表面粗糙非晶矽太陽能電池之三維數值分析及能量轉換效率之最佳化設計

The Optimization of Textured a-Si:H Solar Cells with Three Dimensional Simulation

李君垚(Chun-Yao Lee)

指導教授 ： 吳育任

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.01246

摘要

增加光吸收層的厚度可以有效地改善太陽能電池元件的能源轉換效率。然而氫化非晶矽太陽能電池的擴散長度較短，增加厚度會限制載子在空乏區的傳輸而降低效率。為了解決此困境，特殊紋路表面技術如無規則粗化表面的朗伯特限制、奈米光捕捉結構和光子晶體結構越來越受到重視。此論文分析及研究具有無規則粗化表面之氫化非晶矽太陽能電池定加以提升其能源轉換效率。為了建立無規則粗糙表面，本研究須採用三維數值模擬：三維有限時域差分法及三維波松、電流飄移-擴散方程式加以分析及討論氫化非晶矽太陽能電池之光性及電性表現，並且探究其最佳化結構以求光性及電性特性之平衡。除此之外，我們組合九個具有不同尺度之金字塔結構來建構其無規則粗糙表面，金字塔的尺度主要由實驗樣品量測轉換而得。此金字塔架構之太陽能電池在能源轉換效率上的誤差只有0.10%，足以證實此金字塔架構可以有效代表真實無規則表面。

表面粗糙的特性會影響光性及電性的表現，具有高寬高比的表面結構會增加進光量但同時也會增加元件缺陷密度進而降低載子傳輸，故此論文研究不同粗糙特性如平均粗糙度、方均根粗糙度及粗糙變化的程度等在表面製程上依據的重要參數，並分析其所造成在電性及光性上的的影響。為了達到高效率表現，此論文建議平均粗糙度及方均根粗糙度應為30.60及38.50納米，在此粗糙度下會有最高效率。此外粗糙表面的離均差較小會具有較多的進光量因此可提供較大的短路電流密度。另一方面，吸收層厚度為150納米的氫化非晶矽太陽能電池有較高的開路電壓及較好的填滿因數，故具有最高效率10.48%。

為了改善太陽能電池的電性表面，我們研究其能量損失的成因發現較薄的元件有較強的背部表面複合，需要採取電洞阻擋層來降低背部表面複合。此論文採用階梯式參雜加以形成反向電場加以阻擋在n型非晶矽表面的電洞，而最佳階段參雜的配置為1.0E19/5.0E20 每立方公分，元件在此配置下可減少53.69%的背部表面複合。

以三維數值模擬，我們可以更加準確地計算太陽能元件的光學及電學特性。在基於此論文提出的改善方法下，氫化非晶矽太陽能電池可達到更高之穩定效率表面。

關鍵字

非晶矽 ； 薄膜太陽能電池 ； 紋路化表面 ； 表面粗糙 ； 數值模擬

並列摘要

Increasing the absorber thickness is efficient to improve the energy conversion efficiency of photovoltaic devices. However, it will limit the carrier transport in the depletion region for a-Si:H thin film solar cells due to its short diffusion length. To solve this dilemma, textured surface technologies such as Lambertian limit of randomly textured surface, nano light trapping structures, and photonic structures have gotten more and more attention. This thesis studied the a-Si:H solar cell with randomly rough surface for the high-energy conversion efficiency. The 3D numerical modeling is needed to model the characteristics of randomly rough texture. Therefore, we used the 3D finite-difference time-domain method and 3D Poisson and drift-diffusion solver to discuss and analyze the optical and electrical performance of the a-Si:H solar cell and figure out the optimized configuration for the balance between optical and electrical properties. In addition, we assembled 9 pyramids with different geometry scales determined by measurement results to model the randomly textured surface. And this model with pyramids has been proven that it is efficient enough to represent the real device under small efficiency error of 0.10% in this study.

The characteristic of rough surface will affect the electrical and optical performance dramatically. The surface with high aspect ratio will increase the light absorption but limit the carrier transport due to the increasing of the defect density. So this thesis studied the influence of different roughness scales such as different average height of roughness, and the average deviation of roughness, which are most important parameters for surface manufacturing technologies. For the high-efficiency performance, the proposed value of average roughness and root mean square roughness are 30.60 nm and 38.50 nm which have maximum energy conversion efficiency. Moreover, the surface texture with smaller variation has lager short circuit current density due to the stronger light absorption. On the other hand, because of the better fill factor and open circuit voltage, the proposed average thickness of i-type a-Si:H layer is 150 nm, which has the maximum efficiency of 10.48%.

To improve the electrical performance of solar cells, we investigated the reason for current losses. For the thinner absorber, the hole blocking layer was implemented due to the stronger back surface recombination. The step doping method was implemented into the n-type a-Si:H layer in this thesis to form the back field to block hole carriers. The step doping concentration we proposed is 1.0E19/5.0E20 cm^(-3), which could reduce the back surface recombination by 53.69%.

With our fully 3D numerical modeling without many approximations, we can be more accurately calculating the real device performance. Based on these improvement on both optical and electrical properties, the higher stabilized efficiency of the a-Si:H thin film solar cell could be achieved.

並列關鍵字

Amorphous silicon ； thin film solar cell ； textured surface ； surface roughness ； numerical simulation.

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