在高分子發光二極體中使用主客體□雜系統為發光層，具有材料取得容易與良好的亮度與效率等優點，已被許多文獻所報導。然而當高分子主體與小分子客體形成薄膜後，因主客體間的化學相似性不佳，會產生相當程度上的相分離，影響元件的穩定度。事實上在眾多的文獻當中，並沒有任何文獻對這些以高分子為摻雜主體的發光元件之穩定度有所報導。本論文使用常見的磷光□雜系統，即以非共軛高分子poly(N-vinylcarbazole) (PVK)與銥金屬錯合物fac-tris(2-
Phenypyridine) iridium (Ir(ppy)3)分別做為主體與客體材料，再加入電子傳遞小分子2-(4-tert-Butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD)，來研究相分離與元件穩定度(元件效能)之間的關係。
由光學顯微鏡發現PVK與Ir(ppy)3由溶液旋轉塗佈法形成高分子膜後有相分離產生，Ir(ppy)¬¬3的聚集將導致光激發與電激發下，薄膜的放光不均。在光激發光譜(PL)部份，我們收集PVK/Ir(ppy)3或PVK/PBD於成膜後不同時間下的PL光譜。加入客體Ir(ppy)3，主體(excimer 410 nm)相對於客體的放光強度會隨時間而增加；加入小分子傳輸材料PBD主體放光波長420 nm (PVK-PBD exciplex)會藍移到410 nm(PVK excimer)，並且有新的波峰出現在375及395 nm。這些光譜變化再次證明了添加這些小分子材料於PVK主體中會產生相分離。
元件長時間操作下將造成主體與客體間的相分離，造成元件的不穩定。以PVK-PBD□雜不同□雜濃度Ir(ppy)3(重量比率= 100:40:1、3.5與8 %)為發光層，使用不同陰極結構(Ca/Al, LiF/Ca/Al, and CsF/Al) 下，進行操作壽命測試，顯示所有的元件亮度皆迅速衰退 (起始亮度為1000 cd/m2)由3分鐘到33分鐘；與文獻上小分子主體CBP□雜Ir(ppy)3之元件比較，操作壽命短上數百倍以上。這是由於在施加電場下，客體會快速聚集形成針狀結晶，證據為在PVK-PBD:Ir(ppy)3元件(重量比率= 100:40:8)於定電壓(7V)下操作6天，於光學顯微鏡下觀察到大量針狀結晶。Ir(ppy)3在電場操作下會快速聚集的理由是元件操作過程中電流通過產生的熱，可以促進客體的擴散並使主體PVK側鏈上的carbazole基團局部移動，在薄膜中形成客體可以聚集的通道。客體的聚集使得客體與高分子主體的接觸區域大幅減小，因而大幅降低元件的發光效率與操作壽命。所以雖然PVK-PBD:Ir(ppy)3元件具有非常高的效能(25.9 cd/A and 7400 cd/m2)，但操作時間太短以至於不能應用在高分子發光二極體的顯示器上。這種缺陷來自於PVK與Ir(ppy)3(甚至PBD) 之間的化學相似性不佳與元件操作時的熱造成的Ir(ppy)3聚集，形成的相分離。

In polymer light-emitting diode (PLED), host-guest doping systems as the emitting layers have been extensively investigated in literature because they exhibit many advantages such as high device luminance and efficiency. But, it is easy to observe the occurrence of phase separation after the film was formed on a substrate, and this is attributed to the poor chemical compatibility between host and guest materials. However, there is no report focused on the effect of phase separation on the stability of this type of PLED to date. Therefore, the aim of this thesis is to study the relationship between phase separation and device stability (as well as performance) by using a well-known phosphorescent doping system which consists of poly(N-vinylcarbazole) (PVK, used as host), fac-tris(2-phenypyridine) iridium (Ir(ppy)3, used as guest), and 2-(4-tert-Butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD, used as electron-transporting material).
The phase separation of PVK/Ir(ppy)3 is revealed by optical images of a doping film formed by spin-coating from its polymer solution. In these optical images, the aggregation of Ir(ppy)3 is apparent and results in the non-uniform emission as observed in photoluminescence (PL) and electroluminescence (EL) measurements. In addition, the PL spectra of polymer films of PVK doped with Ir(ppy)3 or PBD are collected at different time periods after polymer films are formed from its polymer solutions. For the case of PVK/Ir(ppy)3, the intensity of PVK emission at 410 nm (excimer emission) increases with time; for PVK/PBD system, the host emission is blue-shifted from 420 nm (exciplex of PVK and PBD) to 410 nm and two additional emission peaks at 375 and 395 nm are generated. These observations again demonstrate the occurrence of phase separation between PVK and Ir(ppy)3 (even to PBD).
The phase separation between host and small-molecule materials implies the happening of device instability while operating with a longer time period. The results of life-time tests of devices based on PVK-PBD:Ir(ppy)3 with various doping ratios (weight ratio of PVK:PBD:Ir(ppy)3 = 100:40:1、3.5 and 8%) and cathode structures (Ca/Al, LiF/Ca/Al, and CsF/Al) show the rapid decays of device luminescence (the initial luminance was set at about 1000 cd/m2 for life-time tests) and life-times are calculated to range from 3 to 33 min. As compared to CBP/Ir(ppy)3-based emitting layer used in organic light-emitting diodes (OLEDs), the observed life-times of PVK-PBD:Ir(ppy)3-based devices are shorter than that of CBP:Ir(ppy)3-based device by a factor larger than one hundred. This is resulted from that Ir(ppy)3 fast aggregates and forms needle crystals upon electric field operation as observed by optic images taken from the device (weight ratio of PVK:PBD:Ir(ppy)3 = 100:40:8) operated under constant voltage at 7 V for 6 days. The aggregation of Ir(ppy)¬3 is attributed to that the heat generated by the pass of electric current in the device not only promotes the diffusion of Ir(ppy)3 but also makes the carbazole moieties of PVK locally move, forming channels which the guest can be gathered. As a result, the aggregates reduce the contact area between PVK and Ir(ppy)3 and, therefore, lower the device efficiency and life-time.
Consequently, although device based on PVK-PBD:Ir(ppy)3 can exhibit high performance (25.9 cd/A and 7400 cd/m2), the life-time of the device is too short to be used in the application of PLED display. The reason for the weakness is the phase separation of PVK and Ir(ppy)3 (even to PBD) due to their bad chemical compatibility, and the aggregation of Ir(ppy)3 will be facilitated by the heat generated by the passing of current in device operation.

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