Multicolored organic/inorganic hybrid perovskite light-emitting diodes

Article abstract of DOI:10.1002/adma.201403751

(Graph Presented).Bright organic/inorganic hybrid perov-skite light-emitting diodes (PrLEDs) are realized by using CH₃NH₃PbBr₃ as an emitting layer and self-organized buffer hole-injection layer (Buf-HIL). The PrLEDs show high luminance, current efficiency, and EQE of 417 cd m^-2, 0.577 cd A^-1, and 0.125%, respectively. Buf-HIL can facilitate hole injection into CH₃NH₃PbBr₃ as well as block exciton quenching.

Full text of DOI:10.1002/adma.201403751

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Multicolored Organic/Inorganic Hybrid Perovskite
Light-Emitting Diodes
Young-Hoon Kim, Himchan Cho, Jin Hyuck Heo, Tae-Sik Kim, NoSoung Myoung,
Chang-Lyoul Lee, Sang Hyuk Im,* and Tae-Woo Lee*
Organic/inorganic hybrid materials have significance in that
superior properties of organic materials (e.g., low cost, flex-
ibility, simple processing, facile tuning of optical and electrical
properties), and inorganic materials (e.g., high charge-car-
be used along with organic semiconductors in light-emitting
diodes (LEDs) because OIHPs have comparable ionization
potential (IP) and electron-affinity levels with those of typical
organic semiconductors, so that similar electron- and hole-
injection barriers in OIHP LEDs to those in organic light-emit-
ting diodes (OLEDs) can be formed. In contrast, colloidal inor-
ganic QDs have deep valence-band (VB) edge (ca. 6–7 eV) and
conduction-band (CB) edge (ca. 4 eV) energy levels, and thus
have high hole-injection barriers (>1 eV) from the organic hole-
rier mobility, mechanical and thermal stability) can be com-
bined.[
1,2]
Organic/inorganic hybrid perovskites (OIHPs) are
especially promising materials for use in photoactive layers of
solar cells because they have unique properties including high
absorption coefficient, balanced electron/hole mobility, pos-
sible low-temperature processing, and smaller exciton binding
energy (37–75 meV) and longer exciton diffusion length (ca.
[
8]
injection layer (HIL) to the QDs.
Despite the OIHPs’ high color purity and comparable elec-
tronic energy levels with organic semiconductors, bright elec-
troluminescence (EL) from OIHPs has been achieved only at
liquid-nitrogen temperature due to critical intrinsic problems
such as significant thermal ionization and delocalization at
1
(
00–1000 nm) than those of organic semiconducting materials
>100 meV and ca. 10 nm).[3]
OIHPs can also be an alternative to conventional organic
emitters and inorganic quantum dot (QD) emitters. Generally,
OIHPs have a three-dimensional (R–NH )MX structure (M:
[
4,5]
room temperature. Achieving bright EL from OIHPs at room
temperature is challenging. Recently, the potential of OIHPs
as an emitter was demonstrated to achieve low-threshold
3
3
metal cation, X: halogen) in which self-organized two-dimen-
sional (2D) planes of organic layers are sandwiched between 2D
−
2
planes of inorganic layers composed of corner-sharing metal
(12 ± 2 J cm ) optically-pumped lasing with a high gain of ca.
halide octahedra (MX6).[
4,5]
Because of a large dielectric con-
250 cm (Chan's method) and ca. 40 cm (Shaklee and Lehe-
ny's method); these values are comparable to those of colloidal
−1
−1
stant difference between their organic (εorganic ≈ 2.4) and inor-
ganic layers (εinorganic ≈ 6.1), OIHPs can confine excitons in the
inorganic layers due to a multi-quantum-well-like structure;
as a result, they generate high color purity [full width at half
maximum (FWHM) ≈ 20 nm] light. Also, the emission spec-
trum can be easily tuned by adjusting the bandgap by simple
[
9]
QDs and conjugated polymer thin films. Very recently, there
was a breakthrough in LEDs based on an organometal halide
perovskite that was used to achieve a bright electrolumines-
−2
[10]
cence of 364 cd m at room temperature. Although the lumi-
nance value is still low, possibly due to the significant lumines-
cence quenching in the device, this result suggests that OIHPs
can be strong candidates as a low-cost emitter in LEDs as well
if one overcome the significant luminescence quenching of
OIHPs at room temperature, because OIHPs have advantages
such as low cost, easy solution processing, appropriate energy
levels for LEDs that are compatible with organic semiconduc-
tors, and very high color purity.
[1]
[6]
substitution of metal cations, inorganic anions, or organic
ligands.[ The color purity of OIHPs is very high with a narrow
spectral width, while organic emitting materials have low color
7]
[4,5]
purity and broad spectral width.
Furthermore, OIHPs can
Y.-H. Kim,[+] H. Cho,[+] T.-S. Kim, Prof. T.-W. Lee
Department of Materials Science and Engineering
Pohang University of Science and Technology (POSTECH)
Pohang, Gyungbuk 790–784, Republic of Korea
E-mail: twlee@postech.ac.kr
When OIHP materials with a deep IP level (≥5.6 eV) and
long exciton diffusion length (ca. 100–1000 nm) are to be
considered as an emitting layer (EML) in LEDs, one must
overcome the large hole-injection barrier and luminescence
quenching. One solution is to use a HIL which can facilitate
hole injection into the EML and block exciton quenching that
occurs on the HIL/EML interface at the same time. However,
the conventional hole-injection conducting polymer, poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS),
has a high hole-injection barrier from the indium tin oxide
[
+]
J. H. Heo , Prof. S. H. Im
Department of Chemical Engineering
College of Engineering
Kyung Hee University
1
Seochon-dong, Giheung-gu
Youngin-si, Gyeonggi-do 446–701, Republic of Korea
E-mail: imromy@khu.ac.kr
Dr. N. Myoung, Dr. C.-L. Lee
Advanced Photonics Research Institute (APRI)
Gwangju Institute of Science & Technology (GIST)
(
ITO) anode to the OIHP layer because the IP level of the
OIHP (5.6–5.9 eV) is much deeper than the work function
[
11]
1
Oryong-dong, Buk-gu, Gwangju 500–712, Republic of Korea
(WF) of PEDOT:PSS (ca. 5.2 eV). This large hole-injection
barrier (0.4–0.7 eV) limits hole injection into OIHPs and thus
limits overall charge balance in the EML. Furthermore, given
[+]These authors contributed equally to this work.
DOI: 10.1002/adma.201403751
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DOI: 10.1002/adma.201403751
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−1
face, we achieved a current efficiency (CE) of 0.577 cd A , an
external quantum efficiency (EQE) of 0.125%, and a maximum
−2
luminance of 417 cd m ; these values represent a more than
3
00-fold improvement over the LED that uses a conventional
−
PEDOT:PSS HIL. In addition, by substituting the bromide (Br )
ion in corner-sharing octahedra with chloride (Cl ) or iodide
−
−
(
I ) ions, we can fabricate multicolored PrLEDs (380 nm ≤
wavelength ≤ 800 nm). We also demonstrated flexible PrLEDs
on a plastic substrate for the first time.
We prepared the CH NH PbBr layer by spin coating an equi-
3
3
3
molar solution mixture of CH NH Br:PbBr in N,N-dimethyl-
3
3
2
formamide (DMF) and annealing the film at 90 °C for 10 min.
The X-ray diffraction (XRD) pattern of the CH NH PbBr fi lm
3
3
3
showed sharp diffraction peaks at 15.51° for the (100) plane,
3
1.36° for (200), and 47.98° for (300); these observations accord
[
14]
with those in previous papers (Figure 2a). These peaks verify
that CH NH PbBr crystals were well-fabricated and highly
3
3
3
[
15]
oriented with α-axis self-assembly. The X-ray photoelectron
spectroscopy (XPS) spectrum exhibited strong peaks due to
bromine (Br) (ca. 68.3 eV), lead (Pb) (ca. 138.3 and 143.1 eV),
carbon (C) (ca. 284.9 eV), and nitrogen (N) (ca. 402 eV), which
also indicate that the crystalline CH NH PbBr layer was well-
3
3
3
fabricated (Figure 2b). The oxidation state of Pb was estimated
to be Pb(II) by deconvoluting the Pb4f spectrum into a sum-
mation of Gaussian–Lorentzian curves (Figure S1, Supporting
Information). Also, the atomic ratio between Pb and Br were
estimated to be 1:2.61 by considering the area of the spectra and
the atomic sensitivity factors of the Pb and Br (see Supporting
Information). We used ultraviolet photoelectron spectroscopy
(
UPS) to measure the IP level of CH NH PbBr (Figure 2c,d).
3 3 3
Figure 1. a) PrLED structure using CH NH PbBr , b) cross-sectional
SEM (scanning electron microscope) images of PrLEDs.
3
3
3
The WF of CH NH PbBr was determined to be 4.9 eV from
3
3
3
secondary cut-off of the UPS spectra, and the VB edge was 1 eV
below WF; therefore the IP was 5.9 eV; this conclusion corre-
a long exciton diffusion length (ca. 100–1000 nm) of OIHPs,
PEDOT:PSS induces significant exciton quenching at the
PEDOT:PSS/OIHP interface, and thereby reduces the device
efficiency by limiting the radiative recombination of charge
carriers.[ Therefore, using a high-WF HIL that can prevent
exciton quenching in LEDs is of prime importance to achieve
high EL by overcoming a high hole-injection barrier and lumi-
nescence quenching.
[
11]
sponds well with previous reports.
[
16]
In our previous report, we found that the CH NH PbI
3
3
3
perovskite decomposed at over 55% relative humidity but
the CH NH PbBr perovskite did not decompose; rather, the
12]
3
3
3
CH NH PbBr perovskite was recovered by drying after expo-
3
3
3
sure in over 55% relative humidity because the CH NH PbBr
3
3
3
perovskite has a more stable cubic phase at room tempera-
ture; the CH NH PbI perovskite has the distorted cubic phase
Here, we report bright organic/inorganic hybrid perovskite
LEDs (PrLEDs) with high spectral purity at room temperature
that are fabricated by using OIHPs (CH NH PbBr ) as an
3
3
3
(tetragonal phase). This result means that the CH NH PbBr
3
3
3
perovskite is relatively more stable than the CH NH PbI per-
3
3
3
3
3
3
EML and a self-organized buffer HIL (Buf-HIL) composed of
PEDOT:PSS and a perfluorinated polymeric acid, tetrafluor-
oethylene-perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid
copolymer (PFI) (Figure 1a,b).[ The WF of the self-organized
gradient Buf-HIL increased gradually from the bottom surface
ovskite. We also tested the stability of CH NH PbBr fi lms in air
3 3 3
by investigating the changes in PL intensity, XRD pattern, and
apparent film quality with increasing exposure time. When the
CH NH PbBr fi lm was exposed to air, the intensity of PL peak
13]
3
3
3
slightly decreased until 10 min. After that, it stopped decreasing
and even started to be recovered with a red-shift of the peak and
was then saturated at about 1 h (Figure 2e, Figure S2a, Sup-
porting Information). The intensity and position of peaks in the
XRD pattern of the CH NH PbBr fi lms did not change with
(ca. 5.2 eV) to the top surface (ca. 5.95 eV) due to self-organiza-
tion of the PFI. This gradually increasing WF can facilitate hole
injection into the OIHPs by reducing the hole-injection energy
barrier more efficiently than can PEDOT:PSS. Furthermore, the
self-organized PFI which is enriched on the top surface of the
Buf-HIL can prevent exciton quenching that occurs at the inter-
face between HIL and EML, and can also block electron flow
into the anode. Therefore, use of the Buf-HIL at the interface
with the EML increases photoluminescence (PL) intensity and
lifetime, and improves the device efficiency. In green-emitting
PrLEDs with Buf-HIL that contained sufficient PFI on the sur-
3
3
3
increasing exposure time, which may imply that the crystal-
linity of CH NH PbBr was not much affected by oxygen and
3
3
3
moisture (Figure 2f, Figure S2b, Supporting Information). Also,
the apparent film quality and color of CH NH PbBr fi lms did
3
3
3
not change with air exposure (Figure 2g).
The Buf-HIL has a gradient WF from the bottom surface (ca.
5.2 eV) to the top surface (ca. 5.95 eV) because the proportion
2
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Figure 2. a) XRD pattern, b) XPS survey spectrum, and c) UPS spectrum of CH NH PbBr . d) Schematic energy-level diagram of ITO/Buf-HIL/
3
3
3
CH NH PbBr , e) PL spectra, f) maximum XRD peak intensities, and g) photographs of CH NH PbBr films on glass with increasing air-exposure time.
3
3
3
3
3
3
of PFI increases gradually from the lower to upper part of the
Buf-HIL because of self-organization due to low surface energy
of PFI (ca. 20 mN m ; Figure 2d). The considerable increase
of the WF of Buf-HIL (ca. 5.95 eV) can reduce the hole-injection
energy barrier more than conventional PEDOT:PSS can (WF ≈
5.2 eV). This reduced hole-injection barrier facilitated hole injec-
tion into CH NH PbBr and electron–hole recombination for
−1
[13]
3
3
3
radiative decay. As the quantity of PFI in the Buf-HIL decreased,
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the WF gradually decreased from 5.95 eV [for 1:1 weight ratio
of PEDOT:PSS to PFI (Buf-HIL11)] to 5.55 eV [for 16:1 weight
ratio of PEDOT:PSS to PFI (Buf-HIL161)] (Table S1, Supporting
Information). These decreased WFs of Buf-HIL increase the
hole-injection energy barriers from Buf-HIL11/CH NH PbBr
3
3
3
(
ca. 0 eV) to Buf-HIL161/CH NH PbBr (ca. 0.35 eV), and
3 3 3
thereby decrease hole-injection capability into OIHPs.
To characterize the hole-injection capability of Buf-HIL, we
fabricated hole-only devices with ITO/Buf-HIL/CH NH PbBr /
3
3
3
tris(4-carbazoyl-9-ylphenyl)amine (TCTA; 50 nm)/MoO (10 nm)/
3
Ag (100 nm) and measured the hole current density (Figure S3,
Supporting Information). As the quantity of PFI in Buf-HILs
increased, the hole current density gradually increased due to the
decreasing hole-injection barrier to the CH NH PbBr emitting
3
3
3
layer that arises from the increasing WF of Buf-HILs (Table S1,
Supporting Information). In contrast, the PEDOT:PSS device
showed high leakage current density in low bias (<1.2 V) and low
current density in high bias (>1.2 V) due to its low WF and the
absence of an electron-blocking self-organized PFI layer.
The luminescence quenching tended to be reduced as the
PFI concentration in the Buf-HIL compositions increased. To
prove the PFI effect on quenching of excitons, we conducted
time-resolved photoluminescence (TR-PL) with Buf-HIL11,
Buf-HIL21, Buf-HIL41, Buf-HIL161, and PEDOT:PSS to obtain
the PL lifetime of CH NH PbBr in a very thin layer (ca.
3
3
3
1
0 nm). We conducted TR-PL with samples [glass/Buf-HIL11,
Buf-HIL21, Buf-HIL41, Buf-HIL161, and PEDOT:PSS (ca.
0 nm)/CH NH PbBr (ca. 10 nm)] (Figure 3a). The PL decay
4
3
3
3
curves were well fitted by a biexponential decay fitting, which
suggests that the PL decay of CH NH PbBr took place through
3
3
3
two relaxation pathways; fast decay, due to the quenching of
free carriers and slow decay, due to radiative decay[ (Table 1).
The average PL lifetime (τavr) of Buf-HIL11/CH NH PbBr
17]
3
3
3
was much longer than that of Buf-HIL21/CH NH PbBr
3
3
3,
Figure 3. a) PL lifetime curves of CH NH PbBr obtained from TR-PL
Buf-HIL41/CH NH PbBr , Buf-HIL161/CH NH PbBr , and
3
3
3
3
3
3
3
3
3
on Buf-HIL11, Buf-HIL21, Buf-HIL41, Buf-HIL161, and PEDOT:PSS.
The samples were excited at 350 nm and their lifetimes were moni-
tored at 530 nm and b) PL intensities of thin CH NH PbBr (ca.
PEDOT:PSS/CH NH PbBr . As the quantity of PEDOT:PSS
3
3
3
increased at the surface of the HIL, PL lifetime gradually
decreased, which indicates that when an exciton is closer
to the PEDOT:PSS, exciton quenching is facilitated due to:
i) exciton dissociation driven by the large energy-level differ-
ence between the IP of CH NH PbBr (5.9 eV) and the WF of
3
3
3
1
0 nm) on Buf-HIL11, Buf-HIL21, Buf-HIL41, Buf-HIL161, and
PEDOT:PSS.
blocking the quenching of excitons at the HIL/CH NH PbBr
3
3
3
3
3
3
the under-layers (HILs), and ii) non-radiative energy transfer
interface.
from CH NH PbBr to PEDOT:PSS.[ The huge increase of
13]
In plots of current density, J (mA cm ), versus voltage of
−2
3
3
3
PL lifetime from Buf-HIL21 to Buf-HIL11 is due to the preven-
tion of exciton dissociation and hole transfer that arises from
the energy-level difference at the interface.
PrLEDs [glass/ITO/Buf-HIL/CH NH PbBr /1,3,5-tris(1-phenyl-
3
3
3
1H-benzimidazol-2-yl)benzene(TPBI)/LiF/Al] with various
Buf-HILs, devices with Buf-HIL11 had a higher current den-
sity than did other devices and the current density of devices
To further verify the effect of PFI quantity on blocking of
exciton quenching, we conducted steady-state PL with various
samples [glass/Buf-HIL11, Buf-HIL21, Buf-HIL41, Buf-HIL161,
and PEDOT:PSS (ca. 40 nm)/CH NH PbBr (ca. 10 nm)] by
Table 1. PL lifetimes obtained from TR-PL of CH3NH3PbBr3 on different
under-layers.
3
3
3
using a spectrofluorometer (Figure 3b). The samples were
excited by using a 360 nm monochromated light source. As the
ratio of PFI to PEDOT:PSS increased, the PL intensity gradually
increased. This trend is due to the increase in the thickness of
a surface-enriched PFI layer on top of the HIL that separates
Hole-Injection Layer
τ1
τ2
τavr
[
ns]
[ns]
[ns]
Buf-HIL11
Buf-HIL21
Buf-HIL41
Buf-HIL161
PEDOT:PSS
2.66
0.22
0.23
0.22
0.24
9.93
1.12
0.73
0.78
0.61
4.7
0.46
0.42
0.39
0.34
excitons generated in the emitting CH NH PbBr from the
3
3
3
quenching PEDOT:PSS. Therefore, increasing the PFI quantity
in the HIL can improve device efficiency not only by facilitating
hole injection into the emitting CH NH PbBr , but also by
3
3
3
4
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gradually decreased as the PFI quantity decreased (Figure 4a).
This trend demonstrates convincingly that as the PFI quantity
increased, WF gradually increased and facilitated hole injection
into the EML by reducing the hole-injection barrier (Table S1,
Supporting Information); this result is consistent with previous
papers which indicate that the surface WF affects hole-injec-
tion capability more than surface conductivity does, although
[
18]
the device has a thin insulating PFI layer. The high current
Figure 4. a) Current density versus voltage characteristics, b) luminance versus voltage characteristics, c) current efficiency versus voltage charac-
teristics, d) external quantum efficiency versus voltage characteristics of PrLEDs with Buf-HIL11, Buf-HIL21, Buf-HIL41, Buf-HIL81, Buf-HIL161, and
PEDOT:PSS, e) electroluminescence spectrum of PrLEDs with Buf-HIL11, f) photograph of green-emitting PrLEDs, g) electroluminescence spectra of
PrLEDs with CH NH PbCl BryI3–x–y, and h) photograph of flexible green-emitting PrLEDs on PET substrate during bending.
3
3
x
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density of devices with PEDOT:PSS is due to the absence of
an electron-blocking self-organized PFI layer. The maximum
Buf-HILs can be a very good solution to achieve bright and effi-
cient PrLEDs. Our strategies suggest that the PrLEDs that use
OIHPs as an EML should consider efficient hole injection and
blocking of exciton quenching at the HIL/EML interface and
these two important issues can be overcome by introducing
good buffer layers to prevent exciton quenching. We also dem-
−2
luminance also gradually increased with PFI from 1.38 cd m
2
(
PEDOT:PSS) to 417 cd m− (Buf-HIL11; a 302-fold improve-
ment) (Figure 4b), which is the highest brightness observed in
PrLEDs at room temperature to date.
Current efficiency (CE) and external quantum efficiency
−
onstrated multicolored PrLEDs by easy substitution of Br ions
−
−
(EQE) were measured for PrLEDs with various Buf-HILs,
with I ions and Cl ions (CH NH PbCl BryI3–x–y). We demon-
3 3 x
and with PEDOT:PSS as a HIL (Figure 4c,d). The CE and
strated flexible PrLEDs on a plastic substrate for the first time.
Although we solved both the problem of hole injection from
the anode into OIHP with deep IP level and the problem of
exciton quenching at HIL/OIHP interface by introducing the
self-organized Buf-HIL, our device performance is still lower
than that of OLEDs and QD LEDs due to their intrinsic thermal
ionization and delocalization in perovskite layers. Thus, fur-
ther studies on tuning of organic ligand, inorganic anion, and
metal cation should be conducted to boost the performance of
PrLEDs by confining exciton and blocking thermal ionization
and delocalization.
Our work may suggest a new route to solve the main issues
of conventional LEDs. Conventional organic or inorganic emit-
ting materials have inherent problems: i) organic emitting
materials have low color purity and need complex chemical syn-
thesis processes to enable them to emit different colors, which
renders the devices expensive for practical applications, and
ii) colloidal inorganic QDs have an impediments to their com-
mercialization in their high hole-injection barrier that arises
from their intrinsically deep VB-edge level, inhomogeneous
color purity that comes from the size distribution due to the
quantum size effect, and the need to fabricate very thin single/
double layers of QDs. Therefore, our use of a thick OIHP layer
as an EML instead of organic emitting materials and inor-
ganic QDs is a valuable method to produce efficient flat-panel
or flexible displays and solid-state lighting with high color
purity with processes suitable for mass production such as
roll-to-roll.
−1
EQE of devices with Buf-HIL11 (0.577 cd A , 0.125%) were
greater than those of devices with other HILs: Buf-HIL21, Buf-
HIL41, Buf-HIL81, Buf-HIL161, and PEDOT:PSS (Table 2).
The improved maximum luminance, CE, and EQE according
to the quantity of PFI indicate that PFI-containing Buf-HILs
have outstanding capability for hole injection due to gradu-
ally increasing WF and blocking of exciton quenching; these
capabilities are very important to achieve high efficiency in our
PrLEDs. The EL spectra showed sharp green emission centered
at 543 nm and high color purity (FWHM ≈ 20 nm; Figure 4e,f).
We also achieved the tuning of EL spectrum by easy substitu-
tion of Br ions with I ions and Cl ions (CH NH PbCl Br I ;
3
3
x
y 3-x–y
Figure 4g). The bandgap tuning of OIHPs arose from the struc-
−
−
tural crystal difference due to the different ionic size of Cl , Br
−
[16]
and I . The EL spectra can be tuned to have maximum peaks
at 393, 543, and 763 nm, which are coincident with the band-
gaps of CH NH PbCl (ca. 3.1 eV) CH NH PbBr (ca. 2.3 eV),
3
3
3
3
3
3
[11,15]
and CH NH PbI (ca. 1.5 eV).
These visible spectra dem-
3
3
3
onstrate the possibility of wide wavelength tunability of OIHPs
by simply mixing with other ions. Finally, we fabricated flexible
PrLEDs (PET/ITO/Buf-HIL/CH NH PbBr /TPBI/LiF/Al) and
3
3
3
demonstrated the flexibility of OIHPs (Figure 4h, Figure S4a–f,
Supporting Information).[
19]
In conclusion, we have demonstrated organic/inorganic
hybrid PrLEDs with a sharp green emission (FWHM ≈ 20 nm)
by using CH NH PbBr as an EML and self-organized Buf-
3
3
3
−
2
HIL, and achieved high luminance, CE, and EQE (417 cd m ,
.577 cd A , 0.125%) values which represent more than a
00-fold improvement over the LED that uses a conventional
−
1
0
3
PEDOT:PSS HIL. The self-organized Buf-HIL not only facilitates
hole injection into the EML by reducing the hole-injection bar-
rier from the ITO anode to the IP of CH NH PbBr , but also
blocks the exciton quenching at the HIL/EML interface. PL
and TR-PL experiments showed that the blocking capability of
exciton quenching of our Buf-HILs increases with PFI quantity.
This excellent capability of blocking exciton quenching in our
Experimental Section
Preparation of CH3NH3PbBr3 Solution: For the synthesis of
3
3
3
CH NH PbBr perovskite, we first synthesized CH NH Br by reacting
3
3
3
3
3
5
0 mL of hydrobromic acid (48% in water, Aldrich) and 30 mL of
methylamine (40% in methanol, Junsei Chemical Co. Ltd.) in a 250-mL
round-bottom flask (RBF) at 0 °C for 2 h while stirring. We collected the
white precipitate by evaporation of solvents at 50 °C for 1 h and purified
the products by dissolving in ethanol, recrystallizing from diethyl ether,
and drying at room temperature in a vacuum oven for 24 h. Finally,
we prepared a 40 wt% CH NH PbBr solution by reacting equimolar
Table 2. Maximum current efficiency, maximum external quantum effi-
ciency, and maximum luminance of PrLEDs with different under-layers.
3
3
3
CH NH Br and PbBr (Aldrich) in DMF at 60 °C for 30 min.
3 3 2
OLED Fabrication: ITO patterned glasses were sonicated twice
in acetone and once in 2-propanol for 15 min each, then boiled in
2-propanol for 30 min and dried in the oven. After these steps, the
glasses were treated with UV-ozone to make the surface hydrophilic.
On the UV-ozone-treated surface, PEDOT:PSS and Buf-HIL were spin
coated to make a layer of 40-nm thickness, then baked at 150 °C for
Hole-Injection
Layer
Max. CE
Max. EQE
[%]
Max. Luminance
[cd A−1
−2
]
[cd m ]
Buf-HIL11
Buf-HIL21
Buf-HIL41
Buf-HIL81
Buf-HIL161
PEDOT:PSS
0.577
0.243
0.125
0.0526
417
159
3
0 min. Each sample was transferred into a glove box and CH NH PbBr
3 3 3
0.0490
0.0180
0.0144
0.00165
0.0106
23.5
12.6
10.1
1.38
solutions dissolved in DMF (40 wt%) were spin coated onto them.
The samples were then baked at 90 °C for 10 min, then TPBI (50 nm),
LiF (1 nm) and Al (100 nm) were thermally deposited in sequence in
0.0039
0.00317
0.000393
−
1
a high-vacuum chamber with a deposition rate of 1, 0.1, and 3 Å s ,
−7
respectively (<10 Torr).
6
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Adv. Mater. 2014,
DOI: 10.1002/adma.201403751

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OLED Characterization: The current–voltage-luminance characteristics
were measured by using a Keithley 236 source measurement and a
Minolta CS2000 spectroradiometer.
Time-Resolved Photoluminescence (TR-PL) Measurement: The excitation
source for TR-PL was a frequency-doubled mode-locked Ti:Sapphire
laser (approximately 150-fs pulse duration and 80 MHz repetition rate);
Chameleon Ultra II, Coherent Inc, equipped with an external pulse-picker
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