Investigation of electronic structures and interfacial chemistries of carrier injection layers in organic light-emitting devices via synchrotron radiation photoemission spectroscopy

本論文利用高解析度的同步輻射光源光電子能譜研究電子及電洞注入層對有機發光元件的影響，並討論介面間的化學反應及電荷注入機制。 首先探討三氧化鉬(MoO3)在有機發光元件中所扮演的角色與能階的結構。在本篇論文的實驗中發現，若將三氧化鉬作為電洞注入層進行加熱處理，三氧化鉬會自分解產生大量的氧空缺，進而產生能隙能階(gap state)，這些能隙能階不僅提供載子的傳輸路徑，也拉近費米能階與電洞傳輸層最高佔用分子軌域的距離，達到降低電洞注入能障的效果並提升電洞注入效率。 再來比較可做為有機發光元件電子注入層的氟化銫(CsF)分別鍍在OXD-7、BCP、金與鋁之間的能階結構變化。由於OXD-7含有苯環與oxadiazole moiety，在鍍上氟化銫後電子結構會改變，導致氟與銫的離子鍵減弱。銫離子會被oxadiazole moiety上氮的孤對電子對(lone pair)吸引形成Cs-OXD金屬錯合物，氟離子則與苯環上面的氫離子形成氫鍵。在OXD-7/氟化銫之上鍍鋁，鋁會與氟離子結合形成氟化鋁(AlF3)，表示氟離子會較銫離子更優先與鋁離子產生鍵結。由於在OXD-7上鍍上氟化銫/鋁會產生一系列的化學變化，電子雲在離子鍵結的過程中會互相重疊，增加電子的傳輸機會，代表Al-F-Cs-OXD錯合物(Al-F-Cs-OXD complex)的產生會增進電子注入效率。另外在BCP與金上鍍上氟化銫/鋁作為對照組，由於BCP與金的分子結構沒有oxadiazole moiety，所以銫離子並不會與BCP產生反應，產生的氟化鋁數量會減少很多，更不會產生能增進電子注入效率的金屬錯合物。由以上的實驗可得知，若以含有oxadiazole moiety的衍生物材料作為有機發光元件的電子傳輸層，再配合氟化銫與鋁可提升有機發光元件的電子注入效率。In this thesis, synchrotron radiation photoemission was used to investigate the interfacial electronic structure at both electron and hole injection layers in organic light-emitting devices (OLEDs). For hole injection layer, the molybdenum trioxide (MoO3) with high work function was employed. We found that upon annealing the MoO3 film the topotactic decomposition occurred to produce a large density of gap states, which was caused by increased the oxygen vacancies. These gap states not only provided the transition path for the carriers from the anode to the hole transporting layers (HTLs), but also induced a close alignment between the Fermi level and the HOMO state. In short, annealed MoO3 reduced the hole injection barrier, thereby enhancing the hole injection efficiency. For electron injection layer, cesium fluoride (CsF) as electron injection layers was inserted between electron transporting materials (OXD-7) and aluminum. Cesium was found to attach the lone pairs of nitrogen, and interacted the oxadiazole moiety of OXD-7, thereby forming the Cs-OXD metal complex. The fluorine atoms bonded with the hydrogen of benzene moiety, giving rise to a weak ionic bond between cesium and fluorine. When aluminum was deposited onto cesium fluoride, the atoms lose electrons and became oxidized. The high polarity of aluminum ions attracted fluorine from the hydrogen of benzene moiety to form AlF3. The sequential chemical interaction among Al, F and Cs-OXD metal complex leaded to higher probability of electron transition and improved the electron injection efficiency