Fabrication and Analysis of Amorphous Microcrystalline Silicon Thin Film Solar Cells under Long-Term Illumination and Flexible Electronics
Date Issued
2012
Date
2012
Author(s)
Yang, Chieh-Hung
Abstract
The hydrogenated amorphous silicon (a-Si:H) has been widely applied in thin film solar cells. The benefit was large area and low cost process than single crystal silicon. However, the light induced degradation in the conductivity of a-Si:H would seriously lower the efficiency of solar cell after long-term illumination. Typically, the material and structure of film would significantly affect the stability of solar cell. Therefore, some kinds of thin film materials are discussed. Firstly, the a-SiC, a-SiGe, and a-SiGeC layers are deposited by conventional plasma enhanced chemical vapor deposition. The effect of Ge and C on the amorphous structure is deduced by Raman spectroscopy and fourier transform infrared spectroscopy. In addition, the variation of the conductivity of the films under long-term illumination is discussed. A better stability of a-SiC layers is obtained when Xg(C)=0.1. The improvement of stability might be attributed to the coupling of Si-C network and Si-H dangling bonds and the accumulated energy of Si-H sites can be easily released.
On the other hand, the microcrystalline silicon (μc-Si) is deposited using SiF4+SiH4+H2 gas mixture. The film morphologies and crystalline structure were studied by Raman spectroscopy, atomic force microscope (AFM) and transmission electron microscopy (TEM). It is found that a subtle SiF4 could significantly improve both the crystalline fraction (85 %) and deposition rate (3.5 nm/min) of microcrystalline silicon. The grain size is 20-30 nm. The surface morphology is also investigated to deduce the influence of adding SiF4 on crystalline structure.
Hot-wire chemical vapor deposition (HWCVD) is also used to deposit a-Si:H and μc-Si films. The extremely high temperature filament would significantly improve the gas dissociation. The a-Si:H and μc-Si films could thus be deposited in a high rate with good quality. Some methods including photo/dark-conductivity, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, UV-visible spectroscopy were used to investigate the film quality. The influence of deposition parameter: filament temperature, gas flow rate and gas-mixture ratio were discussed. Furthermore, the crystallinity of boron and phosphorous doped μc-Si is studied. It is found that under low substrate temperature, the interaction between boron and Si-H would improve the crystal formation. The condition of a-Si:H transit to μc-Si is also investigated.
Numerous materials deposited by PECVD or HWCVD are used in the solar cell fabrication. The degradation behaviors of the solar cells under long-term illumination are systematically investigated. Firstly, a comparison of the degradation behaviors between a-Si:H, a-SiC and a-SiGeC solar cells fabricated by PECVD are reported. Even though there are some different in the structural and electrical properties, however, the degradation behaviors of the three solar cell is very similar. In addition, the p-i-n and n-i-p substrate-type thin film solar cells were fabricated by HWCVD. Graded B- and P-doping was used to modify the electric filed distribution in the i-layer and thus significantly affects the efficiency and stability of solar cells. The graded P-doping causes a stronger electric field near the upper region. Therefore the n-i-p cell has better initial efficiency and stability than the p-i-n cell. Furthermore, the degraded solar cells were annealed to restore the light-induced degradation. Through light-soaking and annealing, the stability of p-i-n solar cell is dramatically improved and the cell would suitable for long-term usage.
Besides the light-induced degradation, another drawback of thin film solar cells is the conversion efficiency is typically low due to the amorphous structure. Surface texturing is thus widely employed on thin film solar cells to enhance the light trapping effect. Several methods have been reported to fabricate textured surface, however, some of processes would damage the plastic substrate. Therefore, a new method is developed to form the textured PI substrate by copying the surface morphology of a rigid substrate as a template. A buffer layer is introduced to moderate the adhesion between rigid substrate and polyimide. The polyimide could therefore be peeled off without damage. A flexible solar cell is fabricated on such textured polyimide substrate. The conversion efficiency is higher than the conventional solar cell in a fraction 17.8 %. In addition, the electrical characteristics of the solar cells are measured under different bending conditions and both tensile and compress stress are applied. When the radius of curvature is equal to 1 cm, the efficiencies of textured solar cells degrade apparently 25-40 %, whereas the non-textured solar cells display a larger degradation under compress stress then tensile stress.
Subjects
Flexible electronic
Thin film solar cell
Microcrystalline silicon
HWCVD
Type
thesis
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