Organic Thin-Film Transistors Based On A Dinaphthpthienophene(DNTT) Derivative

Over the past decades,organic thin film transistors(OTFTs)have received intense attentions due to their great potentials in flexible displays,wearable devices,electronic paper,large-area sensors,radio frequency identification tags,artificial electronic skins and so on.At present,OTFTs have been successfully applied to ultra-high contrast organic liquid crystal display(OLCD)and flexible full-color electronic paper,but it is incapable to drive organic light emitting display(OLED)with OTFTs.Our group is expert in fine pattern processing in organic thin film transistors,and the mobility of OTFTs that we obtained have reached 1.1 cm2/Vs,which is better than amorphous silicon transistors.In order to further increase the refresh rate and the aperture ratio,the higher mobility of OTFTs is demanded.2,9-diphenyl-dinaphtho[2,3-b:2’,3’-f’]-thieno[3,2-b]thiophene(2,9-DPh-DNTT)is a derivative of six-ring-fused dinaphtho-[2,3-b:2’,3’-f]thieno[3,2-b]thio-phene(DNTT).It is a kind of organic small molecule semiconductor material with excellent photoelectric properties and exhibits good stability and high mobility with vacuum-deposited TFT.2,9-DPh-DNTT film was fabricated by using a self-assembled monolayer(SAM)as a gate dielectric,which is not favorable to subsequent photolithography processes and industrial development.To expand the general applicability of the route,we fabricated the 2,9-DPh-DNTT OTFTs by weak epitaxy method.In this dissertation,we adopt the weak epitaxy growth(WEG)method and add buffer layer to obtain the high-performance 2,9-DPh-DNTT OTFTs.We introduce the smooth and continuous inducing films(p-6P and BP2T)which have better lattice-match relationship with 2,9-DPh-DNTT.Continuous large-area,highly ordered and terraced 2,9-DPh-DNTT polycrystalline thin films are obtained.Inserting a buffer layer between the electrode and the organic semiconductor improves the efficiency of the injection of charge carriers and decreases the contact resistance,and thus the 2,9-DPh-DNTT OTFT device performance was improved.The main work of this dissertation is as follows:1.We find the suitable organic rod-like small molecules(p-6P and BP2T)as inducing layer and gain large-area continuous,high-quality 2,9-DPh-DNTT thin films by changing inducing layer conditions.X-ray diffraction patterns show the 2,9-DPh-DNTT films are highly crystalline,and the 2,9-DPh-DNTT film has a standing-up molecular orientation which benefits to the in-plane charge transport.Selected area electron diffraction patterns show that the 2,9-DPh-DNTT films and inducing layers(monolayer and bilayer of p-6P and BP2T)are lattice-matched.A commensurate epitaxial relationship are formed between the inducing layers and 2,9-DPh-DNTT films.Atomic force microscope images show the large-area continuous,high-quality 2,9-DPh-DNTT films are obtained on inducing layers.2.We fabricate four kinds of OTFTs devices with high-quality 2,9-DPh-DNTT polycrystalline films induced by p-6P and BP2T.The 2,9-DPh-DNTT OTFTs prepared with monolayer p-6P film exhibit the best performance.The field-effect mobility reaches up to 6.4 cm2/Vs,which is close to the highest device performance in the literature.But the BP2T/2,9-DPh-DNTT OTFTs show poor performance.The difference in Highest Occupied Molecular Orbital(HOMO)energy levels between two inducing layers and 2,9-DPh-DNTT lead to the different device performance.The HOMO energy levels of BP2T and 2,9-DPh-DNTT are close to each other,making BP2T acted as a transport layer.Therefore the high performance of 2,9-DPh-DNTT polycrystalline film cannot be shown.In a word,the device performance is affected by both the film quality and the inducing layer.3.Adding a buffer layer between the Au electrode and 2,9-DPh-DNTT fabricated a high-performance p-6P/2,9-DPh-DNTT OTFTs successfully.The mobility of the device reaches up to 9.3 cm2/Vs,the threshold voltage is-16.6V,and the on-off current ratio is higher than 106,which is the highest performance of the material.In addition,the contact resistance of p-6P/2,9-DPh-DNTT OTFTs were extracted by transfer-line method(TLM),and analyzed the influence of contact resistance on the mobility and threshold voltage of organic thin film transistors.The device performances improved significantly by adding three buffer layers,and the best performance is the device with F16CoPc buffer layer.The contact resistance of this device is reduced to one tenth of the device without the buffer layer.Finally,we fabricate organic thin-film transistor arrays with a channel length of about 8 microns by photolithography technology.The mobility of the device reaches up to 2.7 cm2/Vs,the threshold voltage is-2.3 V,and the on-off current ratio is higher than 107.It is of great significance for organic thin-film transistors to drive OLEDs.