Highly luminescent and stable layered perovskite as the emitter for light emitting diodes

Article abstract of DOI:10.1002/pssa.201600246

Air instability and poor exciton recombination of 3D perovskites MAPbX₃ (MA = CH₃NH₃, X = halogens) seriously hinder their applications in light emitting diodes. Herein, we report a promising alternative to solve these two critical drawbacks. Layered perovskite OA₂(MA)_n?1Pb_nBr_3n+1 (OA = C₈H₁₇NH₃) has higher binding energy and is passivated by long organic chain, which can be synthesized using a facile method. By increasing the OA⁺ ratio in layered perovskite, strong quantum confinement effect and obvious features of exciton were observed in photoluminescence and UV-Vis absorption spectra. Notably, the photoluminescence quantum yield (PLQY) of (OA)₂(MA)₂Pb₃Br₁₀ (n = 3 layered perovskite) can be up to 67.3% due to the enhanced exciton recombination, significantly higher than its 3D counterpart. Moreover, layered perovskite exhibits promoted stability in air than that of the 3D perovskite. The layered perovskite (OA)₂(MA)₂Pb₃Br₁₀-based perovskite light emitting diodes (PeLEDs) with a maximum current efficiency, a maximum power efficiency and the external quantum efficiency (EQE) of 1.43 cd A^?1, 0.89 lm W^?1, and 0.53% was demonstrated, which can be compared with that of the best-reported perovskite quantum dots LEDs so far. The demonstration of layered perovskite renders its bright future in optoelectronic applications, such as displays and photodetections.

Full text of DOI:10.1002/pssa.201600246

Phys. Status Solidi A, 1–6 (2016) / DOI 10.1002/pssa.201600246
a
www.pss-a.com
applications and materials science
Highly luminescent and stable layered
perovskite as the emitter for light
emitting diodes
1
1
2
2
1,3
2
*,1,3
Mingyang Wei , Weihai Sun , Yang Liu , Zhiwei Liu , Lixin Xiao , Zuqiang Bian , and Zhijian Chen
1
State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P.R. China
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications,
2
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China
Co-Innovation Center for Micro/Nano Optoelectronic Materials and Devices, Chongqing University of Arts and Sciences,
3
Yongchuan Chongqing 402160, P.R. China
Received 30 March 2016, revised 5 May 2016, accepted 25 May 2016
Published online 9 June 2016
Keywords layered perovskite, PeLEDs, quantum wells exciton recombination
*
Corresponding author: e-mail zjchen@pku.edu.cn, Phone: þ86-10-6275-4990, Fax: þ86-10-6275-6567
Air instability and poor exciton recombination of 3D perovskite) can be up to 67.3% due to the enhanced exciton
perovskites MAPbX
3
(MA ¼ CH
3
NH
3
, X ¼ halogens) seri- recombination, significantly higher than its 3D counterpart.
ously hinder their applications in light emitting diodes. Herein, Moreover, layered perovskite exhibits promoted stability in air
we report a promising alternative to solve these two critical than that of the 3D perovskite. The layered perovskite
drawbacks. Layered perovskite OA
) has higher binding energy and is passivated by (PeLEDs) with a maximum current efficiency, a maximum
long organic chain, which can be synthesized using a facile power efficiency and the external quantum efficiency (EQE) of
2
(MA)nꢀ1Pb
n
Br3nþ1 (OA (OA)
2 2 3
(MA) Pb Br10-based perovskite light emitting diodes
¼
8 3
C H17NH
þ
ꢀ1
ꢀ1
method. By increasing the OA ratio in layered perovskite, 1.43 cd A , 0.89 lm W , and 0.53% was demonstrated,
strong quantum confinement effect and obvious features of which can be compared with that of the best-reported
exciton were observed in photoluminescence and UV-Vis perovskite quantum dots LEDs so far. The demonstration of
absorption spectra. Notably, the photoluminescence quantum layered perovskite renders its bright future in optoelectronic
yield (PLQY) of (OA) (MA) Pb Br
10
(n ¼ 3 layered applications, such as displays and photodetections.
2
2
3
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction Organic–inorganic hybrid perov- [15, 16] of perovskite MAPbX . In order to obtain highly
3
skites MAPbX (MA ¼ CH NH , X ¼ halogens) have been efficient PeLEDs, many works have been done to solve the
3
3
3
employed as an important absorber for solar cells [1–3] due first problem such as the synthesis of perovskite quantum
to its low cost and excellent optoelectronic properties such dots (QDs) [17, 18] and the nanograin engineering of
as long charge carrier diffusion length and high charge perovskite thin films [19]. With unremitting efforts, the
carrier mobility (for solution-processed semiconductors) [4, photoluminescence quantum yield (PLQY) of perovskite
5
], which have made the power conversion efficiency (PCE) QDs have already approached unity [18, 20], and the current
ꢀ
1
of perovskite solar cell surpass 20% [6]. In addition, efficiency of PeLEDs with 42.9 cd A could be achieved by
the excellent photoluminescence (PL) properties of the the nanograin engineering [19], which is comparable with
organic–inorganic perovskites also make them as the the phosphorescent OLEDs. However, the stability issue has
potential emitter for light emitting diodes [7]. Thus, it not been apparently improved due to the instable nature of
has been seen the growing interests for perovskite light perovskite QDs and thin films [21]. Thus, methods to
emitting diodes (PeLEDs) since 2014 [7–11].
Unfortunately, two intrinsic problems of perovskite of perovskite at the same time are strongly desired.
have largely limited the potential of PeLEDs, which are The layered perovskite may be a good candidate for
enhance the exciton recombination and improve the stability
weak exciton recombination [12–14] and poor stability chemically stable and high-efficiency PeLEDs. Layered
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
p s s
2
M. Wei et al.: Highly luminescent and stable layered perovskite
perovskite is the 2D counterpart of bulk perovskite (DMF) to form clear precursors with molar ratio of three
MAPbX . It is achieved by partially substituting the short solutes following the structure formula of layered perovskite
3
þ
þ
þ
þ
3
organic cation MA (MA ¼ CH NH ) with the long (OA) (MA) Pb Br
(n ¼ 2, 3, 4, 5, 1). Then 0.1 ml
3
2
nꢀ1
n
3nþ1
þ
þ
3
organic cation such as OA (OA ¼ C H NH ) so that precursor was dipped into 10 ml chlorobenzene under
8
17
the inorganic layers are separated into two-dimensional vigorous stirring to induce the formation of layered
þ
form [22]. When increasing the ratio of OA in layered perovskite. Notably, no oleic acid was injected into
perovskite, each inorganic layer will contain less corner- chlorobenzene during the synthesis of layered perovksite.
shared inorganic octahedral layers as shown in Fig. 1a, For (OA) PbBr , the concentration was ten times smaller in
2
4
leading to the thinner inorganic layer and stronger quantum order to form stable dispersion. (OA) MAPb Br Cl was
2
2
7ꢀx
x
confinement effect [23, 24]. Therefore, layered perovskite synthesized using the same method while the solutes were
can have strong exciton recombination under high changed to MABr, OABr, PbBr , and PbCl with the molar
2
2
þ
OA concentration [24]. Furthermore, the stability of ratio depending on the structure formula of the wanted
layered perovskite is better than 3D perovskite MAPbX3 dispersion. The concentration of (OA) MAPb Br in
due to the large van der Waals force of OA [25]. As a chlorobenzene was controlled by changing the concentration
result, layered perovskite could be an ideal alternative to of precursors in DMF with the following procedure
enhance the exciton recombination and stability of remain the same.
2
2
7
þ
perovskite simultaneously.
In this work, we systematically studied the influence
of OA ratio of layered perovskite on the optical properties Vis spectra were measured using a Shimadzu UV-3150
2.3 Characterization of layered perovskite UV-
þ
and explored its potential for optoelectronic devices. UV-Vis/NIR spectrophotometer. PL spectra (lexc ¼ 350 nm)
þ
By properly controlling OA ratio, the PLQY of were obtained using a FluroMax-2 (Jobin-Yvon-Spex)
layered perovskite can be up to 70 %. More encouragingly, luminescence spectrometer. PLQY was measured on a
the photoluminescence (PL) intensity of layered Nanolog FL3-2iHR spectrometer with an integrating sphere.
perovskite remained almost unchanged without special Structure analysis of layered perovskite samples were
protective measures after a week in chlorobenzene, while carried out by X-ray diffractmeter (XRD, Philips Xpert)
the quenching of 3D perovskite was obvious with time. using Ni filtered CuK (l ¼ 0.154 nm, radiation at 45 KV
a
Finally, we fabricated light emitting diodes using layered and 40 mA). The samples were prepared on quartz by spin-
perovskite as emitter whose device performance has already coating the layered perovskite dispersion (0.02 M in
been comparable with perovskite QDs-based PeLEDs. toluene) at a rate of 3000 rpm for 60 s, and cured at
These results suggest the promising future of layered 110 8C for 10 min.
perovskite for optoelectronic applications.
2
.4 Device fabrication and characterization The
ITO substrates were cleaned by sonication sequentially in
.1 Synthesis of MABr and OABr MABr was deionized water, acetone, and isopropyl alcohol. The
2
2
Experimental
synthesized by reacting methylamine (33 wt.% in ethanol) PEDOT:PSS (Clevios P VP Al 4083) layer was spin-coated
and hydrobromide acid (48 wt.% in water) with the molar on the UV-ozone-treated ITO substrates at a spin rate of
ratio of 1:1 in an ice bath for 2 h with stirring followed by 3000 rpm for 30 s and dried at 140 8C for 10 min in air. Then
vacuum drying and washing with diethyl. OABr were the substrates were transformed into a N -filled glovebox.
2
ꢀ
1
prepared using the same method by mixing n-octylamine The Poly-TPD dissolved in chlorobenzene (10 mg ml
)
and hydrobromic acid in a 1:1 molar ratio.
was also spin-coated on the PEDOT:PSS layer at 2000 rpm
for 60 s, followed by thermal annealing at 110 8C for 30 min.
2
.2 Synthesisoflayeredperovskite MABr, OABr, The layered perovskite layer was then deposited on the ITO/
PbBr (22.02 mg) was dissolved in 1 ml dimethylformamide PEDOT:PSS/poly-TPD layer by spin-coating the layered
2
Figure 1 (a) Structure of layered perovskites
(
OA) (MA) Pb Br
(n¼ 1, 2, 3). (b) The
2
nꢀ1
n
3nþ1
samples of layered perovskite (OA) MAPb Br
2
2
7
(
left) and (OA)
2 2 3
(MA) Pb Br10 (right) in chloro-
benzene under UV excitation.
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
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Phys. Status Solidi A (2016)
3
perovskite dispersion (0.02 M in toluene) at a rate of previous results [23, 24] mentioned in Section 1 and
3
4
000 rpm for 60 s, and cured at 110 8C for 10 min. The structure illustrations shown in Fig. 1a. A more detailed
0 nm TmPyPB, 1 nm LiF, and 100 nm Al layers were discussion can be found in the Supporting Information.
ꢀ
6
thermally deposited under a base pressure of 3 ꢁ 10 Torr
Using the layered perovskite (OA) (MA) Pb Br
2 nꢀ1 n 3nþ1
at deposition rates (monitored with a quartz-oscillator) of (n ¼ 1, 2, 3, 4, 5, 1) dispersions, we studied the effects of
̊
ꢀ1
̊
ꢀ1
̊
ꢀ1
1
A s for TmPyPB, 0.1 A s for LiF, and 4–5 A s for the thickness of each inorganic layer on the optical
2
the Al layer with a device active area of 1 mm . The EL properties by measuring the UV-Vis absorption spectra
spectra were measured with a PR-650 spectrofluorometer by and PL spectra. In Fig. 2a, when n was decreased from 1 to
continually increasing the applied bias voltage using a 1 which means each inorganic layer became thinner, the PL
Keithley 2611 System SourceMeter.
peak of layered perovskite was shifted from 544 to 441 nm.
This blue shift of spectra can be explained by the quantum
3
Results and discussion Layered perovskite confinement effect. Since charge carriers mainly move in
þ
was synthesized under a facile method. OABr, MABr, the inorganic layer of layered perovskite while the OA
PbBr2 (0.06 M) were dissolved in good solvent N, cations are the insulating layer [24], each inorganic layer in
N-dimethylformamide (DMF) to form clear precursors. layered perovskite can be regarded as the quantum well and
The molar ratio of three solutes is determined by the the thickness of each quantum well is tuned by the number
wanted structure formula of layered perovskite of lead bromide octahedral layers each inorganic layer
(
OA) (MA) Pb Br
where n represents the number contains. Thus, thinner inorganic layer will lead to the
2
nꢀ1
n
3nþ1
of lead bromide octahedral layers contained in an individual enhancement of quantum confinement effect which is
inorganic layer. For 3D perovskite, n ¼ 1 while for the common for the quantum well [26], resulting in the blue
thinnest 2D inorganic layer, n ¼ 1. Then 0.1 ml precursors shift of spectra. It is obvious that quantum confinement
were dropped into 10 ml poor solvent chlorobenzene to form effect is the intrinsic property of layered perovskite which
layered perovskite dispersions as shown in Fig. 1b. For can even exist in bulk materials. However, for perovskite
n ¼ 2, 3, 4, 5, 1, layered perovskite dispersions can exist QDs, the quantum confinement effect is arisen from the size
very stably in chlorobenzene. But for n ¼ 1, the solution effect of each particle [18]. Compared with the difficultly of
tended to be cloudy in several minutes under the same controlling the particle size of perovskite QDs, the synthesis
concentration. Only the concentration of (OA) PbBr was of layered perovskite has higher reproducibility.
2
4
ten times smaller, it can exist stably in chlorobenzene.
Notably the synthesis procedure for layered perovskite maximum (FWHM) of PL for layered perovskite (n ¼ 1, 3,
OA) (MA) Pb Br
Furthermore, it can be found that the full width at half
(
is simpler than the fabrication of 4) is very narrow (ꢂ25 nm), which is appropriate for high
2
nꢀ1
n
3nþ1
all-inorganic perovskite CsPbBr QDs where the typical color purity phosphors [21]. But for n ¼ 2 layered
3
reaction temperature is as high as 170 8C [18]. The structure perovskite, two emission peaks appeared in PL spectra
of the layered perovskite synthesized was analyzed by XRD (Fig. 2a). Interestingly, the proportion of the intensity for
as shown in Fig. S1 (see Supporting Information (SI), online two peaks was influenced by the concentration of layered
at: www.pss-a.com), which corresponds well with the perovskite solution (Fig. S2, SI). It has been reported [27]
Figure 2 (a) PL spectra and (b) UV-Vis absorption spectra of layered perovskite (OA)
2
(MA)nꢀ1Pb
n
Br3nþ1 (n ¼ 1, 2, 3, 4, 5, 1).
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a
p s s
4
M. Wei et al.: Highly luminescent and stable layered perovskite
Figure 3 (a) PLQY of layered perovskite (OA) (MA) Pb Br3nþ1, (b) PL intensity of layered perovskite for n ¼ 3 tested every 24 h,
2
nꢀ1
n
comparing with its 3D counterpart.
þ
that the structure distortion can occur when the number of layered perovskite is more suitable than all OA -substituted
stacked inorganic layers is varied. Considering the number layered perovskite for optoelectronic applications consider-
of stacked layers relates closely with the solution ing their exciton features are already clear.
concentration, we speculated two structure phases of
The PLQY measurements were also conducted to find
layered perovskite co-exist in dispersions which caused out whether there is an optimum value of n for
the broadening of PL spectra. However, it needs further optoelectronic applications, as shown in Fig. 3a. For 3D
explanation why such phenomenon happened uniquely for perovskite, its PLQY is less than 1% suggesting poor crystal
n ¼ 2 layered perovskite. The more explicit study is quality and dominant nonradiative recombination. After the
þ
currently underway.
incorporation of OA , the PLQY of layered perovskite
In layered perovskite, electrons and holes are more increases significantly. For n ¼ 4 and 5, PLQYs reach 20%,
tightly bonded than 3D perovskite since they are all confined which may be caused purely by the protective effect of long
in a thin inorganic layer owing to the intrinsic quantum well
structure [26, 28]. Thus, we can expect layered perovskite
has higher exciton binding energy and more significant
exciton recombination than 3D perovskite, which are crucial
for the development of PeLEDs [19]. Indeed as shown in
Fig. 2b, there is no sign of exciton absorption peak near the
band edge of absorption spectra when n ¼ 4, 5, 1 due to the
loose bond, but when we further reduced the thickness of
each quantum well (n ¼ 1, 2, 3), exciton absorption peak can
be clearly observed, suggesting the existence of significant
exciton recombination. Again we can find two exciton
absorption peaks for n ¼ 2 in accordance with the two
emission species in PL spectra. By proper fitting of the
absorption data (Fig. S3, SI), we calculated that the exciton
binding energy for n ¼ 1 is as high as 510 meV, which is
nearly sixfold enlargement for that of 3D perovskite
(84 meV) and much higher than the kinetic energy under
ambient temperature (26 meV) [29]. Even though this high
binding energy is favorable for PeLEDs, the cloudy
phenomenon for the n ¼ 1 layered perovskite under high Figure 4 EL spectra of layered perovskite LEDs under different
concentration in chlorobenzene may indicate n ¼ 2 or n ¼ 3 applied bias voltages.
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
Phys. Status Solidi A (2016)
5
Figure 5 (a) J–V characteristics, luminance–
V characteristics, (b) current efficiency and
power efficiency as a function of voltage for
layered perovskite LEDs.
organic chains [18] since the exciton features are not very designed in the following structure: ITO/PEDOT:PSS/Poly-
clear for n ¼ 4 and 5. But for n ¼ 3 layered perovskite where TPD/(OA) (MA) Pb Br /TmPyPB/LiF/Al. The PEDOT:
2
2
3
10
exciton features start to be exhibited, the strong exciton PSS layer and Poly-TPD layer serve as the hole transporting
recombination and the protective effect of long organic layers (HTLs) when TmPyPB layer serves as the electron
chain may both help the PLQY reach up to 67.3% which is transporting layer (ETL). Layered perovskite layer (0.02M in
already comparable with MAPbBr QD’s and nonshelled toluene) was spin-coated onto the Poly-TPD. This simple
3
CdSe QDs [17]. It needs to note that there is still room to structure is typically used for solution-processed LEDs [30].
improve the PLQY of layered perovskite by purification and The detailed fabrication methods are shown in Section 2. As
surface passivation. However, when n is below 3, the PLQY shown in Fig. 4, we observed green electroluminescence
of perovskite gradually decreased even though the exciton which centered at 508 nm under applied bias voltage higher
binding energy is higher than that of layered perovskite for than 3.5 V. No wavelength shift of EL spectra could be
þ
n ꢃ 3. Therefore, we suggested under high OA ratio, observed under different applied bias voltage and the FWHM
layered perovskite may not maintain very high crystal of EL spectra is also very narrow (27 nm). The maximum
2
quality leading to the rapid quenching.
luminance of 704 cd/m was obtained at 6 V as shown in
For PeLEDs, the stability of perovskite is another most Fig. 5a. After 6 V, the luminance of device gradually faded.
important issue we primarily concern as mentioned in the The current efficiency and power efficiency of the device is
introduction part. Since the strong van der Waals force in exhibited in Fig. 5b. The device reached its maximum current
the existence of long organic chain can effectively stabilize efficiency of 1.43 cd/A and the maximum power efficiency of
perovskite materials [25], we expected (OA) (MA) Pb Br
0.89 lm/W at 4.5 V with the external quantum efficiency
had prolonged longevity compared with 3D perovskite. (EQE) of 0.53%.
Encouragingly, the PL intensity of layered perovskite In Table 1, we compared our device performance with
OA) (MA) Pb Br exhibited no apparent decrease within that of perovskite QDs-based PeLEDs in the literatures
2
2
3
10
(
2
2
3
10
1
week (Fig. 3b). Meanwhile, the PL intensity of 3D
[31–34]. It can be seen that our device performance has
perovskite was only 30% of the original value after 50 h, already been comparable with that of CsPbBr QDs-based
3
indicating the layered perovskite has much better stability PeLEDs and MAPbBr QDs-based PeLEDs reported so
3
than that of 3D perovskite. The enhanced stability as well as far. But considering the high PLQY of perovskite QDs
the stronger exciton recombination of (OA) (MA) Pb Br
[17, 18, 32] and layered perovskite reported in this work,
2
2
3
10
compared with 3D perovskite suggests it can be an ideal none of these devices reached their full potential. The
emitter for PeLEDs in consideration of device lifetime and further research for raising the device performance will
efficiency.
focus on the optimization of surface coverage and balancing
Finally, we fabricated device by using (OA) (MA)2 the injection of charge carriers. Furthermore, the layered
2
Pb Br as the emitter for PeLEDs. The device structure is perovskite (OA) (MA) Pb Br could be an alternative
3
10
2
2
3
10
Table 1 Comparison of device performances of layered perovskite LEDs and perovskite QDs LEDs.
2
emitter
lmax (FWHM) (nm)
V
on (V)
max. EQE (%)
max. CE (cd/A)
max. L (cd/m )
Ref.
MAPbBr QDs
524 (24)
514 (23)
527 (18)
516 (18)
508 (27)
2.9
4.2
3.0
2.5
3.5
1.1
4.5
250
946
407
1377
704
[31]
[32]
[33]
[34]
3
CsPbBr
CsPbBr
CsPbBr
3
3
3
QDs
QDs
QDs
0.12
0.008
0.06
0.53
0.43
0.035
0.19
1.43
(
OA) (MA) Pb Br
this work
2
2
3
10
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ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
p s s
6
M. Wei et al.: Highly luminescent and stable layered perovskite
candidate for other optoelectronic applications such as
photodetectors or phototransistors in consideration of its
long-term stability and strong exciton recombination. The
relevant works are also being conducted.
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4
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perovskite (OA) (MA) Pb Br
by partially replacing MA with OA in perovskite
precursor. By increasing the ratio of OA in perovskite,
(n ¼ 1, 2, 3, 4, 5, 1)
2
nꢀ1
n
3nþ1
þ
þ
þ
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2
3
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[
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[
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(
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as an emitter and obtained the
2
2
3
10
maximum current efficiency of 1.43 cd/A, which has an
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[
[
Supporting Information Additional supporting infor-
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Acknowledgements We acknowledge the financial support
from the National Natural Science Foundation of China (11574009).
M. Wei is also supported by the President’s Fund for Undergraduate
Research of Peking University.
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