White organic light-emitting diodes based on spectral broadening electroluminescence due to formation of interfacial exciplexes

Article abstract of DOI:10.1063/1.1884255

We demonstrate white organic light-emitting diodes (OLEDs) having spectral width of approximately 260 nm in electroluminescence (EL) in a simple bilayer structure, consisting of TPD and zinc benzothiazole, without taking recourse to complex strategies such as blending and doping. The EL is broader than the corresponding photoluminescence (PL) of both component materials and their structures. A deconvolution of PL and EL spectra shows that as large as 60% of the broad EL emission originates from multiple exciplexes formed at the interface during electrical excitation.

Full text of DOI:10.1063/1.1884255

White organic light-emitting diodes based on spectral broadening in
electroluminescence due to formation of interfacial exciplexes
Samarendra P. Singh, Y. N. Mohapatra, M. Qureshi, and S. Sundar Manoharan
Citation: Applied Physics Letters 86, 113505 (2005); doi: 10.1063/1.1884255
View online: http://dx.doi.org/10.1063/1.1884255
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/86/11?ver=pdfcov
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APPLIED PHYSICS LETTERS 86, 113505 ͑2005͒
White organic light-emitting diodes based on spectral broadening
in electroluminescence due to formation of interfacial exciplexes
a͒
Samarendra P. Singh and Y. N. Mohapatra
Samtel Centre for Display Technologies, IIT Kanpur, Kanpur 208016 India and Department of Physics,
Indian Institute of Technology Kanpur 208016 India
M. Qureshi and S. Sundar Manoharan
Samtel Centre for Display Technologies, IIT Kanpur, Kanpur 208016 India and Department of Chemistry,
Indian Institute of Technology Kanpur 2081016 India
͑
Received 2 September 2004; accepted 24 January 2005; published online 9 March 2005͒
We demonstrate white organic light-emitting diodes ͑OLEDs͒ having spectral width of
approximately 260 nm in electroluminescence ͑EL͒ in a simple bilayer structure, consisting of TPD
and zinc benzothiazole, without taking recourse to complex strategies such as blending and doping.
The EL is broader than the corresponding photoluminescence ͑PL͒ of both component materials and
their structures. A deconvolution of PL and EL spectra shows that as large as 60% of the broad EL
emission originates from multiple exciplexes formed at the interface during electrical excitation.
©
2005 American Institute of Physics. ͓DOI: 10.1063/1.1884255͔
Organic light emitting diodes ͑OLEDs͒ have attracted
researchers due to its promise of a viable display technology.
Since the demonstration of an efficient heterostructure
layer Zn͑BZT͒ also acting as electron transport layer ͑ETL͒.
2
A typical device has the structure: ITO/TPD͑50 nm͒/
Zn͑BZT͒ ͑80 nm͒/Al͑100 nm͒. Zinc benzothiazole was pre-
2
1
OLED, many material systems and device structures have
pared from 2- hydroxyphenyl ͑benzothiazole͒ and zinc ac-
been employed for their use in full color display applica-
tions. Recently, white light emitting diodes are steadily gain-
ing importance due to their potential application in full color
displays with the help of color filters, as backlight in LCDs,
etate at 50 °C. Zn ͑BZT͒ was chosen since it’s PL has a
2
peak at 497 nm with a spectral width, full width at half
maxima ͑FWHM͒, of 88 nm. However, much of the proper-
ties of this material relevant to device development have not
been studied in detail and is currently attracting attention as
2
and eventually as lighting sources. Though full color small
1
0
sized OLED displays have already been introduced in the
market, and large size displays are under active development,
white light emitting OLEDs are relatively in the early stage
of development. The production of white OLED is difficult
due to nonavailability of a single active material which emits
light covering whole visible range. A number of strategies
have been reported to make OLEDs emit white light. These
include multilayer OLED structure consisting of two or more
a possible electron transport layer as well. Indium tin oxide
ITO͒ coated glass, as the substrate for OLEDs, was sub-
͑
jected to a routine cleaning procedure prior to loading in a
vacuum chamber for sequential thermal evaporation of the
organic layers, and the top A1 electrode. Thin films of TPD
and Zn͑BZT͒ were deposited at the rate of 1–3 Å/s. In order
2
to improve diode characteristics, in some of the devices, a
thin layer of PEDOT:PSS ͑Bayer͒ was deposited on ITO
prior to vacuum deposition of organic layers.
3
,4
active layers, doping of appropriate amount of dopants in
5
the same host, superposition of differently colored LEDs
The current–voltage–luminance characteristic of a typi-
6
6,7
and smart blends, use of exciplex formation, etc. How-
ever, many problems associated with these approaches can
be overcome if a single emitting material in a simple struc-
ture can give rise to broad emission with desirable color
cal ITO/PEDOT/TPD/Zn ͑BZT͒ /Al device is shown in Fig.
2
1
and the structure is shown in the inset. Good diode char-
8
,9
coordinates. Metal-chelate complexes are good candidates
for such applications and have been reported to yield broad
emission. However, a coherent understanding of their PL and
EL emission, which can enable their full exploitation, is yet
to emerge.
In this letter, we report a white OLED based on zinc
benzothiazole, a metal-chelate complex, giving rise to unusu-
ally large width in electroluminescence. We demonstrate that
the broadened EL spectra owes its origin to exciplexes
formed at the interface of TPD, i.e., N,NЈ–diphenyl-N,NЈ-bis
͑
3-methylphenyl͒-͓1-1Ј bipheny1͔-4-4Ј diamine, and the ac-
tive emissive layer zinc bis-2-͑hydroxyphenyl͒ benzothiazole
Zn͑BZT͒2͔.
A two layer device structure has been used in the study,
͓
with TPD as a hole transport layer ͑HTL͒, and the emissive
FIG. 1. J–V–L characteristics of ITO/PEDOT/TPD/Zn ͑BZT͒ /Al device. A
2
a͒Electronic mail: ynm@iitk.ac.in
typical device structure has been shown in the inset.
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9.142.116.185 On: Wed, 23 Apr 2014 12:10:31
0
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113505-2
Singh et al.
Appl. Phys. Lett. 86, 113505 ͑2005͒
FIG. 2. Normalized PL spectrum of TPD, Zn ͑BZT͒ , TPD+Zn͑BZT͒ thin
2
2
film and EL spectrum of device ITO/TPD/Zn ͑BZT͒ /Al. Note that EL is
2
dominated by long wavelength components absent in PL of materials and
structures.
acteristics were obtained, specially for devices having
PEDOT layer, with typical turn on voltage at ϳ9.1 V. J–V–L
characteristics showed ͑as, in Fig. 1͒ that the luminance
closely follows current–voltage characteristics. Strategies to
improve brightness and efficiency are being worked out
separately along the lines of optimization of conventional
full color OLEDs. However, in this letter, we limit ourselves
to spectral broadening, the most important criteria for a ma-
terial system for white OLED development and device opti-
mization.
Figure 2 shows normalized PL of component materials,
and when assembled as a device structure, and compares it
with normalized EL spectrum of a typical device. Note that
the FWHM of the EL spectra is ϳ260 nm covering major
FIG. 3. ͑a͒ Normalized EL spectrum of device ITO/TPD/Zn ͑BZT͒ /Al ͑at
2
1
.2 mA current͒ shown along with fitting and deconvoluted components; ͑b͒
normalized PL spectrum of ITO/TPD/Zn ͑BZT͒2 thin film structure. The
peaks shown by arrow marks are also observed in EL spectra.
part of the visible range from 447 to 605 nm. This is about
9
1
00 nm wider than the reported comparable devices. On
tion of emitted light to originate from the interface since
most contribution comes from excitons created in the bulk of
each layer. To test this idea, we prepared several multilayered
structures with up to 6 alternating layers ͑ϳ10 nm each͒ in
order to see possible PL broadening due to increase in inter-
faces of the two materials. However, no noticeable broaden-
ing or redshift as compared to bilayer was observed. This
further indicates that the observed spectral broadening occurs
specifically under electrical drive in which oppositely
charged polarons have the maximum chance of meeting each
other at the interface forming charge transfer complexes be-
tween acceptors and donors. The phenomena of broadening
of spectra due to exciplex formation have been observed in
chromaticity diagram the emission shows a Commission In-
ternationale de Eclairage ͑CIE͒ coordinate at ͑0.38 and 0.40͒
and does not show any significant variation with applied
bias.
The most striking feature of our results is the difference
between electroluminescence and photoluminescence spectra
of the device. EL spectrum is highly redshifted with respect
to PL of either material constituents and the combined struc-
ture of TPD and Zn ͑BZT͒ thin films. EL spectrum is broad
2
with the appearance of completely new components absent
in PL spectra in the long wavelength range. In an earlier
9
work on zinc benzothiazole based OLED no difference in
PL and EL was observed, and a modest broadening of about
4
5
150 nm was reported. We attribute the redshifted components
the past in different organic and polymer material systems.
Variants of exciplex-like excited species, variously termed as
in our case to exciplex formation at the interface of TPD and
11
7
Zn͑BZT͒ . In PL one expects to observe only a small frac-
electromers, electroplexes, and their probable formation
2
TABLE I. Spectral components from deconvolution of PL of material and structures, and EL of device.
PL ZN͑BZT͒ film
PL of device
Peak position
EL of device
2
Sr.
Peak position
FWHM
Peak position
FWHM
No.
͑nm͒
͑nm͒
͑nm͒
FWHM͑nm͒
% area
͑nm͒
͑nm͒
% area
Remark
1
2
3
4
5
6
483.83
513.33
560.04
43.19
75.12
399.0
15.46
32.31
7.28
17.77
6.11
447.45
462.26
490.21
551.17
617.65
702.63
20.26
32.18
2.33
3.56
ObservedinPL
420.25
446.30
489.39
544.10
114.56
28.95
65.86
11.10
28.78
26.97
27.25
ObservedinPL
ObservedinPL
EXCIPLEX
EXCIPLEX
102.55
132.70
47.75
20.52
98.67
119.26
186.84
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113505-3
Singh et al.
Appl. Phys. Lett. 86, 113505 ͑2005͒
1
2
under electrical drive, have been also invoked to explain
redshift of spectra in other material systems.
ing the desirable characteristic of broad spectral width.
The authors sincerely thank Girija Samal and Deepak
Panwar for their help in sample preparation and experiments,
and Department of Science and Technology, India, for finan-
cial support.
Figure 3 shows best fitting obtained to PL and EL spec-
tra using Gaussian peaks and unbiased fitting parameters.
The key results of fitting are also summarized in Table I. A
comparison shows several new peaks in EL in addition to
those observed in PL. Specifically noteworthy are the two
broad peaks towards the long wavelength side of the EL
spectrum accounting for about 60% of the area under the
curve. Clearly, the additional peaks are due to formation of
exciplexes or charge transfer complexes across the interface
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1
0
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6
exists as dimers in film form. It appears plausible that
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of exciplexes with different mean energies. The broadening
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In summary, we have demonstrated a white OLED emit-
ting with a spectral width of 260 nm due to formation of
multiple exciplexes at the interface of TPD and zinc ben-
zothiazole under electrical excitation. Our results suggest
strategies for enhancing efficiency of the device without los-
11
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