29-01-2013, 02:06 PM
Blue fluorescent organic light-emitting diodes based on a new electron-accepting polymer
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Abstract
Fluorescent blue polymer light emitting diodes (PLEDs) with a new electronaccepting
poly[(9,9-di-n-octylfluorene-2,7-vinylene)-2,6-diyl-benzo[1,2-d;5,4-d’]
bisoxazole] (PFVcBBO) doped in poly(N-vinyl carbazole) (PVK) are described.
PLEDs with different combinations of two different molecular weights (MWS) of
PFVcBBO (3,600 and 11,100) and PVK (~7.5×104 and 1.1×106) were tested. The
performances of the different PLEDs are in accordance to the surface morphology of
the emitting layer (EML), the smoother the EML’s surface is, the better devices
become. A peak 2.11 Cd/A luminous efficiency was obtained for the combination of
low MW PFVcBBO with the low MW PVK host, which shows the best uniformity of
EML.
Introduction
The performance of organic light-emitting diodes (OLEDs), small molecule- and
polymer-based, has improved dramatically since the first thin film OLEDs were
reported [1]. Intense research activities are undertaken on polymer light emitting
diodes (PLEDs) due to their low-cost and roll-to-roll fabrication potential [2-4].
Within these efforts, blue OLEDs draw particular attention as a key component of
white OLEDs and PLEDs are particularly promising as blue emitting devices. Two
polymer classes that have attracted significant attention are substituted
poly(phenylene vinylene)s (PPVs) and poly(9,9-dialkylfluorene)s (PFOs). For
example, yellow emitting poly[(2-ethylhexyloxy-5-methoxy-1,4-phenylene)vinylene]
(MEH-PPV) is widely used via solution-processing [5]. Similarly, PFOs have been
demonstrated as effective blue emitters with good solubility and high solid-state
photoluminescence (PL) quantum efficiency ηPL ~ 72% [6]. However, like most
conjugated polymers, the electron mobility μe of PFOs is much lower (≥ 100 fold)
than the hole mobility μh [7,8]. To meet the need for balanced hole and electron
transport properties in a single polymer, which is essential to achieving efficient
single-layer OLEDs, a common strategy is to use emissive copolymers containing
both hole-transporting and electron-transporting moieties [9-13]. As an example,
PLEDs based on a copolymer, poly(fluorenevinylene-alt-diphenyloxadiazole), which
contains electron-accepting oxadiazole units and electron-donating triphenylamine
units, have been reported [11].
Results and discussion
As reported previously [14], the fluorescence yield ηPL of PFVcBBO in the solid
state is only ~0.01, however, in the THF solution ηPL is ~0.64, which makes it a
promising candidate for use in guest-host PLEDs.