微晶矽薄膜太陽能電池之元件模擬與分析

Device Modeling and Analysis of Microcrystalline Silicon Thin Film Solar Cells

蔡宛宸(Wan-cheng Tsai)

指導教授 ： 張正陽

國立中央大學/理學院/照明與顯示科技研究所/碩士(2011年)

摘要

近幾年來，各種物理特性對矽基薄膜太陽能電池所產生的影響引起廣泛的研究，尤其在非晶矽與微晶矽薄膜太陽能電池也有著相當多的研究結果。在本論文中，我們利用二維模擬軟體Silvaco TCAD集中於探討各種不同結構參數對於非晶矽與微晶矽薄膜太陽能電池在光電特性上的影響。
首先，我們建立一非晶矽薄膜太陽能電池，其結構為a-SiC:H/a-Si:H/a-Si:H，並針對各種不同參數作特性表現探討，其中參數包含厚度、參雜濃度、以及能態密度。從研究結果中可以發現，當p/i/n厚度依序為40/300/10 (nm) 及參雜濃度為1×1018/1×1017/1×1020 (cm-3) 時為非晶矽薄膜太陽能電池最佳化後的結構參數，其效率可高達9.92%。除此之外，從本質層(吸收層)厚度與懸浮鍵能態密度的關係結果中我們可以做出以下結論：對於一個品質較差(懸浮鍵能態密度較高)的樣品，若其本質層厚度越厚，則光衰退的影響會更趨明顯與嚴重；換言之，我們可利用較薄的本質層來達到更好的電性。
微晶矽薄膜是一種很複雜的材料，由許多小於20-30 nm的結晶粒存在於充滿間隙或缺陷的非晶矽本體所組成。因此，我們建立了一個微晶矽的結構結合可調變結晶度的模型，其中以柱狀形晶粒為考量，並加入晶粒邊界概念於結晶矽與非晶矽的陣列中。我們利用以下的方法來實現一個可調變結晶度(Xc)的微晶矽薄膜太陽電池：以非晶矽塊材放置於一顆晶粒中間；此晶粒的材料參數則與結晶矽材相似。另外，我們再以兩塊具有嚴重缺陷且與非晶矽的材料參數相似之區塊嵌入非晶矽與結晶矽之間做為晶粒邊界以提供更完整的考量。
我們對於微晶矽薄膜太陽能電池電特性表現的討論參數包括：不同的本質層厚度、晶粒大小、結晶度、以及非晶矽區塊中懸浮鍵能態密度都在本論文中討論。根據模擬結果可發現，一個晶粒大小為20 nm、本質層厚度為4 µm的微晶矽薄膜太陽電池，其高結晶度樣本(Xc=90 %)的效率可達到9.02 %；而低結晶度樣本(Xc=10 %)也可達7.55 %。然而，當晶粒大小大於30 nm則具有更好的電性表現，如此的現象是由於當晶粒大小為30 nm時，可視為微晶矽結構轉變成多晶矽結構的臨界點。除此之外，從外部量子效率結果中可發現微晶矽材料所產生的光衰退效應只發生在短波長(400-700 nm)的範圍，換言之則是非晶矽材料對光子具高吸收率的範圍。因此我們可以做出以下結論：對一個低結晶度的微晶矽本質層而言，非晶矽材料是造成微晶矽薄膜太陽能電池中光衰退效應(高能態密度)的主要原因。

關鍵字

非晶矽 ； 太陽能電池模擬 ； 微晶矽

並列摘要

Currently, the effects of physical properties on silicon-based thin film solar cells have been extensively researched as well as numerous investigation results have yielded a considerable amount of information about hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (µc-Si:H) solar cells. In this thesis, we focus on the electrical performances of several structural parameters and two-dimensional device modeling for two types p-i-n thin film solar cells which utilize a-Si:H and µc-Si:H as the material of i-layer (i.e. active layer) are carried out by using Silvaco TCAD simulation program.
First, we demonstrate a a-SiC:H/a-Si:H/a-Si:H thin film solar cell. Some characteristic behaviors are influenced by various parameters, such as the thicknesses, the doping concentration, and the density of states. The optimal efficiency of 9.92% is achieved with thickness of 40/300/10 (nm) and doping concentration of 1×1018/1×1017/1×1020 (cm-3). Nevertheless, from the results of the cells with different i-layer thickness and density of dangling bond states, we can conclude that the defect density is thickness dependent in poor quality samples. The light-induced degradation effect occurs more obviously in the thick samples. As a result, using a cell with thinner active layer thickness will yield better performance with poor quality materials.
µc-Si:H is a complex material composed of microcrystal grains in an amorphous matrix plus voids/cracks with crystal grain sizes smaller than 20-30 nm. A modulated crystalline volume fraction model in µc-Si:H thin film solar cells is established by consideration of the columnar crystal growth, which can be regarded as an array of crystalline and amorphous silicon regions with grain boundaries between them. The approach of utilizing two defective a-Si:H-like material as a columnar grain boundaries and an a-Si:H region which are inserted into the middle of the crystalline-like (namely, highly crystallized µc-Si:H) active layer is realized in order to demonstrate crystalline volume fraction (Xc) in a more realistic and complete way. The electrical properties of microcrystalline silicon solar cells with different i-layer thickness, grain size, crystalline volume fraction (Xc), and density of dangling bond states are presented. According to the simulation results, the efficiency of high and low Xc samples is 9.02 % and 7.55 % with small grain size (=20 nm) when i-layer thickness is 4 µm, respectively. However, the samples with larger grain size (>30 nm) exhibits better performance is due to the fact that it is a critical point for microcrystalline silicon transits to polycrystalline silicon with grain size equals to 30 nm, which. Nevertheless, the higher the density of states and Xc of intrinsic layer is, the less light-induced degradation produced by a-Si:H in solar cell. Some external quantum efficiency results are also presented and indicates that the light-induced degradation effect only occurred in the short wavelength range which corresponds to a-Si:H absorption spectrum and we can further conclude that a-Si:H fraction in µc-Si:H is responsible for this effect.

並列關鍵字

amorphous silicon ； microcrystalline silicon ； silvaco ATLAS ； solar cell

參考文獻

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