Browsing by Author "Lo, CF"

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Effects of tertiary butyl substitution on the charge transporting properties of rubrene-based films
(2004-03-08)
Fong, HH
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So, SK
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Sham, WY
;
Lo, CF
;
Wu, YS
The charge transporting properties of rubrene (5,6,11,12-tetraphenylnaphthacene or RB), and a new rubrene-based complex, tetra(t-butyl)-rubrene [2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene or TBRB], were examined in the form of amorphous films as functions of electric field and temperature by means of time-of-flight technique. At room temperature, the hole mobility mu for RB is 7-9 x 10(-3) cm(2) V-1 s(-1) whereas mu for the more bulky TBRB is about 2 x 10(-3) cm(2) V-1 s(-1). The microscopic conduction mechanism in both materials can be modeled by the Gaussian disorder model in which hopping conduction occurs through a manifold of sites with energetic and positional disorder. The energetic disorder in RB and TBRB is almost identical and is about 78 meV in each case, and is mainly controlled by van der Waals interaction. The t-butyl groups in TBRB induce large fluctuations in the spatial separation among TBRB molecules and result in an increase in the positional disorder, and hence a reduction in the hole mobility. (C) 2003 Elsevier B.V. All rights reserved.
Evidence for the assembly of carboxyphenylethynyl zinc porphyrins on nanocrystalline TiO2 surfaces
(2006)
Lo, CF
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Luo, LY
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Diau, EWG
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Chang, IJ
;
Lin, CY
UV-visible absorption and AFM studies suggest that carboxyphenylethynyl zinc porphyrins aggregate on nanocrystalline TiO2 surfaces in an H-type fashion.
Experimental and theoretical demonstration on the transport properties of fused ring host materials for organic light-emitting diodes
(2006-01-01)
Tse, SC
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So, SK
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Yeung, MY
;
Lo, CF
;
Wen, SW
The charge transport properties of three tertiary-butyl (t-Bu) substituted anthracene derivatives (ADN), critical blue host materials for organic light-emitting diodes (OLEDs), have been investigated experimentally and computationally. From time-of-flight (TOF) measurements, all ADN compounds exhibit ambipolar characters. The hole and electron mobilities are in the range (1-5) x 10(-7) cm(2) V-1 s(-1) Under ail external applied field of about 1 MV cm(-1). Un-substituted ADN has the highest carrier mobilities while heavily t-Bu substituted ADN has the least. The electron and hole conducting properties of are consistent with ab initio calculation, which indicates that the frontier orbitals are localized mainly on the anthracene moiety. t-Bu substitutions in ADN increase the hopping path lengths among the molecules and hence reduce the electron and hole mobilities. The results demonstrate that t-Bu substitution is an effective means of engineering the conductivity of organic charge transporter for OLED applications.
Femtosecond fluorescence dynamics of porphyrin in solution and solid films: The effects of aggregation and interfacial electron transfer between porphyrin and TiO(2)
(2006-01-12)
Luo, LY
;
Lo, CF
;
Lin, CY
;
Chang, IJ
;
Diau, EWG
The excited-state relaxation dynamics of a synthetic porphyrin, ZnCAPEBPP, in solution, coated on a glass substrate as solid films, mixed with PMMA and coated on a glass substrate as solid films, and sensitized on nanocrystalline TiO(2) films were investigated by using femtosecond fluorescence up-conversion spectroscopy with excitation in the Soret band, S(2). We found that the S(2) -> S(1) electronic relaxation of ZnCAPEBPP in solution and on PMMA films occurs in 910 and 690 fs, respectively, but it becomes extremely rapid, < 100 fs, in solid films and TiO(2) films due to formation of porphyrin aggregates. When probed in the S(1) state of porphyrin, the fluorescence transients of the solid films show a biphasic kinetic feature with the rapid and slow components decaying in 1.9 - 2.4 and 19 - 26 ps, respectively. The transients in ZnCAPEBPP/TiO(2) films also feature two relaxation processes but they occur on different time scales, 100 - 300 fs and 0.8 - 4.1 ps, and contain a small offset. According to the variation of relaxation period as a function of molecular density on a TiO(2) surface, we assigned the femtosecond component of the TiO(2) films as due to indirect interfacial electron transfer through a phenylethynyl bridge attached to one of four meso positions of the porphyrin ring, and the picosecond component arising from intermolecular energy transfer among porphyrins. The observed variation of aggregate-induced relaxation periods between solid and TiO(2) films is due mainly to aggregation of two types: J-type aggregation is dominant in the former case whereas H-type aggregation prevails in the latter case.
The role of charge-transfer integral in determining and engineering the carrier mobilities of 9,10-di(2-naphthyl)anthracene compounds
(2006-05-10)
Tse, SC
;
So, SK
;
Yeung, MY
;
Lo, CF
;
Wen, SW
The charge transporting properties of t-butylated 9,10-di(2-naphthyl)anthracene (ADN) compounds have been investigated experimentally and computationally in relation to their molecular structures. The ADN compounds are found to be ambipolar with both electron and hole mobilities in the range of 1-4 x 10(-7) cm(2) V-1 s(-1) (electric field 0.5-0.8 MV/cm). As the degree of t-butylation increases, the carrier mobility decreases progressively. The mobility reduction was examined by Marcus theory of reorganization energies. All ADN compounds possess similar reorganization energies of similar to 0.3 eV. The reduction of carrier mobilities with increasing t-butylation can be attributed to a decrease in the charge-transfer integral or the wavefunction overlap. (c) 2006 Elsevier B.V. All rights reserved.