有機染料分子之結構設計對於聚己基噻吩/二氧化鈦太陽能電池之光伏特性影響研究

Molecular Design of Novel Organic Dye for Efficient Poly(3-hexylthiophene)/Titania Based Solar Cells

游家逢(Chia-Feng Yu)

指導教授 ： 林唯芳

共同指導教授 ： 王立義

國立臺灣大學/工學院/高分子科學與工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.01061

摘要

本論文共分成四個研究主題。在第一部份，我們為了改善有機/無機相容性問題，我們合成出一系列不同LUMO能階的共軛小分子(WL-1~WL-4)做為修飾二氧化鈦之界面改質劑，並應用於聚（3-己烷基噻吩)�中孔性二氧化鈦(P3HT/TiO2)雙層異質界面的太陽能電池之中。從數據中我們發現界面改質劑小分子的LUMO能階，若介在P3HT的LUMO與TiO2的傳導帶之間，可幫助P3HT吸光所產生的激子(exciton)電荷分離並傳導，提昇元件的光電流；另外可藉由共軛小分子本身的偶極矩，提昇TiO2的傳導帶位置，可增加元件的開路電壓(VOC)。我們利用UV-vis吸收光譜、循環伏安法(CV) 和密度泛函數理論(DFT)去計算並獲得各別分子的能階與偶極矩。而在A.M. 1.5G (100 mW/cm2)太陽光模擬光源量測下，其元件電流與電壓皆大幅提昇，從DFT的理論計算中，我們得知元件開路電壓的提昇主要來自於染料分子垂直於TiO2方向的偶極矩所造成；另一方面，由EQE (external quantum efficiency)圖譜的量測，我們發現短路電流(JSC)主要來自於有機共軛小分子與P3HT的貢獻。而在波長570 nm下的EQE值為純粹代表P3HT激子(exciton)貢獻所產生的特徵峰，隨著共軛小分子的LUMO能階越低，此P3HT激子貢獻電流的特徵峰也越來越明顯，其中以WL-4共軛小分子改質過後的元件效果最為顯著，VOC為800 mV、JSC為6.79 mA/cm2、FF為53 %且元件效率為2.87 %。
在第二部份，為了提昇有機/無機混成太陽能電池的電流與元件效率，我們在界面改質劑的結構上引入一系列不同拉電子強度的官能基，討論其如何誘使吸光材料(P3HT)所產生exciton電荷分離，並促進電子注入到無機半導體(TiO2)內。我們利用DFT理論去計算界面改質劑之LUMO能帶軌域分佈圖，以及原子的Mulliken电荷，去分析界面改質劑共軛鏈段上的拉電子能力。另一方面，由螢光圖譜與時間解析光激螢光(tr-PL)圖譜的量測，去分析在不同界面改質劑改質的二氧化碳下，P3HT的螢光瘁滅的情形與其受激發電子轉移的時間(lifetime)，從分析數據中，我們發現除了界面改質劑LUMO能階的位置之外，吸光材料距離界面改質劑LUMO能帶電子軌域分佈的距離，與界面改質劑共軛鏈段上的拉電子能力，皆會影響P3HT電子的轉移，進而影響元件光電流。由UV-vis吸收光譜與EQE圖譜的量測，在波長600 nm下的EQE值主要為P3HT所貢獻的光電流轉換效率，我們發現隨著P3HT距離界面改質劑LUMO能帶電子軌域越近，或界面改質劑共軛鏈段上的拉電子能力越強，P3HT的光電流轉換效率就越大。此發現有助提供有機/無機混成太陽能電池之界面改質劑一個新的設計方向。
在第三部份，我們用與P3HT結構單體類似的己烷基噻吩，進一步延長導電小分子的共軛長度，除了希望能進一步增加作為染料(dye)的小分子貢獻的光電流，也希望能藉由不同位置的己烷基鏈段 (hexyl chain)來修飾二氧化鈦表面，增加與P3HT之間的界面相容性。我們由UV-vis吸收光譜與時間解析光激螢光(tr-PL)圖譜的分析得知染料小分子在二氧化鈦表面聚集(aggregation)的情形以及電子注入到二氧化鈦的效率，並再利用光激螢光(PL)圖譜與電化學阻抗頻譜(EIS)得知TiO2/dye/P3HT等元件內部各界面間的相容性與電荷轉移情形。我們發現導入己烷基鏈段的小分子雖可減少電子、電洞的再結合機率，使電池元件性能表現較為穩定，亦有較高的Voc；但己烷基鏈段的存在會降低固態電洞材料(P3HT)還原有機染料的能力，增加dye/P3HT界面間的傳遞電阻，因而降低了元件的光電流。
在第四部份，我們設計出了兩種含carbazole官能基的新穎有機染料(N-HW與N-3W)，為WL-4有機共軛小分子的衍生物，並製成了以P3HT為電洞傳導材料的全固態染料敏化太陽能電池。我們利用UV-vis、CV 和DFT去計算並獲得各別染料小分子的能階、偶極矩、與在TiO2上聚集的情形，並用PL、tr-PL、EIS等圖譜去觀察各界面間電荷轉移的現象，在A.M. 1.5G (100 mW/cm2)太陽光模擬光源量測下，carbazole官能基以氮原子位置與WL-4小分子連接的衍生物 (N-HW)，其元件有最佳的能量轉換效率(3.17 %)，而carbazole官能基以3號位置與WL-4小分子連接的衍生物(N-3W)，其元件則有最好的VOC (將近1.0 V)。我們從DFT的理論計算中可得知，由於N-HW有較bulky的分子結構，能抑制染料的聚集，可增加染料本身光子轉換成電子的效率，另外，由理論的計算中也可發現N-3W由於其較共平面的結構，能有效延伸共軛長度，所以讓分子本身有極大的偶極矩產生，能有效地提昇TiO2的導電帶，因此增加了元件的開路電壓，但因N-HW較容易聚集的關係，導致界面間電荷再結合的情形嚴重，不利於提昇元件效率。另外，我們可藉由N-HW與N-3W共吸附的方式，大幅提昇元件效率；利用少量的N-3W染料，修飾N-HW改質的TiO2表面，大幅提昇N-HW元件的Voc將近0.1 V以上，而最佳元件效率為3.53 %。

關鍵字

有機無機混成太陽能電池 ； 染料敏化太陽能電池 ； 自組裝單層膜 ； 界面改質劑 ； 能階校準 ； 電荷傳輸 ； 聚己基噻吩

並列摘要

This thesis consists of four topics concerning the heterojunction of poly(3-hexylthiophene) (P3HT) and TiO2 in polymer photovaltaics. In the first part, the interface of P3HT and TiO2 is systematically engineered with four cyanoacrylic acid-containing conjugated molecules with various lowest unoccupied molecular orbital (LUMO) levels, WL-1 to WL-4. The optical characteristics, redox properties and intrinsic dipole moments of these interfacial modifiers (IMs) are examined using UV-vis spectrophotometry, cyclic voltammetry (CV) and density functional theory (DFT) calculations. Using cyanoacrylic acid as a terminal anchoring group in IMs increases the electron affinity in regions close to the titania surface and forms a molecular dipole that is orientated away from the TiO2 surface, enabling both open-circuit voltage (VOC) and short-circuit current density to be increased simultaneously. Photovoltaic measurements demonstrate that VOC increases with the dipole moment of IMs along the molecular backbone. Moreover, the external quantum efficiency (EQE) spectra display a bimodal distribution, revealing that both IMs and P3HT contribute to the photocurrent. The EQE at 570 nm is identified as characteristic of P3HT. More importantly, the LUMO of the IMs decisively determines the dissociation efficiency of P3HT excitons. The device based on P3HT/WL-4/TiO2 exhibits the highest power conversion efficiency of 2.87%.
In the second part, we present four kinds of novel conjugated molecules with different electron-withdrawing ability as the IM of P3HT and TiO2. In order to enhance the electron affinity of the TiO2 and its compatibility to organic materials, an electron-withdrawing moiety was incorporated into the conjugated backbone of IMs to facilitate the electron transfer between P3HT and TiO2. Electronic density distributions of LUMO levels and Mulliken atomic charges about these IMs were examined by density functional theory. From calculations of partial atomic charges by Mulliken population analysis, we could explore the partial negative charge (or electron-withdrawing ability) of each functional groups of IMs. Photoluminescence quenching analysis and time-correlated single photon counting (TCSPC) decay curves revealed that the efficient electron transfer behavior and short electron lifetime were achieved with the stronger electron-withdrawing ability of conjugated backbone of the IMs, or with a shorter distance of LUMO electronic distribution of IMs from P3HT donor material. Photovoltaic measurements and external quantum efficiency spectra also demonstrate that the functionalization of the TiO2 surface with a molecular electron acceptor could promote photoinduced electron transfer from a polymer donor to the TiO2 metal oxide. Our proposed method opens novel IMs design possibilities and provides a new route for fabricating hybrid photovoltaics.
In the third part, the effect of the alkyl chain in organic dyes on the performance of TiO2/dye/P3HT heterojunction solar cells is explored. Herein, a series of WL-4 derived molecules, SL-101 to SL-104, in which the alkyl group is anchored at various positions, were reported and characterized. The UV-vis spectroscopy was employed to measure the absorption spectrum, optical bandgap, and the extent of dye aggregation on the TiO2 film; the energy levels of HOMO and LUMO were determined by cyclic voltammetry; the electron injection efficiencies of dye absorbed on the TiO2 film were calculated by time-correlated single photon counting (TCSPC) technology; the charge transfer behavior in the interface of TiO2/dye/P3HT was also measured by photoluminescence spectroscopy and electrochemical impedance spectroscopy. Our analytical studies reveal that the presence of the hexyl chain may reduce the rate of charge recombination in the interface, thereby improving the VOC and stabilizing the device performance, but reducing the dye-regenerating ability and increasing the charge transfer resistance at the dye/P3HT interface, thus reducing the photocurrent of devices.
In the last part, we applied two novel carbazole derivatives, N-HW and N-3W, as organic dyes in P3HT-based solid-state dye-sensitized solar cells. The photoelectric properties, dye aggregation behavior, dipole moments, electron injection ability of these dyes were characterized by UV, CV, DFT calculations, and TCSPC technology. Then, we used the PL and EIS techniques to analyze the charge transfer phenomena at the TiO2/dye/P3HT interface. Our data revealed that the incorporation of the carbazole moiety with the neighbor thiophene ring through its nitrogen atom, N-HW, may hinder the molecular aggregation upon absorbing onto TiO2 surface, and inhibit the charge recombination at the interface. On the other hand, the N-3W dye, in which the carbazole moiety is bounded to the neighbor thiophene at 3-position, showed a longer conjugation length, leading to the decrease in bandgap and the increase in dipole moment; however, its co-planer structure may promote the intermolecular π-π satcking aggregation and the efficiency of self-quenching upon photoexcitation. Thus, the N-3W device had higher Voc but poor power conversion efficiency
than those of the N-HW device. Additionally, the cell performance was further improved by utiling N-HW and N-3W as co-sensitizer. By replacing a small amount of N-HW with N-3W, the Voc of the N-HW device was dramatically increased,leading to a power conversion efficiency of 3.53%.

並列關鍵字

hybrid solar cell ； solid-state dye-sensitized solar cell ； self-assemble monolayer ； charge transfer ； energy level alignment ； poly(3-hexylthiophene)

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