Photon Management in Si-Based Solar Cells

本篇論文中，首先，利用無遮罩濕式蝕刻方式在矽基板上蝕刻出具調控結構形貌的奈米線陣列，此奈米線結構在波長200奈米-850奈米擁有極低的鏡向反射(<0.1%)。此種顯著的抑制反射歸因於其有優越的抗反射性質，如全方向性，非極化敏性。此奈米線結構也可以有效的抑制散射反射。此種優越的抗反射表現是因為在空氣和奈米線之介面以及奈米線和基板間粗糙介面的幫助，可提供較低的表面奈米線密度以及提供梯度的漸變折射率。矽奈米線的拉曼訊號可以比拋光過後之矽基板的訊號提高超過400倍，再次確認此種奈米線可以增加光的吸收和萃取效率。此第一部份之研究提供了光與奈米結構的交互作用之情形，並可貢獻在不同的結構優化和光電元件上。 其二，晶圓般大的奈米線陣列擁有階層結構，此階層結構包含了奈米線和介面的微米起伏結構，在一次蝕刻製程步驟下完成。以上述結構為基底擁有設計結構之太陽能電池展現出優良的光捕獲特性，如寬頻譜工作範圍，和廣角性在外部量子效應和反射譜中可觀察出。比拋光的矽和傳統奈米線結構，此種含有階層狀之結構存在較好的光電特性，其短路電流密度為32.7 mA/cm2，其轉換效率為11.25 %。此提升之光電特性表現可由理論之有限差分時域方式證實。此一光捕捉現象利用此奈米和微米表面粗化結合之單晶矽太陽能電池在此部分被呈現。 第三，有抗反射性的心殼狀矽奈米線外包覆著二氧化矽，在濕試蝕刻下和退火之過程完成。此心殼狀有好的光捕捉特性如寬頻譜工作範圍，廣角性，和非極化敏性，可歸因於從空氣至基板有平緩的漸變折射率。藉由調變心殼之間的體積比，我們可以得到光侷限的區域不論是在奈米線下的矽基板或是心殼狀中心的矽奈米線，此結構之設計可以助益於平面型或半徑型之p-n介面的太陽能電池結構。此一光管理之現象可被用在奈米的光伏元件中。 最後，粗糙之氧化鋅摻鋁薄膜可被用來提升光的內部散射進而提高光在串聯之矽(非晶矽/微晶矽)薄膜太陽能電池之吸收。透過模擬的優化工作，找出在1.5微米厚度之微晶矽上下電池的吻合電流密度，此轉換效率可比擬3.5微米厚度之微晶矽擁有未粗化之氧化鋅摻鋁薄膜。此模擬之結果可由實驗元件之證實。在較薄的主動層之中可以有更大的提升光散射並由有限差分時域區域方式證實。此一概念和技術在此研究將會使下世代薄膜太陽能電池獲得很大之助益。In this thesis, first, the nanowire array (NWA) layers with controlled structure profiles fabricated by maskless galvanic wet etching on Si substrates are found to exhibit extremely low specular reflectance (< 0.1 %) in the wavelengths of 200-850 nm. The significantly suppressed reflection is accompanied with other favorable antireflection (AR) properties, including omnidirectionality and polarization-insensitivity. The NWA layers are also effective in suppressing the undesired diffuse reflection. These excellent AR performances benefit from the rough interfaces between air/NWA layers and NWA layers/substrate and the decreased nanowire densities, providing the gradient of effective refractive indices. The Raman intensities of Si NWAs were enhanced by up to 400 times as compared with the signal of the polished Si, confirming that the NWA layers enhance both insertion and extraction efficiencies of light. This study provides an insight into the interaction between light and nanostrucutres, and should contribute to the structural optimization of various optoelectronic devices. Second, wafer-scale nanowire arrays (NWAs) with hierarchical structure, combined the nanowire and interface micro-roughness were fabricated by single process of costless wet etching. The NWA based solar cells with designed hierarchical structure demonstrate excellent light-harvesting characteristics, such as broadband working ranges and omnidirectionality in external quantum efficiency and reflectance measurement. Compared to the polished Si and conventional NWAs, the solar cell with hierarchical structure exhibits significantly superior photovoltaic characteristics, i.e., short-circuit current of 32.7 mA/cm2 and conversion efficiency of 11.25 %. The enhanced photovoltaic performances agree with the theoretical analysis based on a finite-difference time-domain method. A viable scheme for light harvesting using the hierarchical structure employing micro-roughness/nanoscale surface textures on single crystalline Si solar cells has been demonstrated. Third, antireflective Si/oxide core-shell nanowire arrays (NWAs) were fabricated by galvanic etching and subsequent annealing process. The excellent light-harvesting characteristics of the core-shell NWAs, such as broadband working ranges, omnidirectionality, and polarization-insensitivity, ascribed to the smooth index transition from air to the substrates, have been demonstrated. By tuning core-shell volume ratios, we obtained enhanced light trapping regions implemented in either the planar Si underneath NWAs or the core regions of NWAs, greatly benefiting the geometry design of planar and radial p-n junction cell structures, respectively. This photon management scheme indicates the potential use in nanostructured photovoltaic applications. Finally, rough AZO films were employed to enhance the internal scattering and consequent optical absorption of thin film (amorphous/polycrystalline Si) tandem solar cells. Through the optimization work by simulations, the matched current densities from the top and the bottom cells were obtained with the device structure containing 1.5-μm roughened polycrystalline Si layer, which produces the efficiencies comparable to those of the 3.5-μm layer without roughening. The simulation results were supported by the device performances measured experimentally. The significantly enhanced light scattering in the thin rough active region was revealed by the calculation results based on finite-difference time-domain method. The concept and technique presented in this study should benefit the development of next generation of thin film solar cells