Enhanced electrical stability of organic thin-film transistors with polymersemiconductor-insulator blended active layersJiyoul Lee, Ji Young Jung, Do Hwan Kim, Joo-Young Kim, Bang-Lin Lee, Jeong-Il Park, Jong Won Chung, JoonSeok Park, Bonwon Koo, Yong Wan Jin, and Sangyoon Lee Citation: Applied Physics Letters 100, 083302 (2012); doi: 10.1063/1.3688177 View online: http://dx.doi.org/10.1063/1.3688177 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Phosphonic acid self-assembled monolayer and amorphous hafnium oxide hybrid dielectric for high performancepolymer thin film transistors on plastic substrates Appl. Phys. Lett. 95, 113305 (2009); 10.1063/1.3231445 Structural origin of the mobility enhancement in a pentacene thin-film transistor with a photocrosslinking insulator J. Appl. Phys. 102, 063508 (2007); 10.1063/1.2780869 Copper phthalocyanine buffer layer to enhance the charge injection in organic thin-film transistors Appl. Phys. Lett. 90, 073504 (2007); 10.1063/1.2535741 Blends of semiconductor polymer and small molecular crystals for improved-performance thin-film transistors Appl. Phys. Lett. 87, 222109 (2005); 10.1063/1.2136356 Humidity effect on electrical performance of organic thin-film transistors Appl. Phys. Lett. 86, 042105 (2005); 10.1063/1.1852708

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Enhanced electrical stability of organic thin-film transistors with polymersemiconductor-insulator blended active layers

Jiyoul Lee, Ji Young Jung, Do Hwan Kim, Joo-Young Kim, Bang-Lin Lee, Jeong-Il Park,Jong Won Chung, Joon Seok Park, Bonwon Koo, Yong Wan Jin,a) and Sangyoon LeeDisplay Device Laboratory, Samsung Advanced Institute of Technology, Yongin-si 446-712, South Korea

(Received 29 November 2011; accepted 6 February 2012; published online 22 February 2012)

We report on an enhanced electrical stability of organic thin-film transistors (OTFTs), where an

organic semiconductor (poly(didodecylquaterthiophene-alt-didodecylbithiazole) (PQTBTz-C12)) and

a polymer insulator (poly(methyl methacrylate) (PMMA)) blended film were used as the active layer,

in comparison with a single PQTBTz-C12 OTFT. While both devices exhibit similar electrical

performance in terms of mobility and ON/OFF ratios, the blended device is less susceptible to

OFF-bias stress. It is suggested that the carboxyl groups of PMMA in the blend may act as

suppressors with regards to hole accumulation in the channel, and thus, the PQTBTz-C12/PMMA

blend based OTFTs exhibit delayed threshold voltage shifts under OFF-bias stress. VC 2012 AmericanInstitute of Physics. [doi:10.1063/1.3688177]

As an alternative to conventional photolithographic

methods, the art of printing the electronic circuitry directly

on substrates, also known as “printed electronics,” has

attracted a lot of attention with regards to low-cost fabrica-

tion process.1,2 For the implementation of printed electron-

ics, polymer semiconductor that can be dissolved in various

organic solvents and functionalized in opto-electronic char-

acteristics is one of the best candidate materials. Polymer

semiconductor has, thus, been actively investigated to obtain

desired electrical properties, and many remarkable advances

in terms of charge carrier mobility have been achieved up to

date.3,4 However, despite these improvements, polymer

semiconductor based electronic devices still encounter seri-

ous issues such as electrical instability under bias stress.5,6

Such a phenomenon has been observed by many research

groups, and although it is still controversial, the associated

device degradation has been attributed to the effect of water,

oxygen, ionization potential, or structural defects. In general,

the degradation of organic thin-film transistors (OTFTs) is

manifested by shifts in the threshold voltage (VT) during

operation.5–8

In this letter, we demonstrate the realization of electri-

cally stable OTFTs by using an active layer that consists of a

blend of a polymer semiconductor and a polymer insulator.

The blending of polymer semiconducting materials is a

widely used technique in ambipolar OTFTs to transport elec-

trons and holes simultaneously by mixing n-type and p-type

semiconductors.9–11 In addition, the blending of organic

semiconductors and polymer insulators has also been used in

OTFTs for at least two purposes: reducing the environment

effects via self-encapsulation and increasing the areal uni-

formity of the device performance by controlling the self-

aggregation process.9,12–14 In such devices that involve the

blending of polymer semiconductors and insulator, the phase

separation direction (lateral or vertical with respect to the

substrate) of the blended polymer after solidification is of

fundamental importance. Generally, a lateral phase separa-

tion of blended polymer materials is suitable in order to con-

trol the crystal growth direction in the OTFT channels

(especially for soluble small-molecule semiconductor–-

polymer semiconductor or insulator) to obtain uniform per-

formance over a large area.9,15,16 On the other hand, vertical

phase separation is more desirable to lessen the susceptibility

of the OTFTs with respect to the environment. Having a

polymer insulator on top of an organic semiconductor would

passivate the latter, thus making it less prone to degradation

upon exposure to the ambient.9,14

In the present work, we rely on the vertical phase sepa-

ration of blended materials having a polymer insulator on

top of an organic semiconductor to enhance the OFF-bias

stability. Rather than by inhibiting the ambient effects, the

major improvement in device stability is based on the sup-

pression of hole accumulation at the channel region, as will

be described later on. In order to fabricate OTFT devices, a

liquid-crystalline copolymer, poly(didodecylquaterthio-

phene-alt-didodecylbithiazole) (PQTBTz-C12)17 was used

as the semiconductor, and poly(methyl methacrylate)

(PMMA) was used as the insulator. Semiconductor/insulator

blend solutions were prepared by mixing 1 wt. % of

PQTBTz-C12 (Synthesized in SAIT, Avg. Mw� 18 000)

and PMMA (Aldrich, Avg. Mw� 93 000) in chlorobenzene

(Aldrich). Thin films with various PQTBTz-C12 and PMMA

ratios were spun onto octadecyltrimethoxysilane (ODTS,

Aldrich) treated silicon oxide substrates by conventional

spin-coating. The thickness of all blended films was con-

firmed by a surface profiler to be approximately 120 nm. Co-

planar (bottom contact and bottom gate) TFT devices were

fabricated, and the electrical characteristics of devices with a

channel width of 1000 lm and a length of 100 lm were eval-

uated as shown in Figure 1. All blending ratios of PQTBTz-

C12 and PMMA (x: 1� x), where x ranges from 0.4 to 0.6,

result in devices that exhibit field-effect characteristics.

Since the transport of charge carriers within the solidified

blend film can occur only in the semiconducting PQTBTz-

C12 layer, it is anticipated that phase-separation of the two

organic components has occurred, with the insulating

PMMA layer forming on top of the stack. Figure 2(a) shows

a)Author to whom correspondence should be addressed. Electronic mail:

ywjin@samsung.com.

0003-6951/2012/100(8)/083302/5/$30.00 VC 2012 American Institute of Physics100, 083302-1

APPLIED PHYSICS LETTERS 100, 083302 (2012)

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time of flight (TOF) secondary ion mass spectrometry

(SIMS) results obtained from a spin-coated film prepared on

a SiO2/Si substrate, with a blend ratio of 4:6 by weight frac-

tion. Figure 2(a) shows that the first signal detected is oxy-

gen, which is highly likely to arise from the side chains of

PMMA. The sulfur signal is also detected at a depth of

approximately 50 nm from the top surface of the film. These

results indicate that vertical phase separation has indeed

occurred in the PQTBTz-C12/PMMA stack. It is suspected

that preferential adsorption of the relatively hydrophobic

PQTBTz-C12 onto the hydrophobic ODTS-treated SiO2 sub-

strate resulted in such phase separation.9 Grazing angle inci-

dent x-ray diffraction (GIXRD) measurements were also

carried out on the blend films to observe the crystalline struc-

ture and molecular orientation, as illustrated in Figure 2(b).

As was shown in former reports for single PQTBTz-C12

films, GIXRD patterns of a PQTBTz-C12/PMMA blend

show the presence of a well-ordered molecular structure with

a perfect edge-on orientation, indicated by narrow spots in

the out of plane direction.17 More evidence on the morphol-

ogy was obtained by tapping-mode atomic force microscopy

(AFM) as shown in Figure 2(c). The PQTBTz-C12/PMMA

blend exhibits a partially polycrystalline flake-like PQTBTz-

C12 (upper side in the AFM image) and a smooth PMMA

phase in the lower side of the AFM image.

Figures 3(a) and 3(b) show representative drain current

(IDS) vs. drain voltage (VDS) plots at different gate voltages

(VGS) for a single-PQTBTz-C12 based TFT (as a reference

sample) and a PQTBTz-C12/PMMA (4:6 weight ratio) based

TFT, respectively. The output characteristics of both transis-

tors display reasonable saturation behavior and little hysteresis

between forward and reverse sweeps. They show well-

resolved linear behavior (ohmic region) in the low drain-

source voltage range (VDS<�1 V). The output current levels

(jIDSj) of both OTFTs are similar, being larger than

0.9� 10�5 A at VGS¼�40 V and VDS¼�40 V. The transfer

characteristics were evaluated by measuring the drain current

as a function of gate voltage for the PQTBTz-C12 based TFT,

and PQTBTz-C12/PMMA based TFT is also shown in Figures

3(c) and 3(d), respectively. From the slope of VGS vs.ffiffiffiffiffiffiIDS

p,

the saturation mobility is calculated to be �0.25 cm2/Vs and

�0.20 cm2/Vs for the PQTBTz-C12 and PQTBTz-C12/

PMMA based TFT, respectively. For both devices, the

ON/OFF ratio is typically over 106, and the turn-on voltage is

FIG. 1. (Color online) Schematic diagram

of bottom-contact OTFTs and chemical

structure of PQTBTz-C12 and PMMA

used in this study (left) and the optical

micrograph (channel length¼ 100lm)

(right).

FIG. 2. (Color online) (a) TOF-SIMS

depth profiles of oxygen and sulfur in

blended PQTBTz-C12 and PMMA. The

graph shows phase separation of PMMA

(upper)/PQTBTz-C12 (lower). (b) High-

resolution x-ray scattering patterns of

PQTBTz-C12 and PMMA blended films.

(c) AFM image of PQTBTz-C12 and

PMMA blended films.

083302-2 Lee et al. Appl. Phys. Lett. 100, 083302 (2012)

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less than a volt with a relatively low sub-threshold swing

(�0.85 V/dec.). The almost identical performance of both

devices may be interpreted in such a way that the lower poly-

mer semiconductor layer formed by the vertical phase separa-

tion in PQTBTz-C12/PMMA based TFT is thick enough to

conduct hole carriers and its crystalline quality also similar to

that of the single PQTBTz-C12 based TFT as supported in

Figure 2(b).

In order to examine the electrical stability of the single-

PQTBTz-C12 based TFT and PQTBTz-C12/PMMA based

TFT, prolonged gate bias experiments were done with

VGS¼þ20 V (OFF-bias) or VGS¼�20 V (ON-bias) at

VDS¼�10 V in air ambient with relative humidity (R.H.)

�40%. Each bias stress was applied for a total duration of 10

000 s, and the stress effects on the threshold voltage were

collected before and after the stress experiments. According

to Chua et al., electrons in p-type organic semiconductor

based devices can be injected under positive gate bias into

the semiconductor even from high-work-function electrodes

such as gold, because organic semiconductor has the intrinsic

ambipolar nature.18,19 Thus, as shown in Figures 4(a) and 4(b),

for both devices, the threshold voltage shifts in the direction

identical to that of the applied gate bias polarity. In the case

of negative bias stress (ON-bias), the amount of threshold

voltage shift (DVT) is relatively small (<5 V) and almost

identical for both devices. However, a relatively large DVT

(>15 V) is observed during positive bias stress (OFF-bias)

for the single PQTBTz-C12 based TFT while the PQTBTz-

C12/PMMA based TFT undergoes smaller shifts (<5 V) in

VT. These differences are more clearly observable when we

display the threshold voltage shifts as a function of time as

shown in Figure 4(c).

In Figure 4(c), experimental results under ON bias stress

(VG¼�20 V, VDS¼�10 V) and OFF bias stress

(VG¼ 20 V, VDS¼�10 V) for both devices are indicated as

scattered points, and these fit well (solid lines) to the

stretched exponential equation,

DVTðtÞ ¼ DV0

�1� exp � t

s

� �b� ��

;

where DV0 ¼ VGS � V0T , V0

T being the threshold voltage

before exerting the bias stress. The characteristic trapping

time (s) and dispersion parameter (b) describe the trapping

phenomenon of carriers and localized energy state distribu-

tion in the channel material, respectively. This equation is, in

general, well accepted to express the associated device deg-

radation upon bias stress.17,19–21 For the ON bias stress on

both devices, the fitted s value and b value are extracted to

be �1.4� 105 and �0.29, respectively. On the other hand,

the s value of the single PQTBTz-C12 based TFT under

OFF-bias stress is �9� 103 and the b value is �0.99, while

the s and b values of the PQTBTz-C12/PMMA blend based

TFT are �5� 104 and �0.89, respectively. For the stretched

exponential function, it should be noted that the parameter sindicates the time at which the function value has reached

63% of the saturation value expected for t!1, and the

time required to reach saturation is much longer when

b� 1.21

FIG. 3. (Color online) Output curve

(VGS: 0 V��40 V) of devices with

W¼ 1000 lm and L¼ 100 lm (a) based

on single PQTBTz-C12 and (b) based on

PQTBTz-C12/PMMA blended films.

Transfer characteristics of (c) the

PQTBTz-C12 based OTFT and (d) the

PQTBTz-C12/PMMA blend based

OTFT.

083302-3 Lee et al. Appl. Phys. Lett. 100, 083302 (2012)

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The bias stress results may lead us to suspect that while

the number of shallow hole traps is identical in both devices

(similar shift in VT during ON bias), the device with the

blended active layer has a smaller number of electron trap

sites at the semiconductor/dielectric interface (smaller shift

in VT during OFF bias). One possible origin of the different

electrical stabilities between the blend film transistor and the

control device involves the different degrees of unintentional

doping caused by ambient air. The blend thin film shows a

vertical phase separation that forms PMMA barriers (which

only covers part of the thin film as shown in the AFM image

of Figure 2(c)) against diffusion of dopants/extrinsic mole-

cules. On the other hand, the control device is vulnerable to

the interaction between the polymer semiconductor under

bias stress and the extrinsic molecules in ambient air. The

higher off-current in the control device may also imply the

presence of unintentional doping. Yet, if one considers the

similar device performance of both devices in terms of field-

effect mobility and the initial threshold voltage or turn-on

voltage, the electron trap density at the semiconductor/

dielectric interface in both devices should also be of the

same magnitude. Therefore, it is believed that for the OFF

bias stress, the difference in VT arises rather from the pres-

ence of the upper PMMA layer, which contains carboxyl

groups. The carboxyl groups in the upper PMMA layer gen-

erally contain strong negative dipoles because of the high

electronegative potential of oxygen atoms (Figure 4(d)). As

a gate field is applied, the negative dipoles would decelerate

the accumulation of majority hole carriers at the channel

region of the lower PQTBTz-C12 active layer. Hence, the

PQTBTz-C12/PMMA based TFT exhibits a somewhat

delayed turn-on behavior compared to the single- PQTBTz-

C12 based TFT, as the gate voltage is swept from positive to

negative values. Negligible changes in the field-effect mobil-

ity values indicate that no additional defect states are created

at the semiconductor/dielectric interface during the bias

stress experiments.

In summary, OTFT devices with enhanced stability

were fabricated by blending the organic semiconductor

(PQTBTz-C12) with a polymer insulator (PMMA), in com-

parison with a single semiconductor PQTBTz-C12 active

layer. While exhibiting similar electrical performance in

terms of field-effect mobility and ON/OFF ratios, the device

with the blended active layer is less susceptible to positive

(OFF) bias stress. Thus, we believe that the carboxyl groups

of PMMA in the blended active layer may act as suppressors

with regards to hole carrier accumulation in the channel

region and are accompanied by delayed turn-on in the trans-

fer characteristics.

The authors gratefully acknowledge helpful technical

support of atomic force microscopy by Ms. Y. Kwon in the

AE group at SAIT, and we also thank Professor Kilwon Cho

and his colleagues at POSTECH for helpful technical sup-

port of synchrotron x-ray diffraction.

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FIG. 4. (Color online) Threshold volt-

age shift (DVT) upon negative gate bias

stress and positive gate bias stress of (a)

the PQTBTz-C12 based OTFT and (b)

the PQTBTz-C12/PMMA blend based

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083302-4 Lee et al. Appl. Phys. Lett. 100, 083302 (2012)

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