiol protecting layer can be
microcontact printed on Au or Ag to prevent the metal
underneath from being etched away to form electrode
patterns
2
. In soft lamination, source and drain electrodes are
deposited on one substrate, then laminated onto another
substrate that already contains the gate, dielectric, and
semiconductor, thus completing the transistor
67
.
Applications
Organics have long been attractive for use in electronics
because of their light weight, flexibility, and low cost
compared with their Si counterparts. Recent increases in
performance, however, have rapidly expanded organic FETs
from niche markets, making them targets for a wider range of
applications.
Organics offer potential advantages in displays, where
TFTs are implemented as switches to activate individual
pixels. Hand-held devices (cell phones, PDAs, etc.) with
ultrathin displays can achieve higher resolution and
information content, while new technologies, such as flexible
displays and electronic paper, are potentially revolutionary
advancements. Integrated smart pixels, with an OTFT
switching an organic light-emitting diode (OLED) pixel, have
been demonstrated, even though actual OTFT active-matrix
OLED displays are yet to be demonstrated
19,20,68,69
.
An alternative to active-matrix flexible displays is an
innovative example by E-ink utilizing an OTFT backplane with
a laminated electronic ink frontplane, consisting of a layer of
electrophoretic microcapsules on a transparent electrode
70,71
.
The OTFT backplane controls the contrast of the display by
moving charged black and white pigments to the transparent
layer, as shown in Fig. 6. In late 2000, E-Ink presented the
world’s first flexible (16 cm x 16 cm) electronic ink display
using an OTFT backplane circuit created by Lucent, consisting
of an array of 256 transistors fabricated using a low-cost,
rubber stamp printing process
2
. The printed transistors from
Lucent and a typical flexible display are presented in Figs. 7
and 8, respectively. Plastic Logic, a company actively
developing ink-jet-printed plastic TFTs, subsequently has
demonstrated a bistable reflective display driven by an
ink-jet-printed active-matrix backplane together with Gyricon
Media, the provider of SmartPaper™ reusable display
material
63
. This first experimental prototype is a display
featuring 3024 pixels (63 x 48) at 50 dpi on a glass substrate.
More recently, Philips and E-Ink jointly demonstrated a
similar electronic ink display driven by OTFTs
3
with 320 x
240 pixels, a diagonal length of 127 mm, a resolution of 85
dpi, and a bending radius of 2 cm. Philips has also announced
September 2004
25
Bottom
electrode
Clear fluid
Transparent
electrode
Fig. 6 Schematic of an E-ink display
70
. An OTFT backplane addresses each element, with encapsulated, charged pigments shifting to the transparent electrode surface.

that it had formed a technology incubator company, Polymer
Vision, to partner with other companies interested in
ultrathin, rollable displays that could double as electronic
paper.
Building upon the basic OTFT component, complementary
metal-oxide-semiconductor (CMOS) technology now shows
promise in organic semiconductors. The ready availability of
both
n
- and
p
-channel semiconductors and recent discoveries
of ambipolar functionality, where a single semiconductor
layer is capable of conducting both electrons and holes, is
enabling the design of robust circuitry with low heat
dissipation
41,72
. This ambipolar behavior has been realized for
an
n
- and
p
-type polymer dispersed layer, and for a single
organic material with a low band gap, and thus a low barrier
for electron and hole injection
73,74
.
RFID cards are made possible with this complementary
circuitry for applications involving identification, verification,
and tracking. Tasks similar to inventory management can be
immensely simplified by exploiting a system of many
transponder circuits with a single reading instrument
75
. Other
low-resolution applications involving logic functions, like
smart cards, or disposable sensors are also made possible
8
.
Summary
In conclusion, the physical properties and relatively facile
processing of organic materials allow for the demonstration
REVIEW FEATURE
September 200426
Fig. 8 The world’s first flexible electronic ink display driven by organic transistors. (Reprinted with permission from
2
. © 2001 National Academy of Sciences USA.)
Fig. 7 A 256-transistor array produced by Lucent using a rubber stamp printing process. (Reprinted with permission from
76
. © 2001 Materials Research Society.)

REVIEW FEATURE
of flexible, low-cost, large-area devices using OTFTs.
Advances in fabrication methods and the development of
higher-performance semiconductor materials have improved
on existing technology and expanded the scope of potentially
realizable applications. Much of this progress has been a
result of the deduction of structure-property relationships of
the active layer component. Molecular tuning based on this
understanding has yielded materials with better electronic
properties and simplified processability. The discovery of
ambipolar functionality of organic materials has increased
OTFT versatility, enabling their incorporation in
complementary logic devices. The numerous applications
presented thus far have already showcased the potential of
OTFTs, which will continue to grow. While challenges exist
for large-scale manufacturing, their rapid development shows
great promise for their future in plastic electronics.
MT
September 2004
27
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