Papers by Author: Tanakorn Osotchan

Abstract: This work presents development of chemiresistor gas sensor based on polyaniline (PANi) blending with poly(methyl methacrylate) (PMMA) thin film for identification of volatile organic compounds (VOCs) relevant to environmental monitoring. The investigated VOCs are including acetone, methanol, ethanol and ammonia. The gas sensors are prepared by solution mixing between non-conducting form of PANi and PMMA in presence of NMP solution. The mixed solution is spun on aluminum interdigitated electrodes and converted into conducting form using HCl doping. Their sensitivity is measured at room temperature by applying constant voltage and measuring response current in the presence of target gases. The results show that PANi/PMMA blend film has higher surface roughness and porous structure leading to higher response to VOCs gases when compared to pure PANi thin film. The PANi/PMMA blend film exhibits the highest response to ammonia and the lowest to acetone vapor. It response shows little different between ethanol and acetone vapor. The different response characteristics to VOCs of PANi/PMMA blend film indicate that it can be used as active layer for room temperature VOCs sensor.

Abstract: Impedance spectroscopy (IS) is a useful technique to investigate the detail of charge transfer in dye sensitized solar cell (DSSC) especially for individual part of cell component. The charge transport in electrolyte, platinum and TiO₂ electrodes can be separately examined by varying the frequency range of IS. In this work, the TiO₂ electrode was modified by various nanostructures and the equivalence circuit parameters extracted from the measured IS were correlated to the power conversion efficiency of fabricated solar cells, in order to describe the origin of decreasing and increasing of the DSSC efficiency. The TiO₂ electrode was prepared at different conditions of mixing with inorganic nanoparticle of CdS, CdSe and nanostructure of C₆₀ at various concentration. Normally, the DSSC with unmodified TiO₂ electrode shows the efficiency about 7.12% at AM 1.5. The dramatically dropping of efficiency was occurred in the cell with very small amount of nanoparticle added in TiO₂ electrode. For most of the cases, the efficiency tended to become increase as increasing the amount of nanoparticle in the TiO₂ electrode. In some structures, the efficiency can become higher than that of the unmodified TiO₂ cell. However the large amount of nanoparticle in TiO₂ electrode leaded to decreasing of cell efficiency in all fabricated types of TiO₂ nanostructure. These efficiency variations were related to the resistance and capacitance values extracted from IS measurement in the frequency range of TiO₂ response. It was found that the extracted value of cell resistance played the important role on the efficiency variation and the capacitance value can be used to describe the life-time of charge carrier in the cells.Keywords: Impedance spectroscopy, dye sensitized solar cell.

Abstract: Microstructure of heterojunction usually has strong influence on its electrical characteristic. In order to study the effect of microstructure, the layers of copper phthalocyanine (CuPc) and tetracyanoquinodimethane (TCNQ) are deposited on indium tin oxide (ITO) coated glass substrate by thermal evaporation method. The modification of microstructure in the organic semiconductor layer with varied underneath layer is studied in the heterojunction between CuPc and TCNQ. Then the effect on its electrical characteristic is examined between ITO and aluminum (Al) electrodes in these alternative structures. By determining the microstructure by atomic force microscope (AFM), the CuPc and TCNQ thin films provide very tiny and large grain, respectively. These large grains in TCNQ thin film connect into line pattern probably due to the slow grain growth. Therefore the CuPc film is deposited on this TCNQ layer, the larger gain size of CuPc film can be achieved. However, the TCNQ layer grown on top of tiny grain of CuPc exhibits relatively small grain size. The variation of X-ray diffraction (XRD) peaks of these alternative structures support the microstructure extracted from AFM image very well. The optical absorption of these various structures exhibits the combination of absorption peaks from each layer. The electrical characteristics of these structures strongly depend on junction between ITO electrode and organic layer. The heterojunction structures of ITO/CuPc/Al and ITO/CuPc/TCNQ/Al show rectifier characteristics while the structures of ITO/TCNQ/Al and ITO/TCNQ/CuPc/Al exhibit similar IV characteristic for both polarities. However the effect of larger grain size in TCNQ layer leads to very much higher current than those other structures. By measuring current down to temperature of 50 K, it is found that the structures with rectifier junction indicate the current decreasing more than five orders of magnitude while the junctions with no rectifier characteristic exhibit only slightly current change even the measured temperature is reduced to very low temperature.

Abstract: In dye sensitized solar cell structure, the transparent electrodes of tin oxide doped fluorine (SnO2:F) was coated by titanium dioxide (TiO2) and platinum (Pt) for cathode and anode electrodes, respectively. In order to achieve high efficiency solar cell, both electrodes are required to have proper crystal structure size and morphology. These can be modified during the annealing process therefore the effects of electrode annealing on their crystal structure and surface modification were investigated in this study. Thick films of TiO2 and Pt were deposited by screen printing method on 3mm thick glass substrate (Nippon Sheet Glass) coated with 500nm thick SnO2:F. The glass substrate has sheet resistance of 20 ohm/square with the optical transmission of about 70%. The mixed TiO2 powder has the diameter of about 20 nm. The screen print structure was heated for drying in the oven at 150oC for 1 hour. Then the TiO2 thick films were annealed at various temperatures from 400 to 550oC for 2 hours, while the Pt films were annealed at lower temperature from 300 to 500oC. The obtained thickness of TiO2 and Pt film after annealing become about 10 and 3 µm, respectively. The crystallinity of the films was examined by x-ray diffraction while the surface morphology of both films was determined by atom force microscopy. To investigate the relation between material structure and the performance of the solar cell, the annealed electrodes at different temperature were used to fabricate the dye sensitized solar cell structure with standard rutherium(II) (N719) dye and then the current voltage characteristic was measured under light with air mass of 1.5. It found that the structure with higher anneal temperature electrode exhibited higher power conversion efficiency originating from the higher short circuit current density of better crystallinity and higher surface area.

Abstract: The layer formation of bovine serum albumin (BSA) on a poly(styrene-co-maleic acid) (PSMA) surface was investigated by using quartz crystal microbalance (QCM) technique at various pH values. The formation of a BSA surface was examined by atomic force microscopy (AFM). To study the effect on the layer formation, the pH of solution was varied from 2 to 7.4 while the concentration of BSA was in the range of 0.01 to 5 mg/ml during the layer absorption. It was found that the BSA adsorption strongly depends on the pH of solution, and the concentration of BSA. The absorption layer occurred maximum at the pH value of 3.5 which resulted from the charge of PSMA and BSA molecules. The layer formation reached the saturate value at the concentration higher than 3 mg/ml. The molecular packing of the BSA layer at different pH values was determined by AFM and total mass change of QCM.

Abstract: Modified poly(3-hexylthiophene) (P3HT) with molecule of 1,4-dithienyl-2,3,5,6-tetrafluorobenzene in the main chain was developed as a new class of conjugated polymer with high stability. The structural and optical characteristics of modified polymer were investigated by x-ray diffraction (XRD) and optical absorption spectroscopy. By comparing to characteristic of P3HT, the XRD pattern of modified polymer showed the diffraction peak shifted from theta of 5.4o to 5.9o with decrease intensity. This indicates that the modified polymer chain has a reducing in structural coplanarity and crystallinity. Since the electrical property can be related to the morphology and structure of thin film, the electrical conduction of modified polymer was studied with the structure of ITO/modified P3HT/Al. The result of I-V characteristic measurement of modified polymer as a function of temperatures showed that the conductance decreased with decreasing temperature. The mobility of copolymer was also evaluated by using time of flight measurement and mobility value of 5x10-4 cm2/Vs was obtained at room temperature.

Abstract: Copper Phthalocyanine (CuPc) thin films have a potential to use as nitrogen dioxide (NO2) gas sensor therefore the study of property modification of CuPc thin film by NO2 gas can lead to a better understanding the sensor operation. CuPc thin films were prepared by thermal vacuum deposition on glass substrate and aluminum interdigitated electrodes. The film characteristics were investigated with and without NO2 gas. These characteristics indicated that the resistance of CuPc film decrease as the NO2 concentration increases. In order to achieve high efficient of sensing properties, the post-deposition film was also annealed. The surface modifications were investigated with different exposed time of NO2 gas. Their properties modifications before and after annealing were investigated on the film morphology and crystalline structure. The surface morphology of the film was examined by atomic force microscopy while the crystalline structure of the film was determined by x-ray diffraction. The electrical measurements of CuPc film exposed to NO2 gas were studied in order to investigate property modification of the CuPc film.

Abstract: The surface of polystyrene (PS) thin films in argon plasma was modified to study the hydrophilicity properties. An inductively coupled plasma (ICP) system was used to generate the argon plasma. In the experiment, the effect of RF power levels, gas flow rate and treatment time was investigated. The surface morphology of PS films was examined by the atomic force microscopy (AFM), also the contact angle goniometry was used for measuring the wettability of PS films before and after plasma treatment. After the plasma treatment, AFM images of PS revealed the increasing of the surface roughness as increasing the power levels and treatment times. Moreover, after treated with argon plasma, the contact angles of polystyrene films also decrease where the power levels and treatment times were increased. It is clear that the effects of power levels and treatment time improve the wettability of PS films. It can also be observed that by placing the sample in air after plasma treatment, the contact angle gradually increases probably due to moisture absorption in the PS films.

Abstract: The optical properties of metal-free and metal phthalocyanine were calculated by using density functional theory with various metals including copper, zinc, cobalt, iron and manganese. The polymorphic form of these crystals was employed only for β from. The molecules were optimized with the symmetry of D4h. For the alignments of the molecule in the crystal structures of this polymorphic form which have not been reported in detail, the variation of total energy was examined as a function of the align angles. The align angle at minimum total energy was used for the band calculation. The density functional theory and plane-wave pseudopotential method were used to calculate the energy band structure and electron density of state. The calculated band structures of various metal phthalocyanines can be divided in two groups according to the peak wavelength of the maximum absorption. The first group with the peak wavelength at about 230 nm consists of β-CuPc, β-H2Pc, and β-MnPc while the wavelength of another group for β-CoPc, β-FePc, and β-ZnPc occurs at 350 nm. From the density of state calculation, it indicates that these two transitions originate from the different band and the ratio of the absorption between these states depending on the type of metal in phthalocyanine. The optical absorption was derived to examine the absorption spectra for various metal compositions while the variation in intrinsic electrical conductivity can be estimated from the shape of the band. The phonon and infrared spectra were also determined in order to investigate the vibration mode of molecule in the crystals

Abstract: Poly(3-hexylthiophene) (P3HT) is one of the most studied conjugated polymer for molecular electronics especially for organic field effect transistors (OFETs) and organic light-emitting devices (OLEDs). This is mainly due to the fact that P3HT provides excellent electrical properties and showed high carrier mobility. In this research we studied the photo generated charge carrier transport of P3HT film by the time-of-flight (TOF) method. For device fabrication, P3HT was dissolved in chloroform with concentration of 8 mg/ml then the solution was spun directly onto an ITO pattern coated on glass substrate. Then the aluminum electrode was prepared on film by thermal evaporation. In TOF measurement, the constant voltage was applied to electrode of sample and the film were photo-excited by irradiation of a short pulsed laser light (λ=650 nm). This caused charge separation within the film. The generated charge carrier was used to calculate the mobility of the film. The TOF mobility was determined as a function of applied voltage and light condition.