Wen-Chao Dan, Hui-Ling Tai, Ya-Dong Jiang, Guang-Zhong Xie, Xian Li, Fang Xu, and Tao Zhu

1.Introduction

The formaldehyde (HCHO) is colorless and strongly stimulating smell gas at room temperature, easily soluble in water, alcohol, and ether, as well an important chemical industrial material and organic solvent[1].HCHO is widely used as an intermediate for the production of various materials such as paints, foams, and polymer products due to its high reactivity and relatively low cost[2].As an economically important chemical, it is very popular in the construction industry and used for building materials[3].However, HCHO is very poisonous, prolonged exposure to low concentrations of HCHO, people feel uncomfortable,headache, pain in the eyes, and respiratory allergies[4].The Occupational Safety and Health Administration (OSHA)has established a 2 ppm short-term (15 min) exposure limit[3],[5],[6].Moreover, HCHO has a relationship with the“sick building syndrome”, which refers to the situation that people living in the sick building for a continuous time suffer health problem, so it is of great necessity to detect the HCHO.

Many methods have been developed to detect HCHO,and the gas sensor gets much attention in this field[7].Compared with other gas sensors, organic thin film transistor (OTFT) sensor has several merits, such as multipleparameters, rapid response, good selectivity, and specially its operation at room temperature[8].So the OTFT device was chosen and developed to detect HCHO in this work.

Based on our early experiments, the P3HT/ZnO composite films with different airbrush mass can be successfully applied to detect HCHO at room temperature,and the OTFT sensor with 1.0 ml composite film behaved the best response and recovery, so this paper studies the effect of P3HT/ZnO composite films with different ratios for detecting HCHO in the condition that the amount of spraying is fixed at 1.0 ml.

2.Experiment

Typically, OTFT devices have two structures:source/drain electrodes locate above and below the organic semiconductor layer, named as top electrode and bottom electrode device configuration, respectively[9].In this paper,bottom electrode structure was applied because of its offering with a more efficient contact[10],[11]to HCHO.Fig.1 shows the structure of the prepared OTFT.Highly doped N-type monocrystalline silicon was used as substrate with the thickness of 450 μm, and the thickness of insulating layer SiO2was about 195 nm.The titanium/aurum thin films were made as the source/drain electrodes,with thicknesses about 20 nm and 50 nm, respectively.The OTFT with the ratio of channel width to length as 160 were chosen to fabricate sensors in this paper.

Fig.1.Structure of prepared OTFT.

Fig.2.Sketch map of spraying instrument.

Fig.3.Output characteristic curve of OTFT with P3HT/ZnO ratios: (a) 3:1, (b) 1:1, and (c) 1:3 in air.

The P3HT (poly-3-hexylthiophene regioregular) was purchased from Luminescence Technology Corp, and the ZnO nanoparticles (40wt%) dispersion was purchased from Sigma-Aldrich.The composite thin film was fabricated as the active layer for formaldehyde by spraying, several different volume ratios for P3HT chloroform solution(3 mg/ml) to ZnO ethanol solution (3 mg/ml) were 3:1, 1:1,and 1:3, and all the solutions were ultrasonic treated nearly 15 minutes to ensure the solute completely dissolved.Fig.2 shows the rough spraying instrument.In the system, the height from device to airbrush was fixed at 9 cm, N2was used as carrier gas, and the pressure of N2was controlled through the reading of reducing pressure valve.The gas flowing time of 1 ml P3HT/ZnO solution was regularly about 80 s.

3.Results and Discussion

3.1 Output Characteristic

The electrical properties of all the prepared OTFT devices including the voltage-current and transfer characteristics were measured by Keithley 4200-SCS source measurement unit in air.The typical drain current(Ⅰds) and drain voltage (Vds) characteristics at different gate biases of sensors with the P3HT/ZnO ratios of 3:1, 1:1, and 1:3 composites are shown in Fig.3 (a), Fig.3 (b), and Fig.3 (c), respectively.It can be seen that all the sensors exhibit the clear p-channel transistor behavior and the OTFT sensors with 1:1 or 1:3 composite film both exhibit the obvious saturation region, but the sensor of 1:1 composite film behaves with a larger drain current.

3.2 HCHO Sensing Properties

The tested HCHO concentration in the experiment was 100 ppm with N2as the carrier gas.The OTFT operated in the accumulation mode, and the working point of the OTFT was that: Vgs= -50 V, Vds= -60 V.After the source-drain current (Ⅰds) of the OTFT was stable in the pure N2, the HCHO was introduced into the sealed chamber.It is interesting that, once the HCHO was introduced, the Ⅰdsdecreased dramatically, as shown in Fig.4 (a) and Fig.4 (b),whereas the Ⅰdsof the device with 1:3 composite film exhibited the opposite changes, as shown in Fig.4 (c).It may be because that, P3HT was typical p-type conducting organic material, while the ZnO nanoparticle was n-type material.With the increasing ratio of n-type ZnO, the major sensing role of p-type P3HT may be reduced.So the OTFT sensor based on the P3HT/ZnO film with the ratio of 1:3 exhibited the opposite response compared with other two devices.It is also noticeable that the recovery was obtained by degassing the chamber with N2for the OTFT sensor with 1:1 composite film, however, the other two prepared sensors could not be reproducibly returned to their original baselines.

The sensing response of the sensor is defined by response values (%)=(Ⅰair-Ⅰgas)/Ⅰair×100%, where Ⅰgasand Ⅰairare the stable drain-source on currents in gas and dry air[12],respectively.Table 1 shows the response values of all the sensors exposed to 100 ppm HCHO at room temperature.It can be seen that the device with P3HT/ZnO ratio of 1:1 had the largest response values.

Fig.4.Response of OTFT with P3HT/ZnO ratios: (a) 3:1, (b) 1:1,and (c) 1:3 to HCHO at room temperature.

Table 1: Response values of all the sensors exposed to 100 ppm HCHO at room temperature

The transfer characteristic curves were recorded when the sensors were exposed in air and 100 ppm HCHO,respectively.Ⅰdsversus Vgscurves are shown in Fig.5 (a),Fig.5 (b), and Fig.5 (c), respectively.

The threshold voltage is acquired by extending the biggest tangent to the Vgs-axis[13]from Fig.5, and the carrier mobility of OTFT sensors can be obtained according to the following formulas[14]:

Fig.5. curve of OTFT with P3HT/ZnO ratios: (a) 3:1,(b) 1:1, and (c) 1:3 in air and 100 ppm HCHO.

Table 2: Carrier mobility and the threshold-voltage of all thesensors exposed to 100 ppm HCHO

Table 2 shows the carrier mobility and the thresholdvoltage of all the sensors exposed to 100 ppm HCHO.It can be seen that the OTFT sensor with the P3HT/ZnO ratio of 1:1 composite has the largest change in threshold voltage and carrier mobility.This result is coincident with the sensing property.

3.3 SEM Images

The surface morphology of the material has great influence on properties of films.The scanning electron microscopy (SEM) images with different ratios of composite films surface are shown in Fig.6.It can be clearly seen that the surface of composite film with the ratio of 1:1 is fluffy enough, providing a more efficient contact sites with HCHO.

Fig.6.SEM images of P3HT/ZnO films with the P3HT/ZnO ratio:(a) 3:1, (b) 1:1, and (c) 1:3.

4.Conclusions

The OTFTs based on sprayed P3HT/ZnO composite with different ratios were fabricated to detect HCHO at room temperature in this paper.The electronic characteristics and sensing properties of three OTFT sensors were studied in detail.The properties had dependence on the ratios of ZnO nanoparticle in composite.The results exhibited that the ratio of 1:1 composite was the optimization by comparing the electrical properties, sensing properties, and SEM images.

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