| Summary: | 碩士 === 國立交通大學 === 應用化學系 === 89 === 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.
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