電荷傳輸基團改質共軛高分子之結構物性及其在發光二極體與光電池元件之應用

Structure–Property Relationship in Charge Transport moiety modified Conjugated Polymers and Applications in Light-Emitting Diodes and Photovoltaic Cells

黃思博(Szu-Po Huang)

指導教授 ： 陳壽安

國立清華大學/工學院/化學工程學系/博士(2006年)

摘要

本論文是將具有電荷傳遞特性的基團引入共軛高分子的結構之中，研究其在發光二極體及光電池元件之應用。PPP系高分子的合成簡單，有利於工業化，且具有深藍色螢光性質。由於文獻上對於PPP系列高分子利用電荷傳遞基團改質未有相關的報導。因此，本研究首先針對電荷傳輸基團改質之PPP系高分子(EHB-PPP-Cz)，探討其結構物性，對其螢光與電激發特性之影響。再以電荷傳輸基團改質之PPP系高分子(Cz100-PPP)，探討其對阻擋磷光小分子三重態的能量傳回高分子三重態及相容性的影響。本研究最後將討論電荷傳遞基團改質之PPV與PF(POPD-MEH-PPV、 T60及OXDPF)應用於光電池元件，探討其對激子解離速率的影響。

側鏈含alkoxy-phenyl之單取代聚對位苯系高分子(EHB-PPP) 與含咔唑 (carbazole) 基團改質對位苯系共聚合體(EHB-PPP-Cz)，利用元素分析法確定高分子中共聚物組成的含量，光譜分析法 (UV-Vis、PL及CV) 來探討EHB-PPP與含EHB-PPP-Cz結構及物性之關係。EHB-PPP，由於苯環基團的導入使能隙增加，使螢光光譜產生藍位移，並且提升了螢光量子效率，它是目前電致發光材料中發光波長最短波長的高分子 (λmax= 403 nm)。
由單一載子元件的分析可知，carbazole基團的導入有助於改進載子不平衡的問題，因此EHB-PPP-Cz具有較好的電洞之注入及傳遞能達到平衡，所以此材料可得到較佳的元件發光量子效率及穩定性，最大發光效率約為1.12 (cd/A)，外部量子效率為2.32%，並且具有穩定深藍光的電致發光(光色不隨電壓而變)，其CIE (Commission Internationale de I’Eclairage) 為 (0.16, 0.05)。

設計了一個共軛高分子應用於磷光發光元件，選定PPP為客體材料(DRO-PPP)，因為其主體放射光譜與客體的吸收光譜重疊性 (overlap) 大，而且主體與客體的HOMO與LUMO匹配。雖然其ET (2.3 eV) 並未高於磷光小分子，但透過淬熄速率常數的量測得知，其側鏈的長度足以阻擋部份磷光小分子三重態的能量傳回高分子的三重態，因此可以得到一個高效率的綠光磷光元件，在元件結構為ITO/CFx/polymer:Ir/TPBI/CsF/Ca/Al，效率為15 cd/A，外部量子效率為3.72%。

更進一步的我們將與小分子互溶性好的carbazole導入高分子的側鏈 (Cz100-PPP)，由淬熄速率常數的量測及AFM可知，此改質過的高分子，除了側鏈長度具有阻擋部份磷光小分子三重態的能量傳回高分子的三重態外，同時因為carbazole基團與小分子的互溶性好，當摻雜8 wt%的綠光銥金屬錯合物時有最佳的綠光元件效能，最大外部量子效率為8.25%，最大發光效率為30 cd/A ( 195 cd/m2, 0.668 mA/cm2)，最大亮度為6127 cd/m2 ( 15 cd/A, 40 mA/cm2)，啟動電場為1.4 MV/cm (膜厚為100 nm時約14 V)。發光波長為520 nm，為目前共軛導電高分子應用於磷光元件中效率最好的綠光發光元件。

同時亦將此高分子分別摻雜紅光與綠光加紅光的磷光小分子，分別得到紅光及白光的磷光元件。當摻雜8 wt%的紅光銥金屬錯合物時有最佳的紅光元件效能，最大外部量子效率為4.34%，最大發光效率為4.5 cd/A ( 323 cd/m2, 7.122 mA/cm2)，最大亮度為829 cd/m2 ( 3.7 cd/A, 22 mA/cm2)，啟動電場為1.4 MV/cm (膜厚為100 nm時約14 V)。當摻雜0.5 wt%的綠光銥金屬錯合物和0.5 wt%的紅光銥金屬錯合物時有最佳的白光元件效能，最大外部量子效率為8.65%最大發光效率為16.8 cd/A ( 84 cd/m2, 0.5 mA/cm2)，最大亮度為344 cd/m2 ( 5.9 cd/A, 5.8 mA/cm2)，啟動電場為1.2 MV/cm (膜厚為100 nm時約12 V)。

為了提升高分子在光電池元件應用中的效率，我們在PPV及PF高分子側鏈導入高陰電性電子傳遞基團，然後製成光電轉換元件進行光電特性之量測。實驗的結果顯示，在MEH-PPV側鏈引進OXD基團(POPD-MEH-PPV (37/63))，BTAZ基團 (T60) 與在PFO側鏈引進OXD基團 (OXD50PFO) 後光電元件的效率有顯著的提升。

利用luminescence decay 和time of flight measurement (TOF)的量測技術來深入了解OXD與BTAZ基團的作用。結果顯示OXD與BTAZ基團可以提升電子的傳送(electron transport)及增加激子解離速率(exciton dissociation rate)。因此，經過OXD與BTAZ基團改質的高分子(POPD-MEH-PPV (37/63) 、T60 和 OXD50PFO)，其光電轉換效率較未改質的高分子(MEH-PPV 和 PFO ) 分別提升了 2.1 、1.8和 29 倍。

關鍵字

發光二極體 ； 太陽能電池 ； 共軛高分子 ； 聚對位苯高分子

並列摘要

This study discloses the introduction of charge transport moieties onto the conjugated polymer structures and their application on light-emitting diodes and solar cells. PPP derivatives are promising candidate deep blue materials for commercialization due to their ease to be synthesized. Since the modification of PPP derivatives through incorporating charge transport moieties are scarce, this study demonstrates for the first time to clarify the structure/property relationship, photoluminescence and electroluminescence of the charge transport moiety modified PPP (EHB-PPP-Cz). Furthermore, we utilize the other charge transport moiety modified PPP (Cz1000-PPP) to elucidate the blocking effects of triplet back energy transfer from triplet state of phosphorescent dopants to that of the polymer and their chemical compatibility. Finally, we show that the charge transport moiety modified polymer derivatives and their application on solar cell to discuss their influences on exciton dissociation rate.

This study discloses the design and synthesis of poly(para-phenylene) derivatives (EHB-PPP and EHB-PPP-Cz), and reports their structure-property relationships. The incorporation of alkoxy-phenyl substitute can adjust the energy level and cause a blue-shifted PL and, therefore promote the fluorescent quantum efficiency and acquire a deeper blue emission. The present copolymer (EHB-PPP-Cz) exhibits the shortest wavelength among the electroluminescent materials (λmax= 403 nm).

From the measurement results of single carrier devices, the introduction of carbazole unit can solve the drawback of charge imbalance of the material. Therefore, EHB-PPP-Cz shows better hole injection capability and balances its charge transport capabilities. The device (ITO/PEDOT/EHB-PPP-Cz/TPBI/CsF/Ca/Al) based on this material exhibits stable blue emission (spectrum remains unchanged upon successive operation) with luminous efficiency of 1.12 (cd/A), external quantum efficiency of 2.32% and CIE (Commission Internationale de I’Eclairage) coordinate (0.16, 0.05).

Here, DRO-PPP is utilized as the host material for electro-phosphorescence PLED application due to its large spectra overlap and excellent HOMO LUMO energy level match with guest materials. Even though the triplet energy of DRO-PPP is lower than phosphorescent material, the long distance of its side chain length can suppress back triplet energy transfer from guest to the host. As a result, the electrophosphorescence PLED based on DRO-PPP exhibits a luminous efficiency of 15 cd/A, external quantum efficiency of 3.72%.

Furthermore, we incorporate the carbazole to the end of the flexible side chain of polymer (Cz100-PPP) due to its excellent chemical compatibility with guest material. From the results of Stern-Volmer and AFM measurements, the modified polymer not only exhibits the suppression of back triplet energy transfer from guest to the host, but also excellent chemical compatibility between host-guest materials. While doping with 8 wt% of Ir-G complex, the device shows performance with the highest external quantum efficiency of 8.25% and luminous efficiency of 30 cd/A (195 cd/m2, 0.668 mA/cm2), the highest luminance of 6127 cd/m2 (15 cd/A, 40 mA/cm2) and turn on electric field of 1.4 MV/cm and EL λmax at 520 nm. To the best of our knowledge, the present device efficiency is the highest one among electro-phosphorescent PLED with green emission to date.

While doping with 8 wt% Ir-R complex, the device exhibits the best performance for red emission with the highest external quantum efficiency of 4.34% and efficiency of 4.5 cd/A (323 cd/m2, 7.122 mA/cm2) the highest luminance of 829 cd/m2 (3.7 cd/A, 22 mA/cm2), turn on electric field of 1.4 MV/cm. Also, when doping with 0.5 wt% Ir-G and 0.5 wt% Ir-R complexes, the device exhibits best performance for white emission with the highest external quantum efficiency of 8.65% and efficiency of 16.8 cd/A (84 cd/m2, 0.5 mA/cm2) the highest luminance of 344 cd/m2 (5.9 cd/A, 5.8 mA/cm2) and turn on electric field of 1.2 MV/cm.

To improve the efficiency of polymer based solar cell, we introduce high electron deficient moiety to the end of side chain of PPV and PF polymers. The devices based on MEH-PPV with OXD moiety (POPD-MEH-PPV (37/63)), BTAZ moiety (T60) and PF with OXD moiety (OXD50PFO) exhibit significant enhanced efficiency than the unmodified analogues.

Moreover, we utilize luminescence decay and time of flight measurement (TOF) measurements to clarify the function of BTAZ and OXD moieties. The experimental results reveal that the OXD and BTAZ moieties can increase exciton dissociation rate and promote electron transport. Hence, the efficiencies of the devices based on OXD and BTAZ modified polymers (POPD-MEH-PPV (37/63), T60 and OXD50PFO) are higher than the unmodified analogues by a factor of 2.1, 1.8 and 29, respectively.

並列關鍵字

PLED ； Solar cell ； Conjugated polymer ； poly(para-phenylene)

參考文獻

[167] 陳彥均， ”聚咔唑高分子衍生物的光學物理以及電致發光特性之研究”， 國立清華大學化工系博士論文研究計畫，民國94年。

[195] A. K. Ghosh, T. Feng, J. Appl. Phys. 49 (1978) 5982.

[144] B. S. Harrison, T. J. Foley, A. S. Knefely, J. K. Mwaura, G. B. Cunningham, T. S. Kang, M. Bouguettaya, J. M. Boncella, J. R. Reynolds,and K. S. Schanze, Chem. Mater. 16 (2004) 2938.

[134] X. Chen, J. L. Liao, and S. A. Chen, in preparation.

[164] J. Jiang, C. Jiang, W. Yang, H. Zhen, F. Huang, Y. Cao, Macromolecules 38 (2005) 4072.