電子施體/受體共軛有機材料於薄膜電晶體與太陽能元件應用

Donor-Acceptor Conjugated Organic Materials for Thin-Film Transistors and Solar cells

李文亞(Wen-Ya Lee)

指導教授 ： 陳文章

國立臺灣大學/工學院/化學工程學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.01517

摘要

電子予體-受體共軛有機材料的光電特性與電子結構性質決定了有機光電元件的性能。在這論文中，我們有系統性的研究電子予體-受體共軛有機材料的合成，光電特性與元件性質。此論文的目的在於研究下列幾個問題：（1）設計與合成新穎電子予體-受體共軛有機材料，（2）釐清電荷傳遞以及化學結構之間的關係，以及（3）分析電子予體-受體共軛有機材料在光電元件上的性質，如薄膜電晶體，光伏打電池或是發光二極體元件。此研究的主題與細節將整理如下：

1. 新穎茀系電子予體-受體共軛高分子之合成，光電性質，與元件應用(第二章)
研究四種帶有不同電子受體（Q, BT, TP）的茀系電子予體-受體共軛高分子 （PFTT, PFDDTQ, PFDTBT與PFDDTTP），發現這些高分子之能隙的順序為PFDDTTP < PFDTBT < PFDDTQ < PFTT，此順序剛好與發光波長以及場效應電荷傳遞率趨勢相反，這是因為帶有強電子受體TP與高共平面的PFDDTTP造成最強的分子內電荷轉移以及最高的電荷傳遞率。此外，PFDDTTP，PFDTBT與PFDDTQ的光伏打電池特性也同樣被研究，光電流以及能量轉換效率的趨勢是PFDTBT > PFDDTQ > PFDDTTP，這結果與上述場效應電荷傳遞率與能隙趨勢相互抵觸。然而，光電流與效率趨勢反而與高分子摻混PCBM後的電荷傳導率的趨勢相符。由於因為電荷轉移發生在PFDTBT/PCBM與PFDDTQ/PCBM摻混物中，使得其電荷傳遞率上升，而PFDDTTP的強分子內電荷轉移能力使得PFDDTTP/PCBM摻混物發生相分離現象，而反而讓電荷傳遞率下降。由於摻混後提高了電荷傳遞率在加上低能隙特性，使得PFDTBT可達到1.1%能量轉換效率。

2. thiophene-thieno[3,4-b]pyrazine交替共聚高分子(PTHTP-C7, PTHTP-C12, 與 PBTHTP-C7) 之電荷傳遞能力與表面型態之研究（第三章）
長側鏈讓PTHTP-C12在HMDS表面上成為纖維化的結構，促使更好的電荷傳遞性質。然而在OTS表面上因為PTHTP的非極性碳鏈與高度非親水表面有很強的作用力，使得PTHTP-C12變成結瘤狀的型態。在高度退火處理後，緊密排列的型態使的PTHTP-C12在OTS處理的二氧化矽表面上可得到高達0.01 cm2V-1s-1的電荷傳遞率。此研究顯示thiophene-based電子予體-受體共軛高分子可經由表面型態的調整來達到高電荷傳遞率。

3. 新穎茀系分子與3,9-或是2,8-取代indolocarbazole交替共聚高分子之合成與特性鑑定（第四章）
利用鉑催化Suzuki coupling反應合成poly(fluorene)-alt-3,9-(indolo[3,2-b]carbazole)) (PF-p-In) and poly(fluorene)-alt-2,8-(indolo[3,2-b]carbazole)) (PF-m-In)，並測試發光二極體與 場效應電晶體的性質。對位結構的PF-p-In使得π電子容易非定域化，因此可以得到較低能隙。PF-p-In與PF-m-In的電致發光亮度相近，但擁有不同的發光顏色：綠色與藍色。從場效應電晶體的分析中，PF-p-In與PF-m-In的電荷傳遞率分別是6.73×10-5 and 1.50×10-4 cm2V-1s-1，這較單聚茀系高分子來得高。此研究顯示了茀系高分子可經由結合不同位置的indolocarbazole來提升光電特性與調整電子結構。

4. 核心氯化NDI衍生物之溶液製程場效應電晶體特性 （第五章）
研究四種帶有不同數目的氯拉電子基之核心氯化NDI衍生物，其中編號各至為1, 2, 3 及4。研究中發現，利用溶液剪應製程製備2之場效應電晶體，可得高電子傳遞率0.17 cm-2V-1s-1，而且擁有很好的空氣中穩定性。另外也發現1與3的界限電位在大氣中會有急速的增加，這是因為1與3的晶粒邊界提供通道，造成氧氣與水氣入侵後捕捉電子載體。從XRD分析中，我們也發現溶液剪應製程會改變結晶方向。此研究顯示核心氯化NDI衍生物將會是個很有潛力的有機電子材料。

關鍵字

共軛有機材料 ； 薄膜電晶體 ； 太陽能元件

並列摘要

Optoelectronic properties of donor-Acceptor conjugated organic materials determine the performances of organic electronics. In this thesis, a systematic study on the synthesis, optoelectronic properties and device characterizations of donor-acceptor conjugated organic materials was investigated. The goals of this thesis is to address the following issues: (1) designing and synthesizing new donor-acceptor organic materials, (2) clarifying the relationship of charge transport and chemical structures, and (3) characterizing the performances of organic electronics, e.g. thin film transistors, photovoltaic cells or light-emitting diodes, based on these studied materials. The objectives and the research findings are summarized as follows:

1. Synthesis, optoelectronic properties, and device applications of new fluorene-based donor-acceptor conjugated polymers (chapter 2): four fluorene-based conjugated polymers, including PFTT, PFDDTQ, PFDTBT, and PFDDTTP, with various acceptor structures, quinoxaline (Q), 2,1,3-benzothiadiazole (BT) and thieno[3,4-b]pyrazine (TP) were explored. It was found that the order of the value in the band gap energy was PFDDTTP < PFDTBT < PFDDTQ < PFTT, which was on the opposite trend of emission maximum and their charge carrier mobility due to the strong electron-withdrawing strength of the TP moiety and the coplanar backbone in PFDDTTP, leading to the highest intramolecular charge transfer and the highest hole mobility among all four polymers. Furthermore, PFDDTQ, PFDTBT, and PFDDTTP were used to fabricate in copolymer/[6,6]-phenyl-C61-butyricacid methyl ester (PCBM) bulk-heterojunction photovoltaic cells. The order in the short-circuit current density (JSC) and power-conversion efficiency (PCE) of the photovoltaic cells was PFDTBT > PFDDTQ > PFDDTTP, which contradicted the order in the band gap energy and mobility. However, the JSC and PCE coincided instead with the order in the mobility of the copolymer/PCBM blend, where the mobility was found to increase in PFDTBT and PFDDTQ devices owing to charge transfer with PCBM, but the mobility deceased in PFDDTTP due to inefficient phase separation resulting from the strong intermole-cular interactions between the polymer chains of PFDDTTP. With its high blended mobility and low band gap, PFDTBT achieved a PCE of 1.1 %.

2. The charge transport characteristics and surface morphology of three thiophene-thieno[3,4-b]pyrazine alternating copolymers (PTHTP-C7, PTHTP-C12, and PBTHTP-C7) (chapter 3). Long alkyl side chains promoted PTHTP-C12 to become a fibrillar-like structure on the hexamethyldisilazane (HMDS)-modified surface and resulted in better charge transport properties than those of the other two copolymers. However, a nodule-like morphology on the octyltrichlorosilane (OTS)-modified surface was observed due to the strong interaction between the non-polar alkyl chains of PTHTP-C12 and highly hydrophobic surface. By further annealing at a higher temperature, a densely packed grain morphology on octyltrichlorosilane (OTS) modified SiO2 surface was observed and led to the field effect mobility of 1.1 × 10-2 cm2V-1s-1. The present study suggests that the thiophene based donor-acceptor conjugated polymers could have a high FET mobility through the manipulation of their morphology.

3. Synthesis, characterization of new alternating copolymers of fluorene and 3,9- or 2,8- substituted di(2-ethylhexyl)-indolo[3,2-b]carbazole (chapter 4). poly(fluorene)-alt-3,9-(indolo[3,2-b]carbazole)) (PF-p-In) and poly(fluorene)-alt-2,8-(indolo[3,2-b]carbazole)) (PF-m-In) were synthesized by palladium-catalyzed Suzuki coupling polymerization and characterized for the applications of light-emitting diodes and field effect transistors (FET). The para-linkage, PF-p-In, facilitates π-electron delocalization and thus has a lower optical band gap and a higher emission maximum than those of the meta linkage, PF-m-In. The electroluminescence devices based on PF-p-In and PF-m-In as the emissive layer show a similar maximum luminance but with different emissive colors of green and blue, respectively. The FET hole mobilities of PF-p-In and PF-m-In are 6.73×10-5 and 1.50×10-4 cm2/V.s, respectively, which are significantly higher than that of polyfluorene. The present study demonstrates the electronic and optoelectronic properties of polyfluorene enhanced by incorporating hole transporting indolocarbazole with different linkages.

4. The solution-processing OTFT characterizations of new core-chlorinated Naphthalene diimide (NDI) derivatives with fluorinated alkyl chains (chapter 5). Four core-chlorinated NDI derivatives, including N,N’-bis(heptafluorobutyl) naphthalene diimide (1), N,N’-bis(heptafluorobutyl)-2,6-dichloro-naphthalene diimide (2), N,N’-bis(nonafluoropentyl)-2,3,6,7-tetrachloro-naphthalene diimide (3) and N,N’-bis(nonafluoropentyl)-2,6-dichloro-naphthalene diimide (4), were studied. A high electron average mobility of 0.17 cm-2V-1s-1 and an excellent ambient stability are observed in OTFT devices made from 2 through solution-sheared deposition method. From charge transport anisotropy measurement, a high mobility of 0.5 cm-2V-1s-1 of 2 was achieved from a perpendicular direction of OTFT. The direction of channel length in the device is vertical to the shearing direction. Furthermore, the threshold voltage of 1 increases dramatically upon exposure to the air because the grain boundaries of 1 provide channels for diffusion of oxygen and moisture, which act as electron traps. From XRD measurement result, it is concluded that the solution-sheared method affect the orientation of crystalline thin films and lower the d-spacing distance, as compared with that of the vapor deposition. This study affords the promising n-type high-performance core-chlorinated NDI derivatives for potential organic electronics.

並列關鍵字

Donor-Acceptor Conjugated Polymer ； Thin-Film Transistors ； Solar cells

參考文獻

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