Perovskite light-emitting diodes based on
solution-processed self-organized multiple
quantum wells
Nana Wang, Lu Cheng, Rui Ge, Shuting Zhang, Yanfeng Miao, Wei Zou, Chang Yi, Yan
Sun, Yu Cao, Rong Yang, Yingqiang Wei, Qiang Guo, You Ke, Maotao Yu, Yizheng Jin,
Yang Liu, Qingqing Ding, Dawei Di, Le Yang, Guichuan Xing, He Tian, Chuanhong Jin,
Feng Gao, Richard H. Friend, Jianpu Wang and Wei Huang
Journal Article
N.B.: When citing this work, cite the original article.
Original Publication:
Nana Wang, Lu Cheng, Rui Ge, Shuting Zhang, Yanfeng Miao, Wei Zou, Chang Yi, Yan Sun,
Yu Cao, Rong Yang, Yingqiang Wei, Qiang Guo, You Ke, Maotao Yu, Yizheng Jin, Yang Liu,
Qingqing Ding, Dawei Di, Le Yang, Guichuan Xing, He Tian, Chuanhong Jin, Feng Gao,
Richard H. Friend, Jianpu Wang and Wei Huang, Perovskite light-emitting diodes based on
solution-processed self-organized multiple quantum wells, Nature Photonics, 2016. 10(11),
pp.699-+.
http://dx.doi.org/10.1038/NPHOTON.2016.185
Copyright: Nature Publishing Group
http://www.nature.com/
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-133002
1
Perovskite Light-Emitting Diodes based on Solution-Processed, Self-Organised
Multiple Quantum Wells
Nana Wang
1*
, Lu Cheng
1*
, Rui Ge
1*
, Shuting Zhang
1
, Yanfeng Miao
1
, Wei Zou
1
,
Chang Yi
1
, Yan Sun
1
, Yu Cao
1
, Rong Yang
1
, Yingqiang Wei
1
, Qiang Guo
1
, You Ke
1
,
Maotao Yu
1
, Yizheng Jin
2
, Yang Liu
3
, Qingqing Ding
4
, Dawei Di
5
, Le Yang
5
,
Guichuan Xing
1
, He Tian
4
, Chuanhong Jin
4
, Feng Gao
6
, Richard H. Friend
5
, Jianpu
Wang
1
and Wei Huang
1,7
1
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),
Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech
University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China
2
Center for Chemistry of High-Performance and Novel Materials, State Key Laboratory of Silicon
Materials, and Department of Chemistry, Zhejiang University, Hangzhou 310027, China
3
State Key Laboratory of Silicon Materials, Center for Chemistry of High-Performance and Novel
Materials, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou
310027, China
4
Center of Electron Microscope, State Key Laboratory of Silicon Material, School of Material
Science & Engineering, Zhejiang University, Hangzhou 310027, China
5
Cavendish Laboratory, Cambridge University, JJ Thomson Avenue, Cambridge, CB3 0HE, UK
6
Biomolecular and Organic Electronics, IFM, Linköping University, Linköping 58183, Sweden
7
Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced
Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials
(SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023,
China
*
These authors contributed equally to this work.
2
Organometal halide perovskites can be processed from solutions at low temperatures
to form crystalline direct-bandgap semiconductors with promising optoelectronic
properties
1–5
. However, the efficiency of their electroluminescence is limited by
nonradiative recombination associated with defects and leakage current due to
incomplete surface coverage
6–9
. Here we demonstrate a solution-processed perovskite
light-emitting diode (LED) based on self-organised multiple quantum wells (MQWs)
with excellent film morphology. The MQW-based LED exhibits a very high external
quantum efficiency of up to 11.7%, good stability and exceptional high-power
performance with an energy conversion efficiency of 5.5% at a current density of
100 mA cm
-2
. This outstanding performance arises because the lower bandgap regions
which generate electroluminescence are effectively confined by perovskite MQWs
with higher energy gaps, giving very efficient radiative decay. Surprisingly, there is no
evidence that the large interfacial areas between different bandgap regions cause
luminescence quenching.
Recently, high-efficiency photovoltaic devices based on three-dimensional (3D)
organometal halide perovskites, such as CH
3
NH
3
PbI
3
and NH
2
CH=NH
2
PbI
3
(FAPbI
3
), have been demonstrated
3–5
. Hybrid 3D perovskites exhibit high
photoluminescence quantum efficiencies (PLQEs) and good charge mobilities,
making them also attractive for electroluminescence (EL) applications
2,8,9
.
Encouraging performance metrics of light-emitting diodes (LEDs) based on 3D
perovskites, such as low turn-on voltages and external quantum efficiencies (EQEs) of
up to 3.5% at high current densities, have been demonstrated
9
. However, the EL
quantum efficiency is far behind the limit predicated by ~70% PLQE of the 3D
perovskites, mainly due to the existence of current losses caused by incomplete
surface coverage of the perovskite films and the fact that the high PLQE can only be
obtained at high excitations
8,9
. By using thick (>300 nm) perovskite films, Cho et al.
obtained LEDs with over 8% EQE
10
. However, for this device, the turn-on voltage is
high and the power efficiency is low, which may result from the thick perovskite layer
used. In order to further enhance the performance of 3D perovskite-based LEDs, it is
3
essential to obtain perovskite thin films with both complete surface coverage and high
PLQE
8–10
. Moreover, device stability, which was proven to be a vital issue in
organic-inorganic halide perovskite-based photovoltaics
11
, has not been addressed in
perovskite LEDs.
The 3D perovskites are actually an extreme case of layered organometal halide
perovskites with a general formula of L
2
(SMX
3
)
n-1
MX
4
, where M, X, L, and S are a
divalent metal cation, a halide, and organic cations with long and short chains,
respectively (Fig. 1a)
12–14
. Here n is the number of semiconducting MX
4
monolayer
sheets within the two organic insulating layers (cation L), with n=∞ corresponding to
the structure of a 3D perovskite SMX
3
. With smaller numbers of MX
4
layers,
quantum confinement effects, such as an increase in bandgap and exciton energy,
become important
6,15
. In consequence, the layered perovskites naturally form
quantum-well structures. At the opposite extreme, when n=1, the layered perovskites
form a monolayer structure of a two-dimensional (2D) perovskite L
2
MX
4
. The 2D
L
2
MX
4
perovskites generally have good film-formation properties
13
. Nevertheless,
the PLQEs of the 2D perovskites are rather low at room temperature, owing to fast
exciton quenching rates
6,7
. LEDs based on the 2D perovskites have been attempted,
while the devices are either very low in efficiency or only operational at cryogenic
temperatures
16–18
. Here we demonstrate very efficient (up to 11.7% EQE) and
high-brightness EL achievable at room temperature by using solution-processed
perovskite multiple quantum wells (MQWs) with an energy cascade, which can
combine the advantages of 2D and 3D perovskites. We note, a relevant perovskite
LED work
19
which shows a peak EQE of 8.8% has been published online during the
revision of this paper.
A precursor solution of 1-naphthylmethylamine iodide (NMAI), formamidinium
iodide (FAI), and PbI
2
with a molar ratio of 2:1:2 dissolved in
N,N-dimethylformamide (DMF) was used to deposit perovskite films (see Methods
for details), which are abbreviated as NFPI
7
films below. Atomic force microscopy
4
(AFM) measurements show that the NFPI
7
film has a smooth and uniform surface
coverage (Fig. 1b). The root-mean-square roughness of the NFPI
7
film, 2.6 nm, is
comparable to that of the 2D (NMA)
2
PbI
4
(n=1) perovskite film, 1.4 nm, and much
lower than that of the 3D FAPbI
3
(n=∞) perovskite film, 18.8 nm (Supplementary Fig.
S1).
We study the optical properties of the NFPI
7
films. Because of the quantum
confinement effects, the absorption and emission features of the layered perovskites
depend on the value of n
6,15
. Previous studies on PbI
4
-based perovskites showed that
the absorption peaks at ~2.4, ~2.2, ~2.0 and ~1.9 eV correspond to excitonic
absorption of QWs with n=1, 2, 3 and 4 respectively
6,20,21
. Our results confirm that the
(NMA)
2
PbI
4
perovskite film, i.e. the n=1 QW film, presents a strong peak at 2.43 eV
(Supplementary Fig. S2). The absorption spectrum of the NFPI
7
film shows a strong
exciton absorption peak at 2.18 eV (Fig. 1c), indicating that the major component of
the perovskite film is (NMA)
2
(FAPbI
3
)PbI
4
, i.e. the n=2 QWs. The shoulders at 2.43
and 1.95 eV suggest the existence of small fractions of n=1 and n=4 perovskite QWs
respectively, while the absorption peak of the n=3 QWs at ~2.0 eV may be hidden by
the absorption tail of the n=2 QWs. Optical features corresponding to QWs with
higher n values are not evident in the absorption spectrum. They become visible in the
PL spectrum, as shown by the dominating peak at 1.62 eV. This emission is ~40 nm
blue-shifted compared to that of bulk 3D (n=∞) FAPbI
3
perovskite (1.54 eV,
Supplementary Fig. S2). The PL spectrum on a semi-log scale reveals several weak
emission peaks at 2.38, 2.14 and 1.91 eV. With the absorption and emission spectra
plotted in the same panel (Fig. 1c), one can tell that absorption and emission peaks for
n≤4 QWs correspond very well to each other. We point out that although n=2 QWs
give the strongest absorption peak in the NFPI
7
films, emission from them is very
weak.
The investigations above suggest that our NFPI
7
film is an ensemble of self-organised
multiple QWs with different exciton energies. The absorption and PL measurements