Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics

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2. Matrerials, Substrates and R2R Printer

2.1. Conducting Silver Ink

For R2R printing gate and drain-source electrodes of TFTs, nanoparticle

based silver inks was formulated by using the following procedures. 40 g

of AgNO

and 15 g of poly(vinylpyrrolidone) was stirred into 1200 mL

of double distilled water under ambient conditions. After completely

dissolving all additives, 14 mL of hydrazine hydrate (reagent grade,

55%, Aldrich) was added to the solution for 1 min. After 5 min, 2800 mL

of acetone was added to the mixture and it was then further stirred for

30 mins. The resulting mixture was centrifuged at 5000 rpm for 5 min

at 10°C. After the solution was slowly decanted, 20 g of silver gel was

attained. The resulting silver gel was used for the manufacture of gravure

inks by adding appropriate amounts of ethylene glycol and hexanol to

control surface tension, as shown Table 1.

The resistivity of printed silver lines after R2R gravure printing on

PET foils using the formulated silver inks, depending the drying

temperatures and times, are shown in Fig 1. Furthermore, the R2R

gravure-printed silver patterns achieved good adhesion (5B) on PET foils

even when dried for ≤10 s using ASTM D3359 test.

2.2. Dielectric Ink

The gate dielectric layer is critical material in R2R printed TFTs since

the gate insulator capacitance will controll density of carriers in the

conducting channel and the electrical chracteristics of TFTs Eq. (1).

Therefore, R2R printed pinhole-free dielectric layers with the high

capacitance are highly desireable to attain high drain current values of

printed TFTs at low gate voltages. Since R2R printing process would

often generate many pin-holes at thinner layers, relatively thick dielectric

layers are mandatory to prevent the possible short. Therefore, high

dielectric materials are needed for providing the comparable capacitance

to the ultra-thin dielectric layers to compensate the R2R printed thick

dielectric layers.

M. Jung et al.

300

In this work, a high κ insulating ink was manufactured with a

viscosity of 200 cp and surface tension of 30 mN/m using various

amounts of barium titanate (BT) nanopowders (50 nm) and 5 g of

poly(methylmethacrylate) (PMMA) with 30 mL of MOE (methoxy

ethanol) through vigorous mechanical stirring for 2 h and then further

dispersed using ultrasonic for 4 h at 30°C. To determine the dielectric

constant of the printed insulating films with a loading ratio of BT,

parallel plate capacitors were fabricated using PET substrates and printed

silver electrodes. Capacitance measurements were carried out using a

LCR meter (Agilent LA5673) from 100 Hz to 100 KHz. Depending on

the amount of BT loaded, a dielectric constant from 13 to 40 could be

attained as shown in Fig. 2(A). Dielectric constant values were calculated

from each of the capacitances C using Eq. (1).The fabricated capacitors

had a 2.45 µm dielectric thickness as measured with a surface profilometer

as shown in Fig. 2(B).

Table 1. The surface tension and viscosity depending on conducting gravure ink

formulation.

Formulation 1 Formulation 2 Formulation 3

Silver nanoparticles 60 wt% 60 wt% 50 wt%

Ethylene glycol 27 wt% 32 wt% 50 wt%

Hexanol 13 wt% 8 wt%

Surface tension 23 mN/m 36 mN/m 44 mN/m

Viscosity 200 cp 200 cp 200 cp

Fig. 1. Temperature and drying time dependence of resistivity for gravure printed silver

patterns on PET foils.

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C = (

r o

)/d (1)

C: Capacitance of gate dielectrics,

: Relative permittivity of dielectric

layer,

: permittivity of free space, d: thickness of dielectric layer,

A: Area.

Considering the brittleness of printed dielectric layers, insulating ink

with 29 wt% BT loading to total loading [A 1:1 ratio of BT to PMMA

is 50% BT and 50% PMMA] was chosen in this R2R printing work.

Furthermore, the insulating ink was stable under ambient conditions as

no precipitation was observed for 20 days.

2.3. Semiconductive Ink

A large number of inorganic and organic materials are available to print

as active layers for TFTs. Among them, if we consider the flexibility and

low temperature curing (<150°C) of R2R printing process, organic

semiconductors are the best option to employ. However, since the

Fig. 2. (A), Dielectric constant measured between 10 Hz to 100 KHz with various

amounts of barium titanate (BT) nanopowders loaded into PMMA solutions and (B), 3-D

image of surface profilometer of R2R gravure-printed dielectric layer on gate electrodes.

M. Jung et al.

302

organic semiconductor based TFTs allow only one type of carrier to

move easily with the other being trapped, the optimal LUMO (lowest

unoccupied molecular orbital) energies for n-channel organic materials

are in the range of 3.8-4.4 eV and the optimal HOMO (highest occupied

molecular orbital) energies for p-channel organic materials are in the

range of 5.0-5.4 eV. Therefore, the electrical performance of printed

p- and n-type TFTs with active layers of organic materials are all

vulnerable to the small change of surroundings. That’s why dispersed

single walled carbon nanotubes (SWNTs) has been considered to

use as semiconductive inks to print TFTs since SWNTs as active

semiconducting layer will be an excellent candidate for printing TFTs

with longer channels since they can offer relatively good electrical

performance even under longer channel length and harsh surroundings.

However, SWNTs is not practical yet to use as the active layer

because the SWNTs has a major intrinsic problem of heterogeneous

bandstructures that span widely from metal to semiconductor when they

synthesized. Therefore, to attain reasonable on-current with a good

on-off ratio of SWNT-TFTs,

4-9

the metallic SWNTs need to be

modulated before employing as a semiconducting layer in printed TFTs.

Fig. 3. (A) Schematic illustration of the process of admicellar polymerization of styrene

on SDS/SWNTs. TEM images for bare SWNT (B) and PS wrapped SWNT (C).

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In order to escape the trade off between high on-state conductance

and switching ability, it is therefore necessary to alleviate the metallic

pathways in printed SWNT-TFTs. As shown in Fig. 3(A), we develop

the wrapping method to disconnect the metallic pathways in SWNT-

TFTs. To wrap SWNTs with ultra-thin polystyrene films, first, various

amounts of purified styrene monomers were added to four different

aqueous solutions (1 mL) of SDS-wrapped SWNTs, and the mixtures

were allowed to equilibrate for 6 h at 25°C while being stirred. After this

time, 0.08 mg (0.5 µmole) of 2,2’-azobis(isobutyronitrile) was added to

the four different mixtures to initiate the polymerization and the mixtures

were heated to 60°C in an oil bath for 12 h with continued stirring. The

resulting PS wrapped SWNTs (PS-SWNTs) were characterized using

TEM (JEM 2100F) (Figs. 3(B) and 3(C)). A sample for TEM analysis

was prepared by placing a single drop of the PS-SWNTs solution onto a

holey carbon coated-copper grid and then, blotting with filter paper to

form a thin deposition. The PS-SWNTs deposited grids were rinsed

5 times to remove surfactants using a consecutive process of dropping

double distilled water onto them and blotting with filter paper. The rinsed

grids were dried under ambient conditions.

2.4. Substrate

For R2R printing process, the substrate must be rollable foils with the

less thermal deformation. Furthermore, since electronic devices with

various inks are printed and cured at maximum 150°C, a stress field

arises due to the difference in thermal contraction between the printed

devices and the substrate upon winding and cooling. Therefore, the

thickness of substrate should be considered with the wettability and

Young’s modulus. If the stress of the printed devices become too large or

the adhesion between the printed devices and the substrate is weak, the

printed devices may crack or peel off the substrate. In our works, R2R

printing is performed on corona treated poly(ethylene terephtalate) (PET)

foils (SKC, Korea) having a thickness of 75 µm and width of 240 mm

with CTE of 30-65 × 10

-6

-1

. The surface roughness of PET was 10 nm

RMS by measuring AFM (Fig. 4).

M. Jung et al.

304

2.5. R2R Gravure

R2R printing techniques have received significant attention for the

manufacturing of various macro-electronic devices such as RFID tags,

sensors,

flexible signage

11,12

and e-paper

12,13

with an ultra-low cost.

Among commercially available R2R printing techniques, R2R gravure

has drawn a lot of attention as a practical production process for printing

macro-electronic devices since R2R gravure has a high throughput with a

mechanically simple process and fewer controlling variables than other

printing processes such as flexography, offset, gravure-offset, screen,

etc.

14-19

To enable the use of gravure printing to realize properly scaled

and integrated electronic systems, however, the microscopic feature

production of R2R gravure printing needs to be studied in detail. As a

typical example, a commercially available R2R gravure should not

generate any problems for printing graphic arts and packaging where

microscopic defects and surface roughness (<100 µm) are acceptable.

However, to print macro-electronic devices, those microscopic defects

will completely damage the devices and will drastically limit circuit yield.

Therefore, the R2R gravure printing process has been first evaluated to

determine whether the R2R gravure is suitable for rendering defect-free,

reliable and constant microscopic uniformity (<100 µm) under the

relationship between the various print, ink, and substrate-related

variables.

In this work, we are employing the evaluated 4 color units of

R2R gravure (Fig. 5) to print TFTs and digital circuits on plastic foils

with the printing speed of 12 m/min, roll pressure of 1.5 MPa on the PET

Fig. 4. AFM image of PET surface.

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web, and a sufficiently high pressure of the doctor blade with an angle

of 60°.

By using R2R gravure in Fig. 5, SWNT-TFTs, inverters, NAND gate,

NOR gate and 4-bit digital circuits were respectively printed on PET

foils by using above-disscussed inks as shown in Scheme 1. At the first

Scheme 1. Schematic illustration of R2R gravure printing process to print SWNT-TFTs

on PET foils.

Fig. 5. Actual image of R2R gravure with 4 color units manufactured by AVACO, Korea.

M. Jung et al.

306

printing unit, silver gate lines were printed with 200 µm width and then,

at the second printing unit, dielectric layers were printed with the

thickness of 3.48 µm and surface roughness of 50 nm RMS. The drain-

source electrodes with thickness of 357 nm and weaveniness of 1 µm

were then selectively printed on printed dielectric layers with OPA of

±50 µm in pararelle to the printing direction and ±10 µm in vertical to

the printing direction. Finally, SWNT networks were inkjet printed on

printed drain-source electrodes. The cross-sectional SEM images for the

three different printed layers are shown in Fig. 6.

3. R2R Gravure Printed Circuits on PET Foils

In this section, the basic operating principles of SWNT-TFT based

p-channel TFT circuits is discussed. Usually Si based complementary

circuits with n-channel and p-channel TFTs were usually considered to

have many advantages over circuits constructed by either only p-channel

TFTs or n-channel TFTs. Among those advantages, better noise margins,

lower power dissipation and stable performances with fluctuated TFTs

are most prominent. However, under current technologies in R2R printed

TFTs on plastic foils, the R2R printed RFID tags can not employ the all

printed complementary circuits because R2R printed n-channel TFTs on

plastic foils are not practically ready yet at this moment. Therefore, in the

following sub-sections, only p-channel SWNT-TFTs based ciruits will be

Fig. 6. The cross-sectional SEM image of all printed gate, gate dielectric layer and drain-

source electrodes.

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discussed with operating principles, design simulations, and performance

characteristics of SWNT network based R2R gravure printed inverters,

ring oscillators, NAND gate, OR gate and D flip-flop circuits on PET

foils. In fact, p-channel based circuits has small benefits that only

one type of semiconductor is needed so that printing and materials

stability issues are minimized. However, it has to sacrifice static power

dissipation and rely on ratioed logic.

3.1. R2R Gravure Printed p-Channel SWNT-TFTs for

Circuit Design Simulations

Typical characteristics of printed SWNT-TFTs used in the circuits

presented here are shown in Fig. 7. Devices have 2.5 µm of gate

dielectrics, channel width of W = 3900 µm, and channel length L = 200 µm.

All electrical measurements of the printed SWNT-TFTs were carried out

under ambient conditions using a semiconductor parameter analyzer

(Keithly 4200). Characteristic p-channel transistor behavior was seen in

the drain-source current (I

) versus drain-source voltage (V

) curve at

various gate voltages (V

) (Fig. 7(A)). I

increased linearly with

saturation at higher V

. The observed field effect originated from

the field dependence of carrier concentration in the PS wrapped

semiconducting SWNTs not from charge accumulation due to water

molecules (Fig. 7(B)). In fact, as shown in Fig. 7(A), almost no

hysteresis effect of SWNT-TFTs was observed under ambient conditions

due to the PS wrapping of the SWNTs.

Fig. 7. (A), I-V characteristics of a printed SWCNT-TFT and (B), the corresponding

transfer characteristics at V

of -5 V.

M. Jung et al.

308

For practical applications, the transistor parameters that govern

the devices’ performance should be evaluated. Among them,

transconductance, mobility, the on/off ratio, subthreshold swing, and

threshold voltage need to be screened for device applications. The

mobility was 0.04 cm

/Vs at an on/off ratio of 10

. Extrapolation of the

linear region of transconductance plot results in a V

of -6.5 V. The slope

in the linear region of I

versus V

gives transconductance g

= d I

= 5.83 nS (Fig. 7(B)), and the subthreshold swing for the printed

SWNT-TFT was 2.6 V/decade.

Since the mobility of the printed SWNT-TFTs show too low to

operate the printed devices at a reasonable switching speed, we reprinted

active layers on printed SWNT-TFTs to increase the current levels of I

but decrease the on-off ratio (Fig. 8). The reprinted SWNT-TFTs show a

mobility of 9.5 cm

/Vs at an on-off ratio of 1000, but they are good

enough to operate circuits at a switching speed of 60 Hz. The slope in the

linear region of I

versus V

gives transconductance g

= d I

/d V

1.399 µS (Fig. 8(B)). The subthreshold swing for the printed SWNT-TFT

was 4.5 V/decade and a threshold voltage was 0.5 V.

Based on those data from Fig. 8, the simulations of SWNT-TFTs

were carried out using Spice software. The software includes various

TFT models for amorphous as well as for poly-silicon. For our purpose

the Spice Level 2 was used, which was originally developed for

amorphous silicon TFTs. The parameters for the amorphous silicon TFT

model were determined in two steps. Firstly, the mobility was calculated

Fig. 8. (A), I-V characteristics of a reprinted active layers in printed SWCNT-TFTs and

(B), the corresponding transfer characteristics at V

of -10 V.