In conclusion, this study proposes a new and simple MD model alternative to complex electron-photon interactions in quantum scale to analyze surface texturization of solar cells.
Three surface texturization shapes are simulated with various angles of incident light. This methodology can easily determine the absorptance differences of various surface texturizations, and suggest better texturization shapes. In the simulations of mono-crystalline silicon, increasing the trench depth, shortening the distance between two trenches, sharpening pyramids and adding more circling to the semicircular structure all help improve the efficiency of solar cells.
In the case of multi-crystalline silicon by alkaline etching, the large tilted angle leads to the lower efficiency of solar cell. To consider the effect of incident angle, a range of high efficiency exists due to the increasing probability of second reflection. Furthermore, the azimuth angle of incident light also affects the efficiency of solar cells.
In simulations of GaAs solar cells, the effects of square texturization are discussed. As the similar results as that of silicon solar cells, larger square’s height and shorter distance between two squares are helpful to increase the probability of second reflection and then enhance the efficiency of solar cells.
From previous studies, we know the emission of heat carriers is an important factor on efficiency. We have interest the effect of phonon emission in our simulation. Thus a modified model with vibration transport among atoms was established. In this model, we found the efficiency of solar cells obviously decreased. The absorptance is 19.07% in perfect surface, obviously lower than the 24.07% of perfect surface without phonon emission. The simulation of trench surface similar with that in section 3-1 shows the absorptance also increases with the height of trenches. But the values are apparently smaller than those in section 3-1.
We also applied this model to the simulation of pyramidal surface texturization in section 3.2. Obviously, the trend is similar with the result from the simulation in section 2-3.
But absorptance is obviously lower than the value from the simulation without phonon emission. The reason is reliable that a part of energy from photon is lost to heat sink through phonon emission. Nozik [53] have mentioned the absorbed photon energy above the semiconductor band gap is lost as heat through electron-phonon scattering and subsequent phonon emission, as the carriers relax to their respective band edges (bottom of conduction band for electrons and top of valence for holes)
Our results agree well with previous studies. This MD model can potentially be used to predict the efficiency promotion in any optical reflection-absorption cases.
In the results of chapter 4, we appropriately modified our model with the concept of phonon emission. The absorptance that was higher than 60% decreased to almost 50%. Thus the results from the simulations with phonon emission were close to reality.
Besides, the results with phonon emission also lead to a new thought that the decreasing of heat loss can increase the efficiency of photovoltaic effect. In the mechanism of PV effect, decreasing phonon’s transportation is one of feasible direction.
The investigations and findings in previous chapters offer an essential knowledge base for the subsequent study. The next work can be continuously focused on another material, such as InGaP, InGaAs and CdTe et al. The more and more potential models are developed, such as different Tersoff potential model for special materials [68, 69]. The other advantage of MD simulations is easily to explore the effect of structures, such as doping, defects and dislocations. Thus we can put the focus on these problems by MD simulations.
To get more precise results, it is necessary to promote the simulation method. Using Monte Carlo method is helpful in our simulations. Monte Carlo method is also helpful to be a contrast to correct our results.
Actually, this MD model is a rough model. Thus, to develop a more precise method is necessary. The method in quantum mechanism such as Born-Oppenheimer Approximation is possible to govern the interaction between photons and electrons in solar cells. If this method can be applied on this problem, this study can make a great contribution to enhance efficiency of solar cells.
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Author
姓名： 鍾孝彥
地址： 桃園縣八德市大忠街 102 號
學歷：
國立交通大學機械系博士班 87/9~97/7
私立中原大學機械系碩士班 85/9~87/7
國立成功大學機械系學士班 78/9~82/7
List of Publications
Journal Paper
1. Chung, H. –Y. Chen, C. –H. and Chu, H. –S., Analysis of Surface Texturization of Solar Cells by Molecular Dynamics Simulations, International Journal of Photoenergy, Volume 2008 (2008), Article ID 540971
2. Chung, H. -Y, Chen, C. –H. and Chu, H. –S., Analysis of pyramidal surface texturization of silicon solar cells by molecular dynamics simulations, International Journal of Photoenergy, 2008, in press.
Conference Paper
1. Chung, H. –Y. and Chu, H. –S., Analysis of Surface Texturization of GaAs Solar Cells by Molecular Dynamics Simulations, in The International Conference on Molecular/Nano-Photochemistry (Solar’08), 2008: Cairo, Egypt
2. Lin, W. –Y., Chung, H. –Y. and Chu, H. –S., Effects of Airflow in Unidirectional Flow Type Clean Room with Heat Source on Workbench, 高 效率能源技術研討會, 2004, Hsin-Chu, Taiwan