有機無機混掺太陽能電池之光學性質與表面形態研究

Abstract

本研究探討共軛高分子P3HT及有機無機混掺材料P3HT:TiO2中，光學性質與薄膜形態(morphology)之關係。我們利用單一溶劑與混合溶劑來控制薄膜的表面形態。高分子在利用混合溶劑所製備的薄膜中展現較高的結晶性，其吸收光譜的振動特徵峰(vibronic features)亦較顯著。從光激發螢光光譜(photoluminescence, PL)與時間解析光激發螢光光譜(Time-resolved PL)中，我們也觀察到高結晶性的高分子具有較窄的螢光頻譜寬度與較短的螢光生命週期(fluorescence lifetime)。根據一系列的實驗結果，我們提出一種可能的機制: 分子鏈間電荷分離(interchain charge separation)，來解釋這個有趣的新發現。而在有機無機混掺材料中，我們研究不同薄膜形態對於P3HT與TiO2之間的電荷轉移(charge transfer)的影響。我們發現使用混合溶劑不只提昇了混掺薄膜中高分子相的結晶性，同時也促進TiO2奈米晶體在薄膜中的分散。這樣的薄膜形態具有較多的P3HT/TiO2界面，因而導致較高的螢光淬熄效率(PL quenching efficiency)與較大的電荷轉移速率常數。此外，高結晶性的高分子與良好的有機無機相分離形態更有利於電荷的傳導。這些分析與探討成功地解釋由混合溶劑製備的太陽能電池所展現的較高效能。

In this study, the optical properties and film morphology of pristine P3HT and P3HT:TiO2 nanorods hybrid have been investigated. The film morphology was controlled by using single solvent and mixed solvent. P3HT films cast from mixed solvent showed higher crystallinity, clear vibronic features in absorption spectra and narrower emission bandwidth in photoluminescence (PL) spectra. The PL dynamics of pristine P3HT also changed with morphology. We propose a mechanism, interchain charge separation, to explain the faster PL decay in the high crystalline pristine P3HT film according to several optical measurements. In P3HT:TiO2 hybrid, the effect of nanoscale morphology on charge transfer process will be discussed. We found that the use of mixed solvent would lead to well-distributed TiO2 in high crystalline P3HT domains. Higher PL quenching efficiency as a consequence of more P3HT/TiO2 interfaces for charge transfer were observed as well as higher charge transfer rate constant. Such bi-continuous phase separation morphology improves not only photocurrent generation efficiency but also charge transport properties, giving rise to significant improvement of photovoltaic devices performance.