29-01-2013, 01:59 PM
High efficiency solution-processed small molecule electrophosphorescent OLEDs
Abstract
We demonstrate very high efficiency (forward power and luminous efficiencies up
to 60 lm/W and 69 Cd/A, respectively) spin-coated small molecule
electrophosphorescent OLEDs (SMOLEDs) based on a green-emitting iridium
complex doped into a 4,4'-bis(9-carbazolyl)-biphenyl (CBP) host. Electron- and holetransporting
molecules were blended with the host to improve the transport balance of
the charge carriers. An additional electron- transporting/hole-blocking BPhen layer
was thermally evaporated on the spin-coated active layer, followed by the LiF/Al
cathode. The peak efficiency of these largely-solution-processed SMOLEDs is higher
than that of any polymer or solution-processed OLED reported to date, and almost as
high as that of the most efficient thermally evaporated (SM)OLED, when excluding
the contribution of outcoupling-enhancing structures such as microlens arrays. When
such outcoupling enhancement is included, the peak power efficiency would be 120
lm/W, essentially the highest of any OLED reported to date. The high efficiency is
attributed to the relatively high carrier mobility in CBP, the enhanced mobility due to
the additional electron- and hole-transporting dopants, and the smoothness of the
doped CBP-based films, whose RMS surface roughness is only ~0.50 nm.
Introduction
Extensive research on organic light-emitting diodes (OLEDs) continues due to their
promise in applications such as flat panel displays and solid state lighting [1-5].
Commonly, thermal high-vacuum evaporation technology is used for fabrication of
small molecule-based OLEDs (SMOLEDs) and solution processing technology is
used for those based on polymers (PLEDs). Thermal evaporation deposition enables
complicated multilayer device architectures and renders excellent devices with high
efficiencies [6,7]. In contrast, solution-based deposition limits fabrication of
composite device structures because the solvent used for one layer can redissolve or
otherwise damage the previous layers [8]. Therefore, thermally evaporated
SMOLEDs are typically more efficient and longer-lived than solution-processed
PLEDs. However, thermal evaporation deposition has its own disadvantages. First, it
requires high vacuum and is consequently much more costly. Second, making multidopant
OLEDs, such as white OLEDs (WOLEDs), requires precise control of the
doping concentration of each dopant in the emitting layer (EML) to obtain the desired
emission [9,10]. These reasons usually lead to a fabrication process of greater
complexity and higher cost. On the other hand, solution processing, such as spincoating,
inkjet printing, and screen printing, is advantageous over thermal evaporation
processing, due to its low-cost and large area manufacturability [10,11]. Additionally,
it is possible to realize co-doping of several dopants by mixing the dopants and host
material in solution. Hence, the fabrication of SMOLEDs via a solution process is of
great importance. To that end, we demonstrate high efficiency (forward power and
luminous efficiencies up to 60 lm/W and 69 Cd/A, respectively) spin-coated
electrophosphorescent SMOLEDs based on green-emitting tris[2-(p-tolyl)pyridine]
iridium(III) (Ir(mppy)3) doped into a 4,4'-bis(9-carbazolyl)-biphenyl (CBP) host,
probably due to the materials and film morphology. This is the highest reported
efficiency of any solution-processed OLED and among the highest of any OLED
without outcoupling enhancement.
Results and discussion
In spin-coated PLEDs, small molecule guests are typically blended with a polymer
host in a suitable solvent as is the case for PVK:Ir(mppy)3 PLEDs [12,13]. However,
when using this approach, phase separation may occur either after some time of
operation or immediately following fabrication due to differences between small
molecules and conjugated polymers in attributes such as viscosity and boiling point
[14]. To address this issue, many other solution-processible organic molecules were
designed and synthesized, including dendrimers, oligomers, spiro-molecules, and
binuclear metal chelates [15-18]. Recently, efficient OLEDs based on solutionprocessed
small molecules have been reported [19–26]. For example, He et al.
reported that fluorescent SMOLEDs fabricated by spin-coating blends of N, N"-bis-
(3-naphthyl)-N, N"-biphenyl-(1,1"-biphenyl)-4,4"-diamine (NPB) and tris-(8-
hydroxyquinoline)-aluminum (Alq3) as the emitting layer exhibited maximum
brightness and luminous efficiency exceeding 10,000 Cd/m2 and 3.8 Cd/A,
respectively [23]. These values are comparable to those of thermally evaporated Alq3-
based devices. Thus, the development of solution-processed SMOLEDs based on
materials used in high-efficient OLEDs fabricated via vacuum deposition is promising.