Fabrication and characterization of high performance ink jet printed OTFT(Organic Thin Film Transistor)

다국어 초록 (Multilingual Abstract)

This dissertation consists of studying on the fabrication and evaluation of high performance OTFTs fabricated using tips pentacene as an active material. All the processes to fabricate OTFTs were done utilizing inkjet process. In spite of many previous efforts, OTFTs fabricated with solution process for a flexible application are not sufficient enough to reach the commercialization because of its lower characteristics and unguaranteed reliability compared with a-Si TFT. Therefore, we carried out a couple of experiments to further improve its performance in terms of the mobility and the passivation scheme.
First of all, we analyzed in detail about Tips pentacene films printed using ink jet process. Then, we attempted to understand a role of crystallinity of Tips pentacene film in determining the mobility by comparing physical and electrical properties of two distinct OTFTs that were fabricated either on good crystalline tips pentacene or bad crystalline tips pentacene. The dendrite grain structure which is a typical garin structure of the solution processed tips pentacene is thought a prime reason to yield a poor electrical transport. The good grain structure consist of big grains gave a 10 times higher mobility and ION current compared to those obtained from the bad grain structure consists of dendrite grains. Thus we concluded that the minimization of coffee strain effect during the solution process is the most critical issue to obtain high performance OTFT.
The first approach to minimize coffee stain effect was to print tips pentacene with discrete drops rather than to print continuous drops. In other words, a desired tips pentacene film was formed by dropping tips pentacene ink one by one in active area, not by consecutively jetting the ink. Throughout aforementioned process, we could eliminate so called dendrite grain structure, thereby obtaining better performance OTFTs.
The other approach was to produce tips pentacene active area on a confined structure which can protect the serious evaporation of solvent at edge regions. We formed active areas having below 10 micrometer of channel length as the confined structure by imprinting Si stamp on PES substrate. We used a capillary phenomenon to form the channel with tips pentancene effectively as it was difficult to form tips pentacene in the confined structure with the solution process. We could confirm that the performances of short channel tips pentacene OTFTs were improved extremely. The average mobility and on/off ratio of OTFTs were 0.25 cm2/Vs and 103, respectively. The threshold voltage of OTFT was -2.5 V.
Finally, we produced PICS(Pattern induced confined structure), and fabricated OTFTs by printing tips pentacene on it. At first, Ag patterns were accurately formed by irradiating UV and board temperature control. Then PICS was obtained by orthogonally printing Ag patterns on underlying PVP layer which was used as a gate dielectric material. The average mobility and on/off ratio of PICS(Pattern-Induced Confined Structure) OTFTs were 0.034 cm2/Vs and 103, respectively. The threshold voltage of OTFT was -2.5 V.
On the other hands, we found out that the passivation layer is necessary for maintaining the OTFT’s properties and improving OTFT’s life time as OTFT’s performance is commonly decreased under influence of H2O and O2 among open air environmental factors. At present, passivation layer deposition using solution process or evaporation process is difficult due to the insufficient study. Therefore, we used a thermosetting epoxy resin film as the passivation layer by attaching on the OTFTs surface. We compared the electrical properties of OTFTs with the passivation layer with those of OTFTs without the passivation layer at specific condition. We verified that the electrical feature of OTFT with passivation layer was well kept with its initial element after keeping the elements inside desiccator which was kept at 25 ˚C with 40 % of relative humidity for 30 days.
We suggest that the poor electrical properties of OTFT can be improved dramatically by eliminating coffee strain effect. The PICS was a good candidate for a solution process, in particular, ink jet process. However, the leakage current should be reduced by modifying the structure of OTFT, Thus, is necessary to further study about the better OTFT structure.

목차 (Table of Contents)

• Table of Contents
• Table of Contents. v
• List of Tables. ix
• List of Equation. ix
• List of Figures. x
• Abstract. xx
• Chapter 1. Introduction. 1
• 1.1Printed Electronics. 1
• 1.1.1 Contact printing techniques. 5
• 1.1.2 Non contact printing technoiques. 9
• 1.1.3 Inkjet printing. 12
• 1.1.3.1 Contiuous inkjet printing. 13
• 1.1.3.2 Drop-on-demand inkjet printing. 15
• 1.1.3.3 Advantage and challenges of inkjet technology. 19
• 1.2 Introduction to printed organic thin film transistor. 21
• Bibliography. 26
• Chapter 2. Theory and operating method of OTFT. 30
• 2.1 Generalities on organic semiconductors. 30
• 2.2 Charge transport in organic materials: models and theories. 33
• 2.3 Organic thin film transistor architecture. 41
• 2.4 Operating mode. 42
• Bibliography. 46
• Chapter 3. Experiment procedure. 47
• 3.1 Material Preparation. 47
• 3.2 Ink jet printing. 49
• 3.3 Characteristic of Poly-4-vinylphenol (PVP). 51
• 3.4 Fabrication of Si stamp. 56
• 3.5 Confined structure formation using imprint process. 59
• 3.6 Surface treatment using UV irradiation. 62
• 3.7 Fabication Process of OTFT. 64
• 3.7.1 Bottom gate OTFT process using shodow mask. 64
• 3.7.2 Ink jet OTFT with imprinted structure. 66
• 3.7.3 PICS(Pattern induced confined structure) OTFT process using all inkjet method. 68
• 3.8 Physical and Electrical Analysis Techniques. 71
• Bibliography. 73
• Chapter 4. Result and discussion. 74
• 4.1 Study on the role of Tips pentacene crystalline of OTFT. 74
• 4.1.1 Analysis of Tips pentacene Morphology and Crystalline. 74
• 4.1.2 Fabrication of OTFTs having different grain structures. 78
• 4.1.3 Electrical Characterization of OTFTs. 81
• 4.2 Characterization of OTFTs with Tips pentacene without Dendrite Grain. 85
• 4.2.1 Tips pentacene film without coffee stain effect obtained using multiple ink print. 85
• 4.2.1.1 Formation of Tips pentacene film without coffee strain effect using multiple ink print. 85
• 4.2.1.2 Analysis of Tips pentacene morphology with respect to droplet count. 87
• 4.2.1.3 Analysis of Tips pentacene crystalline formed with discrete dropping. 90
• 4.2.1.4 Electrical Characterization of discretely jetted Tips pentacene OTFT. 92
• 4.2.2 OTFTs fabricated on Confined Structure using Imprint process. 96
• 4.2.2.1 Tips pentacene film printed on the confined structure. 96
• 4.2.2.2 Electrical Characterization of PVA gate Dielectric film. 101
• 4.2.2.3 Electrical Properties of Confined structure OTFTs fabricated using Ag line Imprinting. 105
• 4.3 PICS (Pattern Induced Confined Structure) OTFT fabrication and Characterization. 109
• 4.3.1 PICS formation using inkjet print. 110
• 4.3.2 Electrical Characterization of PVP dielectric layer. 112
• 4.3.3 Analysis of Tips pentacene film formed on PICS OTFT. 114
• 4.3.4 Electrical Characterization of PICS OTFTs. 118
• 4.4 Study on the electrical property variations of passivated OTFTs. 121
• 4.4.1 OTFT structure before passivation and after passivation. 121
• 4.4.2 Curing of the epoxy resin film. 123
• 4.4.3 Electrical property Characterization of OTFT without passivation layer. 125
• 4.4.4 Electrical Property Characterization of OTFTs with passivation layers. 128
• 4.4.5 Analysis of Electrical Characteristics Recovery. 133
• Bibliography. 142
• Chapter 5. Conclusions and Further Study. 145
• 5.1 Conclusions of This Work. 145
• 5.2 Suggestions for Further Study. 148
• Appendix. 150
• I. Publications. 150
• 1) Journals. 150
• 2) International Conference Proceedings. 152
• 3) Patent. 154