熱交聯電洞傳遞層材料在高分子發光二極體上的應用

Abstract

摘要 我們合成七個以乙烯為熱交聯官能基的三芳香基胺電洞傳導層材料，這些材料不需要任何的起始劑只需加熱即可進行交聯反應，交聯後可以產生相當平整且抗溶劑侵蝕性好的材料，利於旋轉塗佈的製程(因為不會被上層的發光層溶劑侵蝕)。同時，我們在三芳香基胺的對位處加入推拉電子基，來微調材料的HOMO能階。當元件1結構為(ITO/HTLs/PFO/CsF/Al)，在效率上有明顯的提升。因為電洞傳導層材料，幫助形成階梯狀能梯，可以使電洞較容易注入和傳導，同時也有很好的電子阻擋的能力。最重要的是，不用在高熱的環境就可以交聯，所以可以加入PEDOT:PSS當電洞注入層材料，由於PEDOT:PSS有高導電度的功能，可以有效降低元件起始電壓和增加效率。在元件1和2的電激發光圖中，我們發現除了聚芴高分子藍光的放射峰之外，在長波長處多一個放射峰，這可能是fluorenone和我們的電洞傳導層材料之間產生exciplex所放射出來的紅光。在元件3和4使用黃光的發光層材料SY2，由於SY2是屬於電子傳導的材料，在加入我們的電洞傳導層材料後，由於電洞傳導速度比較慢，造成電子和電洞的傳導不平衡，所以元件的效率下降，然而，亮度卻上升。我們預見未來可以利用苯乙烯當作交聯官能基，有效的運用在不同特性的電洞傳導層上，可以有效地提升元件的效率。

Seven tri-arylamine-based hole-transporting materials (HTMs) bearing thermally cross-linkable vinyl groups have been synthesized and characterized. These HTMs could be in situ cross-linked under mild thermal polymerization without any initiator. After cross-linking, the resultant HTMs form robust, smooth, and solvent-resistant networks, which enables the subsequent spin-coating of emissive layer (EML). The HOMO energy levels of the HTMs can be fine-tuned by introducing electron-withdrawing groups or electron-donating groups on the phenyl ring para to the nitrogen. The polymer LED devices with configuration of (ITO/HTLs/PFO/CsF/Al) were fabricated and characterized. The device luminous efficiency is improved because the HTLs can facilitate cascade hole injection and transport, and function as an efficient electron blocker. Most importantly, the milder cross-linking condition for these HTMs allows the commonly used conducting polymer, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS), to be incorporated as the bottom hole-injecting layer to reduce the turn-on voltage and improve the luminous efficiency of the devices. In the electroluminescence spectra, in addition to the blue emission from the polyfluorene, we found that these devices showed an emission band at the longer wavelengths, which is proved to be the exciplex emission between the fluorenone spcies and HTMs. On the other hand, the devices with the configuration of (ITO/HTLs/S2Y/TPBI/LiF/Al) using PPV-type yellow emitting polymer SY2 were also fabricated and evaluated. However, due to the electron-dominating properties of SY2, we found that introduction of the HTLs will slow down the hole transport in the devices, thus resulting in unbalanced charge carriers and reduced luminous efficiency. However, the brightness of the devices is generally improved. We envision that the strategy of using styrene as an efficient cross-linker can also be integrated with various HTMs with different properties for realizing high power efficiency white PLED in the future.