抗濃度驟熄之新型有機發光二極體綠色螢光摻雜材料的設計合成與研究

Abstract

目前應用在有機電激發光二極體（Organic Light-Emitting Diodes，簡稱OLED）的商業產品上，10-(1,3- Benzothiazol-2-yl)-1,1,7,7-tetramethyl- 2,3,6,7-tetrahydro-1H,5H,11H-pyrano-〔2,3-f〕pyrido〔3,2,1-ij〕quinolin-11-one【C-545T】是最被廣泛使用的綠色摻雜物之一。其在製成元件後的光色、發光效率、操作電壓等，皆有相當好的表現，然而此摻雜物並非完美無瑕，如高濃度的驟熄效應，半生期等，仍有可以改善與創新的空間。 根據文獻的記載，C-545T分子中所含的coumarin結構，其3號、4號碳上的雙鍵，是最有可能進行〔2+2〕的固態光化學環化反應的位置，使得雙鍵經反應而破壞了C-545T的發色團，我們推測這樣的反應是導致其在高濃度時發光效率減低的可能原因之一。因此我們想在不影響C-545T發光光色的前提下，降低〔2+2〕環化反應的可能性，於是在C-545T的4號碳位置，接上具有立體阻礙的基團，以阻止分子彼此之間雙鍵的靠近，進而減低〔2+2〕環化反應的可能性。並且研究在此位置接上基團後，其本身及製成元件後性質的變化。 在一連串的有機合成與純化之後，得到接上甲基的C-545MT及接上乙基的C-545ET兩個化合物，經過與C-545T在液態螢光的比較後，發現相對量子效率皆不如C-545T，分別為C-545T的73%與64%。而在固態螢光中，我們將C-545T與C-545MT分別以不同的濃度摻雜在主發光體AlQ3中，觀察其能量轉移的現象，發現最大的發光效率由原先C-545T的摻雜濃度0.5%升高到C-545MT的2.0%，且能量轉移皆十分良好。 進一步製成元件之後，發現C-545MT的最大發光效率出現在摻雜濃度7.5%，遠超過C-545T的0.5%至1.0%，且C-545MT的發光效率在到達了12.5%時，仍呈現一個平緩微降的趨勢。而其他元件效能如亮度、操作電壓、CIE色度變化等在高濃度時依然維持一定水準，此為OLED發展上一嶄新發現。 針對以上現象，我們經由X光繞射儀的結果來做一推論與解釋。經過計算，發現C-545MT發色團的共軛平面比C-545T多扭曲了10.7度，這樣的扭曲可以解釋其量子效率不如C-545T的現象。而藉由分子間堆疊方式的圖形，可以看出C-545T分子間的堆疊方式幾乎是相互平行，而C-545MT則是呈現分子間相互垂直的方式堆疊。兩相比較之下，可以很容易的推測出C-545MT分子間要互相接近的情況比起C-545T是困難許多的，自然減低了驟熄的可能性，才使得元件最大發光效率提昇至7.5%且有平緩微降的趨勢。 雖然C-545MT的最大發光效率仍不及C-545T，但是具有這樣高的抗濃度驟熄效應的特性，是極具商業應用上的潛力，且是在摻雜物發展上前所未見的嶄新發現。

In the history of development of Organic Light-Emitting Diodes (OLED) technology, green emission is the best developed. To date, 10-(1,3- Benzothiazol-2-yl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-pyrano- [2,3-f]pyrido[3,2,1-ij]quinolin-11-one (C-545T) is one of the most widely used green dopants in commercial EL products, which has a high EL efficiency, good green color purity and low drive voltage. However, it is still not a perfect dopant from the view of commercial usage. For example, it tends to decrease rather quickly in efficiency after its’ optimum doping concentration, and the lifetime also needs to be improved. According to the literature, the C3-C4 double bond of C-545T molecule is the most likely position to undergo photo-induced [2+2] cyclization reaction, which destroys the conjugation of the chromophore of C-545T. It is assumed that this reaction dominates the quenching phenomena of C-545T at higher concentration. Hence, the synthetic strategy was to introduce a steric group at C4 position with the purpose of reducing the possibility of the photocyclization reaction and thus, the concentration quenching. In this thesis, we have synthesized and purified two new dopants, C-545MT and C-545ET. Compared with C-545T in solution PL, both the new dopants have lower quantum yield than that of C-545T, which is 73% and 64% of C-545T, respectively. And in the solid state PL, the optimal doping concentration in C-545MT is shifted to 2.0% from 0.5% of C-545T. After the device fabrication, it was found that the maximum efficiency (cd/A) of C-545MT was at doping concentration of 7.5%, and it showed a nearly flat response of EL efficiency with respect to the doping concentration even at concentration as high as 12.5%. And the EL performances such as luminance efficiency, I-V-B curve, CIE color coordinates remains at a good and smooth level from 2.5 to 12.5%. From the analyses of the X-ray crystallography of C-545T and C-545MT, it was found that the plane of conjugation between the coumarin and the benzothiazole rings are both twisted, but C-545MT has an angle of 21.7° which is 10.7° more than that in C-545T. This may give us a clue as to why C-545MT has a lower quantum yield and thermal properties than C-545T. Considering the molecular packing, the difference appears to derive from the fact that in C-545MT the packing pattern is perpendicular to the adjacent molecule and in C-545T every molecule is almost parallel to each other. Thus, the pattern of molecular packing may be important for C-545MT to have such a highly robust characteristic which is of interest in dopant system of OLED.