Browsing by Author "Chiang, Chih-Long"

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Achieving high-efficiency non-doped blue organic light-emitting diodes: charge-balance control of bipolar blue fluorescent materials with reduced hole-mobility
(2009)
Chi, Chih-Chin
;
Chiang, Chih-Long
;
Liu, Shun-Wei
;
Yueh, Han
;
Chen, Chin-Ti
We found an unusual way in improving electroluminescence efficiency of blue organic light-emitting diodes (OLEDs). Two electron deficient 4,5-diazafluorene- or di(2,2'-pyridyl)-containing blue fluorophores, PhSP(N2)DPV (4,5-diaza-2'-diphenylamino-7'-(2,2"-diphenylvinyl)-9,9'-spirobifluorene) and PhF(py2)DPV (N-[7-(2,2- diphenylvinyl)- 9,9'-di(2,2"-pyridyl)-2-fluorenyl]-N,N-diphenylamine), were synthesized and characterized for non-doped blue OLEDs. Whereas PhF(py2)DPV OLED performs ordinarily, PhSP(N2)DPV OLED outperforms previously known PhSPDPV (2-diphenylamino-7-diphenylvinyl-9,9'-spirobifluorene) OLED significantly: maximum external quantum efficiency of similar to 5% (4.6% at 20 mA cm(-2)) and the peak electroluminance of 60510 cd m(-2) (1810 cd m(-2) at 20 mA cm(-2)) versus 3.4% (2.9% at 20 mA cm(-2)) and 33020 cd m(-2) (910 cd m(-2) at 20 mA cm(-2)) of PhSPDPV OLED. We attribute the superior performance of PhSP(N2)DPV OLED to the good charge balancing, which is in turn due to the very low hole mobility of PhSP(N2)DPV. The experimental results reveal that the electron-deficient moiety, 4,5-diazafluorene or di(2,2'-dipyridyl), increases electron affinity but reduces the hole mobility. Electron mobility, determined by time-of-flight (TOF) method, is 5 x 10(-5) and 5 x 10(-4) cm(2) V(-1) s(-1) (at an electric field of 4.9 x 10(5) V cm(-1)) for PhSP(N2)DPV and PhF(py2)DPV, respectively. Surprisingly, they are not higher than 8 x 10(-4) cm(2) V(-1) s(-1) of nonpolar PhSPDPV. On the other hand, hole mobility is 2 x 10(-6) and 2 x 10(-4) cm(2) V(-1) s(-1) for PhSP(N2)DPV and PhF(py2)DPV, respectively, and they are both significantly lower than 6 x 10(-3) cm(2) V(-1) s(-1) of PhSPDPV. For PhSP(N2)DPV and PhF(py2)DPV bipolar blue fluorophores, we have demonstrated that electron-transporting and light-emitting functions involve different molecular halves. The design of such molecular halves greatly facilitates the optical and electronic optimizations of fluorophores for high-performance OLEDs.
Influence of molecular dipoles on the photoluminescence and electroluminescence of dipolar spirobifluorene
(2007-01-01)
Chiang, Chih-Long
;
Tseng, Shih-Min
;
Chen, Chin-Ti
;
Hsu, Chao-Ping
;
Wen, Yuh-Sheng
Here we study a series of strongly fluorescent spirobifluorene compounds, red PhSPDCV and blue PhSPDPV, together with their spiro "dimer" analogs, BisPhSPDCV and blue BisPhSPDPV. We found that the fluorescence quantum yield as well as the OLED performance of the strongly dipolar red PhSPDCV decrease significantly in the "dimeric" analog BisPhSPDCV, but not in the weakly dipolar PhSPDPV and BisPhSPDPV. On the other hand, by doping the highly dipolar PhSPDCV to an isolated environment having low polarity Alq3 as the host matrix and avoiding the emission-quenching dipole from the neighboring molecules, very high performance of saturated yellow OLEDs has been obtained as well.
Influence of molecular dipoles on the photoluminescence and electroluminescence of dipolar spirobifluorenes
(2008-01-24)
Chiang, Chih-Long
;
Tseng, Shih-Min
;
Chen, Chin-Ti
;
Hsu, Chao-Ping
;
Shu, Ching-Fong
This Full Paper investigates a series of strongly fluorescent donor-acceptor-substituted spirobifluorene compounds, red 2-diphenylamino-7-(2,2-dicyanovinyl)-9,9'-spirobifluorene (DCV), green 2-diphenylamino-9,9'-spirobifluorene7-carxoxaldehyde (CHO), and blue 2-diphenylamino-7-(2,2-diphenylvinyl)-9,9'-spirobifluorene (DPV), together with their spiro-linked "dimeric" analogs, 2DCV, 2,2'-bis(diphenylamino)-9,9'-spirobifluorene-7,7'-dicarboxaldehyde (2CHO), and 2,2'-bis(diphenylamino)-7,7'bis(2,2-diphenylvinyl)-9,9'-spirobifluorene (2DPV), respectively. The emission optical density and, hence, the intensity of photoluminescence (PL) or electroluminescence (EL) of the "dimeric" analogs is presumed to increase, which is beneficial for organic light-emitting diode (OLED) applications. The physical properties, including the dipole moments obtained from quantum chemistry calculations, emission solvatochromism, fluorescence quantum yield (Of) as well as the EL of these six spirobifluorene compounds have been examined in detail. We found that Of as well as OLED performance (EL efficiency and intensity) of the strongly dipolar DCV decrease significantly in the "dimeric" analog 2DCV, but less so in the moderately dipolar CHO and 2CHO, and only slightly in the weakly dipolar DPV and 2DPV. This is parallel to the intramolecular dipole moment, which is large for 2DCV, medium for 2CHO, and very small for 2DPV. Here, we show for the first time systematically that the luminescence intensity is closely correlated with the local electric field induced by the molecular dipole. A strong electric field may facilitate radiationless decay channels with a charge-transfer nature, leading to a high quenching rate. Consistent with this conclusion, which is derived from the red DCV/2DCV and green CHO/2CHO, our new blue fluorophore DPV with an essentially zero dipole moment has successfully achieved one of the best electrofluorescent blue OLEDs. At the same time, by doping the highly dipolar DCV into an isolated environment with the low-polarity Alq(3) as the host matrix, we obtained a very high performance of saturated yellow OLEDs as well, This is possibly due to the reduction of emission-quenching dipoles from the neighboring molecules. Our results have provided an important insight in designing luminescent materials, as follows: molecular dipole moments should be kept at a low magnitude to avoid quenching induced by a strong local electric field in the chromophore.
Novel carbazole/fluorene hybrids: Host materials for blue phosphorescent OLEDs
(2006-06-22)
Shih, Ping-I
;
Chiang, Chih-Long
;
Dixit, Ajay Kumar
;
Chen, Ching-Kun
;
Yuan, Mao-Chuan
A series of carbazole/fluorene (CBZm-Fn) hybrids were effectively synthesized through Friedel-Crafts-type substitution of the carbazole rings. These compounds were thermally and morphologically stable host materials for OLED applications. Efficient blue phosphorescent OLEDs were obtained when employing CBZ1-F2 as the host and Firpic as the guest.
Red Fluorenes as the efficient host emitter for non-doped red organic light-emitting diodes
(2005)
Chiang, Chih-Long
;
Wu, Min-Fei
;
Shu, Ching-Fong
;
Chen, Chin-Ti
;
應用化學系
Crystallinic red fluorophores based on donor-acceptor substituted spirofluorene, i.e., PhSPDCV show strong fluorescence in solution (phi(f) similar to 70%) as well as in solid state (phi(f) > 30%). Non-doped red OLEDs fabricated with PhSPDCV exhibit Authentic red (CIE, x = 0.65, y = 0.35) electroluminescence with brightness over 12,000 cd m(-2) (or > 600 cd m(-2) at 20 mA cm(-2)) and remarkable external quantum efficiency as high as 3.6%. On the other hand, the bis-substituted derivatives of spirofluorene BisPhSPDCV show relatively weak fluorescence both in solution (phi(f) < 20%) and in solid state (phi(f) < 10%). Although saturated red electroluminescence (CIE, x = 0.65, y = 0.35) is observed, non-doped red OLED containing BisPhSPDCV exhibit much worse performance compared with that of PhSPDCV OLEDs.
Synthesis and characterization of donor-acceptor-substituted fluorene fluorophores for non-doped red organic light emitting diodes
(2006-12-01)
Chiang, Chih-Long
;
Wen, Ya-Ju
;
Wen, Yuh-Sheng
;
Shu, Ching-Fong
;
Chen, Chin-Ti
A series of fluorene fluorophores bearing bis(4-methoxyphenyl)amino electron donating substitutent as well as bisbenzothiazolylvinyl (AFDB), benzothiazolylcyanovinyl (AFCB), or dicyanovinyl (AFDC) electron accepting substituent were synthesized and characterized. A non-doped red organic light-emitting diode (OLED) device based on AFCB exhibited deep red (lambda(el)(max) = 670 nm, CIE, x = 0.65, y = 0.34) electroluminescence with brightness over 2380 cd/m(2) and an external quantum efficiency higher than 1.2%.