| Summary: | 博士 === 國立中興大學 === 材料科學與工程學系所 === 104 === Amorphous silicon and crystalline silicon heterojunction solar cells have attracted great attention due to their low processing temperature (<200 °C) and high conversion efficiency (>20 %). In fabrication process, the dangling bonds on silicon wafer surfaces and plasma ion bombardment would lead to the increased carrier recombination and reduced device performance. This study applied plasma radical composition analysis technique to silicon heterojunction solar cells.
The self-designed measurement instrument is set for detection of plasma radical distribution during the deposition of intrinsic amorphous silicon films using a plasma enhanced chemical vapor deposition system. The measurement instrument is automatic so the required analyzing time is reduced. Effects of Si* (at 252 nm in plasma spectra), SiH* (414 nm) and H* (656, 722, 772 nm) radical composition on deposition rate, plasma ion bombardment, selective etching and film properties are investigated. The analysis results are further used to assess the uniformity of the amorphous silicon films prepared under various deposition parameters.
The investigate influences of surface texturing morphologies of single crystalline silicon wafers on effective minority carrier lifetime. The pyramidal structure created by etching wafer surfaces are observed using a transmission electron microscope. The mountains and valleys of the pyramids, defined as the turning point in this study, cause deteriorate interface between amorphous silicon passivation layer (5 nm-thick) and single crystalline silicon substrate. When the number of the turning points increases, the carrier recombination rate increases and the wafer surface passivation reduces. The optimal wafer morphology is obtained for the use as a substrate of silicon heterojunction solar cells.
The aforementioned analysis techniques are applied to fabrication of silicon heterojunction solar cells, and it is found that the most effective passivation of surface dangling bonds on wafer surfaces can be obtained when the intrinsic amorphous silicon film has a high hydrogen content (>18 %). Moreover, the plasma radical Si* (252 nm) intensity can be related to the amount of Si-H2 bonds in the deposited films. This bond configuration will result in defects and vacancies appearing at the interface between amorphous silicon and wafer, thereby decreasing the blue light absorption of the fabricated device. As a result, the plasma radical distribution can in-situ evaluate the quality of the deposited amorphous silicon films.
The uniformity of intrinsic amorphous silicon is a critical parameter for device performance, the lage-area it is usually limited by the quality of deposition equipment. High quality deposition systems are expensive. This study adjusts the tilted angle of the substrate during the deposition process, according to the analysis provided by the plasma radical distribution measurement instrument, to reduce the differences of plasma radical composition between various positions. The tilted angle of 1.5° can obtain a small standard deviation of the composition of Si*, SiH* and H* and thus results in improved uniformity. The optimal result shows a large size (15.6 cm × 15.6 cm) silicon heterojunction solar cell with short-circuit current of 35.3 mA/cm2, open-circuit voltage of 0.72 V, fill factor of 0.73 and conversion efficiency of 18.55 %.
The research results of film properties obtained under various process conditions can be saved as a database. The database can give an in-situ evaluation of film properties during film growth, and increase the stability of the deposition system. This study is proper to be used and helpful for semiconductor industry.
Keywords: plasma enhanced chemical vapor deposition, intrinsic amorphous silicon, heterojunction solar cell, plasma radical, plasma radical component measurement instrument
|
|---|
