第四章 實驗設計與結果分析
4.4 實驗三：利用阻擋載子注入以達 LEC 載子平衡之探討
本實驗利用高游離電位的電洞傳輸材料，阻擋電洞注入使之達到載子 平衡的作用 [42] ，其能階圖如圖 30 Device III 所示。
◆ 材料配製
ˇ發光主動層材料：Ru(dtb-bpy)3 (PF6)2
溶液濃度：250 mg/c.c，溶劑：Acetonitrile( CH3CN) ˇ 電洞阻擋層材料：mCP
溶液濃度：10 mg/c.c，溶劑：Chlorobenzene (C6H5Cl)
◆ 元件結構 Device Ⅲ：
ITO (120 nm)/PEDOT:PSS (30 nm)/mCP (20 nm)/ 錯合物 1 號 (450 nm)/Ag (100 nm)
◆ 製程條件
1.PEDOT:PSS 以旋轉塗佈機 4000 rpm 旋轉塗佈 1 min，膜厚約 30 nm 2.PEDOT:PSS 塗佈完以 150 ℃退火 30 min
3.電洞阻擋層以旋轉塗佈機 5000 rpm 旋轉塗佈 1 min，膜厚約 20 nm 4.電洞阻擋層放置低水氧手套箱以 60 ℃退火 6 hr 以上
5.主動發光層以旋轉塗佈機 3000 rpm 旋轉塗佈 1 min，膜厚約 450 nm 6.主動發光層放置低水氧手套箱以 60 ℃退火 6 hr 以上
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4.4.1 電激發光頻譜與複合區位置推估結果分析
載子注入的效率可藉由增加傳輸層，以達到提升載子注入或阻擋載子 注入的功用，使 LEC 元件效率有效的可以被調控的文獻已被發表 [42] ， 然而，解釋元件效率的調整機制卻缺乏了直接的實驗證據 [42] 。為了闡明 LEC 的載子平衡會受調整載子注入效率影響，本實驗利用 mCP 製成 20 nm 之薄膜作為具高游離電位的電洞傳輸層，塗佈在 PEDOT:PSS 層上，接著再 塗佈發光層是為 Device III 結構，其能階示意圖如圖 30 Device III 所示，
接著，相同依前面小節介紹的程序來推估會隨時間變化的複合區位置。圖 36 (a)~(d)，說明了相同以 2.5 V 電壓驅動元件，推估複合區位置在第 12 分 鐘、第 23 分鐘、第 58 分鐘及第 175 分鐘的位置分別為離陰極 365 nm、355 nm、290 nm、273 nm 處。而會隨時間改變而不同之 Device III 複合區移動 位置方向和 Device II 一樣，因為有層薄的傳輸層阻擋電洞注入，故操作過 程中，複合區位置由接近陽極的地方往發光層中央移動。
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圖 36 Device III 之模擬(實心點)和量測(空心點)的電激發光頻譜在以 2.5 V 的電壓驅動下，第(a)12、(b)23、(c)58 及(d)175 分鐘 之情形。複合區位 置(zi)可藉由匹配模擬和量測的電激發光頻譜來推估
500 600 700 800 900
0.0
EL Intensity (a.u.)
Wavelength (nm)
500 600 700 800 900
0.0
EL Intensity (a.u.)
Wavelength (nm)
500 600 700 800 900
0.0
EL Intensity (a.u.)
Wavelength (nm)
500 600 700 800 900
0.0
EL Intensity (a.u.)
Wavelength (nm)
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4.4.2 複合區位置變化與元件效率分析
當摻雜層尚未完全建立之時，由於 PEDOT:PSS/mCP 的接面有著很高 的注入位障，使電洞注入到發光層的量有著很顯著的減少 (參考圖 30 Device III )。因此，和 Device I 相比， Device III 在初期操作時，複合區
會比較接近陽極。當摻雜過程持續進行的同時，在發光層中的陰離子會漂 移到本質 mCP 層，形成了 p 型摻雜 [43] 。隨著時間進行，當摻雜層漸漸 建立時，電洞注入能障也漸漸的降低，注入到電洞的量也明顯增加了，複 合區位置漸漸移到發光層的中央位置。最後，當摻雜層完全建立後(t 大於 200 min)，電子及電洞的注入效率將相對的維持不會改變，複合區位置也穩 定不再變化。以 2.5 V 電壓驅動 10 小時後，隨時間不同而改變的 Device III 外部量子效率如圖 37 所示。開始驅動元件的初期，會有個短暫的峰值出現 (t 小於 200 min)，並且在隨複合區的移動過程中，其元件效率也下降了，
如圖 37 所示，摻雜層的擴張，使複合區也跟著移動，這樣的結果也說明了 激子會在複合區猝熄是因為摻雜層的擴張。然而，在圖 37 的 3~20 分鐘看 到當 Device III 以固定偏壓驅動後，其外部量子效率趨勢在下降後短暫略微 回升。這種有別於前面實驗例子的特殊現象，可歸因於元件運作的初期，
複合區與 p 型摻雜層邊界的距離被改變了所造成的。與非離子型材料的 mCP
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層相比，離子型材料的發光層會有較高的離子遷移率，摻雜層形成速度也 會較快。因此，以固定偏壓驅動元件不久後，p 型摻雜層擴張到發光層中央 的速度會比複合區移離陽極的速度快，主要是由於 p 型摻雜是在 mCP 層中 形成，電洞注入效率被被增強了，複合區和 p 型摻雜層邊界的距離會縮短，
導致發生了明顯的激子猝熄，使外部量子效率在最初有 3~6 分鐘急劇下降，
如圖 37 所示。當 mCP 層中的 p 型摻雜層漸漸形成的同時，電洞注入效率 也漸漸增加，複合區移離陽極的速度也大大增加，使複合區與 p 型摻雜的 邊界之距離也增加了，因此，以固定偏壓驅動元件後，在第 6~20 分鐘可以 觀察到激子猝熄的程度也會降低，元件效率也略為恢復，接著，後面元件 效率之所以下降，則歸因於摻雜層不斷的擴張而使激子在複合區猝熄所造 成。
Device III 達穩態時，外部量子效率為 1.2%，比 Device I 的外部量子
效率 1.8 %還要低，此結果和上小節所提及的 Device II 的外部量子效率會 低於 Device I 有著相同理由，如圖 37 所示，當 Device III 的複合區穩定 不會改變時，約在離陰極 273 nm 處的位置，和 Device I 相比之下，達到穩 定的複合區位置則在離陰極 250 nm 遠處的位置，由於 Device III 的複合區 位置太接近 p 型摻雜層，因此造成了嚴重的激子猝熄，使得元件效率變低。
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External Quantum Efficiency (%)
Time (min)
External Quantum Efficiency (%)
Time (min)
圖 37 Device III 在 2.5 V 電壓驅動下，量測最初 200 分鐘其外部量子效 率及複合區位置變化比較的情形。其內插的圖為完整量測 10 小時後的外部 量子效率變化趨勢
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圖 38 (I)、(II)、(Ⅲ) 分別代表 Device I、II、Ⅲ以固定電壓驅動後之複合 區移動趨勢
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第五章 結論
本論文設計了三種實驗方法來證明三明治結構的有機發光電化學元件 之複合區可以利用微共振腔效應來進行複合區位置的推估，利用此方法可 以得知，使用離子性過渡金屬錯合物的有機發光電化學元件摻雜低能隙的 載子捕捉材料和純膜元件相比，複合區位置較為靠近陽極，導致嚴重的激 子猝熄造成元件效率下降。同樣地，在元件增加一具有高游離電位的電洞 傳輸層以阻擋電洞注入，也可使複合區位置更靠近陽極，當複合區位置接 近陽極時，會使激子猝熄，並可以觀察到元件效率也下降了。以上這些結 果都證明了有直接的實驗證據可以證明有機發光電化學元件的載子平衡可 以藉由增加載子捕捉或者載子注入的方式來調控。此外，微共振腔效應被 證明對於隨時間不同而改變的三明治結構的有機發光電化學元件有很大的 影響，但要以直接量測方式來得知複合區位置是非常困難的，而本實驗設 計的新方法將會是研究有機發光電化學元件之載子平衡的一項利器。未 來，不管調變元件光色或者提高效率，可以利用模擬電激發光頻譜對照量 測出的電激發光頻譜進行匹配，有效的調整元件載子平衡，並可避免激子 猝熄效應發生，使整體元件效能達到理想元件之特性。
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