Studies On The Factors Influencing The Electrical Properties Of Pentacene Organic Thin Film Transistors (OTFTs)

The kind of material and the preparation method of insulator layers of organic thin film transistors(OTFTs) determine the quality of insulator layers and follow-up process, and even the final device performance. The improvement approaches of the transistor device performance are not only materials or structural options, but also the introduction of modified-layers, including insulator layer/active layer interface modification and active layer/source/drain electrodes interface modification. The potential application of OTFTs is for high performance flexible displays. At present, usually electrodes of OTFTs are made from inorganic metals, for example, Au, Al, etc. They are not suitable for flexible applications of OTFTs because these metals are very brittle and with low mechanical strength. An alternative method is to use high conductive polymers as electrodes of OTFTs.Firstly, we built transistor devicesⅠbased on 300nm of SiO2 by electron beam evaporation as the insulator layer and devicesⅡbased on 300nm of SiO2 by thermal Oxidation as the insulator layer. The other parameters of DeviceⅠandⅡare the same. When the VDS is -40V, the mobility of deviceⅠis 9.248×10-3cm2/Vs, on/off current ratio is 6×102, and threshold voltage is -4.1V.When the VDS is -33V, the mobility of deviceⅡis 1.843×10-2cm2/Vs, on/off current ratio is 2.5×103 and threshold voltage is-3V.Through the AFM characterization of pentacene of the two devices,we find that pentacene grew on 300nm thickness SiO2 by silicon thermal oxidation has better quality. Its grain size is relative large which results in the less grain boundaries, fewer defects in the interface and smaller probability of carrier capture,so the properties of deveiceⅠis better than that of deviceⅡ. Second, in order to improve carrier injection of OTFTs, we use LiF to modify the source-drain electrodes.A series deviceⅠ,Ⅱ,Ⅲ,ⅣandⅤcorresponding to the thickness of 0,0.5,1,1.5 and 2.5 nm of LiF layer as an electrode modified-lays respectively are prepared, when VDS is - 40 V, their mobility are 1.84×10-2cm2/Vs, 2.51×10-2cm2/Vs,3.43×10-2cm2/Vs,2.83×10-2cm2/Vs and 2.08×10-2cm2/Vs, respectively. It shows the optimizing thickness of LiF is 1nm. The improvement by LiF modification can be explained as that the hole can well tunneling inject into organic semiconductors when LiF is lnm thickness. In addition LiF can effectively prevent the chemical reaction between Al electrode and pentacene, the organic semiconductor surface metallization due to diffusion, and the appearance of the charge dipole layer.Thus it inhibites further expansion of the hole barrier and improved the device performance.Finally, response to the trend of development of all organic devices, we built 300nm conductive film using PEDOT:PSS solution mixed with 6wt％glycerol in order to be used as the source drain electrodes. The surface resistance is measured as 118.9Ω/口by using four-probe method and its conductivity is 280S/cm. The device structure is bottom contact configuration with top gate. When the voltage of the source and drain is 30 V, the field-effect mobility is 9.208×10-3cm2/Vs, the on/off current ratio is 2.52×103, and threshold voltage is -8.9V. Then we analyze the mechanism that the incorporation of 6wt％glycerol can improve the conductivity of PEDOT:PSS solution,and give the reasons why polymer will do take over metal electrodes.