Investigation of Techniques for Enhancement in Carrier Injection and Interfacial Electronic Structures in Organic Light Emitting Devices

本論文提出並實現三種有效增進有機發光二極體載子注入效率的方法，經由特殊處理一些常見於有機發光二極體的材料，元件電流可被有效提升。此研究同時利用X光與紫外光電子能譜分析載子注入效率增進的原因。利用直接對倒置型有機發光二極體元件進行熱退火的方式，可以催化發生於三(8-羫基喹啉)鋁 (Alq3)、氟化鋰 (LiF)以及鋁電極之間的化學反應，進而提升倒置型陰極結構的電子注入效率。根據光電子能譜顯示，倒置型三(8-羫基喹啉)鋁之陰極結構經過熱退火處理之後，其價帶電子組態與氮原子之氧化態會產生變化，而此改變來自於倒置陰極產生的化學反應。此外本論文提出兩種處理氧化鉬(MoO3)電洞注入層的方法，可以提升電洞經由銦錫氧化物(ITO)陽極注入電洞傳輸層的效率。利用氬離子電漿剝蝕已蒸鍍於基板上的氧化鉬薄膜表面，或是對其進行熱退火處理，都可以有效提升氧化鉬薄膜的電洞注入效果。在離子剝蝕與熱退火處理氧化鉬表面的同時，利用紫外光電子能譜觀察，發現在氧化鉬的能隙中產生大量的能隙能階，這些能隙能階可提供電洞連續的傳輸途徑，使電洞注入效率更好。X光電子能譜的分析則顯示氧化鉬薄膜經過離子剝蝕處理之後，薄膜中的鉬與氧原子比例會由於較多的氧原子被移除而改變，鉬原子也因此具有較低的氧化態。此外發現在熱退火氧化鉬時，類結構分解(topotactic decomposition)會使氧化鉬薄膜釋放所含之部分氧原子，並促使鉬原子產生二聚作用(dimerization)，同時提供大量的能隙能階。經實驗驗證此二法處理過之氧化鉬電洞注入層可用於搭配無法與氧化鉬直接產生化學反應之電洞傳輸材料，如4,4''-環己基二[N,N-二(4-甲基苯基)]苯胺(TAPC)以及N,N''-二苯基-N,N''-二(3-甲基苯基)-1,1''-聯苯-4,4''-二胺 (TPD)，元件採用處理過的氧化鉬薄膜做為電洞注入層可提供較佳的注入電流。Three techniques that enhance the carrier injection efficiency in organic light emitting diodes (OLEDs) are demonstrated in this dissertation. By proper treatments to common OLED materials, the injection current can be effectively improved in devices. The origins of the enhancement in device current and mechanisms regarding these treatments are investigated by ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to provide interpretations. The electron injection in inverted OLEDs is improved by activating the chemical reactions between Tris(8-hydroxyquinolato)aluminum (Alq3), lithium fluoride (LiF), and aluminum cathodes via post-process thermal annealing. By annealing the inverted OLEDs under proper temperatures a better electron injection can be achieved. UPS and XPS spectra reveal the evolution of valence band features as well as oxidation states of nitrogen in Alq3 layers, confirming chemical reactions in the inverted Alq3 tri-layers after proper thermal annealing. Two approaches that enhance the hole injection efficiency from indium tin oxide (ITO) anodes to hole transporting layers (HTLs) are provided with the incorporation of molybdenum oxide (MoO3) hole injecting layers (HILs). By either treating the surfaces of as-deposited MoO3 layers with argon ion sputtering or thermal annealing in high vacuum, efficient hole injection is provided by the modified MoO3 layers. Via UPS, formation of huge amounts of gap states is identified inside the band gap of MoO3 during ion sputtering or thermal annealing. Those gap states provide continuous transition paths for holes to hop through, resulting in superior hole injection efficiency from anodes to HTLs. XPS analysis shows the reduction of Mo atoms due to the removal of oxygen atoms after ion sputtering, providing changes in atomic concentration of the treated surfaces. During thermal annealing, topotactic decompositions of MoO3 release the oxygen inside the films and cause dimerization of Mo atoms, which generate gap states in the band gap. By treating the MoO3 layers, those hole transporting materials (HTMs) that cannot react with as-deposited MoO3 can now also achieve improved hole injection. 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and N,N''-diphenyl-N,N''-bis(3-methylphenyl)-[1,1''-biphenyl]-4,4''-diamine (TPD) HTLs are demonstrated to have better hole injection current on the sputtered and annealed MoO3 layers