Browsing by Author "So, SK"

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Effects of tertiary butyl substitution on the charge transporting properties of rubrene-based films
(2004-03-08)
Fong, HH
;
So, SK
;
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.
Experimental and theoretical demonstration on the transport properties of fused ring host materials for organic light-emitting diodes
(2006-01-01)
Tse, SC
;
So, SK
;
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.
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.