氯改質之高功函數ITO陽極及其高分子發光二極體元件之研究與應用

The Study on Chlorinated-ITO with High Work Function in Polymer Light-Emitting Diodes Application

李昱璇(Yu-Hsuan Lee)

指導教授 ： 陳壽安

國立清華大學/工學院/化學工程學系/碩士(2014年)

摘要

近年來，高分子發光二極體(Polymer Light-Emitting Diodes, PLED)由於適用於溶液製程(solution process)方便製作，能應用於大面積、輕薄或可撓曲顯示器的製作而擁有工業化的潛力，引起了廣泛的研究及注意。相較於傳統陰極射線映象管(cathode ray tube, CRT)笨重且耗電、液晶面板窄視角與反應速度過慢等缺點，PLED具有面板輕薄、高亮度與反應速度快的特點。而目前PLED元件製作上最廣為使用的陽極──具有良好導電性與極佳穿透性質的ITO，然而，其相對低的功函數(4.7 eV)與發光層常用的fluorene-and phenylene-based高分子材料之HOMO (>5.5 eV)相比極不匹配，導致電洞注入能障過大，使得元件的電洞注入量往往不足。因而迫使需要對進行複雜的ITO表面處理或引入其他其他電洞注入層(Hole Injection Layer, HIL)或電洞傳輸層 (Hole Transport Layer, HTL)來降低注入能障、提升電洞注入，常用的方法如 (i) 引入有機中間層(organic interlayer)，常用的材料如電洞傳輸層poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS)、(ii) 利用O2 plasma、ultraviolet (UV)-ozone或CFx plasma處理，以調整ITO表面的元素組成，並同時去除表面髒汙，亦可藉由(iii)插入self-assemble monolayer (SAM)的界面偶極效應(interfacial dipole effect)而提升ITO功函數。然而，多層的元件結構與額外的ITO表面處理不僅複雜化元件製作程序，亦可能增加元件失敗率。
2011年，Lu團隊發表了在UV 照射下，將ITO與o-dichlorobenzene (ODCB)接觸，藉由鹵化反應(chlorination)能讓ITO功函數由4.7大幅提升到6.13 eV，chlorinated-ITO與電洞傳輸層4,4’-N’,N’-dicarbazole-biphenyl (CBP) (HOMO 6.1 eV)之間無電洞注入能障，而得到極佳表現的綠光小分子發光二極體。
本論文分為四個部分，首先就Cl-ITO的製備進行研究，改變製程的步驟、處理時間與處理方法，並探討各個因素其對於chlorinated-ITO (Cl-ITO)之功函數與元素比例的影響。接著分別將Cl-ITO應用於以poly(9,9-dioctylfluorene) (PFO)為發光層的藍光及以poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV)的橘光PLED系統；最後則是使用caesium floride水溶液及1,2-debromobenzene改質ITO以提升功函數，並用於PFO藍光元件。此四部份將分別於第四章至第七章呈現。
第四章中，我們藉由在UV irradiation的照射下，在ITO上覆蓋ODCB，使Cl解離並釋放自由基，而取代ITO表面的氧原子與銦產生鍵結形成Cl-In單分子層成為Cl-ITO，Cl-In所形成的dipole layer與ITO表面元素組成皆會影響ITO功函數的提升，其中，尤以碳含量的移除為關鍵。Ar-etching的結果發現，碳含量大部分分佈於ITO表層，因此我們嘗試以不同方式進行ITO前端處理，前端ITO處理方式中，以UV-ozone (15 min)最為有效除碳且方便控制；後端UV-ozone時間越長越好，十分鐘時碳含量明顯減少且增長時間後差異不大，以此條件製作元件後電洞注入最好。ODCB的處理上，使用量越多、濃度越高則Cl含量越高，Cl-ITO的功函數也越大。綜合以上，pre-UV-ozone (15 min)、ODCB (5 min)、post-UV-ozone (10 min)的Cl-ITO功函數可達5.58 eV。
第五章，由於PFO具有高electroluminescent (EL) 量子效率及高達59 %的photoluminescence quantum efficiencies (PLQEs)，且熱穩定、化學穩定性好、光色穩定，元件製作上，在有機溶劑中極佳的溶解性和良好成膜能力等諸多優點，讓PFO成為發光二極體元件中常用的藍光材料。以溶劑處理過後的β-phase PFO具有electron trapping及提高hole mobility的效果，然而，其HOMO (5.8 eV)與ITO功函數 (4.7 eV)相差甚大，使得電洞能障過大而注入困難，為克服PFO與ITO之間注入能障過大的問題，因此我們將高功函數的Cl-ITO應用在β-PFO的元件上，以改善元件電洞注入量不足的問題而提升元件表現，我們發現Cl-ITO之Cl free radical會有淬熄PFO上exciton的問題，藉由將Cl-ITO以氨水溶液進行處理，改變不同的氨水濃度與處理時間，能夠有效的中和移除radical，而達到減少淬熄的效應，在Cl-ITO/β-PFO/CsF/Al的單層元件中，達到亮度16774 cd/m2與效率2.4 cd/A的元件表現，與未行氨水處理的Cl-ITO元件及ITO/PEDOT:PSS元件相較有明顯提升；此外，我們引入PCn6:K+ (1:3)取代在空氣中不穩定的金屬CsF，並製作Cl-ITO (NH4OH 1 %)/β-PFO (100 nm)/PCn6:K+ (1:3)/Al (100 nm)，元件達到最大亮度28550 cd/m2、最大效率2.6 cd/A，由於PFCn6:K+具有電子傳輸的效果，因此應用於高電洞注入的Cl-ITO陽極的β-PFO且電子載子較多的元件，能夠讓元件擁有更佳的載子平衡，而達到高亮度效率的元件表現。此氨水處理Cl-ITO並用以製作元件的方法，與其他ITO表面處理方法相比電洞注入較大、元件表現較好，不但能夠排除exciton被淬熄的因素，更能減化元件製作、達到單層元件的高效率亮度的表現。
第六章中，我們引入其他化學方法改質ITO──以caesium caesium fluoride (CsF)水溶液及1,2-dibromobenzene改質ITO。使用CsF水溶液改質後的ITO功函數可深達5.75 eV，極適宜作為β-PFO元件之陽極、縮小電洞注入能障而提高電洞注入量，我們製作元件後發現由於ITO上存有若干顆大小不一的particle，推測為CsF鹽類殘留導致元件漏電流，因此元件表現極為不佳。此外，亦延續前一章Cl-ITO的研究，探討鹵素家族中其他鹵素改質ITO的性質，以1,2-dibromobenznen取代ODCB改質ITO，並以Br-ITO為陽極製作β-PFO元件，我們發現ITO上有殘留些許particle造成元件效果不彰，且Br-ITO功函數並無法達到如Cl-ITO高，因此電洞注入效果不如Cl-ITO元件好。
第七章中，我們將Cl-ITO引入MEH-PPV橘光系統中，雖然MEH-PPV之HOMO並不深 (5.10 eV)，使用ITO (5.0 eV)或ITO/PEDOT:PSS (5.2 eV)即無電洞注入問題，但我們利用Cl-ITO高功函數及與ITO相仿的高導電度之特性以提升元件的電洞注入，使用LiF/Al製作Cl-ITO/MEH-PPV/LiF/Al元件達到最大亮度12486 cd/m2、最大效率1.23 cd/A，與使用ITO/PEDOT:PSS相比較電流密度大了一倍，意味著Cl-ITO能有效提升電洞載子的注入；使用CsF/Al作為陰極以提升電子數量後，Cl-ITO/MEH-PPV/CsF/Al達到最大亮度12246 cd/m2、最大效率0.68 cd/A，CsF雖然能提供較多的電子注入，然而由於載子不平衡使得效率較LiF/Al低。兩者的啟動電壓都落在2 V左右，亦表示載子注入順利因此能在很低的電壓下啟動。

關鍵字

Chlorinated ITO ； Cl free radical ； PFO ； PLED

並列摘要

Polymer light-emitting diode (PLED) has drawn great attention due to its potentiality for fabrication of large-area, light-weight and flexible displays by solution process. Compare to the advantages of those conventional cathode ray tube (CRT) with heavy-weight and electricity consuming and liquid crystal display (LCD) with narrow angle of view and low reacting time, PLED owns the characteristics of light-weight, high brightness and fast reacting time. Owing to high electrical conductivity and excellent visible light transmitting property, Indium Tin Oxide (ITO) is widely used as the anode in optical devices. However, its work function (~4.7 eV) is low relative to the highest occupied molecular orbitals (HOMOs) (>5.5 eV) of fluorene-and phenylene-based emitting polymer, which results in a large hole injection barrier. Thus, further surface treatment is required to lower the barrier, such as (i) introducing thin organic interlayer like poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) as hole injection layer (HIL), (ii) utilizing O2 plasma, ultraviolet (UV)-ozone or CFx plasma treatment to adjust surface atomic composition and meanwhile remove the contamination on ITO, or (iii) inserting self-assemble monolayer (SAM) to increase the work function by interfacial dipole effect. However, the multilayer device structure and further surface treatment not only complex the fabrication, but also raise the risk of failure.
Recently, Lu and coworkers reported that the work function of ITO can be raised dramatically from 4.7 to 6.13 eV via exposing ITO to o-dichlorobenzene (ODCB) under UV irradiation at 254 nm. With the chlorination, the hole injection barrier between Cl-ITO (work function 6.13 eV) and the hole transport layer, 4,4’-N’,N’-dicarbazole-biphenyl (CBP) (HOMO 6.1 eV), was eliminated completely and the resulting green phosphorescence organic light-emitting diodes achieved the outstanding performance, 97 lm/W at 100 cd/m2.
This thesis is separated into four parts. To begin with, we study on the fabrication of Cl-ITO, alternate steps of process and discuss different factors affecting the work function and atomic composition of chlorinated-ITO (Cl-ITO). Then, Cl-ITO is introduced into the poly(9,9-dioctylfluorene) (PFO)-based deep blue and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV)-based orange PLED system. Finally, caesium fluoride and 1, 2-dibromoebnzene are used to modify ITO, enhance the work function of ITO and applied to the PFO-based device.
In Chapter. 4, Cl-ITO was fabricated in several ways and its characteristics was carefully studied. For the fabrication of Cl-ITO, the UV-ozone treated ITO was covered with ODCB exposing to UV-irradiation from a low-pressure mercury lamp (SEN Light PL16-110) and the Cl radicals are liberated from solvent, displaced oxygen on the ITO and formed Cl-In monolayer. Both the Cl-In dipole layer and atomic composition of the surface of ITO affecting the enhancement of work function of ITO, the removal of carbon contamination is the most important role, especially. The Ar-etching analysis, to understand the depth element composition profile, reveals that carbon contamination mainly distributed on the surface of ITO. Therefore, we try several ways to reduce the amount of carbon. In the ITO pre-treatment, UV-ozone (15 min) is the most effective and easily control one; in the ITO post-treatment, the longer the UV-ozone treating time, the fewer the carbon. When the treating time up to 10 min, carbon amount drops obviously and remains similar while increasing the treating time, and it also gives the largest hole injection in the device. When about to the ODCB treatment, the much the ODCB amount and the more concentrated of the ODCB solution, the larger amount of the Cl-In and the higher work function of the Cl-ITO. Briefly, the work function of Cl-ITO could be raised to high as 5.58 eV in the process of pre-UV-ozone (15 min), ODCB (5 min) and post-UV-ozone (10 min).
In the Chapter. 5, we introduce the Cl-ITO into the poly(9,9-dioctylfluorene) (PFO)-based deep blue device. Poly(9,9-dioctylfluorene) (PFO) is widely used because of its quite high photoluminescence quantum efficiency (PLQE) 59 % as thin film, good thermal and color stability and excellent film-forming quality. Also, β-PFO got by solvent treated has the capability of electron trapping and enhancement of hole mobility. However, the device suffers from low hole current due to the high hole injection barrier since it possesses much higher HOMO level (~5.8 eV). In this work, we introduce the Cl-ITO into the β-PFO-based device to solve the problem of poor hole injection and improve the device performance. We found that residual chlorine free radicals on the surface of Cl-ITO will quench the exaction on β-PFO and the quenching effect could be reduced by treating Cl-ITO with aqueous ammonia solution to neutralize radicals and remove it with different concentration and treating time. The device Cl-ITO (NH4OH 1 %, 3 min)/β-PFO/CsF/Al achieve the maximum brightness 16773 cd/m2 and maximum luminance efficiency 2.4 cd/A, which are higher than those of the device with untreated Cl-ITO by factors of about 7 and 9, respectively. Also, we also fabricate the device with cathode replacing poly[9, 9’-bis96’-((1, 4, 7, 10, 13, 16)hexaoxacyclooctadecanyl) methoxy] hexyl) fluorenen] (PCn6) chelating to K+ as the electron-injection layer (EIL) with unstable environmentally CsF as the cathode. The device gives the high brightness 28550 cd/m2 and high luminance efficiency 2.60 cd/A. The enhancement is ascribed to the reduction of the electron barrier and facilitation of electron transport provided by PCn6:K+ forming electron –transport channel. This method is not only simple in device fabrication but also owns a dramatic enhancement in the hole injection.
In Chapter. 6, caesium fluoride (CsF) solution and 1,2-dibromobenzene are used to surface modify ITO. On the one hand, after treating with CsF solution (2 mg/ml), the work function of ITO can be raised to 5.75 eV which is suitable for using as anode to lower the injection barrier and increase the hole injection. The device gives poor performance and we ascribe the result to the some particles, may be CsF salt on the ITO leading to leakage current. On the other hand, we replace ODCB with 1,2-dibromoebnzene to treat ITO and discuss the characteristic of other halogenated-ITO. The device with Br-ITO anode exhibited bad performance because of particles on the ITO. Also, the work function of Br-ITO isn’t as high as Br-ITO so that it couldn’t provide larger hole injection into the device.
In Chapter. 7, we introduce Cl-ITO into MEH-PPV-based orange PLED system. Even though there is no problem for hole injection from ITO (5.0 eV) or ITO/PEDOT:PSS (5.2 eV) to MEH-PPV (HOMO 5.1 eV), we utilize the high work function and the comparable high conductivity of Cl-ITO to enhance hole injection and on the other hand, we use CsF/Al as device cathode to increase electron injection. In this way, we aim to increase the chance of recombination, lower the turn-on voltage and enhance the performance. The device Cl-ITO/MEH-PPV/LiF/Al gives the highest brightness 12486 cd/m2, and the highest efficiency 1.23 cd/A. In addition, the device Cl-ITO/MEH-PPV/CsF/Al gives the highest brightness 12246 cd/m2, and the highest efficiency 0.68 cd/A. The lower luminance efficiency with cathode CsF/Al is ascribed to the unbalance of carrier injection due to the surplus of electron injection providing by high work function CsF (2.2 eV).

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