奈米結構之製程與特性以及其於太陽能電池之應用

Abstract

本論文為進行對奈米結構製程與應用的研究，著重於低成本，簡易、大面積化的奈米結構製程技術，包括自組裝奈米結構、奈米球微影以及奈米壓印微影技術。一方面也投入研究自行架設奈米壓印機台，希望能夠建立一個實驗平台，用於做為研究奈米結構應用的實驗技術。藉由結合大面積表面奈米結構與不同類型太陽能電池，探討其結構特性對於元件表現之影響與貢獻。分為以下幾部分： (1) 仿生抗反射奈米結構應用於單晶矽太陽能電池：以次波長蛾眼奈米結構於高分子薄膜表面，並直接壓印於單晶矽太陽能電池試片做為抗反射層。製程上利用奈米球微影技術、軟性材料翻模、反式壓印的方法，證實了利用這樣低成本、快速的製程方式，可直接將奈米結構壓印至表面具有微米結構粗糙化的太陽能電池薄片，且製程過程中不會對試片造成損害，實驗結果有效降低反射率而提升太陽能轉換效率從12.85%至14.2%。 (2) 以奈米壓印形成週期性結構並應用於高分子太陽能電池：為了達到奈米壓印結構之高完整與均勻性，我們架設一個簡易的氣壓式奈米壓印機，並且以紫外光固化的方式進行結構定型，搭配軟模具的使用，可完成壓印大面積的均勻結構，且壓印結構深寬比可達到3:1。接著將奈米壓印製程技術應用於高分子太陽能電池，完成一具有表面週期性結構之透明導電玻璃基板，藉由其繞射效應，使入射光擴散至不同穿透角度，而高穿透角度的散射光可以增加光於吸光材料中的傳遞路徑，增加其光吸收，高分子太陽能電池效率最大增益可從平面的2.75%提升至3.92%。 (3) 奈米結構應用於有機/無機異質接面太陽能電池：利用氯化銫自主裝奈米結構以及紫外光固化型奈米壓印，搭配蝕刻在單晶矽基板表面形成隨機性與週期性奈米結構，再旋塗上導電高分子及金屬電極沉積，形成有機/無機複合材料的新式異質接面太陽能電池。藉由表面奈米結構降低光反射率，增加太陽光吸收而提升光電流，另一方面，由於高分子與單晶矽之間徑向接面的結構，有助於載子的產生與分離。因此，表面奈米結構不僅具備優越的抗反射性效果，亦提升載子傳輸效率，太陽能電池之短路電流最高可提升至32.5mA/cm2，轉換效率可達10.86%。

This thesis focuses on nanostructure fabrication and their applications. The fabrication processes, which were low cost, simple and scalable, include self-assembled nanostructure, nanosphere lithography, and nanoimprint lithography. The content of this thesis is divided into three parts: (1) We demonstrate the implementation of biomimetic nanostructured antireflection coatings with polymethyl methacrylate layer on the micro-textured surface of silicon crystalline solar cells. To reduce cost, the process combines colloidal lithography, cast molding method, and reversal nanoimprint lithography. The technique is simple, low cost, and does not cause damage to the thin and brittle conventional crystalline solar cells. The antireflection properties of this biomimetic nanostructure coating are considered as effective as those of a conventional single-layer SiNx thin film. The resultant structures alone could reduce the reflectance of solar cell and enhance power conversion efficiency (PCE) from 12.85% to 14.2%. (2) In this study, a conductive and transparent substrate with periodic surface structures was demonstrated, which can provid a high light diffusion and high diffraction angles. The nanostructures are fabricated using UV-curing nanoimprint lithography on photoresist followed by coating of the ITO layer and organic materials with morphology that is uniform and conformal. The periodic surface structures are shown to increase the light absorption in poly(3-hexylthiophene) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT :PCBM) solar devices with nanostructures embedded into the ITO layer. The Jsc and PCE of the nanostructured organic solar cells are improved from 7.07 mA/cm2 to 10.76 mA/cm2 and from 2.75% to 3.92%, respectively. The improvement in device performance is attributed to the increase of the effective optical path of incident light due to light trapping and scattering by the nanoimprinted nanostructures. (3) Hybrid solar cells base on nanostructured silicon and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) with excellent PCE was demonstrated by using a simple fabrication process. Self-assembled nanostructure and nanoimprint lithography provide nano- patterns fabricated methods and modify the Si surface to the tapered profile structure by etching processes. The silicon surface nanostructures provide antireflective effect and radial junction architecture that have enhanced light absorption and carrier collection efficiency. The short-circuit current density (Jsc) of the hybrid solar cell with nano-pyramid structures was greatly improved from 24.5 mA/cm2 to 32.5 mA/cm2 in compared to a flat surface device. The highest solar cell efficiency was achieved on a 525 μm thick 2.3 Ω-cm n-type CZ Si substrate with the designated area of 4 cm2.