矽奈米結構/有機混成型太陽能電池與單晶矽薄片製造技術

Si Nanostructure/Organic Hybrid Solar Cells and Fabrication of Single-Crystal Si Thin Foils

許書嘉(Shu-Chia Shiu)

指導教授 ： 林清富

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.10287

摘要

在全球暖化的效應下，減少石化燃料使用並開發新能源成為全球重要議題，太陽能是來源最為豐沛且穩定的潔淨能源，成為極具潛力的再生能源，利用光伏作用能夠有效率地直接將太陽光轉換成電能。結晶矽太陽能電池由於具有高效率與極長的生命週期而廣泛被運用到太陽能發電系統中，更重要的是矽材料為地球蘊藏最豐富的元素之一，可以滿足大量太陽能電池的需求。然而，生產矽晶片與製造太陽能電池元件是高成本且需要消耗大量能量的製程，造成太陽能電力成本過高。
在本論文中，將從三方面研發新型低成本矽太陽能電池，包括抗反射結構製作與研究、矽奈米線/導電有機混成型太陽能電池元件與單晶矽薄片製造技術。首先，我們探究使用金屬輔助化學蝕刻法製作矽奈米線，使用硝酸銀與氫氟酸組成的蝕刻溶液，透過矽表面處理控制沈積在矽表面的銀形貌，進而改變矽奈米線的密度，我們發現高密度的矽奈米線可以達到很好的反射特性，長度僅0.87μm的矽奈米線結構可使矽晶片表面反射率降低至3.3%，要達到相同的反射率，一般的矽奈米線長度必須超過1.8μm。接著，我們將矽奈米線與導電有機材料結合形成軸-鞘結構的混成型太陽能電池，低成本的有機材料溶液製程具有很大的潛力能降低太陽能電池成本，導電有機材料為聚(3,4-乙烯基二氧噻吩):聚(苯乙烯磺酸鹽)(PEDOT:PSS)，經過140℃的低溫退火與n型矽形成異質接面，產生內建電場而能分離照光產生的電子電洞，這種混成型太陽能電池效率達到了9.45%，涵蓋太陽光波長350nm到1100nm。
為了更進一步降低矽太陽能電池成本，我們探究減少矽晶片製造成本的方法，發展出多步驟金屬輔助化學蝕刻技術，蝕刻溶液由過氧化氫與氫氟酸組成，透過控制金屬催化劑銀的氧化與還原，可控制蝕刻方向為非等向性或等向性，可形成矽奈米結構於矽基板上並蝕刻奈米結構的根部，將矽奈米線或矽奈米孔洞脫離矽基板而移植到異質基板上，使原本的矽基板能夠回收利用，大幅降低矽材料成本。除此之外，我們以微影技術圖形化矽表面而限制金屬輔助化學蝕刻的區域，製造出直徑5μm與週期6μm矽微米柱結構，微米柱長度與蝕刻時間成正比，接著以多步驟金屬輔助化學蝕刻能將矽微米柱移植到異質基板上，更重要的是利用此技術能夠生產單晶矽薄片，生產厚度15μm的矽薄片僅消耗相當於厚度6.1μm矽材料，矽材料的損耗遠低於以線鋸法製作矽晶片所產生的損耗量，此作法將使矽晶片的成本降低到原本的十分之一以下，矽薄片的光吸收效率在波長350nm到960nm超過85%，高於一般同厚度的矽晶片。這些技術將提供未來低成本矽太陽能電池的解決方案。

關鍵字

矽太陽能電池 ； 矽奈米線 ； 抗反射結構 ； 金屬輔助化學蝕刻 ； 導電有機材料 ； 異質接面 ； 晶圓切割

並列摘要

Because of global warming, reducing usage of fossil fuel and exploring new energy become important issues in the world. Solar energy has much potential as a renewable energy source because it is the most abundant and cleanest energy. The photovoltaic effect is applied to directly convert sunlight to electricity efficiently. Crystalline Si solar cells are widely used in solar electricity systems due to their high energy conversion efficiency and long cell lifetime. More importantly, Si is one of the most abundant elements in Earth’s crust, satisfying the need for producing a vast number of solar panels. However, producing Si wafers and manufacturing Si solar cells involve high cost and high power consumption processes, resulting into high cost of solar electricity.
In this dissertation, we demonstrate techniques to develop new kinds of low-cost Si solar cells and widen their applications, including fabricating antireflection structures, developing a novel photovoltaic device structure and producing ultrathin Si substrate with low material loss. Initially, we investigate metal-assisted chemical etching method to fabricate Si nanowires. The etching solution consists of AgNO3 and HF. Through manipulating the morphology of the deposited Ag, the density of Si nanowires can be controlled. The highly dense Si nanowires can be fabricated and they can achieve superior antireflection property. This kind of Si nanowires with a length of only 0.87μm can suppress the solar-weighted reflectance to as low as 3.3%. In comparison, the Si nanowires with low density have to be as long as 1.812 μm to achieve the same reflectance. Then we further combine Si nanowires with conducting polymer to form core-sheath hybrid solar cells. The solution process of conducting polymer has great potential to reduce the cost of cell fabrication. The conducting polymer used here is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The Si and PEDOT:PSS can form a compact heterojunction after thermal annealing process at only 140℃. It forms built-in potential in the Si for separation of photo-generated electron-hole pairs. The efficiency achieves 9.45%. The response spectrum covers wavelengths from 350nm to 1100nm.
To further reduce the cost of Si solar cells, we explore methods to reduce the manufacturing cost of Si wafers. A technique of multi-step metal-assisted chemical etching (MS-MacEtch) was developed. The etching solution consists of H2O2 and HF. Through controlling oxidation and reduction of Ag catalyst, the etching direction becomes anisotropic or isotropic. The process forms Si nanostructures on the Si substrate and then removes Si material at the roots of the Si nanostructures. Therefore, these Si nanowires or Si nanoholes can be transferred onto alien substrates from bulk wafer. The original bulk Si wafer is reusable. In addition, using photolithography technique to pattern Si surface can limit the area of metal-assisted chemical etching. We demonstrate fabrications of transferrable large-area Si micro-rod arrays. The diameter and period of the Si micro-rods are 5μm and 6μm, respectively. The length of Si micro-rods increases as the etching time increases. In addition, crystalline Si thin foils with the entire area of a single domain can be lifted off from (100) Si bulk wafer by this technique. The overall MS-MacEtch process takes no more than 20 min and consumes only equivalent-6.1 μm kerf loss to form a Si thin foil with a thickness of 15 μm. The thickness and kerf loss is less than one-tenth of Si wafers fabricated by current technology of wire-sawing. The optical absorption is over 85% from 350 nm to 960 nm, which is higher than conventional Si wafer with the same thickness. These techniques provide routes to achieve low-cost crystalline Si solar cells.

並列關鍵字

Si solar cells ； Si nanowires ； antireflection ； metal-assisted chemical etching ； conducting polymer ； heterojunction ； wafer slicing

參考文獻

[1] NASA, website of Global Climate Change. http://climate.nasa.gov/index.cfm.

[2] Antony, F.; Durschner, C.; Remmers, K. H. Photovoltaics for Professionals. Solarpraxis AG, 2007.

[3] American Society for Testing and Materials. http://www.astm.org/index.shtml.

[4] Timeline of solar cells. From Wikipedia website: http://en.wikipedia.org/wiki/Timeline_of_solar_cells.

[5] Historical timeline. From Bell labs website: http://www.alcatel-lucent.com/wps/portal/BellLabs.