4-1 Moisture Effect to a Novel Porous Pentacene-based TFT
For make sure the water vapor can vastly affect polymer gate dielectrics with OH group, we fabricate a porous pentacene-based TFT with PVP dielectric. We add polystyrene sphere particles on PVP before depositing pentacene film. Use polystyrene spheres as nano-scale shadow mask for pentacene growth. The AFM image of 50-nm-pentacene film depositing on PVP and PS sphere is shown in the Fig. 4-1 (a).The PS spheres (diameter about 200nm) are left on the active layer without affecting device operation. Fig. 4-1 (b) show transfer characteristic of porous OTFT in ambient air. It inconceivably exhibit higher mobility than standard OTFTs with PVP dielectric in ambient air. Assume the novel device could make water vapor or gas flow into PVP surface rapidly instead of slowly diffusing through grain boundaries of pentacene. Be able to examine the moisture effect on PVP dielectric for this porous OTFT.
First, we put the device into micro-fluid system under pure N2 flow. Make PVP surface absorb little water vapor. We can observe mobility and on current vastly decreasing when expose to N2 flow in two minutes. Fig. 4-1 (b) show transfer characteristic of porous OTFT expose to nitrogen. It’s possible due to water vapor through PS sphere to bare surface of PVP without pentacene capping instead of diffusing through grain boundaries. Hence, water vapor has ability to rapidly pass in and out surface of PVP. Further, change hydroxyl group charging quickly and enhance carrier concentration in the channel. Fig. 4-1 (c) show the possible mechanism and reversible chemical reaction as follow:
+ H‐OH + H
3O +
‐OH ‐O ‐
4-2 A Novel Porous Structure Pentacene-based TFTs as Ammonia Gas Sensor
4-2.1 Motivation
It is reported that breath NH3 concentration is higher in cirrhotic patients (0.745 ppm) than that in normal person (0.278 ppm) [25].OTFTs as NH3 gas sensors were reported to have sensitivities in the low-ppm levels, good repeatability, and stable operation in ambient condition at room temperature [26,27]. An OTFT that can detect 0.5-ppm NH3 in a few seconds, however, was not reported. Follow previous section, we could realize the hydroxyl group charging has ability to strongly affect the carrier concentration in the channel. By the concept, a real-time sensitive ammonia sensor based on a porous OTFT is demonstrated.
4-2.2 Sensing Mechanism Comparison
NH3 sensing in conventional OTFT relies on the following two steps: (1) NH3
molecules diffuse through grain boundaries to enter channel area, (2) polar NH3
molecules reduce carrier mobility through charge-dipole interactions and shift device threshold voltage by serving as acceptor-like deep traps. The defect is that reaction is slow, usually irreversible and strongly dependent on the grain structure. To speed up the response to ammonia, a novel structure OTFTs show in Fig. 4-2 to enhance sensing response for NH3.
4-3 Exposed to Low-ppm Ammonia
First, put the device into micro-fluid system under pure N2 flow, in order to make PVP surface absorb little water vapor and pure the sample without unwanted gas molecules. Then let sample expose to 1ppm NH3 flow in one minute. Fig. 4-3 (a) shows in a short time the apparent difference of transfer characteristic between N2 and 1ppm NH3. Fig. 4-2 (b) shows output characteristic of porous device expose to 3ppm NH3 in two minutes and Fig. 4-3 (a) shows transfer characteristic expose to various concentration of NH3 in one minute. It is successful to access apparent difference in a few minute. Porous structures make OTFTs have ability to apply on fast response ammonia gas sensor. Possible mechanism is similar to previous concept when PVP surface interact with ammonia as follow:
4-4 Real-time Response for Low-ppm Ammonia
We try to investigate the porous OTFTs having reversible ability in a short time or not. The real-time measurement is required to observe its behavior and build up a new method for ammonia sensing. As the Chapter 3 is depicted, the gate bias stress effect is strongly affecting the OTFTs operation, including porous OTFTs with PVP dielectric.
So it’s difficult only to observe multiple transfer characteristic of porous OTFTs for NH3 sensing. If take constant gate voltage and drain voltage make device is lie in on state, that sampling current would be suffered severely for bias stress effect as shown in Fig. 4-4 (a).
+ NH
3+ NH
4+
‐OH ‐O ‐
Instead of constant voltage, giving pulse voltage for gate and drain electrodes then sampling in a function time. The method would be benefited to intercept the real-time change for NH3 gas sensing. We use two Keithley 2400 giving pulse in a function time with lab-view program controlling. Real-time measurement expose to N2 is stable without current changing when pulse time is set up to 10 seconds. The sensing responses of porous OTFT are observed by plotting drain current as a function of time exposed to different ambient conditions in Fig. 4-4 (a). The current is stable under N2
flow. As exposing to 3ppm NH3, the current is steeply increasing in a few seconds then go into saturation value. Next, let device under N2 flow and the current also recover fast to original value. Expose to 3ppm NH3 again would get the similar result. It is saying the porous OTFTs have ability for fast response and rapid recover for low-level ammonia sensing. It should be attributed to porous structure make gas molecules go to PVP surface rapidly, and enhance carrier concentration in the channel.
The apparent current difference of porous OTFT is exposed to 3ppm, 1ppm, and 0.5ppm NH3 is shown in Fig. 4-4 (b). Achieve 0.5ppm sensitivity is enough to distinguish cirrhotic patients (0.745 ppm) than that in normal person (0.278 ppm).
4-5 Discussion of Sensing Mechanism and Humidity Effect
For OTFT with PVP dielectric, it is reported that the –OH groups in PVP film dissociate into negative-charged molecules when reacting with water molecules [5].
The negative-charged molecules enhance the accumulation of holes in channel and hence increase carrier mobility and the drain current. As shown in Fig. 4-3, porous OTFT in ambient (relative humidity RH = 60%), in dry N2 (RH < 5%), and in dry N2
dissociation since water molecules are removed. When NH3 molecules enter N2
ambient, they react with –OH groups to form negative-charged molecules and to increase current. When NH3 sensing behavior is measured in humid environment as shown in Fig. 4-5, almost no response can be seen because –OH groups already reacted with water molecules. The result might contribute to H2O is much more than ammonia so it doesn’t have apparent response.
Figures of Chapter 4
-40 -30 -20 -10 0 10 20
10-11 10-10 10-9 10-8 10-7 10-6 10-5
Current ( A )
Gate Voltage ( V )
In the ambient Expose to N2
μ=0.03 cm2/Vs
W1000um/L200um VD=-5V
μ=0.77 cm2/Vs
(b)
Fig. 4-1 (a) AFM image of the PS sphere on the pentacene (b) the transfer characteristics of porous OTFT in different environment (c) possible mechanism.
Fig. 4-2 Schematic device structures (a) Conventional OTFTs (b) Porous OTFTs
Pentacene
PVP
Gate Negative Bias
‐ ‐ ‐ ‐ + + + + + +
+ +
‐O ‐ ‐O ‐ ‐O ‐ ‐O ‐
‐OH
‐OH ‐OH
‐OH
(c)
Glass PVP Gate NH3 NH3NH3
Glass PVP Gate
NH3 NH3 NH3
Pentacene
Gold Polystyrene sphere
Pentacene
(a) (b)
Fig. 4-3 (a) The transfer characteristics of porous OTFT in different environment (b) the transfer characteristics to various NH3 concentration (c) the
output characteristics to N and NH
-40 -30 -20 -10 0 10 20
I
D‐V
Gwith various NH
3for 1min
(b) (c)
Fig. 4-4 Real-time measurement of (a) conventional OTFT is exposed to 5ppm
Exposure Time ( sec ) 5ppm
0 300 600 900 1200 1500
150
Exposure Time ( sec )
Porous OTFT
3ppm NH3
(b)
Fig. 4-5 Porous OTFT is exposed to 3ppm NH3 in humid N2
0 50 100 150 200 250 300 350 400 800
1000 1200 1400 1600 1800
N2+H2O N2 N2+H2O
Drain Current ( nA )
Exposure Time ( sec ) 3ppmNH3 +H2O
Porous OTFT VD=-20V,VG=-35V