Enhancing performances of hybrid perovskite light emitting diodes with thickness controlled PMMA interlayer

Article abstract of DOI:10.1246/bcsj.20180102

Solution processed organic-inorganic halide perovskites emerged as efficient materials for the fabrication of lightemitting diodes (LEDs). Spin coating of perovskites on solid support for device integration leads to poor morphology with pinholes and leakage current through electrical shunt paths thereby decreasing the device efficiency. Here, we report a facile route to improve the performance of MAPbBr₃ perovskite based LEDs by incorporating a poly(methyl methacrylate) (PMMA) interlayer in the device structure at the interface of ZnO and MAPbBr3₃layer. The thickness of PMMA interlayer was varied to achieve optimal device performance by overcoming the leakage current and reduced non-radiative recombination pathways. LEDs with optimal PMMA thickness showed a significant enhancement in device performance comparison to the devices without PMMA interlayer. The perovskite LEDs with ～7 nm PMMA interlayer exhibit a maximum luminance of ～3450 cdm^-2, current efficiency of ～11.88 cdA^-1, external quantum efficiency of ～2.82% and power efficiency of ～4.4 lmW^-1 showing robust LED properties with ³6-fold enhancement compared to a device without PMMA. Our route provides a convenient way to improve the efficiency of perovskite LEDs by controlling device structure with planar PMMA interlayer, which can be extended to other perovskite LEDs.

Full text of DOI:10.1246/bcsj.20180102

Advance Publication Cover Page
Enhancing Performances of Hybrid Perovskite Light Emitting Diodes with Thickness
Controlled PMMA Interlayer
Gundam Sandeep Kumar, Bapi Pradhan, Tapas Kamilya, and Somobrata Acharya*
Advance Publication on the web May 10, 2018
doi:10.1246/bcsj.20180102
©
2018 The Chemical Society of Japan
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Publication manuscripts.

Enhancing Performances of Hybrid Perovskite Light Emitting Diodes with
Thickness Controlled PMMA Interlayer
Gundam Sandeep Kumar, Bapi Pradhan, Tapas Kamilya, and Somobrata Acharya*
Centre for Advanced Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India.
E-mail: camsa2@iacs.res.in
Somobrata Acharya
Somobrata Acharya received his Ph.D. degree from Jadavpur University, India. He is currently Professor
in the Centre for Advanced Materials (CAM), Indian Association for the Cultivation of Science (IACS),
India. He is carrying out research in interdisciplinary areas probing structure–property relationship
and possible applications of semiconductor nanomaterials in the areas of energy generation and
consumption. His research area includes heterostructures, 2D nanostructures, superlattices, supramolecular
assemblies and their suitable applications.
Abstract
Solution processed organic-inorganic halide perovskites
3
MAPbBr based LEDs producing a bright luminance of ~ 364
cdm at room temperature. Recently, high efficiency LEDs
have been fabricated by optimizing the fast crystallization
−2
15
emerged as efficient materials for the fabrication of
light-emitting diodes (LEDs). Spin coating of perovskites on
solid support for device integration leads to poor morphology
with pinholes and leakage current through electrical shunt paths
thereby decreasing the device efficiency. Here, we report a
3
process of MAPbBr through “nanocrystal pinning” (NCP)
2
4
process showing EQE of ~ 8.53%. Yuan et al. and Wang et al.
have achieved near-infrared perovskite LEDs with EQE of ~
8.8% and ~ 11.7% respectively using solution process
2
5,26
facile route to improve the performance of MAPbBr
3
routes.
perovskite grains with longer-chain organic cations to yield
EQE of ~ 10.4%, 9.3% for MAPbI and MAPbBr based
Highly efficient emitters are prepared by coating the
perovskite based LEDs by incorporating a poly(methyl
methacrylate) (PMMA) interlayer in the device structure at the
3
3
2
7
interface of ZnO and MAPbBr
3
layer. The thickness of PMMA
LEDs. Although, high efficiency LEDs have been fabricated
using hybrid halide perovskite nanocrystals, however, direct
spin coating of perovskite precursor solution for the preparation
of active thin film for the device fabrication often contain
non-uniform surface coverage, pin holes and defects. These
lead to leakage current through electrical shunt paths lowering
interlayer was varied to achieve optimal device performance by
overcoming the leakage current and reduced non-radiative
recombination pathways. LEDs with optimal PMMA thickness
showed a significant enhancement in device performance
comparison to the devices without PMMA interlayer. The
perovskite LEDs with ~ 7 nm PMMA interlayer exhibits a
2
8
luminance and overall device efficiency.
-
2
maximum luminance of ~ 3450 cdm , current efficiency of ~
One way to overcome the leakage current and defects is to
use blends of the perovskites with different polymers. Various
composites have been prepared by blending the perovskites
with polymers like ionic conductor (Poly(ethylene oxide)),
poor ionic conductor (PVDF), ionic insulating polymer (PS)
-
1
1
1.88 cdA , external quantum efficiency of ~ 2.82% and power
-
1
efficiency of ~ 4.4 lmW showing robust LED properties with
6-fold enhancement compared to the device without PMMA.
~
Our route provides a convenient way to improve the efficiency
of perovskite LEDs by controlling device structure with planar
PMMA interlayer, which can be extended to other perovskite
LEDs.
2
8,29
and aromatic polyimide (insulator) precursor (PIP).
However, blending insulating polymers with hybrid halide
perovskites for the device fabrication leads to an increase in the
overall resistance of the device, which offers high potential
barrier to the charge carriers. Recently, LEDs of hybrid-mixed
Keywords: Hybrid Halide Perovskites, Light Emitting
Diodes, PMMA Interlayer.
cation perovskite Cs0.87MA0.13PbBr
3
have been fabricated using
insulating polyvinyl pyrrolidine (PVP) polymer in between the
3
0
1
. Introduction
Organic-inorganic halide (MAPbX
Br, Cl) perovskites have received extensive attention in
ZnO and perovskite layer showed EQE of ~ 10.4%. Several
reports have shown that the presence of an additional planar
insulating layer within the device structure improves the device
performance instead of using perovskite-polymer blends as
3
3 3
, MA=CH NH , X=I,
1
-8
optoelectronic device applications.
properties with power conversion efficiencies up to ~ 22% have
been achieved in a short span. In addition to the efficient
3
photovoltaic properties, MAPbX emerged as an excellent light
emitting material showing broad color-tunability, high color
purity with narrow full width at half maxima (FWHM), high
photoluminescence (PL) quantum yield.
Excellent photovoltaic
3
0,31
active emissive layer.
The use of the planar insulating layer
9
with controlled thickness overcomes the leakage current
problem, reduces the non-radiative recombination centers at the
interface and supports balancing the charge injection into
perovskite emissive layer. However, a detailed study by varying
the interlayer thickness at the interface for improving the
device performance is still lacking. Hence, it is necessary to
control and optimize the thickness of the planar insulating
interlayer within the device structure in order to achieve
improved LED performance.
1
0-14
These features
make MAPbX as promising luminescent material for the
3
fabrication of LEDs. Several recent reports showed that the
bright and color-tunable electroluminescence (EL) can be
achieved from hybrid perovskite LEDs, which provides a
platform for the preparation of various kind of displays and
Here we report on the fabrication of efficient MAPbBr
based LEDs by incorporating additional PMMA interlayer
3
1
5-38
lighting applications.
A breakthrough is achieved from the
1

within the device structure. Controlled morphology of
MAPbBr crystals with reduced grain size have been achieved
by NCP process and using excess MABr molar ratio in the
perovskite composite solution. Introduction of optically
transparent and electrically insulating PMMA interlayer in
surface hydrophilic. At first, the ZnO nanoparticles (loading ~
30 mg/ml) were spin coated on ITO – coated glass substrate at
a speed of ~ 2500 rpm for 45 sec and films were annealed at ~
130 ºC for 30 min under N flow to obtain a uniform layer. On
2
top of ZnO layer, an ultrathin layer of PMMA in dichloroethane
solution was spin coated at ~ 3000 rpm for 30 sec and films
3
3
between the ZnO and perovskite MAPbBr emissive layer
suppresses the leakage current, improves the injected charge
density into emissive layer, and significantly reduces the
non-radiative recombination. We optimized the thickness of the
PMMA interlayer in a controllable way to optimize the
performance of the LEDs. The devices with ~ 7 nm PMMA
interlayer thickness shows the optimal device performance with
were annealed at 100 ºC for 10 min under N
the solvent. The surface was rinsed twice with DMF solution.
To prepare MAPbBr thin film, MABr:PbBr composite
solution was spin coated on top of the ITO/ZnO/PMMA layer
by employing the NCP process. The substrates were then
2
flow to evaporate
3
2
annealed at 70 °C for 5 min under continuous N
2
flow.
-
2
maximum luminance of ~ 3450 cdm , current efficiency (CE)
Particularly, MAPbBr nanoparticle layers were fabricated by
3
-
1
of ~ 11.88 cdA , EQE of ~ 2.82%, and power efficiency (PE)
using the NCP process, which use the dripping of chloroform
solution onto the spinning layers during spin-coating of
-
1
of ~ 4.4 lmW . These results represent a way to achieve
improved MAPbBr LED performance using a facile PMMA
interlayer by avoiding complex device architectures.
3
MAPbBr
chlorobenzene solution (10mg/ml) was spin coated on to the
top of MAPbBr layer. The films were annealed at 70 ºC for 5
min under continuous N flow. The PVK layer acts as a hole
3
solution. Further, poly(9-vinylcarbazole) (PVK) –
3
2
. Experimental
Experimental Materials. Hydrobromic acid (HBr, 48%
in water, Merck), Methylamine (CH NH , 40 wt.% in H O, TCI
Chemicals), PbBr (99.999%, Aldrich, trace metals basis),
Poly(9-vinylcarbazole) (PVK, average M ~25,000-50,000,
Aldrich), N,N'-Di-1-naphthyl-N,N'-diphenylbenzidine (NPD,
8%, TCI Chemicals), poly(methyl methacrylate) (PMMA,
average Mw ~350,000, Aldrich) were used as received.
Preparation of MAPbBr Precursor Solution.
2
transport layer and effective electron blocking layer. Further,
the substrates were transferred into a high vacuum thermal
3
2
2
-
6
2
evaporator (~ 10 mbar). The fabrication of perovskite LEDs
successfully completed by thermally evaporating the N,N′
-Di(1-naphthyl)-N,N ′ -diphenyl-(1,1 ′ -biphenyl)-4,4 ′
-diamine (NPB/NPD) (working as a hole injection layer), and
aluminum (Al) top electrode sequentially through a shadow
mask without breaking the high vacuum within the thermal
evaporator deposition chamber. The active device area was 6
w
9
3
Methylammonium Bromide (MABr) was synthesized
according to the reported method with trivial modifications.16
In brief, MABr was synthesized by reacting 50 mL of
hydrobromic acid (48% in water) added dropwise to the 30 mL
of methylamine (40% in methanol) at 0 °C for 2 h while
continuous stirring in a round bottom flask. Following this, the
solvent in the reaction solution was removed by rotary
evaporation at 50 °C for 1 hr. Purification of the products was
conducted by dissolving in ethanol, recrystallizing from diethyl
ether, and drying at room temperature in a vacuum oven for 24
hr. The washing and recrystallization process was repetitively
2
mm (2 mm × 3 mm) defined by the overlapping area of the
ITO and Al electrodes. All the samples were prepared and
device fabrication was done at room temperature in ambient
conditions.
Materials Characterization Techniques. Absorption
spectrum of the MAPbBr
3
thin films deposited on glass
substrates were carried out by using the Agilent UV-vis
spectrophotometer. X-ray-diffraction (XRD) measurements
were carried out by the Bruker D8 Advance powder
diffractometer using Cu Kalpha ( = 1.5418 Å) as the incident
radiation. Field Emission Scanning Electron Microscopy
(FESEM) measurements were performed by using the JEOL
JSM-6700F instrument operating at an accelerating voltage of 5
kV. The PL and EL spectra were recorded with NanoLog
spectrofluorometer (HORIBA JobinYoun). The PL time
resolved spectroscopy measurements were carried out by
Edinburgh PL spectrophotometer (FLS 980) with 373.8 nm
picosecond (ps) pulse diode laser with pulse width of ~ 70.5 ps
(Model EPL 375). The current versus voltage characteristic
measurements of fabricated LEDs were conducted by using a
LABVIEW program based computer-controlled source
measurement unit (Keithley 2420 SMU) and the
luminance-voltage measurements were carried out by Konica
Minolta CS-200 Chroma meter. The EQE measurements were
carried out with help of silicon photodiode from the calibrated
light source. All the LED devices fabricated under room
temperature under ambient conditions until unless mentioned.
done to obtain high-purity crystals. Further, the MAPbBr
precursor solutions (30 wt.%) were prepared by dissolving the
MABr (94.2 mg) and PbBr (205.8 mg) with 1.5:1 molar ratio
3
2
in 1 ml of anhydrous N, N-dimethylformamide (DMF) at 60 °C
while continuous stirring for overnight.
Perovskite Thin Film Preparation
Pinning (NCP) Process. The MAPbBr
-
Nanocrystal
3
thin films were
2
4
prepared by using spin coating technique - NCP process. The
NCP process involves the two steps. At stage one, the
perovskite precursor solution in DMF was loaded onto the
targeted substrate and accelerated to a speed of ~ 500 rpm.
After 10 sec of the 1st stage, the rotation speed was further
accelerated to ~ 3000 rpm and maintained for 90 sec. After
completion of 25 sec, nanocrystal pinning process was initiated
by dripping the chloroform solution onto the spinning substrate.
During the next 65 sec, the residual solvent was evaporated,
which induces the fast crystallization of perovskite nanocrystals.
The films were further annealed at 70 ºC for 5 min under
continuous N
2
flow chamber.
3. Results and Discussion
LED Fabrication using MAPbBr
3
. Initially, the indium
Composition of MABr and PbBr
2
molar ratio have been
tin oxide (ITO) coated glass substrates were patterned
accordingly with the scotch tape, zinc powder and HCl. The
patterned ITOs were cleaned with soap solutions and deionized
reported to plays crucial role in LEDs device
a
2
4,32
performance.
We have prepared MAPbBr
3
precursor
solution (30 wt.%) by dissolving MABr and PbBr
2
at a molar
(
DI) water. Further, the ITOs were cleaned with acetone,
ratio of 1.5:1.0 in anhydrous N, N-dimethylformamide (DMF)
at 60 °C while continuous stirring for 12 hrs. Generally, an
excess amount of Pb atoms remains owing to the incomplete
ethanol and isopropyl alcohol for 10 min sequentially by
sonication process and then placed on a hot plate at 120 °C to
evaporate the residual solvent. The cleaned ITO substrates were
subjected to UV-ozone treatment for 15 min to make the
reaction between MABr and PbBr
2
or unintended losses of Br
atoms when MABr and PbBr are mixed in 1:1 molar ratio.24
2
2

Hence, we have used excess amount of MABr, which
suppresses the exciton quenching and improves the steady-state
deposited from the dichloroethane solution by spin coating
process. We used insulating interlayer of PMMA of different
thicknesses to engineer the device structure to improve
performance. The solution-processed ZnO nanocrystals were
employed as electron-transporting layer (ETL) and
hole-blocking layer due to their high electron mobility,
excellent optical transparency, and a deep valence-band energy
level that effectively blocks the holes. The PVK layer was
2
4,32
PL significantly.
process for the fabrication of MAPbBr
Additionally, we have adopted NCP
2
4
3
thin films. In NCP,
the perovskite precursor in higher boiling point DMF solution
spin coated onto the targeted substrate followed by the
nanocrystal growth initiated by dripping the chloroform
solution. Higher boiling point of the DMF allows a slow
evaporation, which leads to the formation of larger grains.
Hence, excess DMF is washed out by dripping the chloroform
onto the spinning substrate. Faster evaporation rate of the
chloroform initiate pinning of nanocrystals through fast
crystallization process. Hence, reduced grain size and
employed as
a
hole-transporting layer (HTL) and
electron-blocking interlayer. The NPD-layer was used as a hole
injecting layer. Figure 2b shows the energy level band diagram
1
8,34,39
with respect to vacuum level respectively.
The
combination of effective hole-blocking nature of ZnO,
electron-blocking nature of PVK and their appropriate energy
levels allow effective charge carriers injection into the active
uniformity of MAPbBr
solvent evaporation time using chloroform.
3
grains have been achieved by rising
2
4
3
MAPbBr layer for radiative recombination. A cross-sectional
(
a)
(b)
FESEM image of a LED device structure is shown in Figure 2c.
From the cross-sectional FESEM image, the thicknesses of the
3
ZnO, PMMA, MAPbBr , PVK, NPD and Al top-electrode
layers are found to be ~ 70 nm, 7 nm, 100 nm, 30 nm, 30 nm
and 100 nm respectively.
(
a)
(b)
1
0
15 20 25 30 35 40 45 50
 (degree)
20 mm
- 2.3 eV2.4 eV
e
2
Al
3.6 eV
PL
UV-vis
3.7 eV
.7 eV
(
c)
(d)
NPD
PVK
4
.3 eV
4
MAPbBr3
PMMA
ZnO
5
.5 eV
5
.8 eV
5
.9 eV
h
ITO
+
7
.0 eV
2
mm 440
480
520
560
600
640
GLASS
Wavelength (nm)
(
c)
Al
NPD
PVK
MAPbBr3
PMMA
ZnO
Figure 1. (a) XRD of MAPbBr
b-c) FESEM morphology images of MAPbBr
different magnifications. (d) UV-vis absorption (red curve) and
PL (green curve) spectra of MAPbBr thin film.
3
thin film on glass substrate.
(
3
thin films at
3
X-ray diffraction (XRD) pattern of the MAPbBr
sharp peaks corresponding to (100), (110), (111), (200), (210),
211), (220) and (300) planes (Figure 1a) and peak positions
3
films shows
ITO
(
100 nm
Glass
match well with the previously reported cubic Pm
m
1
6,20,21,29,33,34
phase.
The morphology of the thin film was
Figure 2. (a) Schematic representation of device architecture of
the perovskite LEDs. (b) Flat energy-level band diagram of the
multilayered perovskite LED. (c) Cross-sectional FESEM
image showing different layers of a LED device.
analyzed by using Field Emission Scanning Electron
Microscopy (FESEM). Figure 1b-c show FESEM images of
perovskite thin films with different magnification prepared
using the NCP process. FESEM images reveal compact and
uniform perovskite layer covering a large area on substrate
One of the motivations of the present work is to introduce
PMMA layer with an optimum thickness within the device to
reduce the leakage current through shunt paths. We have varied
the thickness of PMMA layer within the LED device to achieve
optimal LED device performance. To quantify the thickness of
the PMMA layer used within the device structure, we have
prepared PMMA thin films on silicon substrate under the same
condition of device fabrication. The PMMA film thickness was
varied by different loading concentrations. Atomic force
microscopy (AFM) images of the PMMA thin films prepared
from the ~ 1.8 mg/ml loading concentration on silicon
substrates are presented in the Figure 3. The thickness of the
films was measured by making an ultrafine scratch on the thin
films (Figure 3a). Figure 3b shows the 3D image of the step of
the scratch drawn on the PMMA film. The line-scan profile
shows a step height of ~ 7 nm (Figure 3c). The PMMA thin
film shows a surface roughness of ~ 1 nm (inset of Figure 3d).
The thickness of the PMMA thin films are found to be ~ 5 nm,
10 nm, 12 nm with the loading concentration of ~ 1.2 mg/ml,
(
Figure 1b). An average dimension of ~ 600 nm of the
nanocrystals is measured from the FESEM image (Figure 1c).
UV-vis absorption spectrum (Figure 1d) shows a peak at ~ 510
nm. We have calculated the band gap of ~ 2.3 eV using Taut
plot by obtaining the threshold of the absorption spectrum
(Supporting Information (SI), Figure S1), which matches with
1
6,33
the previously reported MAPbBr
3
nanocrystals.
The PL
spectrum exhibits an intense peak at ~ 536 nm with a FWHM
of ~ 31 nm. A considerable overlapping between the absorption
and PL spectra is evidenced suggesting a reduced Stokes shift.
We have fabricated LEDs in an inverted structure
configuration using MAPbBr
LED device structure consisting of indium tin oxide (ITO)/
ZnO/PMMA/MAPbBr /PVK(Poly(9-vinylcarbazole))/NPD(N,
N′ -Di(1-naphthyl)-N,N′ -diphenyl-(1,1′ -biphenyl)-4,4′
diamine)/Al (Figure 2a). All layers within the device structure
3
as active emissive layer. The
3
-
were deposited using solution processed route via spin coating
technique except NPD/Al layers. The PMMA layer was
3

2
.7 mg/ml and 3.3 mg/ml respectively (SI, Figure S2).
that the insertion of the additional PMMA layers with an
appropriate thickness helps in reducing the surface defect states
at ZnO/MAPbBr
the MAPbBr
fabrication.
3
interface and enhances emissive properties of
thin film, which is beneficial for LED
3
0,31
3
1
1
7
5
2 nm
0 nm
nm
12 nm
10 nm
7 nm
(
a)
(b)
nm
5 nm
no PMMA
no PMMA
7
nm
4
50
500 550
600 650
700
20
40
60
80 100 120
Wavelength (nm)
Time (ns)
3
Figure 4. (a) Steady-state PL spectra of MAPbBr thin films
Figure 3. (a) The AFM image of the PMMA thin film
deposited on silicon substrate with a loading of ~ 1.8 mg/ml in
dichloroethane. (b) Three-dimensional representation of an
ultrafine scratch on the thin film. (c) Line scanning height
profile of the PMMA thin film showing a step height of ~ 7 nm.
deposited on Glass/ZnO, Glass/ZnO/PMMA with different
thickness of the PMMA layer. Inset shows the PL image of the
MAPbBr
nm UV light excitation. (b) Comparison of PL time decay
curves of MAPbBr thin films on ZnO and ZnO/PMMA with
3
thin films prepared with ~ 7 nm PMMA under 365
3
(d) AFM image of PMMA thin film along with the roughness
different PMMA layer thickness.
profile showing the average roughness of ~ 1 nm (inset).
3
We tested the performance of MAPbBr LEDs containing
We studied the steady-state PL of MAPbBr
deposited at different conditions (Figure 4a). MAPbBr
deposited directly onto ZnO layer shows green emission
peaking at 536 nm. Interestingly, PL spectra of
ZnO/PMMA/MAPbBr with different PMMA layer thickness
enhance the pristine MAPbBr PL intensity. An increase of PL
intensity is observed with increasing layer thickness of PMMA
retaining the PL peak position. Notably, a saturation of PL
intensity is observed upon using ~ 12 nm thick PMMA layer.
The enhancement of PL intensity arises from the reduced
non-radiative recombination centers at the interface of the
3
thin films
PMMA layers with different thicknesses. The LED devices
consisting of ~ 7 nm PMMA layer show the strong green EL at
3
films
~ 540 nm, which replicates PL spectrum of the MAPbBr
3
~
perovskite thin film (Figure 5a). The EL peak intensity
increases significantly with increasing applied bias voltage
while retaining the same peak position. The true color
photographic image of the glowing LED shows EL uniformly
distribute across the entire active device area without any
inhomogeneity (Figure 5b). Corresponding Commission
Internationale de l'éclairage (CIE) chromaticity coordinates
diagram depicts EL at (0.22, 0.72), which closely matches with
3
3
MAPbBr
3
and ZnO layer through the passivation of ZnO
the CIE coordinates (0.21, 0.73) for PL of MAPbBr
(Figure 5c). These observations clearly signify that the EL
solely originates from MAPbBr active layer. Figure 5d shows
3
thin film
3
0
surface defects by PMMA at the interface. These observations
reveal that optimized PMMA thickness is necessary to
passivate the surface defect states of the ZnO layer along with a
simultaneous control over the efficient charge injection to
obtain enhance performance of the LED devices. The
3
the current density and luminance versus voltage characteristics
of device with 7 nm PMMA interlayer. The luminance versus
voltage characteristics of the devices show LED responses with
an effective threshold voltage below 5 V (Figure 5d). Above the
turn-on voltage, the luminance versus voltage plot is mostly
linear (Figure 5d). Luminance is detectable starting from a
threshold voltage of ~ 5 V showing a maximum value of ~
solid-state PL image of the ZnO/PMMA/MAPbBr
3
thin film of
~
7 nm PMMA under ~ 365 nm UV light exposure shows green
emission (Inset, Figure 4a). We carried out Time-Correlated
Single Photon Counting (TCSPC) measurements of the
-
2
-1
MAPbBr
Figure 4b). The time-resolved PL decay curves are best fitted
to bi-exponential function. We observed that the PL lifetime of
the MAPbBr thin films increases upon PMMA passivation.
The average PL lifetime for the ZnO/MAPbBr is calculated to
be ~ 23.92 ns, while the ZnO/PMMA/MAPbBr films show
3
thin films embedded within the device architecture
3450 cdm . The device shows maximum CE of ~ 11.88 cdA
(Figure 5e), EQE of ~ 2.82% (Figure 5f) and PE of ~ 4.4 lmW
(Figure 5g).
-
1
(
3
In addition to the enhancement of the PL, the use of
PMMA interlayer suppresses the flow of leakage current in
between the ETL and HTL which is beneficial for the LED
performance. Because of the presence of PMMA interlayer, the
charge carriers experience a finite potential barrier and the
3
3
lifetimes of ~ 43.29 ns, ~ 54.63 ns, ~ 65.16 ns and ~ 65.93 ns
for PMMA layer thickness of 5 nm, 7 nm, 10 nm and 12 nm
respectively. A similar increase in the PL lifetime of the
perovskite film had been observed previously after PVP
modification, which was attributed to the reduction of bulk
injected charge carrier density into the active MAPbBr
3
2
8,30
emissive
layer
can
be
controlled.
Recently,
polyethyleneimine (PEI) modified ZnO is used as cathode
interlayer for the fabrication of near-infrared (NIR) and visible
3
0
defects. The modification of the interface between the ZnO –
MAPbBr by insertion of a thin PMMA layer increases the
3
1
3
green LEDs. High quality perovskite thin film with crystalline
features, long PL lifetimes, good surface coverage and
improved device performance have been achieved by the
lifetime initially with the layer thickness. However, no
significant change in the lifetime was observed above the
thickness ~ 10–12 nm of PMMA layer. These results indicate
4

(
a)
5 V
(d)
4k
(
b)
2
1
0
1
2
3
7
9
1
1
1
V
V
1 V
3 V
5 V
10
10
3
3
k
k
PL
1
0
2k
2k
PL
EL
(
c) (
0.21, 0.73)
0.22, 0.72)
(
-
1
0
1
5
0
k
-
-
1
1
0
0
00
7 nm
4
80
540
600
660
0
3
6
9
12
15
Wavelength (nm)
Voltage (V)
1
1
0
1
0
1
(
e)
(f)
(g)
1
1
0
0
1
2
3
1
0
0
1
0
-
1
0
0
-
1
1
0
-
-
1
1
0
0
-
1
0
7
nm
7 nm
7 nm
-2
10
0
1
2
1
0
10
10
4
6
8
10 12 14 16
4
6
8
10 12 14 16
2
Current Density (mA/cm )
Voltage (V)
Voltage (V)
Figure 5. (a) Comparison of EL spectra of the devices operated under different bias voltages from ~ 5 V to ~ 15 V with the solid state
PL spectra of MAPbBr thin film. (b) The true color photographic image of the green color LED device under operation. (c) CIE
3
chromaticity coordinates of the solid state PL and EL of LED. (d) Current density versus voltage and luminance versus voltage
curves. (e) Current efficiency versus current density plot. (f) EQE versus voltage plot. (g) Power efficiency versus voltage plot.
additional PEI interlayer. Similarly, a blend of perovskite and a
polyimide precursor dielectric (PIP) solution is used to prepare
the perovskite nanocrystals within a thin-film matrix of PIP for
device efficiency can be improved using polymer interlayer at
the interface of organometallic perovskites. Our modification
using thin PMMA interlayer at the ZnO/perovskite interface
balance charge injection into emissive layer, minimizes leakage
current, reduce the surface defect states and shows comparable
device performance with the above sets of literatures (SI, Table
S1).
2
8
the fabrication of LEDs. This report suggests an improvement
in the device EQE by two orders of magnitude to ~ 1.2% when
PIP was used. Single-layer LEDs fabricated from the
perovskite– poly (ethylene oxide) (PEO) composite thin films
with different weight ratios by spin coating process, exhibited a
We investigated the performance of LEDs by varying the
PMMA interlayer thickness from 5 nm to 12 nm. All the
devices were fabricated in a similar configuration except the
variation of PMMA layer thickness. As the thickness of the
PMMA layer in the device increases, the current density is
decreased gradually at each operating voltage (Figure 6a). The
PMMA layer acts as a finite potential barrier, passivating the
shunt paths and controlling the injected charge density as well
as leakage current. Luminance versus voltage characteristic
-
2
bright-maximum luminance of ~ 4064 cdm , CE of ~ 0.74
-
1
29
cdA and EQE of ~ 0.165%. When PEO is replaced by
PVDF or PS, a drastic reduction in the luminance and increase
in the turn on voltage have been observed. Alternatively, single
layered LEDs fabricated from MAPbBr
3
and PEO composite
thin film prepared by doctor blade technique showed low turn
-
2
on voltage of ~ 2.6 V, maximum luminance of ~ 21014 cdm
3
3
and EQE of ~ 1.1%. However, LEDs with high luminance are
exhibiting lower efficiency possibly because of the presence of
shunt paths and the leakage current. High EQE of ~ 8.53% for
-
2
curve shows a turn ON voltage of ~ 4.5 V (defined at ~ 1cdm )
for the device without PMMA interlayer. The turn ON voltage
is increases slightly upon using PMMA interlayer, which acts
as a finite potential barrier for charge carriers. Luminance from
the devices changes significantly with the PMMA thickness.
MAPbBr
3
LEDs have been obtained using fast crystallization
2
4
NCP process. Recently, LED device performance has been
improved by using
a
composite solution of (PVK)
:
:
-
2
1
,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi)
Luminance gradually increases from ~ 2519 cdm (without
-
2
MAPbBr
3
, which exhibits maximum luminance of ~ 7263
PMMA) to ~ 3450 cdm for ~ 7 nm PMMA interlayer, which
-
2
34
-2
-2
cdm and EQE of ~ 2.28%. These reports show that the LED
drops down to ~ 1402 cdm and ~ 215 cdm for ~ 10 nm and ~
5

5
4
3
2
k
k
k
k
3
2
1
0
1
2
3
1
0
1
2
1
1
1
1
0
0
0
10
(
a)no PMMA
(b)
5 nm
7
1
1
nm
0 nm
2 nm
10
0
-
-
1
0
0
1
0
no PMMA
nm
7 nm
0 nm
5
-
-
1k
0
1
1
0
0
1
-
1
12 nm
-
1
0
1
2
3
0
2
4
6
8
10 12 14 16
10
10
10
10
10
-
2
Voltage (V)
Current Density (mAcm )
1
0
1
2
3
1
0
(
c)
(
d)
0
1
2
3
1
0
1
0
-
1
0
-
1
0
no PMMA
-
-
no PMMA
5 nm
1
1
0
0
-
-
5
7
1
1
nm
nm
0 nm
2 nm
1
1
0
0
7
1
nm
0 nm
12 nm
4
6
8
10
12
14
16
4
6
8
10
12
14
16
Voltage (V)
Voltage (V)
3
Figure 6. Device performance characteristic curves of MAPbBr LEDs fabricated without and with additional PMMA interlayer with
thickness of ~ 5 nm, 7 nm, 10 nm and 12 nm. The colour codes are indicated in the inset. (a) Current density versus voltage and
luminance versus voltage curves. (b) Current efficiency versus current density plots. (c) External quantum efficiency versus voltage
plots. (d) Power efficiency versus voltage plots.
1
2 nm PMMA layers respectively. At higher PMMA thickness,
3
EQE, and PE of the MAPbBr LEDs (Figure 6b-d). All the
the charge carriers experience a higher resistive potential
barrier affecting the charge injection into the emissive
perovskite layer, which results in decrease in overall luminance
of the LED device. The current density reduces and luminance
increases at a specific applied bias voltage with the optimized
thickness of PMMA interlayer. For example, a current density
measured characteristic parameters of the LEDs are presented
in table S2 (SI, Table S2). All the LED parameters like
luminance, CE, EQE and PE are found to increase in presence
of an additional PMMA interlayer with optimum thickness.
Initially, CE is found to be ~ 2.26 cdA without PMMA, which
increases for the optimal PMMA interlayer thickness and
decreases at higher thickness of PMMA. The LED device CE is
-
1
-
2
of ~ 3.0 mAcm is required to produce a luminance of ~ 200
-
2
-2
-1
-1
cdm . However, larger current densities of ~ 18.3 mAcm , ~
1
found to be ~ 2.26 cdA (without PMMA), ~ 3.56 cdA (~ 5
-
2
-2
-2
-1
-1
0.3 mAcm , ~ 4.7 mAcm , ~ 29.3 mAcm are needed for the
nm), ~ 11.88 cdA (~ 7 nm), ~ 5.23 cdA (~ 10 nm), and ~
-
1
devices without PMMA and with other PMMA
thickness-controlled devices like ~ 5 nm, ~ 10 nm and ~ 12 nm
1.32 cdA (~ 12 nm). Similar trend is observed for both the
EQE and PE of the LEDs. The EQE is found to be ~ 0.45%
(without PMMA), ~ 0.78% (~ 5 nm), ~ 2.82% (~ 7 nm), ~
1.12% (~ 10 nm) and ~ 0.28% (~ 12 nm) respectively. Hence a
six-fold enhancement in EQE has been achieved by using ~ 7
nm PMMA interlayer in comparison to the devices without
-
2
respectively to produce a luminance of ~ 200 cdm . Similarly,
higher voltage is required to generate a luminance of ~ 200
-
2
cdm for the device with increased PMMA interlayer. At
optimized PMMA thickness of ~ 7 nm, the device shows
significantly low turn ON voltage, maximum luminance and
less current density compared to other controlled devices
-
1
-1
PMMA. The PE are found to be ~ 0.70 lmW , 1.20 lmW , 4.4
-
1
-1
-1
lmW , 1.73 lmW , and 0.44 lmW for the devices without
PMMA and with 5 nm, 7 nm, 10 nm and 12 nm PMMA layers
respectively. These results emphases comparable device
(Figure 6a). These results suggest that an optimized thickness
of the PMMA interlayer is necessary to achieve improved
performance of the LED devices.
performance with the previously reported MAPbBr
LEDs using a simple PMMA interlayer.
3
based
We further measured the characteristic parameters like CE,
6

4
. Conclusion
14. H. Huang, F. Zhao, L. Liu, F. Zhang, X.-g. Wu, L. Shi, B.
Zou, Q. Pei, H. Zhong, ACS Appl. Mater. Interfaces 2015,
7, 28128-28133.
In conclusion, we have demonstrated a simple and
efficient route to improve the overall performance of solution
processed MAPbBr
3
based LEDs by introducing an insulating
15. Z.-K. Tan, R. S. Moghaddam, M. L. Lai, P. Docampo, R.
Higler, F. Deschler, M. Price, A. Sadhanala, L. M. Pazos,
D. Credgington, F. Hanusch, T. Bein, H. J. Snaith, R. H.
Friend, Nat. Nanotechnol. 2014, 9, 687-692.
16. Y.-H. Kim, H. Cho, J. H. Heo, T.-S. Kim, N. Myoung,
C.-L. Lee, S. H. Im, T.-W. Lee, Adv. Mater. 2015, 27,
1248-1254.
17. N. K. Kumawat, A. Dey, K. L. Narasimhan, D. Kabra,
ACS Photonics 2015, 2, 349-354.
18. R. L. Z. Hoye, M. R. Chua, K. P. Musselman, G. Li,
M.-L. Lai, Z.-K. Tan, N. C. Greenham, J. L.
MacManus-Driscoll, R. H. Friend, D. Credgington, Adv.
Mater. 2015, 27, 1414-1419.
additional planar PMMA interlayer in between the ZnO and
perovskite emissive layer. PMMA interlayer successfully
eliminated the shunt paths and non-radiative defect states,
thereby improving the device efficiency. We optimized the
device structure by controlling the thickness of the PMMA
interlayer to improve the LED device performance. PMMA
interlayer with ~ 7 nm thickness shows a six-fold enhancement
of the device performance in comparison to the devices without
PMMA. The best performing devices showed the improved
-
2
-1
luminance of ~ 3450 cdm , CE of ~ 11.88 cdA , EQE of ~
-
1
2
.82% and PE of ~ 4.4 lmW . Our results using a simple planar
PMMA layer within the device structure is comparable with the
reported MAPbBr based LEDs. Our work demonstrates an
3
19. X. Qin, H. Dong, W. Hu, Sci China Mater 2015, 58,
186-191.
20. J. C. Yu, D. B. Kim, G. Baek, B. R. Lee, E. D. Jung, S.
Lee, J. H. Chu, D.-K. Lee, K. J. Choi, S. Cho, M. H.
Song, Adv. Mater. 2015, 27, 3492-3500.
effective way to reduce the leakage current by controlling the
interface of the device architecture and reduction of surface
states to reduce non-radiative transitions, which could be
beneficial for the development of high performance perovskite
LEDs.
21. J. C. Yu, D. B. Kim, E. D. Jung, B. R. Lee, M. H. Song,
Nanoscale 2016, 8, 7036-7042.
Supporting Information (SI)
The file contains additional Figures, experimental
methods, tables.
22. G. Li, F. W. R. Rivarola, N. J. L. K. Davis, S. Bai, T. C.
Jellicoe, F. de la Peña, S. Hou, C. Ducati, F. Gao, R. H.
Friend, N. C. Greenham, Z.-K. Tan, Adv. Mater. 2016,
2
8, 3528-3534.
Acknowledgement
23. Q. Shan, J. Song, Y. Zou, J. Li, L. Xu, J. Xue, Y. Dong,
B. Han, J. Chen, H. Zeng, Small 2017, 13, 1701770.
24. H. Cho, S.-H. Jeong, M.-H. Park, Y.-H. Kim, C. Wolf,
C.-L. Lee, J. H. Heo, A. Sadhanala, N. Myoung, S. Yoo,
S. H. Im, R. H. Friend, T.-W. Lee, Science 2015, 350,
We acknowledge SERB, DST for the financial support.
The author G.S.K. acknowledges the DST INSPIRE program
for the fellowship. The author T.K. acknowledges the UGC for
the fellowship.
1
222-1225.
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