Atcharawon Gardchareon

Surface Modification of Porous Photoelectrode Using Etching Process for Efficiency Enhancement of ZnO Dye-Sensitized Solar Cells

Surface modification of porous ZnO photoelectrode using one- and two-step etching process is investigated for enhancing power conversion efficiency of ZnO dye-sensitized solar cells. ZnO films are modified by the diluted NH4OH solutions for one-step etching process and used as photoelectrode of dye-sensitized solar cells. Rough porous films are observed after one-step etching process. The fabricated cells based on the optimized one-step etched films show a significant increase in short-circuit current density. The short-circuit current density is directly changed with amount of dye adsorption, which is related to specific surface area. The etched films exhibit higher specific surface area over two times than nonetched films. Thus, the large specific surface area is the key success for increasing amount of dye adsorption. Internal electrochemical property of fabricated cells is also improved, indicating that chemical surface of ZnO films is modified in the same time. The DSSCs fabricated on two-step etched films with NH4OH and mixed acid HCl : HNO3 show the maximum power conversion efficiency of 2.26%. Moreover, fill factor is also increased due to better redox process because of the formation of fine porous structure during the etching process. Therefore, these results implied that the roles of etching processes are improving specific surface area and fine porous formation which can provide better dye adsorption and redox process for dye-sensitized solar cell application.

Ethanol sensing characteristics of sensors based on ZnO:Al nanostructures prepared by thermal oxidation

ZnO and ZnO:Al nanostructure were synthesized and fabricated as ethanol gas sensors. For FE-SEM images, the diameter and length measured at the middle of the wire-like structure were in range of 100-500 nm and several micrometers, respectively. From TEM analysis, it was suggested that the ZnO:Al nanostructure grew along (112̅0) direction on [0001] plane. The Raman spectra of ZnO and ZnO:Al nanostructures can confirm existence of defect effects due to oxygen vacancies and Zn interstitials of ZnO. Besides, it also suggested that the ZnO:Al nanostructures had (112̅0) direction perpendicular to the surface. The ethanol sensors based on ZnO:Al nanostructure sensors can be improved when compare with pure ZnO nanostructure sensor at the ethanol concentrations of 50-1000 ppm. The highest sensitivity of 32 was obtained in ZnO:Al nanostructure sensors with Al 1% by mol compared to 14 of pure ZnO nanostructure sensor at optimum temperature of 300°C. The sensitivity improvement of ZnO:Al sensors can be explained by an increase of oxygen vacancy-related defects which increase the surface depletion layer width as described in sensitivity equation. The larger surface depletion layer width results in higher the potential barrier height at the contacts and finally, sensitivity improvement.

Ethanol Sensor Based on Au-doped ZnO Nanostructures

Ethanol sensing characteristics of ethanol sensor based on Au-doped ZnO nanostructures were studied. Au-doped ZnO nanostructures with 5% gold by weight were prepared by thermal oxidation technique. The sintering time was varied for 6 hours and 24 hours. The wire-like or belt-like nanostructures with the sharp ends outward from microparticles were observed. The diameter and length of ZnO nanostructures are in the range of 250-750 nm and 1.7-7.0 mum, respectively, with the average diameter of 500 nm. EDS spectrum shows Au signal confirming incorporation of Au into ZnO nanostructures. For 24 hours sintering time, the diameter and length of ZnO nanostructures are 170-500 nm and 2.0-7.0 jim, respectively, with the average diameter of 330 nm. The average diameter for 24 hours sintering time is smaller than that of 6 hours sintering time. The response and recovery characteristics of Au-doped ZnO nanostructures upon exposure to ethanol concentration of 1000 ppm at different operating temperatures suggest that the sensitivity depend on operating temperatures. It is found that the highest sensitivity is 88 at 280 degC for 6 hours sintering time and about 170 at 300-320 degC for 24 hours sintering time. The longer sintering times the higher the sensitivity. The enhancement of sensitivity for longer sintering time may be explained in terms of the smaller size of nanostructures due to the increase of effective surface for absorption of ethanol on the surface.

CuO nanostructure by oxidization of copper thin films

CuO nanostructures were synthesized by oxidizing copper thin films. The copper thin film was grown on alumina substrates by evaporation copper powder at pressure of 0.04 mtorr. The copper thin films were then oxidized 800, and 900oC for 12, 24 and 48 hr, respectively. The obtained CuO nanostructures were investigated by Energy Dispersive Spectroscopy (EDS), Field Emission Scanning Electron Microscope (FE-SEM) image, and X-Ray Diffraction (XRD). The diameter of CuO nanostructure is around 100-600 nanometers and it is depends on oxidation reaction time and temperature. These CuO nanostructures have a potential application for nanodevices such as nano gas sensor or dye-sensitized solar cells.