有機無機混摻薄膜之界面工程與其太陽能電池應用

Abstract

由於共軛高分子/無機奈米粒子混摻薄膜在其光伏應用中所展現的優異熱穩定性，此類有機/無機異質接面元件近年來更吸引許多關注。然而，較低的光電轉換效率是此系統在將來的發展以及商業化前的主要課題。因此，合適的製程方式以提高元件轉換效率是近來研究上所迫切需要的。本論文的研究動機在於展示不同的界面工程方法於高分子與無機奈米粒子間之作用，包括對於混摻薄膜表面形態的控制以及針對混摻薄膜內所使用的無機二氧化鈦奈米桿所採用的不同修飾方法。各種混摻薄膜性質的改變以及其在於光伏元件的應用將做為審視各種界面工程方法的依據。 為了能夠在有機無機混摻薄膜中獲得更佳的表面形態控制，於是具有自組裝行為的硬桿‒柔曲嵌段共聚高分子(P3HT‒b‒P2VP)分別做為供體材料以及添加物加入有機無機混摻薄膜。做為供體材料與二氧化鈦(TiO2)奈米粒子的應用中，其長程規則有序結構中可容納的二氧化鈦奈米粒子上限共聚高分子中的P2VP的重量百分比而有所不同，分別為10 wt% TiO2於57 wt% P2VP嵌段共聚高分子的層狀結構、20 wt% TiO2於75 wt% P2VP嵌段共聚高分子的柱體結構以及40 wt% TiO2於86 wt% P2VP嵌段共聚高分子的圓球結構。 此外，由於P3HT‒b‒P2VP各別嵌段對於共軛高分子P3HT (P3HT嵌段)以及二氧化鈦奈米桿(P2VP嵌段) 的良好混溶性，不同比例的P3HT‒b‒P2VP分別做為加工添加物被導入共軛高分子聚3-己基噻吩(P3HT)/二氧化鈦奈米桿混摻薄膜中。在導入1.50 wt% 的P3HT‒P2VP三元系統混摻薄膜中，縮小的共軛高分子P3HT結晶區域尺寸(從88.21 Å 縮小至 85.47 Å)以及較低的光激螢光強度皆表示P3HT‒b‒P2VP作為加工添加物皆能有效改善共軛高分子P3HT以及二氧化鈦奈米桿在混摻薄膜內部的自我團聚現象。 另一方面，各種針對無機奈米粒子的結晶度˴電性以及表面性質的改善方式也被展示於此論文中。經過熟化以及異質摻雜處理，二氧化鈦奈米桿的電子遷移率可分別從原本的6.21×10-5 cm2·V-1·s-1強化至5.27×10-4 cm2·V-1·s-1(熟化)以及2.33×10-4 cm2·V-1·s-1(異質硼摻雜)。另外，利用表面接附共軛修飾分子於無機奈米粒子上以覆蓋其表面晶格缺陷也被視為另一種可行的方法。因此，我們利用二次表面改質處理法，第一階段將三種不同的pyridine衍生物( pyridine, 2, 6‒Lutidine (Lut) and 4‒tert‒butylpyridine (tBP))分別在第一階段表面處理時做為表面改質溶劑以便於在二氧化鈦奈米桿的表面創造不同界面組織，並且幫助了解第二階段表面處理時共軛修飾分子(W4)於不同二氧化鈦奈米桿界面的接附行為。經過量化分析後，tBP被發現能夠有效去除原本存在二氧化鈦奈米桿表面的油酸分子，並且由於在第二階段表面處理時，二氧化鈦奈米桿所接附的共軛修飾分子(W4)數目與裸露的二氧化鈦表面空位有關。因此，經過tBP表面處理的二氧化鈦奈米桿其W4接附量可以達到0.62 mol%，此數值遠高於pyridine (0.38 mol%)以及Lut (0.19 mol%) 處理後的二氧化鈦奈米桿。另一方面，二氧化鈦奈米桿上的載子傳輸速率亦受到其接附的共軛修飾分子數量所影響，因此在二氧化鈦奈米桿溶液製備薄膜以及與P3HT混摻薄膜中，其電子遷移率的改善效果被發現與接附的共軛修飾分子(W4)成正相關。 最後，我們將採用各種界面工程方法所製備的聚三‒己基噻吩/二氧化鈦奈米桿有機無機混摻薄膜應用於光伏元件並且觀察其個別光電轉化效率。根據分別的參照組元件(聚三‒己基噻吩與pyridine處理後的二氧化鈦奈米桿混摻薄膜，其轉換效率約0.40%)，使用柔曲嵌段共聚物P3HT‒b‒P2VP作為加工添加劑以及各種應用於修飾二氧化鈦上的修飾方法，例如熟化、硼摻雜和tBP‒(W4)表面修飾處理，其光電轉化效率上的增幅分別可以達到186%, 31%, 79% 以及 240%。 本論文中所包含的實驗結果闡述了界面工程方法在高分子/無機奈米粒子混摻薄膜的光伏元件應用有著非常重要的效果。本論文所揭露的研究方向被期望能夠在其後續的研究、相關科技的創新以及更進一步的發展和商業應用扮演著承先啟後的角色。

Since the good thermal stability of conjugated polymer/inorganic nanocrystal hybrids in the photovoltaic application, such organic/inorganic bulk‒heterojunction has attracted much attention in recent years. However, the limited power conversion efficiency is the main issue for further development and commercialization. As a result, adequate approaches to improve the photovoltaic performance are urgently required. The objective of this dissertation is to demonstrate different methodologies of interfacial engineering as well as morphology control between polymer and inorganic nanocrystals. The effects of each approach were evaluated by the properties of treated polymer/nanocrystal hybrid thin film and their corresponding photovoltaic performance. To have better morphology control in polymer/nanocrystal hybrid thin film, the rod‒coil block copolymers P3HT‒b‒P2VP with different molecular designs were used as donor materials. The loading limit of TiO2 nanoparticles is found highly correlated to the weight percentage of P2VP segment in the self‒assembled block copolymer (10 wt%, 20 wt% and 40 wt% accommodation limit of TiO¬2 nanoparticles for lamella (57 wt% P2VP), cylindrical (75 wt% P2VP) and spherical (86 wt% P2VP) ordered structures, respectively). In addition, since the good miscibility of block copolymer P3HT‒b‒P2VP to both conjugated homopolymer P3HT (P3HT segment) and TiO2 nanorods (P2VP segment), different small amounts of P3HT‒b‒P2VP was incorporated into the P3HT/TiO2 hybrid thin film as the processing additive for better morphology control. After adding 1.50 wt% P3HT‒b‒P2VP, the reduced P3HT crystalline domain size (from 88.21 Å to 85.47 Å) and low extinction intensity of photoluminescence in P3HT/TiO2/P3HT‒b‒P2VP ternary blend thin films indicate the aggregation of P3HT donor is reduced and the miscibility of TiO2 acceptor in P3HT is improved. On the other hand, techniques aim to improve the crystallinity, electrical properties as well as the surface characteristics of nanocrystals were also adopted in this dissertation. After different treatments such as post‒ripening process and heterogeneous doping of TiO2 nanorods, the electron mobility of corresponding treated nanorods are both significantly enhanced (5.27×10-4 and 2.33×10-4cm2·V-1·s-1 for boron‒doped and ripened nanorod, respectively) compared to the as‒synthesized nanorod (6.21×10-5 cm2·V-1·s-1). Moreover, capping conjugated modifier onto nanocrystal seems to be another feasible approach to cover the pristine surface defects. The existence of surface defect may trap charge carrier during the charge separation and transport. Accordingly, we adopted two-stage ligand exchange process on the surface of TiO2 nanorods: three different pyridine derivatives such as pyridine, 2, 6‒Lutidine (Lut) and 4‒tert‒butylpyridine (tBP) were applied as dispersion solvent in first stage surface modification to create distinct surface characteristics of TiO¬2 nanorod and elucidate the anchoring behavior of conjugated modifier on these surfaces in second stage modification. Additionally, the quantitative studies for category and anchoring amount of ligand on as‒synthesized and three different modified TiO2 nanorods were obtained by elemental analysis, in which the tBP and Lut were proven as the most effective solvent for oleic acid (OA) removal and ligand-anchoring during the ligand exchange process, respectively. After the second stage surface modification using conjugated modifier of 2‒cyano‒3‒(5‒(7‒(thiophen‒2‒yl)‒benzothiadiazol‒4‒yl)thiophen‒2‒yl)acrylic acid (W4), the capping amount is correlated to the number of unbounded sites on the surface of TiO2 nanorods. As a result, the TiO2‒tBP with the lowest amount of anchored ligands shows the highest attaching amount of W4, the bonded W4 number can be reached up to 0.62 mol% compared to those of other two modified TiO2 nanorods (0.38 and 0.19 mol% for TiO2‒Pyridine‒W4 and TiO2‒Lut‒W4, respectively). Additionally, the anchored W4 surface modifier on TiO2 nanorods would also improve the charge carrier transport in TiO2 percolated domains due to its conjugated nature. The enhanced electron mobility (i.e. electron is transported by TiO2 interpenetrating network in P3HT/TiO2 hybrid solar cell) in either solution‒cast TiO2 or hybrid thin film is observed in the sequence of μe, TiO2‒tBP‒W4 > μe, TiO2‒Pyridine‒W4 > μe, TiO2‒Lut‒W4 > μe, TiO2‒OA with the decreasing amount of anchored W4 modifiers. Last, the engineered P3HT/TiO2 nanorod hybrid film was fabricated into solar cell and then its photovoltaic performance was assessed. Compared to the reference hybrid thin film without any treatment (P3HT/TiO2‒Pyridine with about 0.40% of power conversion efficiency), the power conversion efficiency of P3HT/TiO2 nanorod hybrid solar cell are increased by 186%, 31%, 79% and 240% after the incorporation of P3HT‒P2VP additive, as well as ripened, B‒doped, and tBP‒W4 modified TiO2, respectively. The results in this work demonstrate the importance of interfacial engineering for polymer/nanocrystal hybrid thin film in photovoltaic application, and which are useful to boost the technological innovation for relevant commercial exploitation.