Study On Bio-material Based Organic Thin Film Transistors

Thin film transistor(TFT)is the essential part of the backlplane in the organic semiconductor technology based optoelectronic display industry.According to the difference of the functional material,TFT can be devided into two types,which are inorganic and organic TFT(OTFT).Among them,OTFT has attracted more and more attention due to its advantages of wide resources,low cost,and the compatibility to the flexible fabrication process.On the other hand,bio-material is a potential material which not only possesses the advantages of cheap and wide resource,but also the properties of environmental friendly and bio-degradable,which promotes its application in the OTFT field.More and more bio-materials have been adopted to serve as the functional layers in the OTFT device,including substrate,dielectric,and even semiconductor.Meanwhile,to make use of the inherent features of the bio-materials,they have also been employed as the detect layer to the corresponding avtive bio-materials.However,till now,the fabrication process of bio-material is still limited and its application in the OTFT still needs more research.Therefore,to explore the application of the bio-materials in the OTFT field and overcome the problem of the film formation process,this ph.D.thesis analyzes the interface modification mechanism and introduces the spray-coating process;then,mainly focuses on using bio-material to modify the interfaces;finally,focuses on the utilization of bio-material in the OTFT-based sensors.The main works include the following 5 parts:1.Interface modification of OTFT using polymer dielectric was studied.Polymer dielectric material of polymethylmethacrylate(PMMA)as the buffer layer at the interface between the dielectric-electrodes and the semiconductor in bottom-contact structure was introduced.Owing to the uniform and hydrophobic properties of the surface of PMMA buffer layer,the morphology of the semiconductor grown on top of PMMA was optimized.When the thickness of PMMA was optimized to 8 nm,the mobility was increased from 0.0025 cm~2/V s to 0.011 cm~2/V s.This is not only a simple method to optimize the performance of the device,but also paves a way to the application of the polymer dielectric in the organic electronic field.2.The surface modification layer of guanine in OTFT was investigated.Thermally evaporated ultra-thin 2 nm guanine as the surface modification layer upon the silicon dioxide was introduced.The trap density of the dielectric surface was significantly decreased,and the H~+at the surface was also neutralized.The mobility was increased from 0.13 cm~2/V s to 0.42 cm~2/V s.Further,two guanine layers were inserted between two pentacene layers to decrease the cost of the device,and by controlling the film thickness ratio of the guanine and pentacene,the mobility and the on/off ratio of the OTFT device were tuned.In a certain range,by increase the ratio of the semiconductor,the mobility could be enhanced,while increase the rario of guanine layer the on/off ratio could be improved.3.Deoxyribonucleic acid as the hole injection layer in OTFT was analyzed.Bio-material deoxyribonucleic acid(DNA)as the hole injection buffer layer between the semiconductor and the electrodes was introduced.Owing to the dipole formation effect of DNA molecules,the contact between the electrodes and the semiconductor was optimized,leading to the mobility increased from 0.020 cm~2/V s to 0.104 cm~2/V s.The utilization of DNA hole injection layer in different p type materials(including pentacene and?-6 thiophene),as well as different electrode materials(including silver and gold)was analyzed.The stability of the device was also enhanced.4.Effect of DNA solvent on the device performance was studied.Effect of two traditional solvents for DNA,which were methonal:water mix and the alchohol,and the environmental friendly material of water as the solvent for DNA on the performance of OTFTs,was compared.The introduction of the spray-coating method and water solution avoided the phase change brought by the organic solution to the semiconductor,as well as decreased the trap density at the dielectric surface,leading to the performance enhancement.The mobility increased from 0.035 cm~2/V s to 0.153cm~2/V s.5.DNA and guanine based OTFT sensors were realized.Owing to that the efficient carrier channel lay in the few monolayers in the semiconductor upon the dielectric,we sprayed DNA layer between the dielectric and the semiconductor to increase the sensitivity of the OTFT to nitrogen dioxide.Meanwhile,the stability of the OTFT with DNA was also enhanced.Then,using the guanine/semiconductor/guanine/semiconductor structure,we realized the performance enhancement of the OTFT-based nitrogen dioxide sensor.By combining the results of the semiconductor only device and the guanine/semiconductor/guanine/semiconductor structure device,the selectivity between the nitrogen dioxide and five other vapors(including isopropyl alcohol,acetone,ethyl acetate,water,and acetic acid)was achieved.In summary,this thesis focuses on applying the bio-material to the OTFT field.First,the spray-coating method in the fabrication of interface buffer and bio-material was introduced.Then,the utilization of bio-mateirial in the fields of interface modification and OTFT-based sensor was developed.This study paves the way for the application of bio-material in the OTFT and related organic optoelectronic field.