The anomalous gate leakage current effect from xylene solution

Chapter 2 Property of P3HT and Spin-Coating technique

2.5 OTFTs Fabrication by Different Slovents

2.5.2 Result and Discussion

2.5.2.2 The anomalous gate leakage current effect from xylene solution

Current-voltage characteristics of OTFTs were measured in the air with a semiconductor parameter analyzer HP4156. Fig2-8 illustrates source current (IS) and gate leakage current (IG) versus gate voltage where the P3HT OTFT was fabricated by 0.3% xylene solution. The OTFT was turned ON, and the “ON-current” was larger than the gate leakage current by one order. The gate leakage current was comparable to the source current when the device is nearly turned OFF, and finally the gate leakage current dominated the drain current. Due to anomalous gate leakage effect, we cannot measure the ideal I-V characteristics. As a result, the ON-OFF ratio would be affected by anomalous gate leakage current.

Fig2-9(a) shows source current versus drain voltage, where the P3HT OTFT was fabricated by 0.3% xylene solution. Fig2-9(b) shows source current versus drain voltage, where the P3HT OFTF was fabricated by 0.3% chloroform solution. Because of anomalous gate leakage current, the source current at zero bias was above 10^-7 Amp as shown in Fig2-9(a). If the solvent was changed from xylene to chloroform, we could not observe apparent anomalous gate leakage current. From Fig2-9(b), the source current at zero bias was below 10^-8Amp.Therefore, the anomalous gate leakage current was suppressed by chloroform solution.

Because the anomalous leakage current is comparable to the source current of the OTFT with small W/L ratio, the result would be incorrect. Therefore, as the P3HT OTFT was fabricated by 0.3% xylene solution, the anomalous gate leakage current not only influenced the magnitude of source current at zero drain bias, but also the ON-OFF ratio with small W/L. From Fig2-10, if the P3HT OTFT was fabricated by 0.3% xylene solution, ON-OFF ratio was dependent on W/L ratio. But the solvent was changed from xylene to chloroform, the anomalous gate leakage current was suppressed. From Fig2-10, if the P3HT OTFT was fabricated by 0.3% chloroform solution, ON-OFF ratio was independent on W/L ratio.

2.5.2.3 The correlation between field-effect mobility of P3HT OTFT and solvent

The choice of solvents has a very significant impact on the field-effect mobility of P3HT OTFTs. In a recent publication, Bao et al. [12] observed that chloroform was used as a solvent and P3HT organic semiconductor layer was deposited by spin-coating, the field-effect mobility of P3HT OTFT is about 10^-3 cm²/Vs. Xylene was used as a solvent and P3HT organic semiconductor layer was deposited by spin-coating, the field-effect mobility of P3HT OTFT is about 10^-4 cm²/Vs, as shown in Table2-2. From Fig2-11, the field-effect mobility of P3HT OTFT which was fabricated by xylene is about 10^-3 cm²/Vs and the field-effect mobility of P3HT OTFT which was fabricated by chloroform solution is about 10^-4 cm²/Vs. The conclusion was in consistent with the publication [12].

2.6 OTFTs Fabrication by Different Weight Percentages of P3HT

2.6.1 Experiment Detail

In this section, we prepared 0.1%, 0.3%, 0.8%, and 2.0% of P3HT in chloroform. And then the P3HT solution was filtered by a 0.2-µm pore-size PTFE filter and then spun onto the wafer surface. Next, we investigated the electrical characteristics of P3HT OTFTs, such as mobility, threshold voltage and on/off ratio, with different weight percentages of P3HT. Besides, we compared the performance of P3HT OTFTs which were fabricated by new and old P3HT material. New P3HT material, specifically “fresh” P3HT material, means that we purchased it from Aldrich chemical company and were used at once. Old P3HT, although purchased from the same company, had been opened and the remaining chemicals had been storing in air for a year.

2.6.2 Result and Discussion

2.6.2.1 Physical properties of spin-on P3HT film

We used atomic force microscope to observe the surface morphology and topography of deposited P3HT film. Fig2-12~Fig2-15 exhibits the surface morphology of the deposited P3HT film with different weight percentage of 0.1%, 0.3%, 0.8% and 2.0% of P3HT in chloroform. It was found that the deposited films by the low weight percentage of P3HT, such as 0.1% and 0.3%, were very smooth, but the deposited films by high weight percentage of P3HT, such as 0.8% and 2.0%, were very rough. When the weight percentage of P3HT as high as 2.0%, there were apparent pinholes in the deposited film. Table2-3 summarized the surface roughness of P3HT film with respect to weight percentage of P3HT in chloroform. The surface root-mean-square roughness of organic thin film deposited by 0.3% of P3HT is 8.24Å. That is much smoother than RMS roughness of organic thin film deposited in other weight percentages.

2.6.2.2 The bulk current effect from high weight percentage of P3HT

There are two current paths in organic semiconductor layer[13]. One is the channel current (Ich), it comes from source electrode (Pt) and goes through the accumulation holes and into drain electrode (Pt). Using established metal-oxide-field effect transistor (MOSFET) current-voltage relationships, the channel current can be written as:

for the linear regime, where L is the channel length, W is the channel width, Ci is the capacitance per unit area of the insulating layer, Vth is the threshold voltage, and μ is the field effect mobility.

Another leakage path is the bulk current (Ibk). It comes from source electrode (Pt) and goes through conductive layer which is above the accumulation holes and into drain electrode. The bulk current (I_bk) can be represented as

_bk l V_DS L

I =µ×W × × [Equation 2-4]

, where l is the organic semiconductor layer thickness [13].

The P3HT OTFTs were turned ON in the accumulation mode (V_G<0, see Fig1-2(b)) and were turned OFF in the depletion mode (VG>0, see Fig1-2(c)). Fig2-16 shows a typical source current versus drain voltage plot at various gate voltages in both accumulation [Fig.2-16(a)] and depletion [Fig.2-16(b)] modes. Fig2-17 is the same. As the positive voltage increases, the source current decreases. It was shown that the device could be turned OFF. But from Fig2-18, I_S versus VD curve in the depletion mode as a function of weight concentration of P3HT in chloroform, the devices which were fabricated by 0.8%, 2.0 % of P3HT can not be turned OFF. From two aspects of observation, it can be shown that the current is the bulk current. (1) As the organic semiconductor layer thickness increases, the source current increases. (2) As the drain voltage increases, the source current increases. These conclusions are consistent with Equation 2-4.

Either new or old P3HT material, the OTFTs fabricated by high weight percentage of P3HT such as 0.8% and 2.0% can not be observed the ideal I_S-V_G characteristics as shown in Fig2-19~Fig2-20 due to the bulk current effect. As a result, threshold voltage and ON-OFF ratio would be affected by the bulk current. Fig2-21 and Fig2-22 shows that threshold voltage as a function of weight concentration of P3HT in chloroform. Because of the bulk current effect there is a dramatic increase as weight concentration of P3HT is above 0.3%.

2.6.2.3 The correlation between the performances of P3HT OTFT and weight percentage of P3HT

For OTFTs fabricated by fresh P3HT material, the following phenomenon can be observed:

(1) Fig2-22 illustrates the dependence of threshold voltage and various weight percentages of

P3HT. The most appropriate wt% of P3HT is 0.1%-0.3%. (2) Fig2-23 illustrates that the field-effect mobility (weight %) dependence shows a maximum at 0.3%-0.8%. (3) Fig2-24 illustrates that ON-OFF ratio (weight %) dependence show a maximum at 0.1%-0.3%. OTFTs fabricated by 0.8 % of P3HT had a better mobility than the others, such as 0.1%, 0.3%, 2.0%, but they can not have an ideal IS-VG characteristic. Therefore, in order to acquire an OTFT with good mobility, high ON-OFF ratio as well as appropriate threshold voltage, the optimal weight percentage of P3HT would be 0.3%. For OTFTs fabricated by old P3HT material, the foregoing phenomenon could not be observed, except threshold voltage, as shown in Fig2-21, Fig2-25 and Fig2-26. Although similar trends of field-effect mobility and ON-OFF current ratio could not be observed in these figures, the OTFTs fabricated by high weight percentage of P3HT such as 0.8%

and 2.0% can not be observed the ideal IS-VG characteristics, which is the same with fresh material.

2.7 Summary

It was found that chloroform is a good solvent to dissolve P3HT, the anomalous gate leakage current was suppressed by chloroform solution, and the high ON-OFF ratio of about four orders of magnitude and the field-effect mobility of 10^-3 cm²/Vs were attributed to chloroform solution.

The surface root-mean-square roughness of organic thin film deposited by 0.3% of P3HT is 8.24Å. That is much smoother than RMS roughness of organic thin film deposited by others, 0.1%, 0.8% and 2.0%. As weight percentage of P3HT in chloroform is above 0.3%, the bulk current effect would affect IS-VG curves and IS-VD curves. Therefore, in order to acquire an OTFT with good mobility, high ON-OFF current, appropriate threshold voltage, the optimal weight

percentage of P3HT would be 0.3%.

Figure 2-1: Chemical diagram of the polymer poly (3-hexylithiophene).R represents the alkyl chain

Figure2-2:Bottom-contact structure of P3HT OTFTs

Figure2-3:Process flow of bottom-contact OTFTs

Egde-on orientation

Face-on orientation

Figure2-4: Two different orientations of ordered P3HT domains with respect to the FET substrate [9]

(a)

(b)

Figure2-5: Layouts of bottom-contact OTFTs (a) linear type (b) finger type

Source

Drain

-40 -30 -20 -10 0 10 20 30 40 10

^-14

10

^-13

10

^-12

10

^-11

10

^-10

10

^-9

10

^-8

10

^-7 ^10000/10_5000/10

1000/10 1000/15 1000/25 1000/35 1000/50 500/15 500/25 300/35 500/35 500/50

Normalized Source Current I norm(I SL/W Amp)

Gate Voltage V

(volt)

Fig2-6: Normalized drain-source current vs gate voltage

(a)

(b)

Figure2-7:AFM micrograph of P3HT 0.3% in (a) xylene (b) chloroform

-40 -20 0 20 10^-9

10^-8 10^-7 10^-6

Xylene 0.3%

W/L=5000µm/30µm

S ource Current I S (Amp)

Gate Voltage V _G (Volt)

Vd= -7.55V Is I_G

Figure 2-8: Source and gate leakage current versus gate bias

0 -10 -20 -30 -40 -50 -60

Figure2-9:Influence of gate leakage current on output

characteristics I

vs V

:(a) xylene 0.3% (b) chloroform 0.3%

0 200 400 600 800 1000 10¹

10² 10³ 10⁴

Xylene 0.3%

Chloroform 0.3%

O n -Of f Rati o

W/L

Figure2-10: On-Off ratio versus W/L

-40 -30 -20 -10 0 10 20 30 40

Gate Voltage V_G(volt)

Ti/Pt Tox=150nm

Gate Voltage V_G(volt)

Mobilityµ (10-3 cm2 /Vs) Mobilityµ (10 -3cm 2/Vs)

(b)

Figure2-11: Transfer characteristics I_Svs V_Gand field-effect mobility of samples prepared from different solvrnt

(a) W/L=10000/10 µm (b) W/L=5000/10 µm (c) W/L=1000/10 µm (d) W/L=1000/50 µm (continue)

-40 -30 -20 -10 0 10 20 30 40

Gate Voltage V_G(volt) (c)

Gate Voltage V_G(volt)

(d)

Figure2-11: Transfer characteristics I_Svs V_Gand field-effect mobility of samples prepared from different solvent

(a) W/L=10000/10 µm (b) W/L=5000/10 µm (c) W/L=1000/10 µm (d) W/L=1000/50 µm

(a)

(b)

Figure 2-12: AFM micrograph of P3HT 0.1% in chloroform (a) top-view (b) high angle view

(a)

(b)

Figure 2-13:AFM micrograph of P3HT 0.3% in chloroform (a) top-view (b) high angle view

(a)

(b)

Figure 2-14: AFM micrograph of P3HT 0.8% in chloroform (a) top-view (b) high angle view

(a)

(b)

Figure 2-15:AFM micrograph of P3HT 2.0% in chloroform (a) top-view (b) high angle view

0 -10 -20 -30 -40 -50

S/D Metal Pt/Ti T_OX=130nm W/L=1000µm/25µm

S/D MeyalPt/Ti T_OX=130nm W/L=1000µm/25µm accumulation mode (b) in the depletion mode (W/L=1000/25µm)

0 -10 -20 -30 -40 -50 0.0

4.0x10^-8 8.0x10^-8 1.2x10^-7

S/D Metal Pt/Ti T_OX=130nm W/L=1000µm/35µm

Chlorofrom slution=0.3% V_G=-5V V_G=-10V

Figure2-17: IS versus VD curve at different gate voltages (a) in the accumulation mode (b) in the depletion mode (W/L=1000/35µm)

0 -10 -20 -30 -40 -50 0.0

2.0x10^-7 4.0x10^-7

6.0x10^-7 S/D MeyalPt/Ti T_OX=130nm W/L=1000µm/25µm

Figure 2-18: IS versus VD curve in the depletion mode as a function of weight concentration of P3HT in chloroform (a) W/L=1000/25µm (b)

W/L=1000/35µm

-40 -30 -20 -10 0 10 20 30 40

Gate Voltage V_G(volt)

Old material

Gate Voltage V_G(volt)

Old material W/L=1000/35µm

(b)

Figure2-19: Transfer characteristics I_S vs V_G as a function of weight concentration of P3HT in chloroform:

(a)W/L=10000/10µm (b)W/L=1000/35µm (c)W/L=500/15µm (d)W/L=500/25µm (continue)

-30 -20 -10 0 10 20 30 40

Gate Voltage V_G(volt)

Old material

Gate Voltage V_G(volt)

Old material W/L=500/25µm

(d)

Figure2-19: Transfer characteristics IS vs VG as a function of weight concentration of P3HT in chloroform:

(a)W/L=10000/10µm (b)W/L=1000/35µm (c)W/L=500/15µm (d)W/L=500/25µm

-40 -30 -20 -10 0 10 20 30 40

Gate Voltage V_G(volt) (a)

Gate Voltage V_G(volt)

New material W/L=1000/15µm

(b)

Figure2-20: Transfer characteristics I_S vs V_G as a function of weight concentration of P3HT in chloroform:

(a) W/L=10000/10µm (b) W/L=1000/15µm (c) W/L=1000/35µm (d) W/L=500/15µm (e) W/L=500/25µm (continue)

-40 -30 -20 -10 0 10 20 30 40

Gate Voltage V_G(volt) (c)

Gate Voltage V_G(volt)

(d)

Figure2-20: Transfer characteristics I_S vs V_G as a function of weight concentration of P3HT in chloroform:

(a) W/L=10000/10µm (b) W/L=1000/15µm (c) W/L=1000/35µm (d) W/L=500/15µm (e) W/L=500/25µm (continue)

-40 -30 -20 -10 0 10 20 30 40 10^-11

10^-10 10^-9 10^-8 10^-7

10^-6 New material

W/L=500/25µm

0.1% V_DS=-5V 0.3% V_DS=-5V 0.8% V_DS=-5V 2.0% V_DS=-5V

Source Current I S(A)

Gate Voltage V_G(volt)

(e)

Figure2-20: Transfer characteristics IS vs VG as a function of weight concentration of P3HT in chloroform:

(a) W/L=10000/10µm (b) W/L=1000/15µm (c) W/L=1000/35µm (d) W/L=500/15µm (e) W/L=500/25µm

0.0 0.5 1.0 1.5 2.0 0

40 80 120 160 200 240

W/L=10000/10µm W/L=1000/35µm W/L=500/15µm W/L=500/25µm Old material

Threshold Voltage(Volt)