新穎高效率太陽能電池之研究

Abstract

近年來，由於經濟的快速發展與人類的頻繁活動，人們對自然資源的需求與日俱增，其結果造成了各種天然資源日益短缺。為了解決人類永續使用能源的問題，科學界以及工業界正如火如荼地發展各種替代性能源。在這些替代性能源中，太陽光長期以來一直被視作永恆的能量來源，因此與太陽能相關的技術得以迅速地蓬勃發展，而其中以太陽能電池更是被廣泛地研究和討論。在本論文中，我們主要是根據不同電池材料的組成提出許多新穎的結構來提升太陽能電池的轉換效率。首先，我們已經成功地證明結合二維矽奈米柱(線)陣列和斜向銦錫氧化物薄膜的新穎抗反射膜對於入射光有大角度、寬頻譜的強吸收率，因此能有效提升該太陽能電池的轉換效率。第二，我們利用數值模擬方法分析出一不靠外部(雜質)摻雜、僅利用漸變氮化銦鎵的銦含量來製作單一接面滿足全光譜響應的高銦含量三族氮化物n-i-p太陽能電池。最後，我們提出利用硒化鎘量子點調製太陽光譜來提升磷化銦鎵/砷化鎵/鍺串聯式太陽能電池的轉換效率。本論文依照各個章節不同的研究主題和使用方法將摘要進行分類，其分類如下：

1. 利用矽奈米柱(線)陣列搭配斜向銦錫氧化物膜增加太陽能電池光學吸收之應用 矽奈米柱(線)在太陽能電池方面的應用已被廣泛認為是相當具有吸引力的。在本研究中，我們分別利用感應電耦合式乾蝕刻技術和氧化還原-金屬誘導化學蝕刻方式製作出二維矽奈米柱(線)陣列。為了進一步降低發生在空氣和矽奈米柱(線)界面處的菲涅耳反射，我們提出利用斜角濺鍍沉積技術將奈米尺度等級的斜向銦錫氧化物薄膜作為空氣和矽奈米柱(線)間的中間層。由於矽奈米柱(線)能提供遮蔽效應，入射的銦錫氧化物氣流將被優先地沉積在矽奈米柱(線)的頂部，最終我們製作出的斜向銦錫氧化物薄膜可達到幾乎是無損且連續的表面。斜向銦錫氧化物薄膜除了本身擁有低折射率、高透明度外，在快速熱退火450℃的處理下，其薄膜的電阻率約為1.07x10^-3 Ω-cm，其摻雜濃度和載子遷移率分別為3.7x10^20 cm-3和15.8 cm2/V-s，亦可直接拿來當作電池的接觸電極。根據理論計算，該結構的轉換效率相對於單晶矽裸片的太陽能電池約有42％ 的提升，證明上述的奈米結構組合對於入射光有大角度、寬頻譜的強吸收率。然而在實際元件製作上，元件上層與銦錫氧化物接面因極性不匹配以及奈米線的高深寬比導致高的串聯電阻和低的並聯電阻，其結果伴隨著高的逆向飽和電流加劇光生載子在表面復合，進而影響了整體元件的轉換效率。

2. 感應極化摻雜三族氮化物太陽能電池之研究 我們利用理論計算方式來評估並設計出新型感應極化摻雜氮化銦鎵n-i-p太陽能電池。該方法並不使用傳統雜質摻雜，反而是藉由線性增加（0％增至30％）和降低（30％降至0％）氮化銦鎵裡每個單位電池的銦含量所導致的感應極化摻雜來製作太陽能電池的p型和n型區，其中p型和n型區的載子濃度均達到3×10^18 cm-3。在氮化銦鎵n-i-p太陽能電池裡，由於每個單位電池具有大小相同且均勻的極化電荷，將其依銦含量漸變堆疊可預期該元件的電位分佈有平滑的空間變化，這樣一來減緩能帶在異質界面處的不連續性，並有利於光生載子能高效率地流動和收集。最重要的是導電n型和p型區是透過靜電場的離子化而不是熱活化所形成的，該感應極化電場的載子濃度與熱凍結效應無關。因此，感應極化摻雜的三族氮化物n-i-p太陽能電池即使在低溫環境下操作亦可以提供穩定的轉換效率。

3. 使用硒化鎘量子點改善磷化銦鎵/砷化鎵/鍺串聯式太陽能電池之電流匹配與提升其轉換效率之研究 三五族串聯式太陽能電池是最有效提供極高轉換效率的電池結構。然而該元件裡每個子電池之間的電流不匹配問題是引起該電池轉換效率實驗值偏離理論值一顯著挑戰。在本研究中，我們使用硒化鎘量子點來提升被限制的子電池光電流以匹配其他子電池的電流輸出並予以提升整體磷化銦鎵/砷化鎵/鍺串聯型太陽能電池的轉換效率。該限制的光電流被提升的主要原因來自於量子點做為光子轉換器的基本機制。不同尺寸的量子點有調製太陽光譜的獨特能力，因此該太陽能電池提升的效率與選擇量子點的尺寸大小有絕對的關係。本研究結果顯示透過適當地選擇量子點，我們發現佈上直徑4.2 nm、濃度7 mg/ml的硒化鎘量子點在磷化銦鎵/砷化鎵/鍺串聯型太陽能電池上，其轉換效率與沒有佈上任何量子點的電池相比能有效提升10.39％。Recently, both scientific and industrial communities are dedicated to exploring and searching alternative ways of renewable energy due to the inevitable shortage of natural resource. Among them, the solar light was longtime considered as a permanent energy, and that leads to a prompt and intensive development associated with the solar energy technology. In this thesis, we apply several novel structures mainly on the solar cells composed of different based materials, and validate its feasibility in term of the enhanced power conversion efficiency of the devices. First, we propose a brand new structure of antireflection coating (ARC) which combines a low-reflectivity 2-dimensional (2D) Si-based nanorod array, and the slanted indium-tin-oxide (ITO) film simultaneously with excellent electrical (conductive) and lossless optical (transparent) features. Second, as the demand of one single device exhibiting a full-solar-spectrum response is increased, we numerically evaluate the III-nitride solar cells with high indium contents by the grading of indium compositions scheme. Finally, we demonstrate a general strategy by simply casting cadmium selenide (CdSe) quantum dots (QDs) upon InGaP/GaAs/Ge tandem solar cells to tailor the incident solar spectrum, and to achieve current matching between every sub-cells. The highlight of our scientific achievement is briefly described as follows.

1. Use of Si-based nanorods/nanowires solar cells with slanted ITO films to enhance optical absorption for photovoltaic applications The Si-nanorods/nanowires offer a promising architecture that has been widely recognized as attractive devices for photovoltaic applications. We adopt a slanted ITO film as an intermediate layer by using oblique-angle sputtering deposition to further reduce the Fresnel reflection of the device. Besides, the slanted ITO film exhibits the resistivity of 1.07x10^-3 Ω-cm underwent RTA treatment of T=450°C, and the doping concentration and the carrier mobility by Hall measurement amount to 3.7x10^20 cm-3 and 15.8 cm2/V-s, respectively. It is acceptable to perform as a transparent conductive film for photovoltaic applications. Theoretically, the proposed structures exhibit high optical absorption over a broad range of wavelengths and incident angles and an improvement of power conversion efficiency () approximately 42% over that of its bare Si counterpart. Yet the real device of proposed schme shows a low value of  =0.26%, which is mainly attributed to the mis-aligning doped polarity at p-Si/n-ITO interface and the high aspect ratio of Si-nanowires, resulting in large series resistance and small shunt resistance, and excerbating the surface recombination process accompanied with high reverse current characteristics.

2. Polarization-induced doping III-nitride n-i-p solar Cells We numerically evaluate a new type of III-nitride n-i-p solar cells by the so-called polarization-induced doping, which is induced by the graded InxGa1-xN layers of linearly increasing (from x=0% to 30%) and decreasing (from x=30% to 0%) the indium composition to construct the conductive p- and n-type regions, respectively. As the conductive n- and p-type regions are formed by electrostatic field ionization but not by the thermal activation, the concentration of field-induced carriers is independent of thermal freezeout effects, and the device can provide stable power conversion efficiency even operated at low temperatures.

3. Current matching using CdSe QDs to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells We explore a promising strategy using CdSe QDs to enhance the photocurrent of the limited subcell to match with those of the other subcells and to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells. The underlying mechanism is mainly attributed to the photon conversion of the QDs that tailors the incident spectrum of solar light; the enhanced efficiency of the device is therefore strongly dependent on the QD’s dimensions. By appropriately selecting and spreading CdSe QDs upon the InGaP/GaAs/Ge solar cell, the power conversion efficiency shows an enhancement of 10.39% compared to the conventional devices.