FILED OF INVENTION
This invention relates to an improved organic light emitting diode, an
improved organic light emitting diode for tuning the white emission and a
process for fabrication thereof.
PRIOR ART
For general lighting purpose, white light emitting devices are required
that can be used to replace bulbs and fluorescent lamps. In addition,
white light sources are required to provide the backlighting for the LCD
panels. The most studied method is to use phosphor with blue or UV
GaN LEDs. The blue or UV light is absorbed and white light is emitted by
the phosphors. Besides white light can also be generated by combination
of two or three different LEDs. Presently the interest is in developing
solid state light sources with high luminous efficacy (1m/Watt), high
colour rendering index (CRI)- greater than 90 and Commission
Internationale de l*Eclairage (CIE) coordinates of (0.33, 0.33), low cost,
flexible and large area devices. Most commonly used material for the
development of white light emitting devices are inorganic materials such
as crystal Nitride semiconductors, SiC and some oxides that emit in blue
or UV region. But making large area opto-electronics devices with these
materials is not easy. This limitation can be overcome by using organic
light emitting diodes (OLED's) using polymeric as well as small
molecules. They offer various advantages over inorganic LED's easy
processability, large area devices, flexible if made on plastic substrate,
high efficiency, large viewing angle and high brightness. This has also
resulted in an early commercial application of the of the OLED
technology.
The embodiment that can be employed to obtain white light from OLED
are: (a) host-guest system using colour conversion dyes (phosphorescent
or fluorescent) and high energy emitting host material, (b) multilayer
structures, (c) exciplexes, excimers and electroplexes, (d) downconversion
phosphor with blue/UV/NUV OLED, (e) self white light
emitting materials and (f) microcavity structures. In host-guest system,
the host is a wide band gap material compared to the guest molecule and
energy is transferred to the guest. The emission can be from the guest
molecules or the combination of host and guest molecules. Exciplex,
excimers and electroplexes are the charge transfer complexes (consisting
of donor D+ and acceptor A-) that are formed between two excited species
at the interfaces. These charge transfer complexes usually emit a red
shifted broad EL spectrum. In multilayer structures, the different layers
produce different colours and these combine to give desired white colour.
Energy down-conversion using phosphors is similar to the inorganic solid
state lighting devices where blue of UV light is absorbed and white light
is emitted by the phosphors. Problem is that there are only few organic
materials that emit in blue region and it is impossible to get NUV or UV
emission from carbon based polymers due to band gap limitations.
Therefore, new materials need to be invented to produce UV and NUV
OLEDs. Advantage of the UV emission over blue is that existing
fluorescent tube phosphors, that absorbs in UV, can be used. While new
set of phosphors are required to absorb blue and convert it into white
light.
Patents on WOLED
> US 2004/0061107 Al, April 1, 2004; Duggal et al.
> WO 2005/020283 A2, March 3, 2005; EastMan Kodak
Company, Hatwar et al.
> US 2005/0073245 Al, april 7, 2005, X. Gong et al.
With the above goal in mind focus has been made on polysilanes that are
a-conjugated linear Si chain backbone polymers with organic side
groups. These materials exhibit 1-D p-type semiconducting properties.
Polysilanes absorb and emit light in UV or NUV region with small
spectral bandwidth (~20 nm). They exhibit nonlinear optical
characteristics, thermochromism, photoconductivity, high hole mobility
without any dopants (10~3 cm2/Vs) arising from delocalization of o
electrons along the Si backbone. These properties resulted in the use of
polysilanes as hole transport materials in organic multilayer LEDs. They
are also potential candidates as emissive materials in NUV or UV
electroluminescent devices. Recently attempts have been made to utilize
dialkyl, monoalkylaryl and diaryl polysilanes as active materials in LEDs.
However, these reports have shown UV EL from various polysilanes at
low temperatures. Only exception to this is poly[bis(p-butylphenylsilane)]
(PBPS) which has been used by Suzuki el al. to demonstrate an efficient
NUV emission at 407 nm at room temperature.
OBJECTS OF THE INVENTION
The primary object of the present invention is to propose an improved
organic light emitting diode, an improved organic light emitting diode for
tuning the white emission and a process for fabrication thereof.
Another object of the present invention is to propose an improved organic
light emitting diode, an improved organic light emitting diode for tuning
the white emission and a process for fabrication thereof to overcome the
disadvantages of the prior art.
STATEMENT OF THE INVENTION
According to this invention there is provided an improved organic light
emitting diode comprises of a cathode, an improved polysilane layer and
an anode having white emission and/or UV emission at temperature
(about 300K).
Further according to this invention there is provided an improved organic
light emitting diode for tuning the white emission comprises of a cathode,
an improved polysilane layer, an anode and UV phosphors/dye dopants,
wherein the UV emission from the same device is used to tune the white
emission.
Still further according to this invention there is provided a process for
fabrication of an improved light emitting diode comprises steps of: spin
coating of PEDOT:PSS layer over an anode followed by spin coating of an
emitting layer of polysilane from solution in an organic solvent and
evaporation of a cathode on the emitting layer.
Yet further according to this invention there is provided a process for
fabrication of an improved organic light emitting diode for tuning the
white emission comprises steps of spin coating of PEDOT:PSS layer over
an anode followed by spin coating of an emitting layer of polysilane from
solution in an organic solvent and evaporation of a cathode on the
emitting layer wherein phosphor particles are mixed in the polysilane
solution which is spin coated or an extra layer of the phosphor is
deposited on the anode by spin coating or physical deposition.
BRIEF DESCRIPTION OF THE INVENTION WITH REFENRECE TO
ACCOMPANYING DRAWINGS
Further objects and advantages of this invention will be more apparent
from the ensuing description when read in conjunction with the
accompanying drawings and wherein:
Fig. 1. shows the general chemical structures of polysilane
polymers of the present invention.
Fig. 1 (b). PS-4 (Mw 2.0 xlO4) as emitting material in EL device.
Fig. 2 absorption and PL emission spectra of poly(nbutylphenysilane)
(PS-4).
Fig. 3. three different embodiment to generate white light from
polysilane devices.
Fig. 4. EL spectra of the PS-4 device before using phosphor/dyeembodiment
1.
Fig. 5. current-Light-Voltage (I-L-V) characteristic of the
Glass/TO/PEDOT: PSS/PS-4/Ca/Al device (embodiment 1).
Fig.6. Tuning of white emission from polysilane using UV
phosphors/dye dopants EL device-
The CIE chromaticity diagram of PS-4 device and shifting of CIE to pure
white coordinate (0.33, 0.33)
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO
ACCOMPANYING DRAWINGS.
For the first time, a device has been made which emits at deeper UV at
room temperature. In addition, self white emission has been also
reported in the same device made of this new polysilane. This is obtained
without using a phosphor layer.
In this invention OLEDs based on polysilanes D with pheny ring on one
side and the longer chains of alkyl groups on the other side (length
greater than 4) that emits in the deep UV -357 nm (3.48 eV) along with
white light (CIE, x=0.30, y=0.32) at room temperature. The colour
emission of these polysilane EL devices is very close to the white light
CIE coordinates, ideally (0.33, 0.33). Thus with the availability of the UV
emission from the said device, the colour of white light emission can be
tuned by down converting UV light using phosphors or colour covnertin
dyes. In short, this a novel device which emits white light and at the
same time affords ability to colour tune that light, all at room
temperature or higher. Thus no cooling would be required as compared
to polysilane devices reported earlier.
The invention is described hereinunder taking different exemplary
embodiments without restricting the scope of the invention to the same.
Reference is made to figures, Fig l(a) shows the general structure of the
polysilane used in this invention.
Figure 1 (b) shows the chemical structure of the poly(nbutylphenyesilane)
(PS-4) polymer. The details of the synthesis are
reported earlier. The weight-average molecular weight (Mw) of polysiland
is 2.0 X 10 4. This polysilane is a class of n-alkyl aryl polysilane with a
longer chain of n-butyl on one side and a phenyl group on the other side.
The photoluminescence (PL) spectrum is measured by Fluorimeter (Jobin
Yvon). The absorption spectrum is measured by Perkin Elmer UV-VIS
spectrometer. Figure 2 shows the absorption and PL emission spectra,
excited with a monochromatic light of 325 nm from a Xe-lamp, of PS-4
film made from solution in an organic solvent. The maximum peak in
absorption spectrum of Ps-4 is observed at 339 nm (3.6 eV). This peak
wavelength of 339 nm corresponds to the a - a * transition of the Si
backbone. The PL
emission of the film shows a sharp peak of excitonic nature at 357 nm
(3.48 eV), with a small red shift from the absorption peak. The FWHM of
the polysilane is 19 nm which indicates the spectral purity of the
emission.
The inventive idea is to develop OLEDs that provides UV emission at RT
which can be converted into white light using the proposed embodiment
as shown in Figure 3. In Figure 3 the first embodiment is used as self
white emission from polysilane. In the first embodiment the OLED gives
the emission in UV as well as white region, where the white emission
itself can be used for lighting.
The device configuration as shown in the Figure 1 (embodiment -1) is
fabricated on oxygen plasma treated indium tin oxide (ITO/glass). A
PEDOT: PSS layer (40 nm) is spin coated over ITO to remove surface
spikes and increases the work function of the substrate. The PEDOT:PSS
layer is then dried for e.g. at 120 °C in vacuum (~6x 10-6 mbar) for 1
hour. Then an emitting layer of PS-4, approximately 100 nm thick, is
spin coated from a solution in an organic solvent. The PS-4 film is then
dried for e.g. ^t 80 °C in vacuum (~6x 1Q-6 mbar) for 3 hours. However,
the emitting polvsilane layer is dried in vacuum at temperatures higher
than room temperature and lower than glass transition temperature of
the polysilane for an optimized time depending upon the materials.
The Ca (27 nm)/Al (150 nm) cathode is thermally evaporated onto the
emitting layer. The base pressure is -7X10'7 mbar and ~6 X 10-6 mbar
during Ca and Al evaporation respectively. The final structure of the
device is ITO/PEDOT:PSS/polysilane/Ca/Al. The EL spectrum of this
device at room temperature (RT) is shown in the Figure 4. The EL
spectrum comprises of a strong emission in UV light at 357 nm (3.48 eV).
In addition, a broad visible corresponding to white colour spectrum is
emitted from this diode. The CIE coordinate for the broad emission is
(0.30, 0.32). The current-light-voltage (I-L-V) characteristics of the EL
diode are shown in the figure .5. The current increases with increasing
the bias voltage. Correspondingly, the EL intensity, of both UV and white
emission (inset Figure 5), also increases. The turn on voltage is near 8 V
(0.5 MV/cm).
In the second embodiment, the white emission from the polysialne can be
further tuned by using two configuration: embodiment 2 and
embodiment 3 where phosphors/dyes can be used either in dispersed
form in the emission layer or as an extra layer. The dispersed form can
be obtained by mixing the phosphor particles in the polysilane solution
which is spin coated. An extra layer of the phosphor can be deposited by
spin coating or physical deposition. The other steps of fabrication are
similar to embodiment 1 as described above.
The other part of this invention is tanability of white emission obtained
from a single device structure. Figure 6 shows the CIE chromatically
diagram which defines the color seen by human eye in terms of two
coordinates. The sensitivity limit of the eye is represented by the horse
shoe shape, while the parabolic line, called Planckian locus, shows the
coordinates of natural light sources. For the man made lighting sources
it is required to stay close to this locus. Based on this the ideal CIE
coordinates for a white light source (0.33, 0.33), shown by a square. In
fig. 6 plotting has been made of CIE coordinates of the device of
embodiment-1 as a circle. The white colour emission from this device
(0.30, 0.32) is close to equienergy white colour coordinate (0.33, 0.33). In
order to move the CIE coordinate to equienergy point, the embodiment 2
and 3 can be employed. The invention proposed can alter the spectrum
of the device by using the down conversion of the UV emission from the
same device. This can be done by using the down conversion of the UV
emission from the same device. This can be done by using appropriate
phosphors/dyes that will absorb the UV emission and emit in visible
range. Now the final spectrum of the device is addition of the emission
from the polysiliane and emission from the phospors. This will allow to
move from the present CIE coordinates of (0.30, 0.32) to (0.33, 0.33)
along the line shown in Fig. 6.
Where the device structure in all the embodiment can be modified by-
1. changing the cathode- A (single layer), Ca/Al (double layer),
Mg:Ag/Ag (double layer).
2. Incorporating an electron transport layer (ETL) between
polysilane and cathode-A!Q3 [tris-8-hydrroxyquinoline)
Aluminum], PBD [2-(4biphenylyl)-5-(4-tert-butylphenyl)-l, 3, 4-
Oxadizole] etc.
3. Incorporating a hole transport layer (HTL) between Indium Tin
Oxide (ITO) and polysilane layer, using TPD (N, N'-diphenyl-N,
N'-bis-3-methyphenyl-l, 18-biphenyl-4, 4'-diamine), PEDOIT
[polyethylenethioxythiophene] (or any other hole transport layer
for the purpose of efficient hole injection and smbothening the
spikes in the
4. Incorporating an electron injection layer between emissive layer
and cathode-LiF.
5. Incorporating a hole blocking layer between the emissive layer
and cathode-BCP (2,9-dimethyl-4,7-diphenyl-l,10-
phenanthroline).
It is to be noted that the present invention is susceptible to
modifications, adaptations and changes by those skilled in the art. Such
variant embodiments employing the concepts and features of this
invention are intended to be within the scope of the present invention,
which is further set forth under the following

WE CLAIM
1. An improved organic light emitting diode characterized in that a cathode (1), an improved polysilane layer (2) and an anode (3) having white emission and/ or emission at temperature (about 300K).
2. An improved organic light emitting diode as claimed in claim 1 for tuning the white emission comprises of a cathode, an improved polysilane layer, and UV phosphors/dye dopants, wherein the UV emission from the same device is used to tune the white emission.
3. An improved organic light emitting diode as claimed in claim 2 wherein the UV phosphors/dye dopants may be provided in dispersed form in the polysilane layer to obtain desired CIE coordinate, for example perfect white colour, CIE coordinate (0.33, 0.33).
4. An improved organic light emitting diode as claimed in any of the preceding claims, wherein the polysilane layer is poly (n-alkylphenylsilane) with a longer chain of n-alkyl on one side (4-12 carbon) and a phenyl group on the other side.
5. An improved organic light emitting diode as claimed in any of the preceding claims, wherein the cathode comprises an electron injecting layer for example Al/Ca, Al, Ba, Ag/Mg.
6. An improved organic light emitting diode as claimed in any of the preceding claims, wherein anode comprises a hole injecting layer or layers for example of PEDOT:PSS coated an oxygen plasma treated indium tin oxide (ITO).
7. A process for fabrication of an improved emitting diode as claimed in
any of the preceding claims comprising steps of:
- spin coating of PEDOT:PSS layer over an anode followed by spin coating of an emitting layer of polysilane from an toluene solution and evaporation of a cathode on the emitting layer.
8. A process for fabrication of an improved organic light emitting diode as claimed in any of the preceding claims for tuning the white emission comprises steps of spin coating of PEDOT:PSS layer over an anode followed by spin coating of an emitting layer of polysilane from an toluene solution and evaporation of a cathode on the emitting layer wherein phosphor particles are mixed in the polysilane solution which is spin coated an extra layer of the phosphor is deposited on the anode by spin coating or physical deposition
9. The process for fabrication as claimed in claim 7 and 8 wherein the emitting polysilane layer is dried in vacuum at temperatures higher than room temperatures and lower than glass transition temperature of the polysilane for an optimized time.
10. The diode as claimed in claims 1, 2, 5 and 6, 8 wherein additional electron transport layer (ETL) such as A1Q3, PBD may be provided between polysilane and cathode.
11. The diode as claimed in claims 1, 2, 4, 5, 6, 7, 8 and 10 wherein
additional hole transport layer (HTL) for example TPD, PEDOT may be
provided between Indium Tin Oxide (ITO) and polysaline layer.
12. The diode as claimed in claims 1, 2, 4, 5, 6, 7, 8, 10 and 11 wherein additional hole blocking layer for example BCP may be provided between the cathode/ETL and emissive layer.
13. The diode as claimed in claims 1, 2, 4, 5, 6, 7, 8, 10, 11 and 12 wherein additional electron injection layer for example LiF may be provided between the cathode and emissive layer.
14. An improved organic light emitting diode for white emission and/or UV emission and a process for fabrication thereof substantially as herein described with reference to the accompanying drawings.