可重複利用基板的單晶矽晶圓薄化技術與矽奈米結構/有機混成型太陽能電池的效率提升

Abstract

能源短缺以及由石化燃料產生排放的溫室效應氣體已成為近十年全球重要的議題。因此，太陽能電池成為極佳的石化燃料替代能源。然而以目前主流的晶矽太陽能電池來看，它的成本依舊很高，且大部分來自矽晶圓與電池製造的成本。一些可能的替代方案如薄膜太陽電池由於相較起晶矽太陽能電池效率仍然很低，因此現階段仍無法取代晶矽電池的地位。所以，我們的目標便是降低晶矽的成本以及製作較便宜的高效率太陽能電池，以期能超越現今的市電同價成本。 在本論文中，我們著重兩部份的改進。第一，我們嘗試用薄化與重複利用矽晶圓來降低材料成本。我們並以溶液為本的金屬輔助化學蝕刻法 (MacEtch) 來制備矽奈米結構與薄膜。透過這個晶圓薄化技術，原則上一片單一的矽晶圓可以被使用來製作超過70片以上的薄膜。有了矽奈米結構的輔助，矽薄膜在很薄的矽材料中呈現出非常突出的光捕捉效應。實驗結果顯示我們的薄膜在經X光繞射(XRD)量測後有單晶特性，並且6μm得厚度就可以擁有超過98%的吸收率(此吸收率是經由積分球量測出來的穿透率和反射率計算而得)。 再者，我們的薄膜在經過矽晶圓重複使用後仍能達到高效率的光吸收。透過蝕刻加熱法或增加雙氧水/氫氟酸的體積比例來拋光晶圓，矽晶圓可以再重複使用。我們的結果顯示雙氧水/氫氟酸的體積比在1:1時，薄膜的材料利用率可達93%以便減少材料損耗。 第二，我們可以藉由使用有機材料來降低晶矽太陽能電池的製程成本。矽與有機混合的太陽能電池具有低溫製程的優勢，且使用薄又透明的有機材料可以使矽材料有更多的吸光。在實驗過程中，我們透過不同催化劑銀的分佈來改變矽奈米結構的填充率，可以有效降低奈米結構的反射率。實驗結果顯示52.2%高填充率的矽奈米緞帶有7.7%的低平均反射率及8.7%的功率轉換效率(η)。這裡和矽奈米結構搭配的是聚(3,4-乙烯二氧噻吩)-聚苯乙烯磺酸(PEDOT:PSS)/ITO結構。當奈米結構的填充率增加到60%以上時會形成奈米洞的結構。從結果來看，用深度淺的矽奈米洞可以達到高短路電流(Jsc)以及高功率轉換效率是因為奈米洞極佳的光捕捉。最佳的矽奈米洞/PEDOT:PSS元件表現為37.8 mA/cm2的短路電流與10.8%的效率。在開路電壓(Voc)與等效少數載子生命周期(τ)的研究上顯示在矽表面上的載子複合與元件表現有很緊密的關係，並經由分析找出奈米洞光捕捉與表面載子複合的平衡。在未來可以透過我們可重複利用晶圓的薄化技術與矽奈米洞/有機材料搭配的結構來達成具有前瞻性的高效率低成本太陽能電池！

Energy shortage and green-house gas emission from fossil fuels has been the top global issue over the last few decades. Due to this concern, solar cell becomes a very good alternative for power generation to replace fossil fuels. However, the cost of current mainstream crystalline silicon solar cell is still high, consisting of large part of Si wafer and cell processing costs. Some possible alternatives like thin film solar cells still suffer from low efficiency compared to crystalline silicon, and thus cannot replace much of crystalline Si solar cell for now. Therefore, our aim is to lower the crystalline Si cost and fabricate cheaper high-efficiency solar cell so that we may surpass the current grid parity level. In this thesis, we focus on two parts. First, we try to lower the material cost by thinning Si and reusing Si substrate. The solution based metal-assisted chemical etching (MacEtch) is used to produce Si nanostructure and thin film. In this wafer thinning method, one single Si wafer can be exploited to more than 70 thin films in principle. Together with the assistance of Si nanostructure, the Si thin film provides excellent light-trapping effect with thin Si layer. The result shows that our thin film has single crystalline quality from X-ray diffraction measurement, and holds high absorption over 98% with thin 6μm thickness, which is calculated from thin-film transmittance and reflectance by integrating sphere. Moreover, the high absorption of thin film is achieved even when Si wafer is reused. The wafer reusing method can be completed by either heating treatment during etching or the higher H2O2/HF volume ratio for wafer polishing. With 1:1 H2O2/HF volume ratio, we obtain high-efficiency thin-film material utilization of 93% to reduce material consumption. Second, the processing cost of crystalline Si solar cell can be reduced by applying organic materials to form Si/organic hybrid solar cells. Si/organic hybrid solar cells have the advantage with low-temperature process, and the light absorption of Si can be improved with highly-transparent organic material. In the experiments, we change the filling fraction of Si nanostructure through different Ag catalyst distribution to lower the reflectance. The results show that with high filling fraction of 52.2%, where Si is in the shape of nano-ribbon, has the low average reflectance of 7.7% and achieve power conversion efficiency (η) of 8.7% with Si/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (also PEDOT:PSS) /ITO structure. Moreover, when the filling fraction is much increased to more than 60%, nanohole structure is formed. From our results, large short-circuit current (Jsc) and high power conversion efficiency can be achieved with shallow Si nanohole structure because of the excellent light trapping of nanohole. The best performance of Si/ PEDOT: PSS /ITO device exhibits Jsc of 37.8 mA/cm2 and η of 10.8%. The investigation of open circuit voltage (Voc) and effective minority carrier lifetime (τ) shows the Si surface carrier recombination has very close relationship with nanohole depth and corresponding device performance, and shallow nanohole depth has lower carrier recombination rate and higher τ. As a result, our high-efficiency solar cell is achieved with the appropriate nanohole depth, where light trapping effect and surface recombination should be balanced. In the future, high-efficiency and low-cost solar cell is promising with wafer thinning technology on reusable substrates and Si nanohole/organic structure.