| Summary: | 博士 === 國立交通大學 === 光電工程研究所 === 106 === In this thesis, we have developed and optimized the hydrogenated amorphous silicon-oxide (a-SiOx:H) and hydrogenated microcrystalline silicon-oxide (μc-SiOy:H) films for single-junction and multi-junction solar cell applications. The high efficiency silicon thin-film solar cell were achieved by integrating two approaches, as following. The first approach is developing the wide-bandgap n-type a-SiOx:H and n-type μc-SiOy:H films to form a double n-layer structure in cell for improving the light management. The second approach is developing the high-quality undoped hydrogenated amorphous silicon-oxide (a-SiOx:H(i)) film as graded p/i buffer layer in cell for enhancing carrier transport. All silicon thin films and solar cells were prepared by radio-frequency plasma-enhanced chemical vapor deposition (RF PECVD) with 27.12 MHz.
In the first part, we developed a-SiOx:H(n) and μc-SiOy:H(n) films as n-type layer, intermediate reflecting layer (IRL), and back-reflecting layer (BRL) to improve the light management in silicon thin-film solar cells. In the development of a-SiOx:H(n) and μc-SiOy:H(n) films, we have optimized the optical and electrical properties by properly adjusting RF power, PH3 concentration, CO2 concentration and H2 concentration. After optimization, the optical bandgap of a-SiOx:H(n) and μc-SiOy:H(n) can be increased while maintaining sufficient conductivity. In a-Si:H single-junction cells, employing n-type a-SiOx:H (oxygen content of 3.6 at.%) as the replacement for a-Si:H(n) resulted in a relative efficiency enhancement of 7.2% due to the reduced parasitic absorption loss. Besides, we have also found that μc-SiOy:H(n) can replace back ITO layer as BRL by all in-situ PECVD process, resulting in obtaining the high efficiency of 9.4% and simplifying the fabrication process. For a-Si:H/a-Si1-zGez:H tandem cell, employing μc-SiOy:H(n) as IRL increased the current density of top cell, leading to the current matching. In addition, employing a-SiOx:H(n) as a replacement of a-Si:H(n) in the top cell increased the current density of bottom cell due to the reduction of absorption loss. Combining all the improvements, the a-Si:H/a-Si1-zGez:H tandem cell with efficiency of 10.5%, VOC of 1.58 V, JSC of 9.68 mA/cm2, and FF of 68.4% was obtained.
In the second part, we have developed the high-quality a-SiOx:H(i) films with variable bandgap as p/i buffer layer in silicon thin-film solar cells for reducing the carrier recombination loss at interface. In development of a-SiOx:H(i) films, the high-quality a-SiOx:H(i) films was achieved by adjusting the H2 concentration and CO2 concentration. After optimization, the a-SiOx:H(i) films with oxygen content from 4 to 7 at.% exhibited high photo-response of over 105. By employing a single-bandgap a-SiOx:H(i) as p/i buffer layer in the a-Si:H single-junction cell, the improved VOC from 0.85 to 0.90 V and the increased short-wavelength from 400 to 550 nm response were attributed to the improved p/i band offset. Further employing the graded-bandgap a-SiOx:H buffer layer with 4 graded-steps in a-Si:H single-junction cell, the enhanced visible-wavelength from 350 to 660 nm response and the reduced reverse saturation current density were due to the improved carrier transport and the reduced carrier recombination, which resulted in a relative efficiency enhancement of 11.2%. By combining the a-Si:H top cell having the graded-bandgap a-SiOx:H buffer layer together with the hydrogenated microcrystalline silicon-germanium (μc-Si1-aGea:H) bottom cell, the total current density significantly increased from 22.06 to 23.20 mA/cm2 compared to the tandem cell without buffer layer. As a result, compared to the cell without buffer layer, the efficiency of a-Si:H/μc-Si1-aGea:H tandem cell with graded-bandgap a-SiOx:H buffer layer increased from 9.91% to 11.04%, which had a relative enhancement of 11.44%, Furthermore, the tandem cell performance of VOC of 1.33 V, JSC of 11.6 mA/cm2 and FF of 71.6% were obtained.
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