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Study On The Preparation And Gas Sensitive Property Of Organic Thin Film Transistors

Posted on:2017-03-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:T JieFull Text:PDF
GTID:1108330485985078Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Organic thin film transistor(OTFT) has promising applications in the future by reason of low cast, scalability, simple preparation and mechanical flexibility. Recently,OTFTs are promising candidates for practical applications such as organic light-emitting diode(OLED) displays, radio frequency identification(RFID) tags, and sensors.Especially in the area of gas sensor, the OTFT gas sensors own their special advantages such as multiparameter, easy integration and bend. It is peculiar in OTFT gas sensors that the transistor channel materials are also acting as sensitive materials. Therefore, the improvement of OTFT gas sensor is the focus of most concern through the selection and modification of active layer film. In order to increase the responses of sensors, OTFTs with a bottom gate, bottom contact configuration were adopted. This configuration can make the sensitive film to contact the gas molecules. The fabrication of OTFTs based on the sensitive films such as Poly(3-hexylthiophene)(P3HT) film, P3HT- Zinc Oxide(Zn O) nanorods composite film, P3HT/ Reduced Graphene Oxide(RGO) bilayer film and so on was presented in this dissertation. The electrical properties and gas sensing characteristics of prepared OTFTs were systematically studied. The gas sensing mechanisms were explored further according to the characteristics of sensitive films.The details of this work are described as below:1. The OTFT gas sensors based on P3 HT film for nitrogen dioxide(NO2) detection were fabricated. The sensing mechanism was analyzed, and the optimizing parameters of sensitive film were obtained. The results indicated the proper decrease of sensitive film thickness can promote sensor properties. The sensitive film was fabricated by technology of spray-deposited film, and the thickness of film can be controlled by the volume of spray. The carriers condition in the channel was controlled by the gate voltage. The sensing mechanism, “doping effect”, was verified through different gate voltages operated. In order to optimize properties of OTFT gas sensor, the thickness of sensitive film was changed. The properties of OTFTs based on different thickness films were compared, and the results were listed as follow:(1) The surface potential at the interface would increase with the thickness of sensitive film. The higher surface potential at the interface for thicker films is helpful for more holes to be attracted to the interface to form the channel at the same Vgs. Therefore, the devices exhibited morepositive Vth and higher mobility for the thicker film than the thinner film.(2) The response to NO2 increased as the decrease of sensitive film thickness which can be explained by band models. In the thin film, the air/ P3 HT surface was near to the P3HT/Si O2 interface so that the surface doping level perturbed carrier transport by the NO2 made the response of the thin film larger than that of the thick film.2. OTFT gas sensors with P3HT-Zn O nanorods composite films as gas sensing layers were prepared for NO2 detection. Two methods were proposed to rapidly and exactly evaluate the performance of OTFT gas sensors according to the sensing mechanism. The proposed evaluating methods were capable of dealing with the response bias of OTFT gas sensors arising from slow recovery. The effects of Zn O nanorods on the properties of OTFT gas sensors were detailed investigated. Zn O nanorods improved the crystallinity of P3 HT and optimized orientation of polymer chains. Therefore, the carriers in the channel of OTFT would transport smoothly, and the properties of OTFTs would be improve. Moreover, the heterojunction formed by P3 HT and Zn O nanorods can affect the distribution of carriers and improve properties of devices. The gas sensing properties of gas sensors when the sensors exposed to NO2 were tested at room temperature. The prepared sensors were sensitive to NO2, and the limit of detection achieve to order of ppb. However, the poor recovery of OTFT gas sensor led to the shift of the gas response curves at the high concentration of NO2 so that the accuracy of test data would be affected. Here two methods were proposed to evaluate the performance of OTFT gas sensors. The first method used the variation of Vth as output signal of gas sensor. The variations of Vth were near identical for several tests at the same concentration of NO2. The second method was a new signal processing method based on the gas adsorption behavior. This method owned high reliability by proving experimentation.3. Molybdenum Disulfide(Mo S2) was added in P3 HT as sensitive materials for ammonia(NH3) detection first. The different structures of sensitive film were fabricated.The recovery time of OTFT gas sensor based on P3HT-Mo S2 composite film obviously reduced. Mo S2 is a typical semiconductor with high mobility of carriers. The electrical properties had a distinct change in output characteristic curve due to Mo S2 doping. The saturation region was not obvious caused by properties of Mo S2. According to analysis results of film microscopic structure, the distance of the P3 HT lamellar layer structurewas close as a consequence of interaction between P3 HT and Mo S2. The closer interlayer d-spacing of the P3 HT chains resulted in more rapid charge transfer.Therefore, the more rapid charge transfer brought about the improvement in sensor recoverability. In addition, the sensing mechanism was discussed deeply. The different carrier gas was operated in test process. It can be found that when dry air served as a carrier gas, the oxygen-containing molecules would absorb on sensitive film from the strong to the weak adsorption sites. As a result, once the device was exposed to NH3, the NH3 molecules would replace the oxygen molecules and absorb on the adsorption sites.The recovery of sensor was a reverse process.4. OTFT gas sensor based on P3HT/RGO bilayer film which was composed of new gas sensitive material RGO and P3 HT was fabricated. The sensitivity of OTFT gas sensor to NO2 increased clearly due to the RGO as bottom layer, and the selectivity of sensor based on RGO film was improved. The investigated results of electrical properties indicated the unsaturation current was obvious in the output characteristics curve because of the high concentration carriers and carrier mobility of RGO. It seriously affected the electrical performance of OTFT, while the bilayer film structure,RGO as bottom layer and P3 HT as top layer, can increase the response and sensitivity of gas sensor to NO2. This effect was due to the 2D structure of RGO. Every carbon atom in RGO was a surface atom so that there would be more electron transport between the target gas molecules and the sensitive material. The further research results of response-recovery curve indicated the response can be broken into two parts, the rapid response and the slow response, by the reason of the different absorption sites on the surface of RGO. Moreover, P3 HT as the top layer can prevent other gas from contacting the bottom layer RGO which can increase the selectivity of sensor based on RGO.
Keywords/Search Tags:organic thin film transistor, gas sensor, P3HT film, composite film, bilayer film, sensing mechanism
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