Taweewat Krajangsang

ZnO Films with Very High Haze Value for Use as Front Transparent Conductive Oxide Films in Thin-Film Silicon Solar Cells

We successfully increased the haze value of zinc oxide (ZnO) films fabricated using metal–organic chemical vapor deposition (MOCVD) by conducting glass-substrate etching before film deposition. It was found that with increasing the glass treatment time, the surface morphology of ZnO films changed from conventional pyramid-like single texture to greater cauliflower-like multi texture. Further, the rms roughness and the haze value of the films increased remarkably. Using ZnO films with a high haze value as front transparent conductive oxide (TCO) films in hydrogenated microcrystalline silicon (µc-Si:H) solar cells, we improved the quantum efficiency of these cells particularly in the long-wavelength region.

Effect of light intensity on performance of silicon-based thin film solar cells

Theoretical and experimental studies were performed to explore the effect of light intensity on the performance of silicon-based thin film solar cells. The theoretical study was conducted using AMPS-1D (Analysis of Microelectronic and Photonic Structures) to analyze how the device structure affects the performance of silicon-based solar cells under various concentration ratios of sunlight. We calculated a-Si:H and μc-Si:H p-i-n type single-junction solar cells, and also experimentally evaluated a-Si:H, a-SiO:H and μc-Si:H solar cells. From both simulation and measurement results, we confirmed that the Voc logarithmically increases with increasing the light intensity. Moreover, from simulation results, we also observed that the defect density and thickness of i-layer strongly influence the light-intensity dependence of a-Si:H solar cells.

Development of high-efficiency tandem silicon solar cells on W-textured Zinc oxide-coated soda-lime glass substrates

In this study, we have investigated the effect of refractive index of Soda-lime glass (SLG) substrates on its surface roughness after being etched by Reactive Ion Etching (RIE). It was found that there was a relationship between the refractive index of the glass and etched surface roughness. Higher roughness was obtained on SLG with lower refractive index. Based on this finding, we can easily control the glass etching process and achieve its good repeatability. Furthermore, ZnO films have been deposited on this etched SLG to achieve W-textured TCO substrates. As a result, a-Si:H/μc-Si:H tandem solar cell fabricated on this developed substrate has a high efficiency of 12.40% showing that this new morphology control of glass etching is a promising method for development of high-efficiency thin-film solar cell.

Effect of high-haze zinc oxide films fabricated on soda-lime glass substrate for thin-film silicon solar cells

We have successfully fabricated zinc oxide (ZnO) films with a very high haze value using metal organic chemical vapor deposition (MOCVD) technique by carrying out a unique glass-substrate treatment before ZnO film deposition. Effects of soda-lime glass treatment conditions on the properties of ZnO films were investigated. It was found the larger craters surface and high rms roughness of soda-lime glass could be obtained when the treatment gas pressure was 13 Pa. It was also found that the surface morphology of ZnO films can be modified by adjusting the glass treatment conditions with preserving their good electrical properties. By employing newly developed ZnO film as front TCO films in conventional p-i-n-type a-Si:H single junction solar cells, we obtained an efficiency of 9.87%. Therefore, the obtained ZnO films deposited on etched soda-lime glass have a lot of potentials to be used as the front TCO films in thin-film silicon solar cells.

High efficiency a-Si:H/a-SiGe:H tandem solar cells fabricated with the combination of V- and U-shaped band gap profiling techniques

Hydrogenated amorphous silicon germanium (a-SiGe:H) films prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a mixture of SiH4, H2, and GeH4 were investigated for their use as the bottom cell of amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem solar cell structures. Narrow optical band gaps (Eopt) in the range of 1.5 to 1.6 eV were obtained by varying the GeH4/(SiH4 + GeH4) gas flow rate ratio in low-temperature deposition. The a-SiGe:H films deposited with various GeH4/(SiH4 + GeH4) gas flow rate ratios were used as intrinsic layers for the a-Si:H/a-SiGe:H tandem solar cells with different graded band gaps: V-, VU-, and U-shapes. It was found that using the VU-shape improves the solar cell efficiency owing to a higher Jsc when compared with using V-shape. The VU-shape’s Voc and FF are also improved when compared with the U-shape’s Voc and FF. As a result, a high efficiency of 11.0% (Voc = 1.74 V, Jsc = 9.07 mA/cm2, and FF = 0.70) was successfully achieved with the solar cells fabricated using the VU-shape graded band gap technique.

Wide-Gap p-c-:H Films and Their Application to Amorphous Silicon Solar Cells

Optimization of p-type hydrogenated microcrystalline silicon oxide thin films (p-c-:H) by very high frequency plasma enhanced chemical vapor deposition 40 MHz method for use as a p-layer of a-Si:H solar cells was performed. The properties of p-c-:H films were characterized by conductivity, Raman scattering spectroscopy, and spectroscopic ellipsometry. The wide optical band gap p-c-:H films were optimized by CO2/SiH4 ratio and H2/SiH4 dilution. Besides, the effects of wide-gap p-c-:H layer on the performance of a-Si:H solar cells with various optical band gaps of p-layer were also investigated. Furthermore, improvements of open circuit voltage, short circuit current, and performance of the solar cells by using the effective wide-gap p-c-:H were observed in this study. These results indicate that wide-gap p-c-:H is promising to use as window layer in a-Si:H solar cells.

Effect of the CO2/SiH4 Ratio in the p-μc-SiO:H Emitter Layer on the Performance of Crystalline Silicon Heterojunction Solar Cells

This paper reports the preparation of wide gap p-type hydrogenated microcrystalline silicon oxide (p-c-SiO:H) films using a 40 MHz very high frequency plasma enhanced chemical vapor deposition technique. The reported work focused on the effects of the CO2/SiH4 ratio on the properties of p-c-SiO:H films and the effectiveness of the films as an emitter layer of crystalline silicon heterojunction (c-Si-HJ) solar cells. A p-c-SiO:H film with a wide optical band gap (E04), 2.1 eV, can be obtained by increasing the CO2/SiH4 ratio; however, the tradeoff between E04 and dark conductivity must be considered. The CO2/SiH4 ratio of the p-c-SiO:H emitter layer also significantly affects the performance of the solar cells. Compared to the cell using p-c-Si:H (CO2/SiH4 = 0), the cell with the p-c-SiO:H emitter layer performs more efficiently. We have achieved the highest efficiency of 18.3% with an open-circuit voltage () of 692 mV from the cell using the p-c-SiO:H layer. The enhancement in the and the efficiency of the solar cells verified the potential of the p-c-SiO:H films for use as the emitter layer in c-Si-HJ solar cells.

Advantages of N-Type Hydrogenated Microcrystalline Silicon Oxide Films for Micromorph Silicon Solar Cells

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide films (n μc-SiO:H) in hydrogenated amorphous silicon oxide/hydrogenated microcrystalline silicon (a-SiO:H/μc-Si:H) micromorph solar cells. The n μc-SiO:H films with high optical bandgap and low refractive index could be obtained when a ratio of carbon dioxide (CO2) to silane (SiH4) flow rate was raised; however, a trade-off against electrical property was observed. We applied the n μc-SiO:H films in the top a-SiO:H cell and investigated the changes in cell performance with respect to the electrical and optical properties of the films. It was found that all photovoltaic parameters of the micromorph silicon solar cells using the n top μc-SiO:H layer enhanced with increasing the CO2/SiH4 ratio up to 0.23, where the highest initial cell efficiency of 10.7% was achieved. The enhancement of the open circuit voltage () was likely to be due to a reduction of reverse bias at subcell connection—n top/p bottom interface—and a better tunnel recombination junction contributed to the improvement in the fill factor (FF). Furthermore, the quantum efficiency (QE) results also have demonstrated intermediate-reflector function of the n μc-SiO:H films.

Quantitative Analysis of Surface Morphology of Boron-Doped Zinc Oxide for Microcrystalline Silicon Solar Cells

The optimization of textured boron-doped conductive zinc oxide (ZnO:B) films deposited by metal organic chemical vapor deposition (MOCVD) for hydrogenated microcrystalline silicon (µc-Si:H) solar cells was performed. We found that the argon (Ar) plasma treatment of a textured ZnO:B substrate is effective in improving the open-circuit voltage and fill factor of µc-Si:H solar cells. We proposed (tan θ)/λ2 as an evaluation factor and found that there is a clear correlation between the (tan θ)/λ2 of textured ZnO:B films and the open-circuit voltage of µc-Si:H solar cells. Moreover, the effect of the surface morphology of a textured ZnO:B substrate on the growth of intrinsic hydrogenated amorphous silicon (i-a-Si:H) top layers was examined to obtain the optimum conditions for preparing a-Si:H/µc-Si:H tandem solar cells. We confirmed that our proposed evaluation factor (tan θ)/λ2 could be used to obtain the surface morphology of i-a-Si:H top layers that are appropriate for µc-Si:H bottom-cell deposition.

Performance of multi-junction silicon-based thin film solar cells under concentrated sunlight

Multi-junction silicon-based thin-film concentrator solar cells are promising candidate to achieve both low-cost and high-efficiency. For the application of silicon-based thin film solar cells to concentrator photovoltaics, it is required to be revealed the light intensity dependence of the performance of silicon-based thin film solar cells. From these reasons, in this study both calculation and experimental studies were conducted with several types of single-junction and multi-junction tandem solar cells. From both simulation and measurement results, we observed that double- and triple-junction solar cells achieve high open-circuit voltage and large logarithmic increment in open-circuit voltage with increasing light intensities. On the other hand, it became clear that the drop of fill factor is required to be improved for the realization of the multi-junction silicon-based thin-film solar cells with very high efficiency under low concentration ratios of sunlight.