Highly Efficient Halide Perovskite Light-Emitting Diodes via Molecular Passivation

Article abstract of DOI:10.1002/anie.202100243

Metal halide perovskites are promising for applications in light-emitting diodes (LEDs), but still suffer from defects-mediated nonradiative losses, which represent a major efficiency-limiting factor in perovskite-based LEDs (PeLEDs). Reported here is a strategy to synthesize molecular passivators with different anchoring groups for defects passivation. The passivated perovskite thin films exhibit improved optoelectronic properties as well as reduced grain size and surface roughness, thus enable highly efficient PeLEDs with an external quantum efficiency of 15.6 % using an imidazolium terminated passivator. Further demonstrated is that the in situ formation of low-dimensional perovskite phase on the surface of three-dimensional perovskite nanograins is responsible for surface defects passivation, which leads to significantly enhanced device performance. Our results provide new fundamental insights into the role of organic molecular passivators in boosting the performance of PeLEDs.

Full text of DOI:10.1002/anie.202100243

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 8337–8343
International Edition: doi.org/10.1002/anie.202100243
German Edition: doi.org/10.1002/ange.202100243
Light-Emitting Diodes
Hot Paper
Highly Efficient Halide Perovskite Light-Emitting Diodes via
Molecular Passivation
Aihui Liang+, Kang Wang+, Yao Gao, Blake P. Finkenauer, Chenhui Zhu, Linrui Jin,
Libai Huang, and Letian Dou*
Abstract: Metal halide perovskites are promising for applica-
tions in light-emitting diodes (LEDs), but still suffer from
defects-mediated nonradiative losses, which represent a major
efficiency-limiting factor in perovskite-based LEDs (Pe-
LEDs). Reported here is a strategy to synthesize molecular
passivators with different anchoring groups for defects passi-
vation. The passivated perovskite thin films exhibit improved
optoelectronic properties as well as reduced grain size and
surface roughness, thus enable highly efficient PeLEDs with an
external quantum efficiency of 15.6% using an imidazolium
terminated passivator. Further demonstrated is that the in situ
formation of low-dimensional perovskite phase on the surface
of three-dimensional perovskite nanograins is responsible for
surface defects passivation, which leads to significantly en-
hanced device performance. Our results provide new funda-
mental insights into the role of organic molecular passivators in
boosting the performance of PeLEDs.
light-emitting devices,[6] photodetectors[7] and field-effect
transistors.[8] Particularly, owing to the high color purity with
narrow spectra emission (full-width at half-maximum
(FWHM) ꢀ 20 nm),[9] and broad range of color tunability
(400–1000 nm),[10] MHPs are widely used in light-emitting
diodes. In 2014, Tan et al. first reported the perovskite light-
emitting diodes (PeLEDs) using three-dimensional (3D)
perovskite CH3NH3PbI3 (MAPbI3) and CH3NH3PbBr3
(MAPbBr3) as emitting layers, which showed an external
quantum efficiency (EQE) of 0.76% and 0.1%, respective-
ly.[6a] Since then, the EQE of PeLEDs has been boosted to
over 20% by adopting two different approaches.[11] One way
is to enhance radiative recombination rates by confining the
electrons and holes, which include the use of ultra-thin
emissive layers,[12] the fabrication of nanoscale polycrystalline
features,[13] the design of low-dimensional or multiple quan-
tum well structures,[14] and the synthesis of perovskite
quantum dots.[15] The other way is to suppress the defects-
related nonradiative recombination, which have been identi-
fied as a major efficiency-limiting factor for PeLEDs.[16]
In fact, the state-of-the-art solution-processed perovskite
materials suffer from severe defect-mediated nonradiative
losses, which are generally believed to be associated with ionic
defects, such as lead vacancies and halide vacancies.[17] At the
same time, the ionic nature of the perovskite lattice enables
molecular passivation through coordinate binding based on
Lewis acid-base chemistry.[18] Organic molecules with differ-
ent functional groups have been employed as molecular
passivators to passivate perovskite surface defects.[18a,19] For
instance, in 2017, Xiao et al. incorporated large-group n-
butylammonium halides (BAX, X = I, Br) into the perovskite
precursor solution acting as a surfactant that dramatically
constrains the growth of 3D perovskite grains and decreases
film roughness, which increased the EQE of MAPbI3 and
MAPbBr3 based PeLEDs from 1.0% to 10.4% and 0.03% to
9.3%, respectively.[13] Later in 2019, several bulky organo-
ammonium ligands such as 4-fluorophenylmethylamonium
iodide (4-F-PMAI) and 4,5-fluorophenylmethylamonium io-
dide (4,5-F-PMAI) were used, and a highly efficient MAPbI3-
based PeLEDs with EQE ꢁ 15% was achieved.[20] However,
most of the currently used passivators are based on primary
ammonium terminated molecules, which still suffer from
limited device efficiency and stability, and there is a lack of
deep understanding of how the molecules interact with the
perovskites.
Introduction
Metal halide perovskites (MHPs) are emerging as a new
class of semiconducting materials with many remarkable
optoelectronic properties such as facile solution processabil-
ity,[1] tunable band gap,[2] balanced charge transport proper-
ties with long diffusion lengths,[3] and high photoluminescence
quantum yield (PLQY),[4] which have attracted considerable
interests in optoelectronic applications, such as solar cells,[5]
[*] Dr. A. Liang[+]
College of Chemistry and Chemical Engineering, Jiangxi Normal
University
Nanchang, 330022 (P. R. China)
Dr. A. Liang,[+] Dr. K. Wang,[+] Dr. Y. Gao, B. P. Finkenauer, Prof. L. Dou
Davidson School of Chemical Engineering, Purdue University
West Lafayette, IN 47907 (USA)
E-mail: dou10@purdue.edu
Prof. C. Zhu
Advanced Light Source, Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
L. Jin, Prof. L. Huang
Department of Chemistry, Purdue University
West Lafayette, IN 47907 (USA)
Prof. L. Dou
Birck Nanotechnology Center, Purdue University
West Lafayette, IN (USA)
[+] These authors contributed equally to this work.
In this work, we synthesized a series of new phenyl and
thienyl-based molecular passivators with ammonium, forma-
midinium and imidazolium terminal groups, to examine the
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100243.
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effects of different anchoring groups on defect passivation.
After introducing the passivators, the as-prepared perovskite
thin films display improved optoelectronic properties with
low nonradiative recombination rate and high PLQY as well
as reduced grain size and surface roughness. On this basis, we
demonstrate highly efficient PeLEDs with an EQE of 15.6%
using a novel imidazolium terminated passivator. Employing
grazing incidence wide-angle X-ray scattering technique and
single-crystal analysis, we further confirm that the in situ
formation of low-dimensional perovskite phase on the surface
of 3D perovskite nanograins is responsible for the surface
defects passivation, which lead to significantly enhanced
device performances. Our results suggest imidazolium can
serve as highly efficient passivation group for high-perfor-
mance PeLEDs.
imidazolium (Im+)) to passivate the potential surface defects
of MAPbI3 films. The molecular structures of these passiva-
tors are shown in the right panel in Figure 1a, where
phenylmethylammonium iodide (PMAI) and thienylmethy-
lammonium iodide (TMAI) were purchased from commercial
sources. The other passivators, including phenylformamidi-
nium iodide (PFAI), thienylformamidinium iodide (TFAI),
phenylimidazolium iodide (PImI) and thienylimidazolium
iodide (TImI) are synthesized in our lab and their synthetic
routes are shown in the Methods section found in the
Supporting Information. PFAI and TFAI were prepared from
benzonitrile and thiophene-2-carbonitrile, respectively. The
nitrile is reacted with sodium methoxide in methanol followed
by treatment with acetic acid and ammonium chloride, and
then treated with aqueous hydrogen iodide to give the desired
molecules. PImI and TImI were obtained by using aqueous
hydrogen iodide to deprotect tert-butyl 4-phenyl-1H-imida-
zole-1-carboxylate and tert-butyl 4-(thiophen-2-yl)-1H-imi-
dazole-1-carboxylate, which were synthesized from phenyl-
boronic acids and tributyl(thiophen-2-yl)stannane reacted
with tert-butyl 4-bromo-1H-imidazole-1-carboxylate by Su-
zuki–Miyaura and Stille cross-coupling reactions, respective-
ly. All the chemical structures of the target passivators were
identified by 1H and 13C nuclear magnetic resonance (NMR)
spectroscopy and mass spectrometry (MS).
In order to find the optimal molar ratio of molecular
passivators, we primarily sought to fabricate the PeLEDs by
introducing different amounts of PMAI into MAPbI3 pre-
cursor solutions. As displayed in Figure S1 in the Supporting
Information, the EQE of the PeLEDs shows an increasing
trend when the molar ratio of PMAI increased from 5% to
20%, while further increasing the molar ratio to 30% results
in an EQE decrease. All these results are consistent with
previous works.[13,20] As a result, we set the molar ratio of all
the molecular passivators at 20% for further spectroscopic
and morphologic studies. The absorption and photolumines-
cence (PL) spectra of perovskite films with different passiva-
tors (PMAI, PFAI, PImI, TMAI, TFAI, and TImI) at the
molar ratios of 20% are shown in Figure 2a and b. Relative to
Results and Discussion
We fabricated the PeLEDs based on a device structure of
ITO/Poly-TPD/perovskite/TPBi/LiF/Al (Figure 1a) with en-
ergy diagrams shown in Figure 1b. Poly[N,N’-bis(4-butyl-
phenyl)-N,N’-bis(phenyl)-benzidine] (Poly-TPD) was used as
the hole-transporting layer, 1,3,5-tris(1-phenyl-1H-benzimi-
dazol-2-yl)benzene (TPBi) was used as the electron-trans-
porting layer, 3D organic-inorganic hybrid perovskite (MAP-
bI3) was used as the emitting layer, and indium tin oxide
(ITO) and LiF/Al were used as the anode and cathode,
respectively. Besides those conventional ammonium tails,
formamidinium and imidazolium also can anchor to the Pb-I
framework through coordination with the Pb ions or hydro-
gen bonding with the iodide ions.[21] Moreover, benzene and
thiophene rings are hydrophobic and their p-conjugation
structure favors charge transport. Organic molecules with
phenyl or certain thienyl substituents exhibited very different
anchoring interactions and passivation effects with perov-
skites.[22] Hence, we introduced several phenyl and thienyl-
based bulky molecular passivators with different anchoring
tails (methylammonium (MA+), formamidinium (FA+) and
Figure 1. a) Device structure of the PeLEDs. The library of molecular passivators with different terminal groups and conjugated heads is shown on
the right. b) Energy level diagrams of MAPbI3-based PeLEDs. The energy levels of the perovskite and other layers are taken from literature.
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Figure 2. a) Absorption, b) PL spectra, and c) TRPL lifetime of MAPbI3 thin films with or without 20% molar excess of different passivators in the
precursor solutions.
that of the control MAPbI3 film without passivators, the
absorption edges and PL peaks of passivated perovskite films
exhibit a small blue-shift, which could be corresponding to the
emergence of low-dimensional perovskite grains and the
reduction in crystallite size, thus increasing the band energy
by quantum confinement effect.[13] The PLQY and time-
resolved photoluminescence (TRPL) were then conducted on
these perovskite films to examine their carrier recombination
dynamics. As shown in Table S1 in the Supporting Informa-
tion, the PLQY of the perovskite films with passivators are
dramatically increased compared with the control MAPbI3
film. Specifically, passivators with phenyl group show higher
PLQY than the corresponding passivators with thienyl group.
This should be attributed to the existence of heavy sulfur
atom in thienyl-based molecules and the efficient solid-state
packing characteristic, which will limit the fluorescent quan-
tum yields in both solution and the solid state.[23]
Based on the TRPL curves, the average lifetime (tav) of
the control MAPbI3 film is fitted to be 47.5 ns (Figure 2c and
Table S1, Supporting Information). In contrast, the perovskite
films with the passivators exhibited longer average carrier
lifetimes, which implies that defect-assisted non-radiative
processes are effectively suppressed.[4a] Accordingly, the
radiative and nonradiative recombination rate (kr and knr)
of those perovskite films were calculated using the expres-
sions PLQY= kr/(kr + knr) and tav = (kr + knr)À1. The results
are summarized in Table S1 in the Supporting Information. In
comparison with the control MAPbI3 film, the perovskite film
with the passivators showed a smaller knr, which further
confirms the passivation effects of the as-obtained molecular
passivators. As shown in Table S1, the perovskite films with
phenyl-based passivators showed higher kr than the corre-
sponding perovskite films with thienyl-based passivators.
Furthermore, the film containing 20% molar excess PImI
yielded the highest kr (21.3 ” 105 sÀ1), suggesting that the
PLQY improvement was primarily due to enhanced radiative
decay processes. Interestingly, both phenyl and thienyl group
based passivators with different tails exhibit the kr in the order
of Im+ > MA+ > FA+, which suggest varied passivation effect
of different terminal groups. Such enhanced radiative recom-
bination together with suppressed non-radiative recombina-
tion makes PImI the most promising passivator for high-
performance PeLEDs.
Notably, these phenyl- and thienyl-based passivators with
different terminal group are too large to fit into the corner of
[PbI6]4À octahedral layers due to the restriction of tolerance
factor, and therefore induce the formation of low-dimensional
perovskite phase, which will impede the growth of 3D
perovskite grains during the film formation process. To
investigate the influence of the chemical structure of organic
molecular passivators on film morphology, we further conduct
atomic force microscope (AFM) measurements. Compared to
the control MAPbI3 film, the crystallite size and the film root
mean square roughness (r.m.s) were significantly reduced
when adding the passivators (PMAI, TMAI, PImI and TImI)
into the perovskite films. However, the r.m.s is a little lager
while PFAI and TFAI were added into the perovskite films
(Figure 3a–g). The PImI passivated MAPbI3 film (Figure 3d)
is the most uniform and smooth one with a r.m.s. of ꢁ 1.14 nm,
which is among the most flat and uniform perovskite films
reported so far.[13,24] The same influence of grain size and
smooth surface of the perovskite film after incorporating of
passivators can also be visualized by scanning electron
microscopy (SEM) (Figure S2, Supporting Information).
The smaller perovskite grain size can increase the exciton
binding energy to improve the PLQY, and the reduced
surface roughness can help decrease the trap density and
avoid current leakage, both of which are favorable for LED
applications.
Figure S3–S7 present the device performances of PeLEDs
with different molecular passivators (PFAI, PImI, TMAI,
TFAI, and TImI) as the molar ratio increased from 5% to
30%. The optimal molar ratio of all passivators doped in the
perovskite layer are determined to be 20%, which are
consistent with previous reports.[13] The current density-
voltage (J–V) curves of the champion PeLEDs with different
passivators are shown in Figure 4a. The passivated films
maintain their current densities at a very low level below 2 V,
suggesting negligible current leakage in our PeLED devices.
The abrupt increase of current densities after 2 V indicates
the injection of charge carrier into perovskite materials. As
shown in Figure 4b, the EQEs of PeLEDs with phenyl-based
passivators are much higher than the corresponding thienyl-
based passivators. More interestingly, the PeLEDs perform-
ances of phenyl- or thienyl-based passivators with different
tails are in the order of Im+ > MA+ > FA+ (detailed in
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Figure 3. AFM height image of MAPbI3 thin films with or without 20% molar excess of different passivators in the precursor solutions. a) MAPbI3;
b) PMAI-MAPbI3; c) PFAI-MAPbI3; d) PImI-MAPbI3; e) TMAI-MAPbI3; f) TFAI-MAPbI3; g) TMAI-MAPbI3. The scan area of AFM images is
5 mm”5 mm.
Figure 4. a) Current density (J)-voltage (V)-radiance (R) curves, b) EQE versus current density, and c) normalized EL spectra of PeLEDs with or
without 20% molar excess of different molecular passivators. d) Operational stabilities of the device with PMAI and PImI passivators tested at
a constant current density of 10 mAcmÀ2
.
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Table S2, Supporting Information), which is in good agree-
graphic studies with thin film X-ray diffraction (XRD), two-
ment with the results of spectroscopy and morphology studies.
Most importantly, the device with PImI passivator shows the
highest EQE of 15.6%, which is ten-fold higher than that of
the control device and among the highest value for MAPbI3
based PeLEDs (Figure S8 and S9, Supporting Information).
The electroluminescent (EL) peaks of all perovskite
devices displayed blue-shift with increasing amounts of
molecular passivators. The blue-shift of EL spectra can be
attributed to the terminal group of passivators, which results
from the change of crystalline structure, film morphology, and
the surface trap passivation effect.[25] Fortunately, no wave-
length shift of EL spectra was observed under different
applied bias voltage, indicating good color-stability of the
PeLEDs (Figure S3–S7, Supporting Information). Figure 4c
plots the normalized EL spectra of PeLEDs with different
molecular passivators at a molar ratio of 20%, where EL
wavelength is increasingly blue-shifted from MAPbI3, FA+-
passivated, MA+-passivated, to Im+-passivated PeLEDs. This
can be ascribed to the decreased grain size. In addition to the
remarkably enhanced device efficiency, the PeLEDs based on
the PImI passivator also exhibit considerably better opera-
tional stability. The EQE of the PImI-based device dropped
to half of the initial value after 2 h under a constant current
density of 10 mAcmÀ2 (Figure 4d). This result demonstrates
that the stability of PImI-based device is significantly
improved even compared with the PMAI-based device, which
can only survive for about 6 minutes under the same
conditions. All of these results suggest that the LED device
performances regarding EQE and EL wavelengths are
strongly related to the terminal anchoring group of those
molecular passivators.
dimensional synchrotron-based grazing incidence wide-angle
X-ray scattering (GIWAXS), and single-crystal analysis. As
shown in Figure 5a, the passivated MAPbI3 films exhibit
similar XRD spectra with two diffraction peaks around 148
and 288, corresponding to (110) and (220) planes of 3D
perovskite, respectively.[26] No diffraction peaks correspond-
ing to low-dimensional perovskites are clearly observed after
the introduction of molecular passivators, which indicate that
the as-formed films with passivators are nearly indistinguish-
able from the control 3D perovskite crystal structure. How-
ever, those XRD peaks become broad after adding the
passivators into the perovskite film. According to Scherrer
Equation (1),[27] the larger FWHM corresponds to a smaller
average crystallite size.
0:9l
D2qcosq
1
L
where L is the mean size of the perovskite crystalline
domains, l is the wavelength of incident X-ray, D2q is the
full width at half maximum of the peak, and q is the Bragg
angle. After adding passivators, the grain size decreased from
ꢁ 46 nm for MAPbI3 to 15 nm for PImI-MAPbI3 (see
Table S1, Supporting Information for more details), which is
consistent with the SEM and AFM results.
The recorded GIWAXS patterns of the control and of the
excess passivators incorporated MAPbI3 perovskite films are
shown in Figure 5b–h. The control MAPbI3 film exhibits
nearly isotropic Bragg rings at q = 1.0, 1.4, 1.65, 1.7, and
À1
2.0 Š corresponding to the (110), (112), (211), (202), and
(220) crystal planes of 3D perovskite phase (Figure 5b),
In order to determine the origin of the EQE enhancement
with PImI and further understand the interaction of molec-
ular passivators with perovskites, we carried out crystallo-
indicating a random crystallite orientation.[28] With passiva-
À1
tors, the diffraction patterns appear in the q < 1.0 Š range,
which implies the formation of low-dimensional perovskite
Figure 5. a) XRD profile, and b–h) GIWAXS pattern of perovskite thin films with or without 20% molar excess of different passivators for
b) MAPbI3, c) PMAI-MAPbI3, d) PFAI-MAPbI3, e) PImI-MAPbI3, f) TMAI-MAPbI3, g) TFAI-MAPbI3, h) TImI-MAPbI3. The Miller indices of prominent
diffraction peaks are marked in each pattern. The color scale is proportional to the intensity of X-ray scattering. LD, low-dimensional perovskite
phase.
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structures (Figure 5c–h).[28a] For those PMAI, TMAI, PImI or
defects passivation, but also guides the beginning of a new
realm of molecular engineering for the development of high-
performance perovskite-based optoelectronic devices.
TImI passivated MAPbI3 films, the diffraction rings in the q <
À1
1.0 Š can be indexed into different crystal planes of their
corresponding low-dimensional perovskite structures (See
their single crystal structures in Figure S10, S11, and Table S3,
Supporting Information), which have been marked in Fig-
ure 5c, e, f, and h, respectively. Such random orientations and
the absence of low-dimension diffraction in thin film XRD
pattern indicate the in situ formation of low-dimensional
perovskite phase on the surface of 3D perovskite nanograins,
which can passivate the surface defects of perovskite and also
protect 3D perovskites from being damaged easily, thus
leading to enhanced device efficiency and stability. Mean-
while, those resulting low-dimensional perovskites from
PMAI and PImI both exhibit that Pb-I bond lengths are
around 3.21 Š, while I-Pb-I bond angle is about 1588 for
PMAI and 908 for PImI, which makes PImI-based low-
dimensional perovskites share more similar lattice parame-
ters to those of MAPbI3 single crystals (Pb-I bond length
3.16 Š, I-Pb-I angle 908).[29] This means that the formation of
mixed-phase perovskite nanograins from PImI passivation
should have smaller lattice strain than that in PMAI case.
Therefore, it is more favorable to passivate the defects and
reduce non-radiative loss.[30] For PFAI and TFAI (Figure 5d
and g), the perovskite films exhibit a distinct orientation in
the direction perpendicular to the substrate and some
periodic diffraction spots derived from the low-dimensional
perovskites (See its single crystal structures in Figure S12 and
Table S3, Supporting Information). In this case, PFAI and
TFAI no longer play the role of surface passivator and
crystallite terminator, but rather participate in crystallization,
which was detrimental to the morphology of our perovskite
films (also see SEM and AFM results), and thus could reduce
device performances.[20]
Acknowledgements
This work is supported by Office of Naval Research (Grant
No. N00014-19-1-2296, Program Manager: Dr. Joe Parker and
Dr. Paul Armistead) and Davidson School of Chemical
Engineering of Purdue University. B.P.F. acknowledges sup-
port from the Purdue Process Safety & Assurance Center.
Conflict of interest
The authors declare no conflict of interest.
Keywords: light-emitting diodes ·mixed-phase nanograins ·
molecular passivator ·organic-inorganic hybrid perovskite ·
surface engineering
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Conclusion
In summary, we have synthesized a series of phenyl and
thienyl-based molecular passivators with different anchoring
groups, such as methylammonium, formamidinium and imi-
dazolium, for defects passivation. The passivators are favor-
able to passivate the surface defects of perovskites, which led
to improved optoelectronic properties regarding suppressed
nonradiative losses and enhanced radiative recombination.
Morphology studies reveal that those molecular passivators
can impede the growth of 3D perovskite nanograins during
the film crystallization process, which result in smaller grain
size and reduced roughness of the perovskite films. On this
basis, we fabricated highly efficient PeLEDs with an EQE of
15.6% using an imidazolium terminated passivator, which are
among the best MAPbI3-based LEDs reported so far. Further
by employing grazing incidence wide-angle X-ray scattering
technique and single-crystal analysis, we clearly demonstrated
that the in situ formation of mixed-phase perovskite nano-
grains is possibly responsible for the surface defects passiva-
tion, thus leading to significantly enhanced device perform-
ances. This work not only provides us a deep insight into the
interaction of organic molecules with perovskite lattice for
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Manuscript received: January 6, 2021
Accepted manuscript online: January 26, 2021
Version of record online: March 4, 2021
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