利用氧化銦錫奈米柱提升有機太陽能電池轉換效率

Abstract

在此研究當中，我們發展出一種氧化銦錫的奈米結構作為埋入式的電極應用在有機高分子太陽能電池上。這種氧化銦錫柱乃是利用電子槍斜向蒸鍍法，在體異質接面(bulk heterojunctions)中提供三維電荷傳導路徑，可對低載子遷移率的電洞作有效率的收集，進而達到光電流的增益，及元件壽命的延長。本篇論文分三個部分:(1)氧化銦錫奈米柱材料特性分析 (2)使用「旋塗法」將PEDOT:PSS塗佈於氧化銦錫奈米柱製作埋入式電極 (3)和利用「電化學沉積」將PEDOT鍍在奈米電極上以完成元件的製作。 第一部分中，我們根據本實驗室之前的成果證實，氧化銦錫奈米柱成長機制乃由Self-Catalyst VLS的方式所長成的，藉由內部的錫扮演著催化的角色，使材料本身達到利於生成奈米結構的條件。而由於有機太陽能電池屬於薄膜型元件，因此，我們所選用的奈米柱高度僅僅100nm~150nm左右，但根據TEM結果顯示，此奈米結構仍然有結晶特性。而根據四點探針的量測，成長在氧化銦錫薄膜上，並不會造成本身電性劣化，另外，我們從事Conductive AFM的分析，也證明了ITO奈米柱擁有相當程度的導電特性，且由於本身的柱狀結構所產生的局域性(localized)電場，可以增加元件載子的收集效率。所以氧化銦錫奈米結構擁有相當的潛力應用於有機太陽能電池上。 如何有效的將電洞傳輸材料覆蓋在奈米結構上形成奈米電極，是一個關鍵的問題，而本論文比較了旋轉塗佈及電化學沉積的方式，所製成的奈米電極之元件特性。第二部分內，我們選用旋轉塗布的方式，將PEDOT:PSS覆蓋在氧化銦錫奈米柱上，而擁有此結構的元件，在一倍強度(one-sun)及五倍強度(five-sun)的標準光源照射下，其轉換效率分別達到3.4%及4.4%；相對於傳統的薄膜平面電極的有機太陽能電池，其轉換效率提昇了10%及36%。此外，在五倍強度的光源照射下，元件壽命延長兩倍，達到約110分鐘。然而，旋轉塗佈方式易造成PEDOT:PSS厚度過厚，將奈米結構給填平，易造成元件無法達到預期的效果，因此我們選用PEDOT，利用電化學聚合的方式，達到均勻沉積在奈米結構上，形成良好的奈米電極。 而使用電化學沉積法將PEDOT均勻覆蓋在奈米柱上，並形成埋入奈米電極的元件，相對於傳統薄膜型電極而言，電流能夠達到10.1%的提升，而效率擁有13.1%的改善；根據SCLC模型可知，光電流的提升乃是由於電洞遷移率的提升，並達到電子電洞遷移率平衡所致。從IQE分析中我們了解，這奈米電極擁有全波段的提升 (400nm~650nm)。且由變角度材料吸收頻譜和變角度效率分析，我們可以證實，此奈米結構更可以解決由於太陽運行所造成元件電荷收集的劣化，並展現此氧化銦錫結構在現實元件應用的潛力。

In this paper, ITO nanorods are employed to serve as buried electrodes for organic solar cells. The embedded nano-electrodes allow three-dimensional conducting pathways for low-mobility holes, offering a highly scaffolded cell architecture in addition to bulk heterojunctions. According to different subjects, this thesis will be classified into three part: (1) The material analysis and characteristics of ITO nano-columns. (2) The properties of the device with embedded nano-electrodes, which are coated by PEDOT:PSS via spin-casting. (3) The results of analysis for the device with ITO nano-columns, uniformly covered by PEDOT via electrochemical deposition. Firstly, the growth mechanism of ITO nanorods was previously demonstrated to be self-catalyst vapor-liquid-solid (VLS). The tin component in the ITO nanorods plays a catalyst to help the growth of nano-columns. Since the organic solar cells are thin-film photovoltaics, the height of ITO nano-columns is around 100nm~150nm. Despite the short heights, TEM analysis still showed the crystalline inside the ITO rods. Besides, based on the four-point measurement, the ITO nano-columns don’t degrade the original electrical performance of ITO film. Moreover, CAFM proved that the ITO nano-columns even show the excellent capability of carrier collection. Hence, ITO nano-columns exhibit great potentials for the application to organic solar cells. How to effectively cover hole conducting layer onto the nanostructured electrodes is an essential issue. In this thesis, we compare two methods, spin-casting and electrochemical deposition, to fabricate the organic photovoltaics. In the second part, the power conversion efficiency of a organic cell employing spin-coating is increased to about 3.4% and 4.4% under one-sun and five-sun illumination conditions, respectively, representing an enhancement factor of up to 10% and 36% compared to a conventional counterpart. Also, the corresponding device lifetime is prolonged twice as much to about 110 min under five-sun illumination. However, spin-coating is easy to be planarize the nanostructure, eliminating the benefits of nano-electrodes. In the last, we present evidence of balanced electron and hole transport in polymer-fullerene based solar cells by means of embedded indium-tin-oxide nano-electrodes. Enabled by a controllable electrochemical deposition, the individual nano-electrodes are uniformly enclosed by a PEDOT hole-conducting layer, allowing a relatively short route for holes to reach the anode and hence increasing the effective hole mobility. Besides, devices with ITO nano-columns show broadband enhancement in IQE against conventional planar electrodes. Moreover, nano-electrodes (NEs) even exhibit the wide-angle carrier collection, proved by higher Jsc at large incident angles and slower degradation in angular response of IQE, compared to the conventional planar electrodes. Hence, free-standing ITO NEs can’t only overcome the spatial restraint from the location of photo-generated carriers but achieve omnidirectional carrier harvesting, exhibiting the potential for the practical solar cell application.