29-01-2013, 02:00 PM
Indium-tin-oxide-free tris(8-hydroxyquinoline) Al OLEDs with 80% enhanced power efficiency
Abstract
Efficient indium tin oxide (ITO)-free small molecule organic light-emitting diodes
(SMOLEDs) with multilayered highly conductive poly(3,4-ethylenedioxy thiophene):
poly(styrenesulfonate) (PEDOTSS) as the anode are demonstrated. SMOLEDs with
the structure PEDOTSS/MoO3/N,N’-diphenyl- N,N’-bis(1-naphthylphenyl)-1,1’-
biphenyl-4,4’-diamine (NPD)/tris(8- hydroxyquinoline) Al (Alq3)/ 4,7-diphenyl-1,10-
phenanthroline (BPhen)/LiF/Al exhibited a peak power efficiency of 3.82 lm/W,
which is 81% higher than that of similar ITO-based SMOLEDs (2.11 lm/W). The
increase in the device performance is believed to be due to the advantageous higher
workfunction, lower refractive index and decreased surface roughness of PEDOTSS
as compared to ITO, as well as to Ohmic hole injection from the PEDOTSS to the
NPD layer via the MoO3 interlayer. The results demonstrate that a polymeric anode
has the potential to substitute ITO in OLEDs with strongly improved device
performance.
Introduction
Transparent electrodes are crucial for organic optoelectronics, in particular for
organic light-emitting diodes (OLEDs) and organic solar cells (OSCs) [1-2].
Traditionally, this role has been well-served by indium tin oxide (ITO). However,
ITO has key issues: (i) Its ever-increasing cost due to the short supply of indium, (ii)
its relatively inefficient deposition processes [3], (iii) its fragility and inflexibility that
is due to its ceramic nature, which limits the processing advantages of organic devices
[4], and (iv) its relatively high index of refraction (nITO ∼ 2.0), which is higher than
norg ~ 1.7 of the organic materials and ngl ~ 1.5 of the conventional glass substrate.
The high nITO contributes to the high unwanted total internal reflection at the
ITO/glass interface [5]. Thus, these issues beg for alternatives to ITO. Conducting
polymers [6-8], carbon nanotubes (CNTs) [9], grapheme [10], thin metals layers [11]
and printable metal grids [12] are being investigated for this purpose. Conducting
polymers, especially poly(3,4-ethylenedioxythiophene) (PEDOT) stabilized by
aqueous poly(styrenesulfonate) acid (PSS), were used as electrodes in organic
electronic devices since their inception [13,14].
Results and discussion
Devices with 1 to 4 PEDOTSS layers were evaluated and the device architecture
and molecular structures of the various organic materials are depicted in Fig. 4-1.
Table 1 summarizes the thickness, T550, and R□ of the PDEOTSS and ITO films