Paper presented at IMID 2012 regarding the development of high mobility organic semiconductors for application to Flexible Displays

1. M.A.Cowin
IMID 2012 DIGEST
Advances in High Performance Organic Semiconductors for Flexible Displays
M.A.Cowin, R.Griffiths, K.Crowley.
SmartKem Ltd, Hexagon Tower, Delaunays Road, Blackley, Manchester, M9 8ZS United Kingdom.
Tel.:+44 (0)161 795 3157, E-mail: m.cowin@smartkem.com
Keywords: Mobility, Organic Semiconductor, Flexible, Display.
Abstract
We report on high mobility, high stability, solution
processed p-type organic semiconductor (OSC) inks
used in flexible Organic Thin Film Transistors (OTFTs).
The results demonstrate that the semiconductor
materials are readily processed at room temperature and
in ambient conditions whilst affording high levels of
OTFT performance, field effect mobilities up to ~ 4
cm2
/Vs were measured. This high performance OSC is
well suited to both the manufacturing process
requirements and application performance requirements
for future flexible OTFT backplane applications.
1. Introduction
Organic thin film transistors (OTFTs) have the potential for
use as an alternative to silicon based or Indium Gallium
Zinc Oxide based (IZGO) TFT backplanes for a wide range
of display technologies. The solution processability of
organic semiconductor inks, combined with the multitude
of options to modify the formulation and print onto low
cost polymer substrates makes organic semiconductors an
attractive prospect to display manufacturers.
There is currently significant market growth and technology
development in the field of Active Matrix Organic Light
Emitting Diode (AMOLED) displays, with the Samsung
GalaxyTM
and WaveTM
Smartphones being some recent
product examples. Most commercially available AMOLED
displays tend to be driven by low temperature
polycrystalline silicon (LTPS) backplanes or in more recent
display demonstrators, by IZGO backplanes. However
neither of these inorganic backplane approaches lend
themselves to developing fully flexible AMOLED displays.
Recent work by Sony [1], has shown that incorporating a
relatively low mobility OSC, peri-xanthenoxanthene (PXX)
OTFT backplane into their so-called rollable OLED
displays did in fact result in a fully flexible AMOLED
device. So whilst it is generally accepted that organic
semiconductors are unlikely to exceed the field effect
mobilities that can be achieved from LTPS and IZGO,
organic semiconductors are able to attain adequate mobility
to drive AMOLEDs and offer significantly more flexible
form factors than the inorganic alternatives.
For an OTFT backplane to be considered as a viable
candidate for driving such a current driven technology, field
effect mobilities greater than 2 cm2
/Vs at channel lengths
of ~ 5 microns are desirable. In this work we focus on the
spin coating method to deposit our high stability OSC ink
and show how to fabricate OTFTs on a plastic substrate
having mobilities greater than 3 cm2
/Vs at short channel
length. When considering the application of OTFTs to
AMOLED displays, organic transistor materials have the
capacity to offer many of the same advantages as OLEDs
such as light weight, inherent mechanical flexibility, and
compatibility with flexible substrates. Organic transistors
are made often with similar tools and processes used for
OLEDs, so a combined fabrication process may be more
easily implemented and thus may offer additional
manufacturing advantages [2]. Here we demonstrate a
novel, high performance class of OSC inks with mobility
superior to a-Si, good electrical stability and thermal
stability in excess of 250ºC.
2. Fabrication of Devices
The p-type OTFT consisted of a top gate with bottom
contact source and drain. The source and drain electrode
metals were deposited by sputtering of 5nm of Ti as an
adhesion layer followed by 50nm of gold onto a PEN host
layer, laminated to a glass carrier substrate. The source and
drain pattern was subsequently defined by standard
photolithographic techniques and wet etch. Prior to
deposition of the surface treatments or semiconductor
material no UV ozone or plasma treatment was required.
Prior to deposition of the OSC and OGI layer the source
and drain contacts were pre-treated with a SAM treatment
of 2,3,4,5,6-pentafluorobenzenethiol (PFBT). A p-type
OSC solution formulated in tetralin was used to deposit a
semiconductor layer via spin coating in air. A fluorinated
gate insulator polymer was subsequently deposited by spin
coating and dried. The final step was definition of the gate
electrodes by thermal evaporation of Au via shadow mask.
Fig. 1. Typical OTFT 100x100mm backplane
fabricated on PEN laminated to glass carrier.

2. M.A.Cowin
IMID 2012 DIGEST
All solution processing was undertaken in ambient
conditions at room temperature. It should be noted that no
isolation step of the individual OTFT devices was
performed and that the array was unpassivated. The OTFT
array reported was 100x100mm and contained 360
transistors, as illustrated in Figure 1 above.
3. Results & Discussion
A summary of device characteristics for a channel width
of 15000μm are outlined in Table 1. It may be seen from
Figure 2 that the field effect mobility in the linear regime
was approaching 4.9cm2
/Vs at a channel length of 100μm
and 4.2cm2
/Vs at a channel length of 4μm. It may therefore
be observed that for this SmartKem OSC ink there was no
significant decrease in mobility with channel length
reduction. The on/off ratio for each device, despite having
no OSC isolation, is in the order of 106
.
Table 1. Summary OTFT results
Channel
Length
Mobility
cm2
/V.s
ON/OFF
ratio
Vth
100 4-5 106
1-4V
10 3-5 106-7
2-4V
4 3-4 106-7
2-6V
Fig. 2. Transfer and mobility curves for OTFT with
L=4μm, 100μm both at W=15000μm.
The electrical stability of the devices were tested under
negative bias voltage of VGD= -30V with VD= -5V to
simulate the drive conditions for a typical pixel drive
transistor in an OLED display. It may be observed from
Figure 3 that this resulted in less than 4% shift in drain
current during this period. The respective shift in threshold
voltage was ~0.15V. Typical dual sweep output
characteristics are illustrated in Figure 4. It may be seen
that minimal hysteresis is exhibited between the forward
and reverse sweep demonstrating repeatable OTFT
performance. It should be noted that no passivation was
employed with these devices. The temperature stability of
the OSC material was tested by thermogravimetric analysis
and found to have stability in excess of 250ºC with no
weight loss until 410ºC.
Fig.3. Percentage shift in Id under negative bias
stress conditions for 7hrs for L=4μm, W=15000μm.
Fig.4. Device output characteristics forward and
reverse sweep (L=100μm W =15000μm)
4. Summary
An air-stable p-type semiconductor has been demonstrated
that offers high intrinsic thermal stability and excellent
electrical stability in OTFT devices. The material also
exhibits high field effect mobilities, in the order of 4-5
cm2
/Vs and is compatible with standard
printing/processing techniques in air. It is expected that
this new class of versatile, air-stable OSC materials will
enable present and future AMOLED display
manufacturers to achieve the desired levels of physical
flexibility, whilst maintaining acceptable electrical
performance for the first time.
References
1. N. Kawashima et.al., SID 2009, Digest, pp.25-27
2. V.Vaidya, S.Soggs, J.Kim, A.Haldi, J.N. Haddock,
B.Kippelen, and D.M. Wilson, IEEE Transactions
on Circuits and Systems, 55(5), pp1177 (2008).