R ESULT AND D ISCUSSION

For a pure cross-linked PVP film, the dielectric constant is 4.3 at 1 kHz, which close to the value reported earlier [27,30]. From Table 3-1, we can see that the dielectric constant increases with the amount of the TiO nanoparticle embedded in the thin films. For the dielectric film with 15 wt% of TiO nanoparticle, the dielectric constant increased to 10.8, due to the higher solubility of the organosiloxane surface modified TiO fillers, compared to that reported earlier [27].

Several theoretical models have been investigated to predict the dielectric constant of the nanocomposite [31,32]:

2

respectively, and υ is the filler volume percentage. Compared the modeling results with the experimental data, as shown in Fig. 3-2, the Model B, called Effective Medium Theory (EMT), which considers the average permittivity around the fillers

εc , ε1 and ε2 are the composite, base and filler dielectric cons

expanding from Bruggeman’s theory, can better and accurately predict the dielectric constant of the nano-composite.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Volume fraction of TiO2 % Experimental data

Experimental data with P8 EMT

Bruggrman L oglaw Rayleigh

Fig. 3-2 Calculated and experimental dielectric constants

lectrical parameters of the OTFTs in this study.

μ_sat (cm²/Vs)

From the experimental data, the dielectric constant of PVP film blended with 15%wt TiO2 was smaller than that of the film further modified with PαMS. It is suspected that the higher leakage current, resulted from the blending of nanoparticles, reduces the effective dielectric behavior of the dielectrics. After

PαMS treatment, the value of dielectric constant (κ=11.7) is closer to the value predicted from EMT model (κ=12.2). It is also found that at high filler volume regions, the dielectric constant increases exponentially with filler volume. Based on our calculation, it is anticipated that this method is promising for increasing the dielectric constant of organic insulators as long as we can load more fillers into the matrix.

(a) (b)

characteristics of OTFTs with (a) a composite insulator with 15 wt% TiO₂

The drain– rrent (IDS) vs. drain–source voltage (VDS FT ifferent TiO2 concentrations incorporated in the gate insulators is shown in Fig. 3-3.

Th

Fig. 3-3 The output a neat PVP insulator (b)

source cu ) of OT s with

d

e carrier mobility was calculated in the saturation regime using the following equation:

IDS = (WCi/2L)μ(VG-VTH)² eq. (3-2)

where Ci is the capacitance per unit area of the insulator, and VTH is the threshold

voltage. For the device with a neat PVP gate insulator, (Fig. 3-3(a)) the mobility in the saturation region and the threshold voltage of the OTFT are 0.42 cm² V^-1 s^-1 and -5.2 V, respectively. The on–off ratio is more than 10⁴. With 15 wt% of TiO2

nanoparticles blended into the dielectric layer, (Fig. 3-3(b)) the device exhibits more than triplet the field-induced current compared with that of the device using the pure PVP insulator which is attributed to the higher surface capacitance. Fig. 3-3 reveals he content of TiO2

anoparticles in the gate insulators. The parameters of the dielectric materials as ell as the corresponding electrical characteristics of the OTFTs with different mount of TiO2 nanoparticles embedded in the gate insulators are summarized in able 3-1. On the other hand, we also observe that the threshold voltage (VTH) ecreases and then increases when more nanoparticles were added (Table 3-1). From e surface morphology study by AFM, the insulator roughness increased with the creasing concentration of TiO2 blended. Consequently, the shift of VTH to higher alues may be the result of the insulators surface roughness. The interface between e organic semiconductor and the insulator is affected by the incorporation of TiO2

anoparticles. Additionally, we can also find that the on/off ratio decreases while the

conce evice

with 15 wt% TiO2 h that with 1 wt%

anoparticles. The leakage problem is probably due to the low-band gap of TiO2. In ad

). In order to rectify this problem, the insulator layer was covered with a of cross-linked that the drain–source current increased by increasing t

n

ntration of TiO2 increases (Table 3-1). Fig. 3-4 shows clearly that the d as much higher leakage current than

n

dition, structure defects induced by the present of high concentration TiO2 might also result in the higher leakage current, which has been confirmed from the fact that the surface roughness of the insulators increased with the content of nanoparticles (Table 3-1

poly(α-methylstrylene) (PαMS) layer. Due to the robustness

polymers, the underlayer was not affected by this process. As shown in Fig. 3-4, the

device off current is dramatically suppressed after spin-coating ~3 nm PαMS on the nanoparticle/cross-linked PVP insulator. The over-coating of another interfacial layer can reduce the concentration of surface defects of the dielectrics and smooth the dielectric surface. In addition the dielectric constant of the insulator modified with PαMS is higher than that without modification. This further supports the fact that PαMS can inhibit the leakage current and enhance the dielectric strength of the composite polymer. On the other hand, the smooth dielectric surface might also induce the formation of a more orderly crystalline pentacene film, and subsequently, increase the device mobility as shown in Table 3-1.

Fig. 3-4 The transfer characteristic at constant VDS= -30V for OTFTs with 1wt%, 15wt% of TiO2 nanocomposite insulators, and nanocomposite insulator with PαMS interfacial layer

Table 3-1 apparently shows that the surface roughness of the dielectric layer affects the device mobility in the saturation regime. While the concentration of TiO2

is more than 5 wt%, the mobility drops dramatically. The rough dielectric surface probably interferes with the formation of an ordered crystal structure. Fig. 3-5 shows the surface morphology of pentacene deposited on different dielectrics. In contrast to the clear crystal formation on the neat crosslinked PVP, the grain size of pentacene on the 15 wt% TiO2 filled-PVP film is rather small. The higher concentration of grain boundary might limit the charge transport in the organic films. However, after over-coating the PαMS layer, typical lamella morphology appears again, which

plies the formation of an ordered crystal structure.

(a) (b) (c)

Fig. 3-5 AFM height-mode images of pentacene deposited on the surface of (a) neat cross-linked PVP; (b) cross-linked PVP blended with 15 wt% TiO₂ nanoparticles; (c) cross-linked PVP blended with 15 wt% TiO₂ nanoparticles and further modification with PαMS interfacial layer.

im

r gate insulator.

The inset shows the corresponding output characteristic from V_GS = 0 V to -10 V

In summary, the PαMS layer not only suppresses the leakage current by reducing the concentration of defects in the dielectric layer, but also induces pentacene to form a more ordered molecular conformation thus maintaining the high mobility in the conducting channel. Since the leakage problem has been overcome by incorporating an interfacial layer, thinner dielectric layers will be allowed to achieve a greater capacitance value. Fig. 3-6 shows the output characteristics for an OTFT

with TiO2

filled te is

10.5, which is slightly lower than that of previous one. The device exhibits mobility

2 /Vs. The subthreshold swing is 1.0 V/decade and the threshold voltage . The on–off ratio is more than 3.0 x 10⁴. From Fig. 3-6, it is apparent that s can be fabricated by using nanocomposite dielectric polymers Fig. 3-6 The transfer characteristics of the OTFT with a thinne

a 270 nm nanocomposite insulator, consisting of one layer of 15 wt%

PVP film and another thin PαMS. The dielectric constant of this composi

of ~ 0.4 cm

with simple and solution-processable processes.

We also tried to make the devices onto flexible PET substrate. The device tructure was same as pure PVP gate insulator device (700 nm PVP gate insulator).

ue to the limitation temperature of PET substrate, we changed the PVP curing ondition from 200⁰C, 20 min to 100⁰C, 12 hour. Fig. 3-7 shows the bendable evices and output curves. The averaged mobility was around 0.1 cm²/Vs. The

exible organic TFTs were demonstrated by using polymer-based gate insulators.

.4 Conclusion

In conclusion, high performance organic thin-film transistors incorporated with the dielectric layers have been demonstrated

succe the

nanocomposite insulators, has been overcome using over-coat of another thin in

s D c d fl

3

high dielectric nanoparticles in

ssfully. Moreover, the problem of leakage current of OTFTs, while using

terfacial layer. This method offers a feasible and economic way to deposit gate insulators for OTFTs with high capacitance without the complications associated with sputtering of high dielectric materials and high temperature annealing. Finally, one low-voltage OTFT, which can operate within 10 V, has also been achieved by this method.

Fig. 3-7 (a) Device structure and bending view of flexible OTFTs. (b) Output ristics (IDS-VDS) of the flexible OTFT.

characte

Chapter 4

Organic thin-film transistors with reduced photosensitivity

4.1 Part. A Organic thin-film transistors with reduced photosensitivity

4.1.1 Introduction

Currently, there is increased interest in thin-film transistors made of organic materials due to their great potential applications in low-cost and flexible electronics, such as smart cards, radio-frequency identification (RFID) tags and paper-like displays [17,33- 37]. A level of performance by organic thin film transistors (OTFTs) comparable to that of amorphous silicon (a-Si) has been achieved. For example, a field-effect mobility higher than 1 cm²/Vs and with several orders of on–off ratio has been demonstrated for OTFTs based on pentacene. While the intrinsic properties of organic sem

extensively, their reliability and photoresponse levels have not received much attention until recently [38- 43]. Moreover, methods to lower photosensitivity and to improve device stability have received considerably less attention. On the other hand, the photosensitivity of OTFTs is a critical issue for driving applications in displays, such as liquid–crystal displays (LCDs) [43]. For instance, pentacene has high level of photo absorption in the visible range, which is due to its high oscillator strength and low energy between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO): around 2.3 eV. Therefore, iconductors and the device physics of OTFTs have been studied

light from backlight modules may pass through the OTFTs and cause a serious threshold voltage (V t [43]. As a result, OTFTs usually cannot be turned off effectively when the gate bias is set at zero under illumination [42,43]. In this work,

shift is minimized and the output current becomes be attributed to nters induced by the TiO2 nanoparticles. Moreover, by reducing the photosensitivity, the pentacene-based OTFTs have the potential to drive the circuits without a noticeable VTH shift and with no light shield. Finally, the device re

TH) shif

one method to reduce photosensitivity and to enhance device stability has been reported. To put it simply, by blending the polymer dielectrics with titanium dioxide (TiO2) nanoparticles, the VTH

more stable under white light illumination. This improvement can the recombination ce

ported in this work will greatly facilitate the making of more reliable transparent organic electronics: an area which has received much attention recently [44,45].

(a) (b)

Fig. 4-1 (a) The device structure of the OTFTs in this study. (b)Energy band diagram of pentacene and TiO2

Pentacene

4.1.2 Experiment

s thermally vaporated as the source and drain electrodes. The channel length (L) and width (W)

suppress the degradation of e on–off ratio after adding the nanoparticles, the insulator was further overcoated ith a very thin-layer of poly(a-methylstrylene) (PαMS) by spin-coating from a iluted toluene solution (0.1 wt%). Due to the robustness of the cross-linked olymers, the underlayer was not affected by this process [46]. The film thickness nd roughness were measured using a DI 3100 series atomic force microscope

FM). The current–voltage (I–V) characteristics of the OTFTs were measured with

a HP as a

stand s were performed under the atmosphere.

.1.3 Result and Discussion

The transfer characteristics of the devices before and after the modification of the Titanium oxide with a rutile structure (κ = 114) was used as the high-dielectric component in the nanocomposite dielectrics [27,46]. The device structure is shown in Fig. 5-1(a). Poly-4-vinylphenol (PVP) and poly(melamine-co-formaldehyde) methylated were dissolved in propylene glycol monomethyl ether acetate (PGMEA), blended with TiO2 nanoparticles, whose surface was further modified with organosiloxane to enhance their solubility (Ishihara Sangyo Kaisha LTD., Japan) [46]. The solution was then spin-coated onto the indium–tin-oxide (ITO) patterned glass substrates and ITO was used as the gate electrodes. The resulting film was further thermally annealed to 200 ⁰C. Then, the thermally evaporated pentacene was used as the semiconducting layer for the OTFTs. Next, gold wa

e

of the devices were 100 and 2000 μm, respectively. To th

4156A semiconductor parameter analyzer. The illumination light source w ard Hg lamp. All measurement

4

PαMS are shown in Fig. 4-3(a). The extracted mobility of the PVP device following the conventional field effect transistor model is 0.1 cm² /V s. The threshold voltage (VTH) and subthreshold swing (S) are 0.5 V and 2.9 V/ decade, respectively. Initially, the on/off ratio was about 1.0 x 10⁴, but after the addition of a thin layer of the low-κ material, PαMS, the S became smaller, the on/off ratio was improved by one order;

while the mobility was almost unchanged. The controllable VTH and turn-on voltages (VTO) in pentancene TFTs have been demonstrated by using different organosilanes with various functional groups as the self-assembled monolayers (SAMs) on the SiO2 insulators [47,48]. To quantify the device characteristics, the VTO is defined as the gate voltage at which the drain current starts to increase exponentially; while the transistor is in a flat-band position [48,49]. The built-in electric field resulting from the polar SAM molecules induces mobile charges which alters the threshold voltage [49].

Table 4-1 Water contact angle of different surface

Insulator Modified material Water angle

(a) (b)

69

⁰

86.7

⁰

0

0

PVP PαMS

Fig. 4-2 (a) Water contact angle of PVP; (b) PαMS

In this study, the water contact angle on the cross-linked PVP surface is 69.0 , while the angle on PαMS is 83.7 (Table 4-1 and Fig. 4-2). This indicates that cross-linked PVP is much more polar than PαMS. Similarly, the high polarity of cross-linked PVP, which is attributable to its hydroxyl groups, may also explain the high surface polarization which causes the higher mobile charge density.

Consequently, as shown in Fig. 4-3(a), the VTH and VTO are more positive when compared with those of devices with a PαMS thin-film. Such phenomena suggest the presence of a dipole field at the pentacene/dielectrics interface [48]. However, after applying a low-κ PαMS onto the PVP, the VTO shifts from 7.5 V to 2.5 V. This change implies a lower free charge density at the interface. The non-polar properties of PαMS prevent surface polarization, thereby reducing the charge density in the conducting channel at VGS = 0 V. While the device was under white illumination (10 mW/cm²), the VTH and VTO of both the devices shifted toward the position direction, the off-current became stronger (Fig. 4-3(a)). In addition, the S also increased. It has been suggested that the photo-generated electron-hole pairs increase the off-state current dramatically [42,43]. However, while the holes can flow out through the drain electrode under the electrical field, the electrons may be trapped at the grain

boundaries of the pentacene, and/or the insulator/pentacene interface, resulting in a positive shift in the VTH [42,43].

(a)

(b)

Fig. 4-3 Transfer curves for devices in the dark and under illumination (10mW) with (a) cross-linked PVP and cross-linked PVP/PαMS double layers as the dielectric insulators and (b) PVP + 15 wt% TiO2 as the gate insulator.

A change in the behavior of the devices was observed upon blending the TiO2

nanoparticles into the gate dielectrics. Fig. 4-3(b) shows the transfer characteristics of the device in the dark and under illumination. In the dark, the VTH becomes lower because of the use of an insulator with a higher dielectric constant. While the S also decreases to about 1 V/decade. Such phenomena are quite consistent with previous reports on OTFTs with adopted high-κ insulators [36]. Upon illumination, instead of a positive shift, the VTO comes closer to 0 V, the S becomes even better. The changes of the VTH and S are exactly opposite to those of the devices with PVP and PVP/PαMS (Fig. 4-3(a)) as the gate dielectrics. The electric parameters of the devices have been summarized in Table 4-2.

Table 4-2 Electrical parameters in this study

To explain the different behaviors of the devices represented in Fig. 4-3, several experiments have been conducted which attempt to identify the possible mechanisms involved. First, from the surface morphology analysis using an AFM, it has been shown that the m

both t MS film (Fig.

-4) [46]. Furthermore, the X-ray diffraction patterns of the pentacene deposited on e neat crosslinked PVP, the PαMS, the nanocomposite with additional PαMS, all

Device Mobility

PVP dark 0.1 4.3 290nm 13.1 0.51 2.85 3.86*10¹² 9*10³

PVP light 0.1 4.3 290nm 13.1 4.5 3.19 4.33*10¹² 4*10³

PVP/PaMS dark 0.13 4.2 290nm 12.8 -4.34 1.76 2.30*10¹² 8*10⁴

PVP/PaMS light 0.14 4.2 290nm 12.8 0.23 2.13 2.80*10¹² 5*10⁴

PVP/15wt%TiO2/P8 dark 0.4 10.5 270nm 34.1 -2.83 1.09 3.71*10¹² 5*10⁴

PVP/15wt%TiO2/P8 light 0.4 10.5 270nm 34.1 -2.89 0.85 2.85*10¹² 2*10⁴

orphology of pentacene is quite similar on the surfaces of he neat cross-linked PVP and the nanocomposite with the thin Pα

4 th

reflect the so-called ‘‘thinfilm’’ phase structure (Fig. 4-5) [50].

Fig. 4-4 AFM images of pentacene on (a) cross-linked PVP, (b)

blended with 15 wt% TiO2 nanoparticles and further modification with PαMS.

PαMS/cross-linked PVP and (c) PαMS/TiO₂ nanoparticles + cross-linked PVP

Fig. 4-5 The x-ray diffraction pattern of pentacene deposited on Si/SiO₂ substrates modified with (a)PαMS; (b)cross-linked PVP; (c) cross-linked PVP

The peaks marked with asterisks are due to the structure of TiO₂ nanoparticles.

Consequently, the morphology change is not likely to be the mechanism which

0 5 10 15 20 25 30 35 40 45

(c)

(b)

lo g( Inte nsi ty) (a.u.)

2 θ ( deg. )

SiO₂/PαMS SiO₂/PVP SiO₂/PVP+15wt%TiO₂/PαMS 5.77⁰

11.51⁰ 17.27⁰

* *

(a)

causes the different device behaviors upon blending the nanoparticles. The above entioned observation is also probably due to the processes occurring at the ITO–insulator interface. In order to determine whether the effect is due to the interface, heavy n-type doped Si and SiO2 were used in the devices as the gate electrode and the dielectrics, respectively (Fig. 4-6).

m

Fig. 4-6 Transfer characteristics of the device with n-doped Si/SiO2 as the substrate in the dark and under illumination (10mW). The inset shows the device structure.

Th 2 in

PVP) vior

(that is, a negligible VTH shift), which implies that the mechanism is not relevant to the different processes of the gate electrodes. On the other hand, the present of TiO nanoparticles in the gate insulator seems essential to minimize the effect of white

e SiO2 surface was further modified with a nanocomposite (15 wt% TiO /PαMS bi-layer. However, the device with TiO2 exhibited similar beha

2

illumination for the device. The results strongly indicate that the blending of TiO2

nanoparticles is an important part of the mechanism. The present of TiO2

nanoparticles probably induces the recombination of the centers in the channel. As seen in Fig. 4-1(b), because of the conduction band of TiO2 and/or the energy levels induced locate between the HOMO and LUMO of the pentacene, they may behave like recombination centers; thereby relea ing the trapped electrons. Consequently, upon illumination, the excess electrons can be eliminated more easily and the VTH

shift is suppressed. Stress tests on the three different devices were also performed to further understand the effects of the TiO2 nanoparticles. The typical stress-test results of the devices, both in the dark and under illumination, used in this study are shown in Fig. 4-7. For the device with a neat cross-linked PVP as an insulator, the field-induced current and/or photocurrent increased steadily over time. This result was consistent with the previous report [39,43]. It has already been indicated that the absorption of water molecules in cross-linked PVP enhances surface polarization, thus the accumulation of extra charges increases the current and results in a positive shift of VTH [39]. However, after modification with PαMS, the current increased initially, but then, decreased with time. Since PαMS is non-polar, the surface polarization probably was found to be inhibited. In addition, it was also found that

the h the

initial sharp increase, the increase of photocurrent is relieved and the current ecreases again, probably due to the natural device decay [39]. On the other hand, a m

s

ydrophobic PαMS may also retard water absorption. Consequently, after

d

ore stable current was observed after the introduction of TiO2 nanoparticles in the

ore stable current was observed after the introduction of TiO2 nanoparticles in the

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