以斜向入射電子束蒸鍍法製作抗反射層之特性研究

Abstract

在太陽能電池上，抗反射層是減少入射太陽光反射和增加太陽能電池效率的重要元件。但對於傳統太陽能電池上的單層抗反射層，低反射波段太窄；而且此種抗反射層，無法有效減少斜向入射光的反射。漸進式抗反射層為一種改良式的抗反射層，此種抗反射層擁有寬波段的低反射率，並且可以有效減少斜向入射光線的反射率。漸進式抗反射層中薄膜的折射率非定值，而是漸進的變化，因此， 此種抗反射層不易製作。在本論文中，我們在矽基板上，使用一種叫做斜向入射電子束蒸鍍法的方法，製作出漸進式折射率抗反射層。斜向入射電子束蒸鍍法可蒸鍍出奈米孔洞薄膜，並且可藉由改變入射電子束和薄膜垂直方向間的角度，或是稱做蒸鍍角角度，來控制鍍出薄膜的折射率。 我們在電子槍蒸鍍的腔體內設計特殊的載台，藉由斜向入射蒸鍍，此載台可同時蒸鍍出不同蒸鍍角的奈米孔洞薄膜。因為氧化銦錫具有高穿透性和傳導性，我們使用氧化銦錫進行斜向入射蒸鍍，並在蒸鍍完成後，使用不同的熱退火條件來增加多孔洞氧化銦錫薄膜的穿透。我們量測出不同斜向蒸鍍角和不同熱退火條件的氧化銦錫薄膜的穿透率和折射率，並且對於氧化銦錫薄膜的片電阻也進行了量測。 藉由以上對氧化銦錫薄膜特性的研究，我們設計出了在矽基板上的多層漸進式抗反射層，此多層漸進式抗反射層中，有三層是利用斜向入射電子束蒸鍍法蒸鍍出的氧化銦錫氧化銦錫薄膜，另外一層是用非晶系的矽來進行蒸鍍。此抗反射層反射率<5%的區域大約是從520 nm到960 nm，最低折射率的地方在800 nm，大約將矽基板的反射率由33%降到1.5%。對於斜向入射的光源而言，反射率在光入射角為60o之前，反射率都小於10%，此抗反射層有接近500 nm的低反射區域，且對於斜向入射光源也有很好的抗反射效果。在本論文的最後，我們也對多層奈米孔洞薄膜的表面特性和氧化對於此抗反射層造成的影響，進行了研究和討論。

Anti-reflection (AR) coating on crystalline silicon solar cell can reduce the reflection of incident sun light and increase the efficiency of solar cell. However, a typical single layer AR on crystalline silicon solar cell coating has narrow low reflection band and sensitive to incident angle of light. A gradient-index AR coating is a modified AR coating with broad band low reflection and insensitive to incident angle of light. The refractive index of gradient-index AR coating is gradually varying as the thickness of coating. Hence, it is difficult to fabricate gradient-index AR coating. In this thesis, a fabrication technique called electron-beam (e-beam) oblique-angle deposition is used for fabricating gradient-index AR coating on silicon. E-beam oblique-angle deposition is used e-beam evaporation system only. And this technique can produce nanoporous thin film with a specific refractive index by varying the angle between incident vapor flux of e-beam system and normal of substrate or so called deposition angle. In our work, a specific holder is designed for oblique-angle deposition. It can produce nanoporous thin films with different deposition angle at the same time. Indium tin oxide (ITO) is chosen as deposition source for oblique-angle deposition for the transparency and conductivity. And different thermal annealing parameters are applied after oblique-angle deposition for increasing transparency of ITO thin film. Optical properties like transmittance and refractive index of nanoporous ITO thin films with different deposition angle and different thermal annealing parameters are measured and discussed. The sheet resistance of ITO nanoporous thin films with different deposition angle and thermal annealing parameters is also measured in this thesis. A multi-layer gradient-index AR coating on silicon is designed. This AR coating has 3 layers of ITO nanoporous thin films by oblique-angle deposition and 1 layer of amorphous silicon. Reflection < 5% for our multi-layer gradient-index AR coating is from about 520 nm to 960 nm. The lowest reflection of our AR coating is at 800 nm and reflection reduces from 33% for bare silicon to 1.5%. For oblique incident light, the reflection is still smaller than 10% at 60o of oblique incident light. Degradation for oxidation in amorphous silicon layer in our AR coating is also discussed. Finally, different between morphology of single layer and multi-layer of nanoporous thin film by oblique-angle deposition is observed and discussed.