Fabrication and Analysis of High Performance Alternative Electrode for Organic Thin Film Transistor and Dye Sensitized Solar Cell

Abstract

Many groups have studied the electrode materials, which have been the essential component for all electronic device including organic thin film transistor (OTFT), organic photovoltaic (OPV), dye sensitized solar cell (DSSC), organic light emitting device (OLED), to improve electrical properties and replace silicon based electronic device in real industry with low cost materials and processes. The interface properties as well as bulk properties of materials are crucial factor for electrode, because the contact resistance highly depends on interface properties. The difference between work function of electrode and HOMO level of semiconductor determines contact resistance at electrode /semiconductor interface of OTFT and the catalytic activity of catalysis counter electrode highly influences charge transfer at the electrolyte/electrode interface of DSSC. Therefore, in this study, the film properties such as resistivity, work function, morphology, catalytic activity and so on were investigated. Besides, the pentacene-TFTs and dye-sensitized solar cells (DSSC) using alternative electrode as a source/drain (S/D) electrode and catalysis counter electrode (CCE) were actually fabricated respectively and their performance were compared.In chapter 1, items needed for the optimization of the metal&#8211

semiconductor interface and methods to measure interface properties are reviewed. Optimization of the interface between the source/drain electrode and the organic semiconductor is one of the important factors for organic thin film transistor (OTFT) performance along with the insulator&#8211

semiconductor interface. Various electrode materials are compared in terms of the work function, contact resistance and the pentacene OTFT performance. The effect of surface modification is also discussed.In chapter 2, in-situ ultraviolet photoemission spectroscopy was used during the pentacene layer growth on Ru film on Ti adhesion layer (Ru/Ti) to measure the energy barrier in the metal-semiconductor contact. Ru surface was treated with 4-florothiophenol and 4-methylbenzenethiol self assembled monolayer to improve the interface properties between the metal electrode and the pentacene layer. The chemical bonding state of the self-assembled monolayer (SAM) on the surface of Ru/Ti film was confirmed using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The Ru/Ti film showed excellent electrical properties as a source and drain electrode of the pentacene thin film transistor (TFT) owing to the high work function of 4.64 eV and low resistivity of ~160 μΩ-cm. It was confirmed that 4-florothiophenol SAM treatment on Ru/Ti film increased the work function of the metal to 5.1 eV and the grain size of the pentacene layer grown on SAM was also increased. With the reduction in the barrier height, the pentacene TFTs with 4-florothiophenol treated Ru/Ti film showed higher mobility of 1.03 cm2/Vs and on-off ratio of 5×106 than pentacene TFT with bare Ru/Ti (μ: 0.205 cm2/Vs, on/off ratio: 106) or bare Au/Ti (μ: 0.275 cm2/Vs, on/off ratio: 106) of the same structure. In chapter 3, Ru/Ti and RuO2/Ti films were deposited on SiO2 (300nm)/N++Si substrate by controlling the Ar/O2 gas flow ratio in the radio frequency (rf) sputtering system with titanium and ruthenium targets. The performances of Ru/Ti and RuO2/Ti films as a S/D electrodes were studied and compared with Au/Ti electrode. The RuO2/Ti film showed excellent electrical properties as a source/drain (S/D) electrode in the pentacene thin film transistor (TFT) due to its high work function of 4.92 eV and low resistivity of ~350 μΩ&#8226

cm. The in-situ ultraviolet photoelectron spectroscopy during pentacene deposition showed that RuO2/Ti film formed lower hole-injection barrier than Ru/Ti film or Au/Ti film with pentacene. Pentacene TFT with RuO2/Ti film showed higher mobility of 0.435cm2/Vs and on-off ratio of 106 than bare Ru/Ti (μ: 0.205cm2/Vs, on/off ratio: 106) or bare Au/Ti (μ: 0.275 cm2/Vs, on/off ratio: 106) of the same structure. In chapter 4, highly conductive multiwalled carbon nanotube (MWNT)/Poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) films were prepared by spin coating a mixture solution. The solution was prepared by dispersing MWNT in the PEDOT:PSS solution in water using ultrasonication without any oxidation process. The effect of the MWNT loading in the solution on the film properties such as surface roughness, work function, surface energy, optical transparency and conductivity was studied. The conductivity of MWNT/PEDOT:PSS composite film was increased with higher MWNT loading and the high conductivity of MWNT/PEDOT:PSS films enabled them to be used as a source/drain electrode in organic thin film transistor (OTFT). The pentacene TFT with MWNT/PEDOT:PSS S/D electrode showed much higher performance with mobility about 0.2 cm2/Vs and on/off ratio about 5×105 compared to that with PEDOT:PSS S/D electrode (~ 0.05 cm2/Vs, 105). The complementary inverters exhibited excellent characteristics including high gain value of about 30.In chapter 5, the performance of MWNT/PEDOT:PSS composite films as a counter electrode of dye sensitized solar cells (DSSCs) were characterized. The high conductive and catalytic MWNT/PEDOT: PSS films was prepared by dispersing MWNT in the PEDOT:PSS solution using ultrasonication without any oxidation process. The addition of MWNT in the PEDOT:PSS solution induced higher conductivity and catalytic activity compared to the sole PEDOT:PSS. The MWNT/PEDOT:PSS film shows much lower sheet resistance (0.2 MWNT: ~482 Ω/□ and 0.3 MWNT: ~162 Ω/□) and larger roughness (0.2 MWNT: 25.4 nm and 0.3 MWNT: 111 nm) than those (215 kΩ and 0.815 nm) of PEDOT:PSS film, and the high conductivity and large roughness of MWNT/PEDOT:PSS films enables them to be used as a high performance catalysis counter electrode in DSSCs. The interface resistance between MWNT/PEDOT:PSS film and electrolyte decreases and the performance of DSSC with MWNT/PEDOT:PSS was improved, as a function of MWNT concentration in PEDOT:PSS solution. The MWNT/PEDOT:PSS well acts as a CCE in DSSCs and especially, The MWNT/PEDOT:PSS well acts as a CCE in DSSCs and especially, the DSSCs with high concentration of MWNT/PEDOT:PSS CCE and DSSC with FTO-free MWNT/PEDOT:PSS composite counter electrode show excellent efficiency of 6.0 and 5.4 %, respectively. Those values were comparable to the efficiency (7.1 %) of DSSC with Pt CCE, owing to high conductivity and F.F.