CHAPTER 5 COLOR FILTERING FUNCTIONAL OTFTS
5.1 P ART . A O RGANIC THIN - FILM TRANSISTORS WITH COLOR FILTERING FUNCTIONAL
5.1.1 Introduction
Organic electroactive materials have received much attention recently because they could offer low-cost approaches, such as low temperature and printing methods, fo
) (PEDOT:PSS), could be used as the active material for lectrochemical transistors, electrochromic displays, push-button input devices and atteries in a system-on-a-sheet label [68]. As a result, only one step is required to rint the PEDOT:PSS layer for all the devices in the electronic label. Herein, we ported organic thin-film transistors (OTFTs) with an additional function of olor-filtering. The colored polymer insulators not only serve as the dielectric r the manufacture of electronic products [61]. Additionally, the devices made of organic compounds have great potential for applications on flexible electronics such as smart cards, radio-frequency identification tags, and paper-like displays [62- 67,24]. On the other hand, from the viewpoint of the value structure of printing technology, it is necessary to reduce the number of printing steps to fabricate truly low-cost products [61]. Therefore, to accomplish printed electronics, one rather promising approach is to use a multifunctional material (ink) as the common component of different devices in parallel in a certain electronic system [61]. For example, the polymer material, poly(3,4-ethylenedioxythiophene): poly (styrenesulphonate
materials for field-ef sistors, which could be the driving-circuits, but also as color filters for liquid crystal displays (LCDs). Traditional LCDs compose of a
cycling ratio, and minimizing energy/waste have to be taken unctional ce color filters significantly contribute the bulk of material cost, integrating color filters and gate insulators is also an effective method for reducing the overall cost of LCDs and process steps.
5.1.2 Experiment
fect tran
backlight module, 2 polarized films, a TFT circuit array, a liquid crystal layer, and a color filter array. For saving the earth, reducing the number of process steps, improving material re
into consideration. This work represents one potential example for multif organic electronics. Further, sin
Typical colorant inks are composed of dispersants, nano-sized pigments, styrene, acrylic acid, and azo-bisisobutyronitrile. To achieve multiple functions, we further introduced a polymeric material, poly(2,2,2-trifluoroethyl methacrylate) (PTFMA) [69,70] (Fig.5-1), whose dielectric constant (κ) is equal to 6.0, to modify the surface of the color filters. The PTFMA layer smoothes the surface of the color filters, facilitating the crystallization of the semiconducting molecules, pentacene, and its higher dielectric constant helps to induce more field-effect charges, increasing the output current and driving capability. The molecular structure of PTFMA is similar the that of polymethylmethacrylate (PMMA) (Fig. 5-1). Although PMMA has excellent film formation properties, the low dielectric constant of PMMA (κ = 2.7) [71] usually results in lower output current. Therefore, we replaced the hydrogens on the terminals of the side chains with trifluoromethyl (-CF3) groups. Owing to its high polarity, the CF3 group increased the dielectric constant from 2.7 to 6.0. As a result, the use of PTFMA could increase the capacitance of the dielectric insulators.
Further, from the viewpoint of optical properties, PTFMA has limited absorption in the visible regime. Therefore, the absorption spectra and the corresponding CIE coordinates of the multilayer insulators remain unchanged.
(b) Chemical structure of polymethylmethacrylate (PMMA) (c) Chemical
The cross-section of the OTFTs in this work is illustrated in Fig. 5-1. The devices were fabricated on indium tin oxide (ITO) patterned glass substrates and the 100 nm thick ITO was used as the gate electrodes. Commercial colorant inks (Everlight Chemical Industrial Corporation) [46] were spin-coated on the substrates, and the colored films were subsequently cured at 90 C for 15 min and then at 230 C for 40 min. PTFMA dissolved in propylene glycol monomethyl ether acetate (PGMEA) (9.0 wt%) was spin-coated on the as-prepared color filters and then cured at 100 C
Fig. 5-1 (a) The cross sectional illustration of the OTFT with a bi-layer colored dielectric insulator consisting of a commercial color filter and PTFMA.
structure of poly(2,2,2-trifluoroethyl methacrylate) (PTFMA). (d) Chemical structure of poly(α-methylstrylene) (PαMS).
o o
for 1 hr. For some devices, an additional layer of poly(α-methylstrylene) (PαMS) (5 nm) was further coated from toluene solutions (0.1 wt%). After the preparation of the dielectric layers, pentacene was thermally evaporated on the insulators as the emiconductors of the devices. Finally, gold was thermally deposited as the source ) and drain (D) electrodes through a shadow mask. The channel length (L) and idth (W) of the pentacene OTFTs were 130 and 2000 μm, respectively. The evices with a metal-insulator-metal (MIM) structure, consisting of the dielectric yers sandwiched between ITO and Al, were used for capacitance measurements.
he calculated dielectric constants were 3.5, 4.7, and 4.0 for red, green, and blue ielectric layers, respectively. The capacitance measurements were conducted with a P 4284A Precision LCR meter. The transmittance spectra were obtained by a erkin Elmer Lambda 650 spectrometer. The CIE coordinates were measured by a
onoScope (Autronic-Melchers, GmbH). The film thickness and roughness were easured using a DI 3100 series atomic force microscope (AFM). The electrical
chara ctor
param
.1.3 Result and discussion
Initially, the colorant materials were used directly to serve as the dielectric layers.
However, limited field-effect and larger hysteresis were observed. The poor device performance was probably owing to the high polarity of the surface. On the other hand, after the modification of PTFMA, hysteresis was inhibited and larger output current was obtained. Fig. 5-2(a) show the transfer characteristics of color filtering OTFTs at room temperature. The extracted motilities in the saturation region following the conventional field effect model were 0.31, 0.21, and 0.42 cm2/Vs for s
cteristics of the OTFTs were measured with a Keithley 4200 semicondu eter analyzer in a light-shielded ambient environment.
5
red, green, and blue devices, respectively. On/off current ratios for all the devices were around 105. We also discovered that the red device modified with a second thin layer of PαMS layer had an even higher mobility (~0.51 cm2Vs) [Fig. 5-2(a)].
Fig. 5-2 (a) The transfer and (b) output characteristics of the color filtering functional devices.
Further, Fig. 5-2(b) shows the typical output characteristics of colored devices (with a blue colored dielectric insulator in this case). Apparently, when the gate voltage was reversely swept, very limited hysteresis was observed, suggesting very stable device characteristics.
To further identify the function of the buffer layer, PTFMA, the surface morphologies of the colored films and the pentacene thin films on different
dielectric surfaces were examined by AFM. The typical AFM images are displayed in Fig. 5-3.
Fig. 5-3 (a) The AFM image of the surface of the red color filter. The surface
es might trap great amount of charges, iting charge transport and resulting in significant hystresis. On the other hand, the grain size o
morphology of the pentacene layers on (b) the red color filter; (c) the red color filter/PTFMA insulator; (d) the tri-layer red color filter/PTFMA/PαMS insulator.
The surface of the color filters was quite rough as shown in Fig. 5-3(a). Therefore, pentacene molecules were not able to grow well on the rough surface. The grain size of pentacene on the neat color film was very small, thereby leading poor device performance [Fig. 5-3(b)]. The grain boundari
lim
n the surface of the PTFMA modified bilayer insulator became larger
[Fig. 5-3(c)]. The PTFMA significantly smoothed the surface and further changed the surface energy of the colored film, facilitating the crystallization of pentacene olecules. Further, for the devices with tri-layer insulators, pentacene also grew ell on PαMS. [Fig. 5-3(d)]. The more “compact” grains of the pentacene film robably reduced the density of charge traps at the grain boundaries, leading to even igher device mobility [Fig. 5-2(a)]. The nonpolar nature of PαMS might improve e crystal growth of pentacene. The results of the morphology study were onsistence well with the aforementioned electrical characteristics.
m w p h th c
Fig. 5-4 Optical properties of red, green, and blue functional OTFTs: (a) The transmission spectra and (b) CIE 1931 coordinates.
Since the light absorption of organic materials increases with the film thickness, thick color filter layer usually have better filtering performance. On the other hand, the capacitance of the gate dielectric decreases with the increasing insulator
thickness. Therefore, the smaller capacitance would lead to lower density of field-effect charge carriers and, therefore, higher operating voltages. As a result, the optimum thicknesses of red, green, and blue colorant films with PTFMA bi-layer were 1.36, 1.20, and 1.37 μm, respectively. Fig. 5-4(a) shows the transmission spectra of these colored devices. The light transmitted through ITO glasses, the color-filtering and the PTFMA layers. Since PTFMA has limited absorption in the visible regime, the absorption spectra were almost unchanged after the addition of the PTFMA layer. Fig. 5-4(b) shows the corresponding CIE coordinates, which were .64, 0.34), (0.36, 0.54) and (0.14, 0.15) for red, green and blue devices, respectively, covering 49.2% National Television Systems Committee (NTSC) standard. From the above results, it is proved that the devices not only have high electrical performance, but also have satisfied optical properties.
5.1.4 Conclusion
In summary, we demonstrated color filtering OTFTs with multilayer gate insulators exhibiting high field effect mobilities, on-off current ratios as well as color filter functions. The PTFMA polymer smoothed the surface of the colored films and improved the crystallization of pentacene molecules, thereby enhancing the device performance. This study provides an alternative approach to integrate
gate i ping
unique materials which having color-filtering, insulating, and polarizing abilities.
Finally, the current work also represents one practical example for multifunctional organic electronics.
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nsulators and color filters in LCDs, and even open a new vision for develo