新穎高效率太陽能電池之研究

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(1)國立臺灣師範大學光電科技研究所博士論文 Institute of Electro-Optical Science and Technology National Taiwan Normal University. 新穎高效率太陽能電池之研究 A Study of Novel Structures on the Enhanced Power Conversion Efficiency of Solar Cells. 研究生：姚詠祺指導教授：李亞儒博士. 中華民國. 一○四. 年二月.

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(3) 謝誌光陰似箭、歲月如梭，歷經四年半終於走到提筆寫下謝誌的時候。首先我要感謝我的指導教授─李亞儒老師，若沒有您的指導我想我的研究之路不會走的這麼順遂，您讓我看見一個題目從看 paper、討論、發想、實驗(或模擬)、再討論、再實驗一直到撰文乃至於公開發表，每個步驟其實都是需要相當縝密的規劃和富邏輯性的思考才能一一完成。再來我要感謝我的口試委員盧廷昌、張守進、林泰源、陳永芳、許進恭、胡淑芬以及吳謙讓老師在我口試期間均給予我相當保貴的意見，使我在爾後思考問題上能更加謹慎。接著我要感謝這四年半來共事過的學長、夥伴以及學弟妹們。我記得當時暑假剛進研究室自行架設霍爾量測系統時，就是第一屆家榮和之皓學長協助我焊接轉接頭以及扒開吸得牢牢的永久磁鐵，這對剛進研究所的我而言的無疑是最印象最深刻的事情。再來是第二屆的一清、MOMO 以及立維學長，除了平時就教你們的問題都能耐心的回答以外，在你們畢業之際我們共同研究的成果稍後亦成功地發表在國際期刊上，相當令人振奮。第三屆我的夥伴家豪，無論是畫光罩或是做實驗都非常有 sense，論資排輩你對於實驗室的貢獻也算是數一數二；另外一位夥伴柏緯，雖然你平時有點白爛，但是模擬該衝的時候你也是會咬緊牙關向前進，所以你們倆是我最佳的研究夥伴。接著第四屆是僑人、青雲和麗蓮，僑人是一位相當認真的學弟，即便他被分配到合作的實驗室，也都會與我們保持聯絡並討論在研究上遇到的瓶頸；另外青雲和麗蓮學妹在接下實驗室的模擬工作後，對於新的模擬軟體也花了相當多的時間來努力學習並加以應用，對於後來我再接手使用非常有幫助。再來是人數最多的第五屆，除了優秀的安帆、詠安與庭萱先後與我有共同的研究成果發表外，忠翰、志忠以及幸樺也都有很好的研究成果產出並且發表在各大期刊上，實屬佳績。接著是第六屆的易倫、韋辰、晨凱和旭展，雖然我們在研究上的合作時間不多，但是你們的努力我相信對於實驗室的現在和未來將會有許多的貢獻，特別是易倫，你的辛苦我都看在眼裡。另外在我畢業之際相遇的第七屆學弟妹顯榮、靖羽、張翔、詣凱、怡恩以及很有緣的吉助、高義及冠霖，我想對你們說，給機會磨練自己，我相信在研究所這兩年時間你們會收穫滿滿。.

(4) 另外，我也要謝謝我這四年半來在我身邊的好友和貴人。首先是品光，謝謝你對於我們實驗室的幫忙，沒有你我們學弟的一些量測可能沒辦法進行的那麼順利。再來是台大積學館 SEM 的紀小姐，每次要把我們的試片拍出能放在 paper 上的 SEM 影像都要下相當的功夫和耐心，真的是非常感謝。接著是東銳的財哥，即便打從買 sputter 我們就開始認識，但是只要機台一有問題您總是不辭辛勞親臨現場解決，相當給力。還有我在師大住宿期間的歷任室友鈞閔、Qureshout 及小笠原，雖然我回宿舍的時間都不早，但有時我們卻能碰上並聊上幾句，甚至有些畢業後依然保持聯繫，我真的非常珍惜這樣的緣分。當然還有更多的好友無法一一列舉，但是你們都已在我心中。最後我要感謝我的家人，首先是我的父母親，你們除了在壽險事業上表現亮眼，對於我更是不遺餘力的用心栽培，你們辛勤的付出是我努力的一大動力，真的非常感謝。再來是我的弟弟祥喆，你在民俗、陣頭和舞蹈之間找到非常棒的詮釋方式，再加上你的研究也是緊扣你的演出主題，讓我由心底相當佩服我這位可愛的小老弟。因為你們在各自崗位上的盡心盡力，讓我在研究所這段期間能全心全力地投入並且順利地完成我的博士學位。在此向所有關心我的師長、朋友以及家人獻上我最深的感謝與致意！. 姚詠祺謹誌於國立臺灣師範大學光電科技研究所 2015 年 01 月.

(5) 中文摘要近年來，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 3.7x1020 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. I.

(6) 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 3×1018 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％。關鍵字：太陽能電池、斜角沉積技術、奈米結構製程、極化、量子點 II.

(7) Abstract 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 III.

(8) 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 3.7x1020 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 IV.

(9) 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.. Keywords: Solar cell; Oblique-angle deposition; Nanostructure fabrication; Polarization; Quantum dot. V.

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(11) Contents 中文摘要 .......................................................................................................................... I Abstract ........................................................................................................................ III Contents ....................................................................................................................... VII List of Figures……………………………………………………………………….. IX Chapter 1 Introduction .................................................................................................. 1 1.1 Energy Demand and Photovoltaics Background ................................................ 1 1.2 Classification of Solar Cells ............................................................................... 3 1.2.1 Classification of Solar Cells Depending on Various PV Materials ......... 3 1.2.2 Development of Diversified Solar Cells Efficiencies.............................. 7 1.3 Research Motivation and Literature Review ...................................................... 9 1.3.1 Si-based Nanorods/Nanowires Solar Cells with Slanted ITO Films ....... 9 1.3.2 Polarization-induced Doping III-nitride n-i-p Solar Cells..................... 11 1.3.3 Current Matching Using CdSe Quantum Dots to Enhance the Power Conversion Efficiency of InGaP/GaAs/Ge Tandem Solar Cells .................... 12 References .............................................................................................................. 15 Chapter 2 Solar Cell Characteristics .......................................................................... 23 2.1 Air Mass and Solar Spectrum ........................................................................... 23 2.2 Optical Properties of Solar Cell-Materials ....................................................... 26 2.2.1 Absorption and Photon-Induced Current ............................................... 26 2.2.2 Absorption Coefficient .......................................................................... 27 2.2.3 Solar Cell Bandgap ................................................................................ 30 2.2.4 Reflectance and Transmittance .............................................................. 32 2.3 Solar Cell Equations ......................................................................................... 35 2.3.1 I-V Characteristic .................................................................................. 35 2.3.2 Open-Circuit Voltage and I-V Characteristic ......................................... 36 2.3.3 Fill Factor and Power Conversion Efficiency ....................................... 37 2.3.4 Quantum Efficiency............................................................................... 39 References .............................................................................................................. 42 Chapter 3 Si-based Nanorods/Nanowires Solar Cells with Slanted ITO Films ...... 43 3.1 Introduction ...................................................................................................... 43 3.1.1 Nanostructured Solar Cells .................................................................... 43 3.1.2 Oblique-Angle Deposition..................................................................... 45 3.1.3 Antireflection Coating & Thickness Determination .............................. 48 VII.

(12) 3.1.4 van der Pauw and Hall Measurements .................................................. 51 3.2 Use of Two-dimensional Nanorod Arrays with Slanted ITO Film to Enhance Optical Absorption for Photovoltaic Applications ................................................. 55 3.2.1 The Fabrication Process for Proposed ARC ......................................... 55 3.2.2 Results and Discussion ......................................................................... 57 3.2.3 Conclusions ........................................................................................... 67 3.3 Direct Electrical Contact of Slanted ITO Film on Axial p-n Junction Silicon Nanowire Solar Cells ............................................................................................. 68 3.3.1 The Method of Redox Reaction & Metal-Induced Chemical Etching.. 68 3.3.2 Results and Discussion ......................................................................... 69 3.3.3 Conclusions ........................................................................................... 76 References .............................................................................................................. 77 Chapter 4 Polarization-induced Doping III-nitride n-i-p Solar Cells ..................... 85 4.1 Introduction ...................................................................................................... 85 4.1.1 The Advantage of III-V nitride Semiconductor Compound Materials . 85 4.1.2 Spontaneous and Piezoelectric Polarization ......................................... 86 4.2 Physical Mechanism and Device Structure ...................................................... 89 4.2.1 Polarization-Induced Doping ................................................................ 89 4.2.2 Device Structure.................................................................................... 92 4.3 Results and Discussion .................................................................................... 95 4.4 Conclusions ...................................................................................................... 99 References ............................................................................................................ 100 Chapter 5 Current Matching Using CdSe Quantum Dots to Enhance the Power Conversion Efficiency of InGaP/GaAs/Ge Tandem Solar Cells ............................ 103 5.1 Introduction .................................................................................................... 103 5.1.1 Multi-junction Solar Cells in Tandem ................................................. 103 5.1.2 Limiting-current Subcell Issue ............................................................ 104 5.1.3 Quantum Dots adopted as Light Downonverters ................................ 106 5.2 Structure Model and Analysis Method .......................................................... 108 5.3 Results and Discussion .................................................................................. 112 5.4 Conclusions .................................................................................................... 120 References ............................................................................................................ 121 Chapter 6 Conclusions and Future Work ................................................................ 123 Appendix A ................................................................................................................. 125 Appendix B ................................................................................................................. 128. VIII.

(13) List of Figures Fig. 1. 1 Classification of various solar cell technologies. ............................................... 3 Fig. 1. 2 Pie charts of various type of solar cells market share. ....................................... 7 Fig. 1. 3 Conversion efficiencies of best research solar PV cells (NREL)....................... 8. Fig. 2. 1 (a) The path length in units of Air Mass (AM), changes with the zenith angle. (b) The terrestrial solar spectral at AM 1.5 for a 37o tilted surface. ............................. 23 Fig. 2. 2 (a) ASTMG173-03 Reference Solar Spectral Irradiances: AM0 and AM1.5. (b) Solar spectrum at AM1.5 for corresponding photon flux density. ......................... 25 Fig. 2. 3 Optical constant n and  for silicon. ................................................................ 27 Fig. 2. 4 Silicon optical absorption coefficient. .............................................................. 28 Fig. 2. 5 Interband transitions in materials: (a) direct bandgap, (b) indirect bandgap. .. 30 Fig. 2. 6 An indirect bandgap plot for Si. ....................................................................... 31 Fig. 2. 7 The optical diagram of EM wave traveling between two media...................... 33 Fig. 2. 8 A setup for the reflectance and transmittance measurements........................... 34 Fig. 2. 9 The side view schematic of a solar cell. ........................................................... 35 Fig. 2. 10 The equivalent circuit diagram for an ideal p-n junction solar cell. .............. 36 Fig. 2. 11 The typical solar cell J-V characteristics. ....................................................... 38 Fig. 2. 12 The quantum efficiency (QE) of a Si solar cell. ............................................. 40. Fig. 3. 1 (a) Efficiency of Si SC w/ and w/o nanopillar vs. minority carrier diffusion length. (b) The coupled incident light guided in the nanostructure. ....................... 43 Fig. 3. 2 (a) Photographs and SEM images of a-Si:H w/o and w/ nanostructures. (b) Schematics. (c) The effective refractive index profiles. Absorption (d) at normal incidence, and (e) over different angles of incidence. ............................................ 44 Fig. 3. 3 (a) Availability of materials with suitable refractive indices is very limited. (b) Shadowing effect. (c) Schematic of oblique-angle deposition.. ............................. 46 Fig. 3. 4 (a) SEM images for a SiO2 nanorod. (b) Refractive indices of SiO2 and ITO can be controlled by the deposition angle. .................................................................... 47 Fig. 3. 5 (a) Typical reflectance, transmittance and absorption for a bare Si. (b) Refecctance of Si wafers coated w/o and w/ ARCs. .............................................. 49 Fig. 3. 6 Geometry for the absorption from the (a) polished and (b) textured solar-absorber materials. ......................................................................................... 50 Fig. 3. 7 (a) van der Pauw measurement method. (b) Plot of f vs. RA/RB .. .................... 51 Fig. 3. 8 (a) Schematic of Hall effect. (b) Sample geometries for Hall effect measurements. (c) Hall voltage measurement configurations. ............................... 53 IX.

(14) Fig. 3. 9 (a) Schematic of the fabrication of 2D Si-nanorod array with slanted ITO film. (b) SEM image of nano-sized Ni. (c) SEM (left hand) and AFM (right hand) images of the slanted ITO film. .......................................................................................... 55 Fig. 3. 10 Measured complex refractive indices of (a) bare Si and 2D Si-nanorod array, and (b) normal and slanted deposited ITO films. .................................................. 57 Fig. 3. 11 Cross-sectional SEM images of (a) normally deposited ITO film, 2D Si-nanorod arrays (b) without, and (c) with the slanted ITO film. ........................ 59 Fig. 3. 12 (a) Measured reflectivity as a function of normal-incident wavelength for all samples. (b) Calculated reflectivity. (c) Calculated absorption. ............................ 62 Fig. 3. 13 (a) Calculated J-V curves for all samples. (b) A plot of Aavg calculations corresponding to each absorption throughout the day. .......................................... 65 Fig. 3. 14 (a) Primary fabrication scheme of vertically aligned axial p-n junction SiNW SC. (b) Cross-sectional SEM image of the proposed SiNW SC............................ 68 Fig. 3. 15 (a) XRD patterns of the slanted ITO film for different RTA temperature. (b) FWHM of (222) XRD diffraction peak and the measured resistivity of slanted ITO film vs. RTA temperature. ...................................................................................... 70 Fig. 3. 16 (a) Photographs of all samples. (b) (Top) Measured reflectivity as a function of normal-incident wavelength for all samples. (Bottom) Angular-dependent reflectivity of all samples. (c) Optical absorption for all samples. ........................ 71 Fig. 3. 17 (a) Semi-log plot of J-V behavior both in the dark and under AM 1.5G illumination. (b) The same J-V current plotted in a linear scale. ........................... 74. Fig. 4. 1 Energy bandgap vs. lattice constant of III-V nitride semiconductors. ............ 85 Fig. 4. 2 Surface charges and direction of electric and polarization field for spontaneous and piezoelectric polarization in III-V nitrides materials. (b) Magnitude and direction of spontaneous and piezoelectric polarization in GaInN and AlGaN grown pseudomorphically on relaxed GaN....................................................................... 87 Fig. 4. 3 Schematic illustration of polarization-induced doping III-nitride n-i-p solar cell. (a) Sheets of charge dipoles in every unit cell of the crystal. (b) Net bound charge density. (c) The corresponding energy-band diagram. ........................................... 90 Fig. 4. 4 (a) Conventional solar cell structure, and (b) its Ein and EP vs. doping concentrations. (c) The proposed structure, and (d) the corresponding polarization-induced doping density and electric field. ......................................... 93 Fig. 4. 5 Calculated strain profile of (a) impurity and (b) polarization-induced doping solar cells. .............................................................................................................. 95 Fig. 4. 6 Calculated (a) energy band diagrams, and (b) distributions of photogenerated carriers for both solar cells under 1-sun illumination. ........................................... 97 Fig. 4. 7 Temperature-dependent J-V curves of (a) impurity doping p-i-n and (b) X.

(15) polarization-induced doping n-i-p solar cells. ........................................................ 98. Fig. 5. 1 (a) Schematic of a multi-junction solar cell in tandem. (b) Different PV materials w/ different energy bandgap. (c) Top cells of tandem solar cells have a larger energy bandgap than anothers in the following cells. ...................................................... 103 Fig. 5. 2 limiting-current subcell causes current mismatching restricting the maximum power conversion efficiency of tandem solar cells. ............................................. 105 Fig. 5. 3 (a) The maximum power conversion efficiency in GaInP/GaAs/Ge tandem solar cell. (b) Quantum efficiencies of individual materials in tandem solar cell. ........ 105 Fig. 5. 4 (a) Enhanced conversion efficiency in QD solar cells by impact ionization. (b) Electronic energy levels and optical spectrum depend on the QD’s dimension. .. 106 Fig. 5. 5 (a) Mechanism of QDs as a downconverter. (b) The absorbance and photoluminescence spectra of InGaP/ZnS core/shell QDs in toluene. ................. 107 Fig. 5. 6 (a) Schematic plot of an InGaP/GaAs/Ge triple-junction solar cell with CdSe QDs. (b) Simplified structure for facilitating optical calculations. ...................... 109 Fig. 5. 7 (a) Absorption and (b) photoluminescence spectra of CdSe QDs of different sizes in toluene. .................................................................................................... 113 Fig. 5. 8 Calculated light intensity of the solar spectrum and quantum efficiency for the device (a) w/o and (b) w/ CdSe QDs with D=2.1 nm. ......................................... 114 Fig. 5. 9 Calculated J-V characteristics of each subcell for the device (a) w/o and (b) w/ CdSe QDs with D=2.1 nm. ................................................................................... 115 Fig. 5. 10 (a) Calculated JSC of each subcell and (b) the overall PCE of the device (bottom) as a function of the CdSe QD’s diameter.............................................................. 116 Fig. 5. 11 (a) Electrical performance of the device after dispensing 7 mg/mL of CdSe QDs with D=4.2 nm compared to the one w/o QDs under AM1.5G illumination. (b) J-V characteristics with different concentration of CdSe QDs. ............................ 117 Fig. 5. 12 Reflectance of the devices dispensing CdSe QDs with various concentrations. .............................................................................................................................. 119. XI.

(16) XII.

(17) Chapter 1 Introduction 1.1 Energy Demand and Photovoltaics Background Energy is indispensible in everyone’s life, and plays an essential role for the sustained development of all mankind. Throughout the history of civilization revolution, our demand for energy has increased unprecedentedly with the growth of the population, urbanization, and modernization [1.1]. Fossil fuels, including coals, natural gas, and oils, which supply the day-to-day use of electrical power, have been the primary energy source for a sustaining of human society. Yet the environmental stock of such fossil fuels is gradually diminished due to the over exploiting of natural resources all around the world. It was predicted that only a few hundred years supply of fossil fuels (or shorter) are available estimated by the current energy consumption rate. Meanwhile, It was also reported by the United Nations Intergovernmental Panel on Climate Change (IPCC) that the global warming caused by the consumption of fossil fuels in the next coming 50 years will have devastating effects on our living environments and economic developments [1.2]. One possibility weaning away from fossil fuels is to use the so-called renewable or alternative energy, which directly converts solar light, wind, and waterpower into the electricity. Specifically, by considering the fact the energy produced by the natural photosynthesis in the earth is more than eight times the need for sustaining the humanity, the energy provided by the solar light itself is sufficient for our daily use, even we assume the converting efficiency of solar energy is only 10% and the sun’s illumination area on the earth is around 1% [1.3]. Employment of solar technologies, including a sunny area of square 161 km per year, would produce the energy equivalent to that used annually in the entire United States [1.4]. Throughout a. 1.

(18) region where energy is demanded, the illuminated area could be centrally located, or distributed on the rooftops of buildings. The challenges are how to harvest this solar energy, and how to make use of it in a cost-effective manner. During the past several decades, scientists have developed solar technologies and learned how to use proper materials to convert the solar energy. In general, the solar converters are the so-called photovoltaic (PV) solar cells or solar cells which transfer the incident solar radiation into electricity. Typical solar cells use a solid-state p-n junction that includes one region to conduct positive charge carriers (i.e. holes) and another region to conduct negative charge carriers (i.e. electrons). Electron-hole pairs created via the absorption of light in a semiconductor are separated by the builded-in electrical field, and eventually diffuse into the two types layers of solar cells by different diffusion rates. External contacts allow electrical currents flowing through the solar cell to the load. Industry PV modules produce direct-current (dc) electricity and their actually output power depends on the intensity of sunlight, operating temperature, and other factors. Additional components like electrical switches, diode-protection circuits, inverters, and batteries, connect the PV output with electrical load. The resulting assembly of components is known as the PV system. The main constrain of popular capacity of solar cells is their installation cost much higher than that of traditional electricity sources, and recently the government subsides and incentives can help reduce such issue. In general, types of various solar cells have been classified into crystalline silicon (Si), amorphous Si, cadmium telluride (CdTe), gallium arsenide (GaAs), and copper indium gallium diselenide (CIGS), and etc. With sustained interests and investments, it is expected to continue in the development of solar cells towards high power conversion efficiency and low fabrication costs.. 2.

(19) 1.2 Classification of Solar Cells 1.2.1 Classification of Solar Cells Depending on Various PV Materials In general, the solar cells are mainly classified into few categories (Fig. 1.1), according to the quality, overall thickness and the fabricated approach of the devices [1.5]: . Wafer. based. crystalline. silicon. solar. cells,. which. include. mono. (single)-crystalline and poly (multi)-crystalline silicon. . Thin-film solar cells, which include amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) etc.. . Emerging PV technologies, which include concentrating photovoltaics, dye sensitized solar cells, polymer organic solar cells etc.. Fig. 1. 1 Classification of various solar cell technologies.. Wafer-based crystalline silicon solar cell technology So far, most technologies used for the solar cell fabrication rely heavily on the microelectronics industry, and that can be mainly classified into two categories as (i). 3.

(20) Mono (single)-crystalline, and (ii) Poly (multi-)-crystalline silicon solar cells. (i) Mono (single)-crystalline silicon solar cell This is the most established and efficient solar cell technologies till date, which have module efficiency of 15-18%. The cell and module fabrication technology is well developed and reliable. These cells are manufactured from single silicon crystal, by process called Czochralski process. During the manufacturing, c-Si crystals are cut from cylindrical ingots, they do not completely cover a square solar cell module. (ii) Poly (multi)-crystalline silicon solar cell The production of poly-crystalline silicon (poly-Si) cells is more cost-efficient which are manufactured by cooling a graphite mould filled with molten silicon. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. These cells have module efficiency of around 12-14%.. Thin film solar cell technology Thin film solar cells are also called second-generation photovoltaic cells. In this approach thin layers of semiconductor material are deposited onto a supporting substrate, such as a large sheet of glass. Typically, less than a micron thickness of semiconductor material is required. Some of the thin film solar cells in use are as follows: (i) Amorphous silicon (a -Si), (ii) Cadmium telluride (CdTe), and (iii) Copper indium gallium (di)selenide (CIS, CIGS). (i) Amorphous silicon thin film solar cell Amorphous Silicon (a-Si) modules are the first thin film solar module to be commercially produced and at present has the maximum market share out of all thin. 4.

(21) film solar cell technologies. A-Si solar can be fabricated at a lower deposition temperature hence permits the use of various low cost flexible substrates by easier processing technique. The major concern of a-Si solar cells is their low stabilized efficiency. The overall efficiency drops inevitably at module level and at present the efficiencies of commercial modules are in the range of 4-8%. (ii) Cadmium telluride (CdTe) thin film solar cell Being a crystalline compound Cadmium Telluride is a direct band gap semiconductor, which is a strong solar cell material. It is usually sandwiched with cadmium sulfide to form a pn junction PV solar cell. CdTe with laboratory efficiency as high as 16% have been developed at NREL. Multitudes of manufacturing techniques are main advantage of these solar cells which are suitable for large scale production. Limited availability of cadmium and pollution problem associated with cadmium is main concerns with this technology. (iii) Copper indium gallium diselenide (CIGS) solar cells This is a new semiconductor material comprising copper, indium, gallium and selenium in a specific order, which is used for solar cell manufacturing. It is one of the most promising thin film technologies due to their high-attained efficiency and low material costs. Amongst thin film solar cells, the advantage of CIGS solar cell is its extended operational lifetime without significant degradation. The inherent properties of CIGS also provide an opportunity for maximizing the efficiency.. Emerging PV technologies Emerging PV technologies are the techniques which represent the most outstanding ongoing developments, advances, and innovations in PV fields. The following lists some of current emerging PV technologies, which contains (i) concentrating 5.

(22) photovoltaics, (ii) dye sensitized solar cells, and (iii) organic solar cells. (i) Concentrating photovoltaics In concentrating photovoltaic system, lenses or mirrors are used to focus sunlight onto a much smaller area of photovoltaic panel. In general, the system uses a tracking device to follow the sun as it moves across the sky. The system’s ability to concentrate sunlight dramatically decreases the size of the receiver; hence it is a good candidate for more expensive, higher efficiency receiver materials including stacks with multiple layers of semiconductors that can absorb much more of the solar spectrum. (ii) Dye sensitized solar cells A dye-sensitized solar cell (DSSC) is based on a semiconductor formed between a photo-sensitized anode and an electrolyte. The DSSC has a number of attractive features including being easy for conventional roll-printing techniques, semi-flexible and semi-transparent. However, its liquid electrolyte is limited to the weather condition in the practical applications. Nevertheless, its price/performance ratio should be good enough to allow them to compete with fossil fuel electrical generation by achieving grid parity. (iii) Organic solar cells An organic solar cell is a type of polymer solar cell that uses organic electronics for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect. Despite the device used polyacetylene as the organic layer, its charge collection efficiency was only 0.3% with Al and graphite at the early stage; recently there is a breakthrough in development that a three-layer fullerene-free stack achieved a conversion efficiency of 8.4% with an open-circuit voltage around 1 V.. 6.

(23) Fig. 1. 2 Pie charts showing the various type of solar cells market share from 2008 to 2010.. As can be easily seen from the pie charts in Fig. 1.2, wafer-based crystalline silicon solar cells dominated the solar market by a large margin from 2008 to 2010. Even to 2013, wafer-based crystalline silicon solar cells were still up to 90% market share [1.6]. The thin film solar cells’ (including a-Si, ribbon Si, CdTe, CIGS etc.) share for all thin film technologies was only 14% in 2010. wafer-based crystalline silicon solar cells’ share has been rapidly increasing the last few years as Chinese manufacturers have come on strong; however, the thin film’s market share is forecast to decline further to 7% by 2017 according to Solarbuzz [1.6].. 1.2.2 Development of Diversified Solar Cells Efficiencies In the aspect of conversion efficiencies of best research solar cells worldwide, which are surveyed by National Renewable Energy Laboratory (NREL), from 1976 through 2014 for various photovoltaic technologies as shown in Fig. 1.3. Cell efficiency results are provided within different families of semiconductors: (1) multijunction cells, (2) single-junction gallium arsenide cells, (3) crystalline silicon cells, (4) thin-film 7.

(24) technologies, and (5) emerging photovoltaics. Some 26 different subcategories are indicated by distinctive colored symbols.. Fig. 1. 3 Conversion efficiencies of best research solar PV cells worldwide for various photovoltaic technologies since 1976 [1.7].. The most recent world record for each technology is highlighted along the right edge in a flag that contains the efficiency and the symbol of the technology. The company or group that fabricated the device for each most-recent record is bolded on the plot. In 2014, three companies broke the record of 25.6% for a silicon solar cell. Panasonic’s was the most efficient. The company moved the front contacts to the rear of the panel, eliminating shaded areas. In addition they applied thin silicon films to the (high quality silicon) wafer’s front and back to eliminate defects at or near the wafer surface [1.8]. In the part of thin-film solar cells, which made from these materials tend to be less efficient than bulk silicon, but are less expensive to produce. Their quantum efficiency is also lower due to reduced number of collected charge carriers per incident photon. The performance and potential of thin-film materials are high, reaching cell efficiencies of 12–20%; prototype module efficiencies of 7–13%; and production 8.

(25) modules in the range of 9% [1.9]. The thin film cell prototype with the best efficiency yields 21% (First Solar) in 2014 [1.10]. On the other hand, multi-junction (tandem) solar cells have achieved amazing efficiency levels. For example, in 2013, the highest efficiency of triple-junction solar cell certified by NREL is a created by SHARP, with 44.4% efficiency [1.11], [1.12]. In that same year, NREL reports 31.1% efficiency for III-V solar cell which marks a world record for a two-junction solar cell measured under one-sun illumination [1.13]. Recent years, the progress in perovskite solar cells should not be underestimated. Since 2009, organolead halide perovskites have been used for solar cells. As of June 10th 2014, the certified record power conversion efficiency of 17.9% was achieved by the Korean Research Institute of Chemical Technology (KRICT), which was certified by NREL [1.14].. 1.3 Research Motivation and Literature Review 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. The brief introduction of our research topics in this thesis is described as follows.. 1.3.1 Si-based Nanorods/Nanowires Solar Cells with Slanted ITO Films The main constraint of widespread utilization of Si solar cell stems from the high cost 9.

(26) of pure materials that supports sufficient long diffusion length of minority carrier and effective collection of photogenerated carrier [1.15], [1.16]. To overcome the cost issue, several approaches, such as the use of thin-film architecture and nanostructured geometry, have been widely investigated [1.17]–[1.23]. Among them, the Si nanowires (SiNW) has attracted much attention due to its natural profile associated with the core-shell geometry. However, as compared to its planar counterpart, the core-shell SiNW solar cells generally exhibit the smaller VOC value [1.24]–[1.27], and that hinders the ultimate performance of power conversion efficiency. Recently Wong et al. have reported that the photogenerated carriers are mainly concentrated in the nanowire itself, primarily attributable to the confinement effect of incident photons [1.28]. Therefore, even for SiNW solar cells embedded with axial p-n junction, the collection of charge-carrier is facilitated without compromising of VOC value, as the excess recombination of charged-carrier associated with trap density is significantly reduced. Yet for such axial p-n junction SiNW solar cells configuration, making an electrical contact related to depositing a continuous conductive film on the top of SiNWs is difficult, as its junction sidewall is completely exposed to the air and thus easily induces undesirable leakage electrical path of charged-carrier during cell fabrication [1.29]. In this work, we demonstrate a general strategy involving oblique-angle deposition scheme that directly integrates SiNW solar cells embedded with axial p-n junction into photovoltaic devices. Additionally, the slanted ITO film applied to the top of SiNWs can be considered as an intermediate refractive index layer. The Fresnel reflection that occurs at the interface between the SiNWs and the air caused by the large difference in their refractive indices is hence reduced. As a result, the proposed nanostructures show strong absorption over a broadband spectrum and a variety of incidence angles, as well as high short-circuit current density and power generation 10.

(27) efficiency obtained by the numerical calculation.. 1.3.2 Polarization-induced Doping III-nitride n-i-p Solar Cells Due to the advantage of the wide range of bandgap tunability by elemental compositions (from 0.64 eV for InN to 3.4 for GaN) [1.30] and of the high optical absorption (~105 cm-1) [1.31] by the direct bandgap property, the III-nitride (InxGa1-xN) material system, used to be the fundamental of light-emitting diodes, now is revolutionizing the photovoltaic industry. The inherent characteristics possessed by III-nitride materials such as high drift velocity, high carrier mobility, and high thermal conductivity, all of them contributes to the requirement of highly efficient solar devices under concentrated sunlight. Therefore, the III-nitride material system has been predicted as a potential and promising candidate for full-solar spectrum photovoltaic applications with extremely high resistance of photon irradiation for harsh environments [1.32]. In general, III-nitride solar cells were mainly achieved by the double hetero-junction structures similar to the fundamental structures of III-nitride light-emitting diodes, i.e. the p-GaN/i-InGaN/n-GaN layers (from top to bottom) grown by the metal organic chemical vapor deposition (MOCVD), where an intrinsic InGaN layer or the multiple quantum-well (MQW) was sandwiched between p- and n-type GaN for the absorption of solar light [1.30]-[1.36]. For such a stacked p-i-n configuration, the epitaxial strain caused by the lattice-mismatch between the InGaN and GaN hetero-junctions is the most challenge issue, which limits the power conversion efficiency of the device. In addition to the material defects acting as trapping centers of photogenerated carriers, the epitaxial strain significantly hinders the efficient collections of photogenerated carriers because it induces piezoelectric. 11.

(28) polarization dipoles within the InGaN layer having electric field opposite to the direction of built-in electric field generated by typical p-n junction [1.37], [1.38].Theoretically, rearranging the order of stacked layers as n-GaN/i-InGaN/p-GaN structure can mitigate the adverse impact of piezoelectric effect. Yet, for the standard MOCVD equipped with impurity doping system, the growth of such a III-nitride n-i-p solar cell is practically infeasible due to the outgassing-prone of dopant acceptor (Mg) in p-GaN and its strong memory effect [1.39]. Additionally, the main applications of III-nitride solar cells are possible for systems of the outer space, where the lowest temperature is down to a few degrees of Kelvin. In this work, we numerically evaluate the effect of polarization engineering in III-nitrides by grading the composition of InGaN materials to form a new type of n-i-p solar cells. In the absence of any impurity doping, our proposed device’s built-in electric field across the absorption layer is equivalent of its polarization-induced electric field, beneficial for the electric drifting and efficient collection of photogenerated carriers. Furthermore, as the conductive nand p-type regions are formed by electrostatic field ionization but not by the thermal activation [1.40]-[1.42], the concentration of filed-induced carriers is irrelevant to temperature variations. Such polarization-induced doping III-nitride n-i-p solar cells can hence provide stable power conversion efficiency, even are operated under the low temperature environment.. 1.3.3 Current Matching Using CdSe Quantum Dots to Enhance the Power Conversion Efficiency of InGaP/GaAs/Ge Tandem Solar Cells The past few years have witnessed an explosive growth in research that addresses different aspects of the use of semiconductor materials in varied configurations for photovoltaic applications. Among them, III-V compound tandem solar cells, which take. 12.

(29) advantage of the bandgap tunability by elemental multi-junction compositions and of the high optical absorption by direct bandgap materials, have attracted increasing attention for their extremely high conversion efficiency [1.43]-[1.46]. Ideally, a calculated power conversion efficiency as high as η=50.1% (under AM1.5G, 1000 sun) is achievable for a series-connected InGaP/GaAs/Ge triple-junction solar cell, which is far beyond the theoretical limit of a single-junction solar cell estimated by the Shockley-Queisser’s calculation scheme [1.47], [1.48]. In practice, an appropriate alignment of the bandgap energy of multi-stacking layers that provides current matching between each subcell is the most challenging issue in this tandem architecture, which restricts the maximum power conversion efficiency of the device and the potential applications in the photovoltaic industry. More specifically, the GaAs middle subcell generally limits the overall photocurrent of a InGaP/GaAs/Ge tandem solar cell. To overcome the issue of current mismatching, several approaches, such as the use of a quaternary AlGaInP top subcell and the substitution of the middle subcell with an InGaAs material, have been widely investigated [1.49]. However, an introduction of Al content into the InGaP top subcell causes a significant photocurrent droop due to the associated oxygen contamination on minority-carrier properties [1.50]. In addition, the substitution of a fraction of the gallium atoms with indium in the middle subcell accompanies a lattice mismatch and requires a complicated growth scheme such as the graded buffer layers to avoid a large dislocation density that also reduces the photocurrent of the device [1.51]. Hence, for InGaP/GaAs/Ge tandem solar cells, an approach that does not adversely affect the device’s performance and that is capable of resolving the current-mismatching issue is necessary. Recently, semiconductor nanoparticles, known as quantum dots (QDs), have been intensively studied and utilized to generate multiple carrier excitations from one incident photon by the 13.

(30) so-called impact ionization [1.52]-[1.54]. Such a nonlinear phenomenon can cause a solar cell’s quantum efficiency to be greater than 100%, primarily due to the discrete carrier density of states and the strong quantum confinement effect [1.55]. Additionally, as the electronic energy levels and the optical spectrum strongly depend on the QD’s dimension, its effective bandgap energy can be tunable. For the same reason, the semiconductor QDs are also adopted as downconverter materials to help harvest the ultraviolet regime of solar energy in silicon solar cells [1.56]. In this study, we recognize the photon conversion aspect of nanocrystal QDs and explore a novel strategy using CdSe QDs to tailor the incident spectrum of solar light to enhance the photocurrent of a limited subcell in InGaP/GaAs/Ge tandem solar cells and to enhance the overall power conversion efficiency of the cell. We demonstrate the ability of CdSe QDs to enhance the performance of the device, not only by theoretical calculations based on the fundamental of material optics but also by directly measuring the device’s electrical characteristic. The device exhibits an enhancement of 10.39% in the power conversion efficiency compared to the device’s counterpart without integrating QDs. The theoretical and experimental results validate that the CdSe QDs have promising potential for efficient solar spectrum utilization in InGaP/GaAs/Ge tandem solar cells.. 14.

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(39) Chapter 2 Solar Cell Characteristics 2.1 Air Mass and Solar Spectrum Here we introduce the source of the energy─the sun. For the purpose of solar cell studies, two parameters are most important: the irradiance and the spectra characteristics of the light. The irradiance value outside the Earth’s atmosphere is called the solar constant (1365 W/m2). After being filtered through the Earth’s atmosphere, several portions of the solar spectrum diminish, and peak solar irradiance is lowered to approximately 1000 W/m2. If one were be track the sun for eight hours, the average daily solar irradiance would be approximately 333 W/ m2 [2.1].. Fig. 2. 1 (a) The elevation of the sun above the horizon, or, conversely, the angle of the sun from the vertical (or zenith) determines what is called air mass (AM); (b) The geometry that defines the standard for the terrestrial solar spectral irradiance tables at AM 1.5 for a 37-deg tilted surface [2.1].. The solar spectrum and irradiance is established by the air mass. Air mass (AM) refers to the amount of air beam of sunlight must go through before reaching the solar converter. It is determined by the angle,  that the sun makes with a vertical (zenith) line perpendicular to the horizontal plane, as shown in Fig. 2.1(a); and further, Fig. 2.1(b) shows the geometry that defines the standard for the terrestrial solar spectral. 23.

(40) irradiance tables at AM 1.5 for a 37-deg tilted surface. It is given by AM  number  . 1 cos . (2. 1). The solar spectrum outside the atmosphere, i.e. AM0, is similar to the Plank’s law of black-body radiation with an effective temperature of approximately 5743K, which corresponds to an irradiative intensity (extraterrestrial irradiance, Etr.) of 1365 W/m2 at the center of mean distance between the sun and the earth. The spectrum of black-body radiation at T= 5743K is plotted in Fig. 2.2(a). AM1.0 refers to the thickness of atmosphere sunlight passes through if the beam is directly overhead. For purposes of standard solar cell measurements, an average solar spectrum at AM1.5 is used (= 48.19o). It should be noted that the total irradiance used for AM1.5 is often normalized to 1000 W/m2 (= 100 mW/cm2) in recent work. An attempt to replicate the AM1.5 spectrum is made in standardized solar simulators. The red line in Fig. 2.2(a) shows the solar spectrum at AM1.5, which is from solar disk plus sky diffuse and diffuse reflected from ground on south facing surface tilted 37 deg from horizontal (as shown in Fig. 2.1(b)). The integral over the wavelength yields the total irradiance, 1000 W/m2 (area under the red curve in Fig. 2.2(a)). The many notches in the spectrum are attributed to the absorption bands of various atmospheric gases such as H2O, CO2, O3, and O2. Absorption by ozone is essentially complete below a wavelength of 300 nm. The relatively large attenuation below 800 nm is due to scattering of molecules and particulates. Theses scattering processes become weaker at longer wavelengths, as has been shown by both theory and observation. When analyzing the performance of solar cell systems, the cell output is usually assumed to be proportional to the solar radiation intensity with little regard to the variations in the spectral distributions. The solar spectrum discussed above can be used to determine the number of photons that can 24.

(41) Fig. 2. 2 (a) ASTMG173-03 Reference Solar Spectral Irradiances: (Black line) AM0: Extraterrestrial Radiation (solar spectrum which obtained form the Plank blackbody equation) at mean Earth-Sun distance; (Red line) AM1.5: Spectral radiation from solar disk plus sky diffuse and diffuse reflected from ground on south facing surface tilted 37 deg from horizontal [2.2]. (b) The solar spectrum at AM1.5 for corresponding photon flux density.. produce electrons in the solar cell. The wavelength sale on the solar spectrum can be converted to photon energy from the relationship Photon Energy = h =. hc. . . 1240 [eV]  (nm). (2. 2). Thus, a photon at a wavelength of 550 nm has an energy of approximately 2.2 eV. Knowing the energy per photon at each wavelength, the irradiance of solar spectrum shown in Fig. 2.2(a) can be converted to a number of photons per second per unit area, i.e., photon flux density, P; such plot is shown in Fig. 2.2(b). To convert the incoming solar irradiance, Iin, the photon flux density uses the relationship W I in [ 2 ] photons m P[ ]= sec  m 2 h [eV]. (2. 3). A plot of photon flux density as shown in Fig. 2.2(b) is useful in establishing the limits on the photocurrent from a solar cell.. 25.

(42) 2.2 Optical Properties of Solar Cell-Materials 2.2.1 Absorption and Photon-Induced Current One of the most fundamental questions is how many photons of the solar spectrum can be absorbed by a solar cell. There are several useful optical parameters to be considered when characterizing a solar cell or solar cell material. Some of fundamental constants of material, others are lumped parameters that only characterize the particular device or solar geometry in question. Here we list the common fundamental constants which include the complex index of refraction, the extinction coefficient, the absorption coefficient, and the absorption. When determining a solar cell’s light absorption, it’s the optical parameter called the absorption that is most useful when assessing potential absorber materials for solar cells, or when optimizing a given absorber material for a solar cell. The quantum absorption is the fraction of incoming light at a given photon energy, h, that is absorbed by the material to produce an excited state such as an electron-hole pair. It’s measured and calculated, as a function of the photon energy, yielding A(); it also can be expressed as a function of the photon wavelength, A(). The absorption, A(), can be multiplied by the incoming photon flux, P() (=P × (unit area) =Pin()/ h ()), to determine how many electron-hole pairs can be produced. Multiplying this result by the elemental charge, q, and integrating over the internal quantum efficiency, IQE(), then yields the upper limit for how much current (i.e., photon-induced current, Iph, or short-circuit current, ISC) can be extracted from a device made with the solar-absorber material. The above-mentioned results can be expressed as following relationship f. I ph  I SC =q  IQE ( )  A( )  P ( )d , i. 26. (2. 4).

(43) J SC =. q f   IQE( )  A( )  I AM 1.5 ( )d , hc i. (2. 5). where IAM1.5(), which equals to Pin()/(unit area), is the solar spectral irradiance at AM1.5, hence the JSC means the short-circuit current density. The symbols of h and c represent Plank’s constant and speed of light in vacuum, respectively. The absorption, A(), can be measured directly, or it can be calculated using basic optical properties that are constant for a material.. 2.2.2 Absorption Coefficient. Fig. 2. 3 Optical constant n and  for silicon as real and imaginary parts of the complex refractive index.. The optical properties of a material depend on the complex index of refraction, n , given by the relationship. n = n - i ,. (2. 6). where the imaginary part of n is the extinction coefficient, . The real part of n is the refractive index, n. Figure 2.3 shows the n and  values for silicon. Typically, n, and 27.

(44)  are determined by using ellipsometry. This is an optical-based technique for the in-situ nondestructive characterization of interfaces using the change in a light probe’s state of polarization. It relied on the fact that linearly polarized light is elliptically polarized when reflected from a dielectric material, such as a semiconductor, used in solar cell.. Fig. 2. 4 Silicon optical absorption coefficient, .. The extinction coefficient is related to the absorption coefficient by the relationship.  ( )=. 4 ( ). . ,. (2. 7). Figure 2.4 shows the absorption coefficient of silicon as a function of wavelength, (). The absorption coefficient, n and  are all a function of the wavelength of the light. For example, for silicon at a wavelength of approximately 800 nm, = 103 cm-1, and so= 0.006. It should point out that the absorption coefficient for a material, although a function of wavelength, is not affected by the thickness of it. However, the optical. 28.

(45) absorption is a strong function of the thickness and the geometry of the solar cell. The absorption coefficient describes the decrease in light intensity as a beam of light propagates through a material (e.g., solar cell). The change in photon flux density P as a function of position is given by d P    dz , P. (2. 8). where z is the position in the absorbing material along the beam. Here, neglecting the reflection, and only the light that has already entered the light-absorbing material is considered. By integrating the above equation, (2.8), one obtains the number of unabsorbed photons (the remaining photons). This is given by  P  0exp(    d ) ,. (2. 9). where d is the thickness of the material, and 0 is the number of photons that initially entered the material. The transmittance, T, is therefore equal to P/0. This equation is one of the form of the Beer’s Law. Many are familiar with this equation as it applies to the absorption of solution. If an absorbing medium has thickness d, the front and back of its surfaces have equal reflectance R, then the transmittance T is related to absorption coefficient by the relationship T  (1  R)2 exp(  d ) ,. (2. 10). The absorption of an optical medium can also be quantified in terms of the optical density (O.D.). This is sometimes called the absorbance, as is defined as O.D.   log10 (.  P (d )  d )  0.434   d , 0 ln(10). It is apparent that the O.D. is directly related to the absorption coefficient . 29. (2. 11).