1
Comprehensive defect suppression in
perovskite nanocrystals for high-efficiency
light-emitting diodes
Young-Hoon Kim
1,2†
, Sungjin Kim
1,2†
, Arvin Kakekhani
3†
, Jinwoo Park
1,2
, Jaehyeok Park
4
,
Sung Heo
5
, Yong-Hee Lee
1
, Hengxing Xu
6
, Satyawan Nagane
7
, Dongwook Lee
5
, Robert B.
Wexler
3
, Dong-Hyeok Kim
1,2
, Seung Hyeon Jo
1,2
, Laura Martínez-Sarti
8
, Peng Tan
3,9
,
Aditya
Sadhanala
7,10
, Gyeong-Su Park
1
, Young-Woon Kim
1
, Bin-Hu
6
, Henk J. Bolink
8
, Seunghyup
Yoo
4
, Richard H. Friend
7
, Andrew M. Rappe
3*
, and Tae-Woo Lee
1,2*
1
Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro,
Gwanak-gu, Seoul 08826, Republic of Korea
2
School of Chemical and Biological Engineering, Institute of Engineering Research, Research
Institute of Advanced Materials, Nano Systems Institute (NSI), Seoul National University, 1
Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
3
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104,
USA
4
School of Electrical Engineering, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 34141, Republic of Korea.
5
Group for Molecular Engineering of Functional Materials, Ecole Polytechnique Fédérale de
Lausanne, CH-1951 Sion, Switzerland
6
Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN
37996, USA
2
7
Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE,
U.K.
8
Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedrático José Beltrán, 2,
46980 Paterna, Spain
9
Department of Physics, Harbin Institute of Technology, Harbin 150001, China
10
Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford
OX1 3PU, UK.
†
These authors contributed equally to this work.
*Authors to whom correspondence should be addressed: E-mail: twlees@snu.ac.kr,
taewlees@gmail.com, rappe@sas.upenn.edu
3
Electroluminescence efficiencies of metal halide perovskite nanocrystals (PNCs) are
limited by lack of material strategies that can both suppress formation of defects and
enhance charge carrier confinement. Here, we report a one-dopant alloying strategy that
generates smaller, monodisperse colloidal particles (confining electrons and holes and
boosting radiative recombination) with fewer surface defects (reducing nonradiative
recombination). Doping of guanidinium (GA) into formamidinium lead bromide PNCs
yields limited bulk solubility while creating an entropy-stabilized phase in the PNCs and
leading to smaller PNCs with more carrier confinement. The extra GA segregates to the
surface and stabilizes the under-coordinated sites. Additionally, a surface stabilizing
1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene was applied as a Br vacancy healing agent.
The result is highly efficient PNC-based light-emitting diodes that have current efficiency
of 108 cd⋅A
-1
(external quantum efficiency (EQE) of 23.4 %) which rises to 205 cd⋅A
-1
(EQE of 45.5 %) with a hemispherical lens.
Metal halide perovskites (MHPs) with the general ABX
3
formula (A = organic or inorganic
cation, B = metal cation, X = halide anion) have narrow emission spectra (full width at half
maximum ≈ 20 nm) that can achieve high color-purity, tunable emission wavelength range (400
nm ≤ λ ≤ 780 nm) and low-cost solution processability
1–6
. As a result, they have been regarded
as promising light emitters
5–12
. This has also led to perovskite light-emitting diodes (PeLEDs),
which have shown a tremendous increase in electroluminescence (EL) efficiencies
13–15
. These
improvements have been obtained by a) increasing radiative recombination rate
16
, and b)
lowering non-radiative recombination rate
17
. The radiative recombination rate of the charge
carriers has been increased by spatially confining the electrons and holes in small perovskite
polycrystalline nano-grains
3
, low-dimensional crystals
18–20
, or colloidal perovskite
nanocrystals (PNCs)
4,5
. To reach beyond the state of the art and further increase the EL
efficiency, more effective strategies for suppression of defects and associated non-radiative
4
recombination are required.
In perovskite polycrystalline bulk films, stoichiometry control of precursors
3
, use of
passivation agents
18–21
and post-treatments
14,22,23
were shown to lead to the highest current
efficiency (CE) = 78 cd⋅A
-1
(external quantum efficiency (EQE) of 20.3% ph/el based on
Lambertian assumption)
18
. In colloidal PNCs, non-radiative recombination and defects are
reduced by surface-binding ligands. However, this method has limitations: a) the dynamic
nature of ligand-surface bonds, and b) steric hindrance effects that can leave under-coordinated
sites unpassivated and prone to defect formation
8,9
. B-site cation engineering
10
, X-site anion
exchange
11
, and surface-binding ligand engineering
12
have also been used, leading to the
highest CE of 76.8 cd⋅A
-1
(EQE of 17.1% ph/el) in green emission
24
and CE of 10.6 cd⋅A
-1
(EQE of 21.3% ph/el considering angular EL distribution) in red emission
11
in PeLEDs based
on CsPbX
3
.
The majority of research on PeLEDs is based on all-inorganic PNCs. It has been shown that
the orientational freedom of the liquid-like dipoles associated with the organic cations can
reduce the charge recombination rates
25–28
. While beneficial for photovoltaic applications, this
is undesirable for LEDs
29
. The all-inorganic MHPs contain spherical atomic A-site cations with
zero dipole moment, leading to enhancement of charge-recombination. Nevertheless, there are
shortcomings associated with the atomic A-site cations: a small phase space for tunability and
a lack of lattice-stabilizing directional hydrogen bonds. At the same time, there has been
lacking in comprehensive material design strategy to passivate the surface defects and confine
charge carriers inside the nanocrystals.
Here, we propose a simple and rational PNC design to stabilize the under-coordinated sites
at the surface and improve the charge carriers’ confinement inside the nanocrystals
simultaneously by employing zero-dipole guanidinium cation (CH
6
N
3
+
; GA
+
)
30,31
that provides
lattice-stabilizing effect of hydrogen bonds. We exploit fine substitutional doping of
5
formamidinium (CH
5
N
2
+
; FA
+
) lead bromide (FAPbBr
3
) by single GA
+
cation in colloidal
PNCs instead of cation alloying approaches in perovskite polycrystalline bulk films that have
been used in solar cells
32,33
. The increased surface stability is driven by the extra amino group
in the GA
+
due to its extra hydrogen bonds
34
and more uniformly distributed positive charge
35
.
Decylamine and oleic acid are also used as surface-binding ligands (Fig. 1a), providing an
additional level of surface stabilization. We show that although from an internal energy point
of view adding GA to the particle is penalized due to its larger size, the gain in configurational
entropy
36
stabilizes low concentrations of GA. We further reduce non-radiative recombination
by applying 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (TBTB) overcoat, which can heal
the leftover halide vacancies. Ultimately, we achieved a CE of 108 cd⋅A
-1
, further increased to
205 cd⋅A
-1
by employing a hemispherical lens.
Results
Structural properties
We begin by studying how GA doping influences the FAPbBr
3
structure. We construct a
computational model of FAPbBr
3
PNCs (see Supplementary Fig. 1 and Methods for details).
The fully ab-initio extended bulk model (Fig. 1b) can represent the FAPbBr
3
PNCs and
reproduce their experimental X-ray diffraction (XRD) patterns (Fig. 1c). We calculate the
formation free energy (from precursors) of FA
1-x
GA
x
PbBr
3
as x increases from 0 to 1, taking
into account both enthalpic and configurational entropic contributions (Fig. 1d). GA is larger
than FA and beyond the tolerance of the (lead bromide based) perovskite structure
37–39
, so
enthalpy does not preferentially drive GA to the inside of the perovskite. Nonetheless, small
concentrations (≈12.5%) of GA can still be dissolved in the structure due to entropy
stabilization
36
. Beyond ≈12.5%, enthalpy wins over the entropy and drives the surplus GA to
