Efficient 1.54 μm light emitting diode with nanometer thick polycrystalline Si anode and organic sandwich structure

Article abstract of DOI:10.1063/1.2220483

This letter reports that the 1.54 μm electroluminescence efficiency of the organic light emitting diode (OLED) with a structure of nanometer thick polycrystalline silicon (NTPS)/NPB/ErQ/AlQ/Al is two orders of magnitude higher than that of the OLED with a structure of thick crystalline silicon/NPB/ErQ/Al, which is similar to the OLED reported in literature [Curry et al., Appl. Phys. Lett. 77, 2271 (2000)]. Such an improvement is mainly attributed to the fact that hole injection is controlled by NTPS anode and holes are blocked by AlQ to match electron injection, and a higher light out coupling as well.

Full text of DOI:10.1063/1.2220483

Efficient 1.54 μ m light emitting diode with nanometer thick polycrystalline Si anode
and organic sandwich structure
W. Q. Zhao, G. Z. Ran, G. L. Ma, W. J. Xu, L. Dai, W. M. Liu, P. F. Wang, and G. G. Qin
Citation: Applied Physics Letters 89, 022109 (2006); doi: 10.1063/1.2220483
View online: http://dx.doi.org/10.1063/1.2220483
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/2?ver=pdfcov
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APPLIED PHYSICS LETTERS 89, 022109 ͑2006͒
Efficient 1.54 ␮m light emitting diode with nanometer thick polycrystalline
Si anode and organic sandwich structure
W. Q. Zhao, G. Z. Ran,a͒ G. L. Ma, W. J. Xu, and L. Dai
State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China
W. M. Liu and P. F. Wang
Technical Institute of Physics and Chemistry, CAS, Beijing 100101, China
G. G. Qinb͒
State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China
and Key Laboratory of Semiconductor Materials, CAS, Beijing 100083, China
͑Received 15 March 2006; accepted 5 June 2006; published online 11 July 2006͒
This letter reports that the 1.54 ␮m electroluminescence efficiency of the organic light emitting
diode ͑OLED͒ with a structure of nanometer thick polycrystalline silicon ͑NTPS͒/NPB/ErQ/AlQ/Al
is two orders of magnitude higher than that of the OLED with a structure of thick crystalline
silicon/NPB/ErQ/Al, which is similar to the OLED reported in literature ͓Curry et al., Appl. Phys.
Lett. 77, 2271 ͑2000͔͒. Such an improvement is mainly attributed to the fact that hole injection is
controlled by NTPS anode and holes are blocked by AlQ to match electron injection, and a higher
light out coupling as well. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2220483͔
In recent years, a monolithic integration of optics and
electronics in a single Si chip has been pursued. However, Si
is a poor light emitter due to its indirect band structure.
Many approaches have been suggested to gain light from
Si.1–3 1.54 ␮m Si-based light emission is of particular
interest and realized by cooperation of organic electrolumi-
nescence ͑EL͒ technology with Si technology recently, where
the emitters are erbium complexes.4–6 Curry et al. have
demonstrated an effective 1.54 ␮m organic light emitting
Refs. 8 and 9. The sheet resistance of NTPS was measured to
be ϳ250 ⍀/ᮀ and the hole mobility was ϳ3 cm2/V s. The
c-Si anodes were prepared as reported previously,10,11 and Al
contacts were formed on their backsides each with a window
for light collection. The organic materials and the metals
were thermal evaporated in a chamber with a base pressure
of 5ϫ10−6 Torr, and the typical deposition rate was
1–2 Å/s, monitored by a quartz crystal oscillator.
ErQ was synthesized by mixing erbium ͑III͒ chloride in
80%
ethanol
and
20%
water
solution
with
diode ͑OLED͒ with
a
structure of crystalline Si
8-hydroxyquinoline in ethanol. The infrared luminescence
was measured by a liquid-nitrogen cooled Ge detector at
room temperature.
͑c-Si͒/N,N -diphenyl-N,N -bis͑3-methyl͒-1,1 -biphenyl-
Ј
Ј
Ј
4,4 -diamine
͑TPD͒/tis͑8-hydroxyquinoline͒
erbium
Ј
͑ErQ͒/Al.6 However, both the device current and voltage are
high, resulting in a poor EL efficiency.
In the following, three types of devices will be studied.
Type A: 500 ␮m c-Si/60 nm NPB/60 nm ErQ/50 nm Al, in
which the c-Si anode has an electrical resistivity of 0.01, 1,
or 40 ⍀ cm, and so the resulting devices are, respectively,
referred to as 0.01, 1, and 40 ⍀cm device. Type A devices
are similar in structure to the OLED ͑ϳ0.01 ⍀ cm c
-Si/40 nm TPD/50 nm ErQ/Al͒ reported in Ref. 6. Type B:
75 nm NTPS/60 nm NPB/60 nm ErQ/50 nm Al, i.e., the
OLED having a NTPS anode and two organic layers. It is
referred to as the NTPS-2 device. Type C: 75 nm
NTPS/60 nm NPB/10 nm ErQ/50 nm AlQ/50 nm Al, i.e.,
the OLED having a NTPS anode and three organic layers. It
is referred to as the NTPS-3 device. The c-Si anode or the
NTPS anode is transparent for 1.54 ␮m light. The NPB layer
is the hole-transport layer and the Al layer is the cathode in
all the three types of devices. The 60 nm ErQ layer is used as
the emissive and electron-transport layer in types A and B
devices. A 10 nm ErQ layer served as the emissive layer and
a 50 nm AlQ layer as the electron-transport layer in type C
device.
In this letter, we investigated how the following three
factors impact the I-V characteristic and EL efficiencies
of the 1.54 ␮m Si-based OLED: ͑1͒ electrical resistivity
of the p-type c-Si anode, ͑2͒ replacing the c-Si anode
with a nanometer thick polycrystalline Si ͑NTPS͒ anode,
and
͑3͒
replacing
organic
double
layers
Ј
of
N,N -bis-͑1-naphthl͒-diphenyl-1,1 -biphenyl-4,4 -diamine
Ј
Ј
͑NPB͒/ErQ with triple layers of NPB/ErQ/tris͑8-
hydroxyquinoline͒ aluminum ͑AlQ͒.
The NTPS film was fabricated with the magnetron
sputtering technology followed by the nickel-induced-
crystallization process as follow. In a chamber with a base
pressure of 1ϫ10−8 Torr, 20 nm SiO2, 75 nm Si, and 2 nm
nickel layers were successively sputter deposited on the K9
glass substrates. The SiO2 is to block the diffusion of impu-
rities of glass into the Si layer. The sputtered Si layer has the
same boron concentration as the 0.001 ⍀ cm boron doped Si
target.7 Then the samples were annealed at 500 °C in nitro-
gen atmosphere to form NTPS. We had no attempt to remove
the native oxide on the NTPS surface, just as reported in
Figure 1͑a͒ depicts the EL spectra for the 0.01, 1, and
40 ⍀ cm c-Si devices, and Fig. 1͑b͒ for the 40 ⍀ cm c-Si,
NTPS-2, and NTPS-3 devices at the same current density of
2 mA/mm2. In all the EL spectra, the peaks center at
1.54 ␮m, which is the characteristic emission of Er ions. It is
a͒
Electronic mail: rangz@pku.edu.cn
Electronic mail: qingg@pku.edu.cn
b͒
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0003-6951/2006/89͑2͒/022109/3/$23.00 89, 022109-1 © 2006 American Institute of Physics
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022109-2
Zhao et al.
Appl. Phys. Lett. 89, 022109 ͑2006͒
FIG. 1. ͑a͒ The EL spectra for the 0.01, 1, and 40 ⍀ cm devices at the same
current density of 2 mA/mm2. ͑b͒ The EL spectra for the 40 ⍀ cm, NTPS-2,
and NTPS-3 devices at the same current density of 2 mA/mm2.
generally accepted that this luminescence originates from a
mechanism called the antenna effect.12 Intersystem crossing
occurs from the excited singlet formed by recombination of
holes and electrons to a triplet state on the ligands, and then
the energy of the triple state is transferred efficiently to the
excited 4f states of the Er ions via a Dexter transfer. Finally,
4
the intratransitions I13/2-4I15/2 within Er ions produce the
1.54 ␮m luminescence.12–14
The current density, the infrared EL intensity, the current
efficiency, and the power efficiency versus applied voltage
are shown in Figs. 2͑a͒–2͑d͒, respectively, for all the devices.
It can be found that for the c-Si devices, the current density
decreases with the increase of the electrical resistivity of
c-Si anode, because the hole injection from the c-Si anode is
reduced with increasing resistivity.10 Moreover, the EL effi-
ciency increases with increasing electrical resistivity of the
c-Si anode, because the balance between electron and hole
injections is improved by reducing hole injection. As a result,
the 40 ⍀ cm device gives the best EL current and power
efficiencies compared to the 0.01 and 1 ⍀ cm devices.
The infrared EL performance can be improved by using
the 75 nm NTPS anode ͑NTPS-2 device͒ to replace the
c-Si anode ͑40 ⍀ cm c-Si device͒ as shown in Figs.
2͑a͒–2͑d͒. When the applied voltage is below 8 V, the cur-
rent density of the NTPS-2 device is close to that of the
40 ⍀ cm device. When the applied voltage is between 8 and
15 V, a negative resistance occurs, the reason of which is not
clear now and will be discussed elsewhere. When the applied
voltage is above 15 V, the current density of the NTPS-2
device is markedly lower than that of the 40 ⍀ cm device.
The maximum current and power efficiencies of the NTPS-2
device are 22 and 18 times, respectively, higher than that of
the 40 ⍀ cm device with a c-Si anode.
FIG. 2. ͑a͒ The current density, ͑b͒ the infrared EL intensity, ͑c͒ the current
efficiency, and ͑d͒ the power efficiency vs applied voltage for 0.01 ⍀ cm,
1 ⍀ cm, 40 ⍀ cm, NTPS-2, and NTPS-3 devices.
with the NTPS or the c-Si anodes by replacing the Al cath-
ode with a much higher work function metal Au.15 As shown
in Fig. 3, the hole current of device with NTPS anode is
much smaller than those of any devices with c-Si anodes,
that is, the hole injection ability of NTPS is much smaller
One reason for the significant EL enhancement is that
the balance between electron and hole currents in the OLED
is improved. It is due to a low hole injection ability of NTPS,
caused by the small carriers mobility. To show the hole in-
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jection ability of NTPS, we fabricated “hole-only” devices
than that of c-Si.
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022109-3
Zhao et al.
Appl. Phys. Lett. 89, 022109 ͑2006͒
͑LUMO͒ and electronic structure of ErQ are similar to those
of AlQ.17 As a result, the emissive layer ErQ can efficiently
accepts holes from NPB and electrons from AlQ, just like the
AlQ layer in a conventional NPB/AlQ ͑emissive layer͒/AlQ
͑electron-transport layer͒ structure.
In summary, an efficient 1.54 ␮m Si-based OLED with a
structure of NTPS/NPB/ErQ/AlQ/Al is demonstrated. Its
current and power efficiencies have been improved by two
orders of magnitude, compared to the structure of
c-Si/NPB/ErQ/Al, which is similar to that in a pioneering
work.6 The large improvement is attributed to ͑1͒ the NTPS
anode controlling the hole injection, ͑2͒ the AlQ layer block-
ing the holes, and ͑3͒ the enhanced infrared light
out coupling.
This work was supported by the National Natural
Science Foundation of China.
FIG. 3. The I-V characteristics of “hole-only” devices each with a NTPS or
c-Si anode. ͑᭢͒ 75 nm NTPS/60 nm NPB/60 nm ErQ/50 nm Au, ͑᭡͒
c-Si ͑40 ⍀ cm͒/60 nm NPB/60 nm ErQ/50 nm Au, ͑b͒ c-Si
͑1 ⍀ cm͒/60 nm NPB/60 nm ErQ/50 nm Au, and ͑᭿͒ c-Si
͑0.01 ⍀ cm͒/60 nm NPB/60 nm ErQ/50 nm Au.
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further by improving the structure of organic layers. Figures
2͑a͒–2͑d͒ demonstrate that, compared to NTPS-2 device, the
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