Thipwan Fangsuwannarak

Structural and optical characterizations of n-type doped ZnO by sol-gel method for photovoltaic

This paper presents the effect of Bi doping concentration and annealing temperature on structural and optical properties of zinc oxide thin films. The samples in this study were prepared by sol-gel technique. These thin films were characterized by X-ray diffractometer technique. XRD analysis revealed that the Bi-doped thin films after annealing indicated good preferential orientation along c-axis perpendicular to the substrate, in particular very low concentration of Bi dopant. As a result, it indicated that Bi atoms acting as donor impurities can be in phase of ZnO. The surface morphology was characterized by using SEM (Scanning Electron Microscope). The visible region was measured by UV-VIS spectrophotometer. Finally, the optical band gap of undoped and Bi-doped ZnO thin films were obtained by the Tauc plot. Varying the Bi doped in the range of 0.2–0.6 at.% under annealing temperature at 600 °C caused a no significant change in optical band gap. In contrast, the annealing procedure produced significant changes from 3.27eV to 3.32eV in the optical band gap in order to higher blue responsibility for photovoltaic.

Locally contacted rear surface passivated solar cells by inkjet printing

Inkjet printing of photoresist material may provide a new route for low-cost fabrication of patterned oxide passivation layer of solar cells that require fine patterning and simple process. However, printing by liquid-based, environmentally friendly ink and printing device required development efforts aimed at achieving a fine patterning and long used inkjet nozzles under corrosive influence. This work was demonstrated a concept for grooved silicon oxide patterning for rear localized contact of p-n junction solar cells by chemical etching after photoresist patterning obtained. This article reviews the silicon dioxide fabrication on p-Si substrate from sol-gel technique for oxide passivation layer of solar cells. The aluminium was deposited on the patterned oxide layer and then heated at its Al-Si eutectic temperature. Finally, an aluminium-induced solid-phase epitaxial growth of p+ forming into the openings of the oxide passivation layer was presented. The sheet resistance of n-emitter layer, carrier life-time and surface recombination velocity values are investigated. Photoconductive measurements were performed on the prepared samples after each thermal process to measure the effective lifetime of the minority carriers. Carrier lifetime up to 60 microseconds has been measured on c-Si wafer passivated by the opened SiO2 layer. It was shown that the patterned SiO2 passivation has obtained high passivation quality making by the proposed inkjet printing method.

Boron Doping Effects on the Structural and Optical Properties of Sol-Gel Transparent ZnO Films

In this study, undoped and B-doped ZnO thin films were successfully deposited on the glass substrates by a sol-gel spin coating method. The influence of doping concentrations and the annealing temperature effects on the structural and optical properties of ZnO thin films were investigated. All of films exhibited polycrystalline structure, with a preferential growth along the c-axis plane and the optical transparency with visible transmittance was higher than 90%. The crystallite size was calculated using a well-known Scherrer’s formula and found to be in the range of 11-18 nm. The optical band gap of these films were determined and compared with those obtained for undoped ZnO thin film.

Optical Band Gaps and Electrical Conductance of Si Nanocrystals in SiO2 Matrix for Optoelectronic Applications

In this study, silicon nanocrystal (Si-nc) films were synthesized by compositing of Si-nc powder embedded in silicon oxide phase. The Si-nc film produced by the spin-coating methode using Tetraethylorthosilicate, ethanol, phosphoric acid, and Si-nc powder as suspension precursors. The variation in structural and optical properties of Si-nc sol films with the amounts of Si-nc powder have been characterized. Atomic force microscopy (AFM) shows that low density level of Si-nc power can result in the amount of porosity in the Si-nc films. It is found that when the Si-nc films have the higher Si-nc density, the small pores in the SiO2 phase were removed. In addition, optical energy gap (Eg) of Si-nc samples was evaluated by the Tauc plot method. It is a crucial attribute for a promising photonic device. The obtained optical bang gap values were extended from 1.10 eV to 1.40 eV as compared with the typical Si bulk. In addition, density of Si-nc clusters has a considerable effect on the electrical conductance of the Si-nc films measured at room temperature.

Aluminum-Induced Crystallization of p+ Silicon Pinholes for the Formation of Rear Passivation Contact in Solar Cell

Formation of nano-crystalline p+ silicon (Si) in pinholes through a silicon dioxide layer was achieved by pinning of aluminum through the thin silicon dioxide (SiO2) layer. In addition to opening holes of SiO2 layer by aluminum (Al) pining, amorphous silicon (a-Si) was subsequent deposited on the Al layer and another heated at low temperature (500°C) to allow solid- phase epitaxial growth of p+ Si in the pinholes due to the Al induced layer exchange process. The poly-crystalline p+ Si obtains lower effective surface recombination than the Al back surface field (BSF). The technique demonstrated to result in ohmic contacts with low contact resistance. The evaluation of Al-induced crystallization of a-Si in a-Si/Al bilayer was studied by X-ray diffraction. In this paper, the influence of a-Si/Al thickness ratio on the specific conductivity value and crystalline grain size of the p+ Si thin film is discussed. The obtained results are helpful for a further design of the rear passivation contact in solar cell.

Surface Passivation of Point-Contacted Solar Cells by Inkjet Printing

The oxide passivation of the localized contacts for p-n junction solar cells by inkjet printing is studied by combining the useful of spin-on-doping (SOD) concept for producing n-Si emitter layer. The phosphorus SOD is a precursor for a dopant source as phosphosilicate glass (PSG). The sheet resistance of the n-emitter layer was obtained in the range of 900–20 Ω/square by increasing the diffusion temperature from 800°C to 1000°C. In addition, an inkjet printing method for patterning a SiO2 dielectric on n-Si emitter layer was demonstrated. With using typical printer, the groove and circular patterns of openings were achieved by the modified printer dropping acetone solution as a resist solvent. The SiO2 layer underneath the resist polymer layer was etching in the region of the groove or circular openings for forming the patterns of SiO2 openings. The formed openings in the thin SiO2 can be produced as point contacts for reducing carrier recombination process of silicon solar cell.

Effect of annealing temperature on dark current density of silicon nanocrystals embedded in a nitride matrix for photovoltaic application

The purpose of using high density nano-crystalline silicon embedded in insulator matrices is the energy confinement of Si based quantum dot nanostructures. This approach aims to engineer wide band gap Si materials to be used as upper cell elements in Si based tandem cells in order to increased efficiency and low cost thin film processes. One of the main challenges is to obtain sufficient carrier mobility and hence a reasonably conductivity for photovoltaic application. The results of current density as a function of thermal annealing show the evolution of SiQD formation in Si3N4 matrix. As deposited film fabricated by using dual-mode PECVD has composition of ordered Si rich nitride (Si3+xN4) arrays.