Blue Perovskite LEDs: Reducing Architecture Limitations for Efficient Blue Perovskite Light‐Emitting Diodes (Adv. Mater. 20/2018)

Abstract

Blue perovskite light‐emitting diodes (LEDs) have lagged significantly behind red and green ones. In article number 1706226, Daniel N. Congreve and co‐workers show that the device architecture plays a key role in this lag, and they propose an alternate structure that maintains robust nanocrystal emission. Devices with this architecture show external quantum efficiencies of 0.50% at 469 nm. This architecture enables efficient devices across the entire blue–green portion of the spectrum.

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Reducing Architecture Limitations for Efficient Blue
Perovskite Light-Emitting Diodes
Mahesh K. Gangishetty, Shaocong Hou, Qimin Quan, and Daniel N. Congreve*
Dr. M. K. Gangishetty, Dr. S. Hou, Dr. Q. Quan, Dr. D. N. Congreve
Rowland Institute at Harvard University
Cambridge, MA 02142, USA
E-mail: congreve@rowland.harvard.edu
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201706226.
DOI: 10.1002/adma.201706226
behind the more robust green and red
ones.[15] Song et al. demonstrated a
quantum efficiency of 0.07% with a bright-
ness as high as 742 cd m−2 with a 455 nm
emission peak.[10] Pan et al. reported effi-
ciencies of 1.9% as blue as 490 nm.[12]
Recently, Yao et al. demonstrated a bright
blue device based on a nickel oxide (NiOx)
hole-transport layer (HTL) and clever sol-
vent engineering with a peak efficiency of
0.07%.[20] In order for perovskite LEDs to
become commercially relevant, a solution
to these low quantum efficiencies must be
found.
In this work, we synthesize CsPbBrxCl3−x
perovskite nanocrystals and demonstrate
that their emission efficiency and lifetime
are significantly impaired when used with
HTLs such as NiOx. By developing a device architecture that
does not affect the emission of the nanocrystals, we are able to
increase device performance to a maximum EQE of 0.50% and a
brightness of 111 cd m−2 at an emission wavelength of 469 nm.
Finally, we demonstrate that these device improvements can be
beneficial across the blue-green visible spectrum, with bright,
efficient devices spanning from 511 to 469 nm.
We synthesize CsPbBrxCl3−x nanocrystals following the Pro-
tesescu et al. method[9] but at lower temperature to favor an
asymmetric crystal growth in order to achieve slight quantum
confinement.[23] The yielded nanocrystals are ≈20 nm in the lat-
eral dimension and 5 nm thick, Figure 1b. The absorption and
emission properties are presented in Figure 1c, demonstrating
a narrow emission of 23 nm full width at half maxi mum
(FWHM). These nanocrystals were robust to purification pro-
cess by antisolvent (ethyl acetate) washing, and the washing
was found to preserve their exciton confinement and emission
but enable better charge injection.
The lagging blue device efficiencies of these materials, cou-
pled with their relatively strong photoluminescence quantum
yield (PLQY),[9] led us to suspect the device environment was
playing a crucial role, especially in these thin nanocrystal layers
where the emission layer is near to the interface. Therefore, we
turned our focus to the surrounding layers, with a particular
eye toward the HTL. In order to judge the effect of the local
environment, we monitored the transient decay of a thin film
of nanocrystals as a function of underlying layer. All data were
measured in air with 379 nm excitation using a Hamamatsu
streak camera integrated across the emission wavelength of the
nanocrystals.
In Figure 2a, we compare the transient photoluminescence
of nanocrystals spuncast on top of glass (red) and NiOx (orange)
Light-emitting diodes utilizing perovskite nanocrystals have generated strong
interest in the past several years, with green and red devices showing high
efficiencies. Blue devices, however, have lagged significantly behind. Here,
it is shown that the device architecture plays a key role in this lag and that
NiOx, a transport layer in one of the highest efficiency devices to date, causes
a significant reduction in perovskite luminescence lifetime. An alternate
transport layer structure which maintains robust nanocrystal emission is
proposed. Devices with this architecture show external quantum efficiencies
of 0.50% at 469 nm, seven times higher than state-of-the-art devices at that
wavelength. Finally, it is demonstrated that this architecture enables efficient
devices across the entire blue-green portion of the spectrum. The improve-
ments demonstrated here open the door to efficient blue perovskite light-
emitting diodes.
Blue Perovskite LEDs
Inorganic–organic perovskites have exploded in interest over
the last several years, due to extremely strong photovoltaic per-
formance.[1,2] The long diffusion lengths[3] and low trap densi-
ties[4] that drive this performance also mark these materials as
strong candidates for light emitting diodes (LEDs).[5,6] Recent
work has shown that green LEDs fabricated from these mate-
rials can reach external quantum efficiencies (EQEs) as high as
9.3%[7] and current efficiencies of 42.9 cd A−1.[8]
Significant recent attention has turned toward high stability
all-inorganic perovskite materials, with nanocrystals a particular
interest. Following the development of a facile synthesis by Pro-
tesescu et al.,[9] interest in cesium-based lead halide perovskites
has exploded due to their high photoluminescence quantum
yields[9] and wide variety of applications. After an initial publica-
tion by Song et al. showed modest efficiencies around 0.1%,[10]
work from several groups have demonstrated green LEDs with
quantum efficiencies greater than 1%,[11–13] with the highest EQE
of 8.73%[13] and brightness over 15 000 cd m−2.[11] Similarly, red
devices based on cesium lead iodide nanocrystals have shown
high performances with quantum efficiencies over 5%.[14,15]
Despite finding some success in organic–inorganic perovs-
kites, with EQEs up to 1.38%,[16,17] and 2D nanoplatelet per-
ovskites,[18,19] inorganic blue devices have consistently lagged
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in the first nanosecond after excitation. A clear decrease in
lifetime is observed in the presence of NiOx, which, when
combined with the strong reduction in photoluminescence
(see the Supporting Information), indicates the appearance
of a nonradiative decay channel, possibly decaying through
defect states in the NiOx or via a charge transfer process.[24,25]
Thus, in order to obtain optimal emission efficiencies from
the nanocrystals, it is necessary to find an HTL which does not
introduce any nonradiative decay channels.
To improve the emission from the device, we turn to an
HTL constructed of a bilayer of poly[(9,9-dioctylfluorenyl-2,7-
diyl)-co-(4,4′-(N-(4-sec-butylphenyl) diphenylamine)] (TFB)
and Nafion perfluorinated ionomer (PFI). TFB, an electron-
blocking, hole-transport polymer with a high hole mobility
of 0.01 cm2 (Vs)−1,[26] has been shown to be successful as
an HTL in Cs-based perovskite LEDs.[15] PFI has been used
successfully as part of a buffer hole injection layer leading
to high brightness devices.[22,27] The strong surface dipole
induced by PFI leads to a band bending of the underneath
HTL layer to a higher work function, favorable for hole injec-
tion, while the isolation of the perovskite layers helps to
reduce exciton quenching. Thus, we hoped that the combi-
nation of the two materials could lead to high performance
blue devices.
Unfortunately for our transient photoluminescence (PL)
study, the luminescence of TFB overlaps that of the nanocrys-
tals, so in order to better understand their dynamics we first
had to subtract the TFB-only emission from the samples. The
details of this subtraction can be found in the Supporting Infor-
mation. The final subtracted data are plotted in Figure 2b, blue.
We observe that the transient decay is virtually identical to the
emission from glass, demonstrating that the nanocrystals are
not significantly perturbed by the presence of the HTL, and
thus this HTL could substantially improve the emission from
the nanocrystals relative to current HTLs. This improvement
is further reflected in the increased steady state emission from
the nanocrystals as compared to a NiOx underlayer, see the
Supporting Information.
To more clearly quantify the benefits of this HTL switch,
we fabricate devices utilizing both NiOx and TFB/PFI as the
HTL, following the device structures in Figure 1e,f. Indium
tin oxide (ITO)-coated glass is cleaned via solvent washing and
plasma cleaning immediately before sequential spincoating
of the HTL layers, either poly(3,4-ethylenedioxythiophene)-
poly(styrenesulfonate) (PEDOT:PSS), TFB, and PFI, or the
NiOx precursor (followed by heating to form NiOx). We found
the PFI layer to be crucial to maintaining nanocrystal emis-
sion; without it, electroluminescence (EL) from mostly TFB
Adv. Mater. 2018, 30, 1706226
Figure 1. a) The crystalline structure of our CsPbBrxCl3−x nanocrystals. The ligand is a mix of oleic acid, oleylamine, and trioctylphosphine. b) Trans-
mission electron microscopy (TEM) image of the synthesized nanocrystals showing nanocrystals of ≈20 nm lateral dimension and 5 nm thickness.
c) Absorption (solid) and emission (dashed) spectra of the 469 nm nanocrystals show a narrow emission FWHM of 23 nm. d) Structure of the mole-
cules used in this work. e,f) Energy level diagram of the fabricated LEDs. Either NiOx or PEDOT:PSS/TFB/PFI are used as the HTL. PFI is represented
here by band bending to a deeper work function, as it has been shown to increase the surface work function.[21,22] TPBi is used as the electron-transport
layer. Energy levels of the nanocrystals are from XPS measurements (see Figure S10 in the Supporting Information). Energy levels of the transport
layers are from refs. [15,20].

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is observed (see the Supporting Information), agreeing with
previous reports of improved film quality on top of PFI with
low surface energy.[21] The nanocrystals are then spuncast from
octane followed by transfer to a thermal evaporator, where
40 nm of 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene
(TPBi) is evaporated, followed by LiF/Al to form the top con-
tact and pattern the device with a diameter of 2 mm. All device
fabrication is performed inside a glovebox; all device testing is
done on unpackaged devices in air.
After device fabrication, we observe clear electrolumines-
cence from the perovskite nanocrystal layer, Figure 3a, in
both structures. The electroluminescence spectra from the
two devices are nearly identical, with a peak of 469 nm and a
FWHM of 24 nm for the NiOx and 25 nm for the TFB/PFI.
The emission is unchanged as a function of applied voltage, see
the Supporting Information.
In Figure 3b,c we present the electrical characteristics of the
devices. The NiOx device, a much more conductive structure,
demonstrates a low turn-on voltage and high brightness, but
is limited by high dark current and the nonradiative recom-
bination of the nanocrystals discussed previously, leading
to a maximum EQE of 0.03% in our measurements. This
is similar to the 0.07% measured by Yao et al.[20] and Song
et al.[10] The TFB/PFI device, in contrast, demonstrates a
higher turn-on voltage but a much lower dark current. This
dark current reduction likely results from better film forma-
tion and reduced pinholes, see Figure S8 in the Supporting
Information. The EQE achieves a maximum 0.50% before
rolling off at higher current densities. This high quantum effi-
ciency clearly demonstrates the value of TFB/PFI as an HTL
and represents a new efficiency standard for blue inorganic
perovskite nanocrystals.
This efficiency gain is likely an accumulation of several ben-
efits in addition to the increased luminescence demonstrated in
Figure 2. Devices with the HTL material poly(9-vinylcarbazole),
which has a much lower hole mobility, show a maximum EQE
of 0.063%, indicating that the high hole mobility of TFB helps
to balance the injected charges (Figure S4, Supporting Informa-
tion). Further, a strong reduction in film roughness is observed
moving from NiOx to TFB/PFI, indicating a more uniform film
with reduced pinholes (Figure S8, Supporting Information).
The additive effects of these gains results in the high overall
efficiency of the TFB/PFI device.
Finally, to demonstrate the universality of the TFB/PFI
device structure, we tune the bromide to chloride ratio in order
to adjust the emission wavelength as shown in Figure 4a, while
keeping the device structure constant. The fabricated devices
Adv. Mater. 2018, 30, 1706226
Figure 2. a) Time-resolved photoluminescence data show the clear
emergence of a nonradiative channel in nanocrystals on top of NiOx as
compared with glass. Data taken from the integration of streak camera
data. b) Nanocrystals on TFB/PFI demonstrates dynamics identical to
those of nanocrystals on glass, enabling unaltered emission inside the
device structure.
Figure 3. a) Electroluminescence, b) J–V–L, and c) EQE curves for LEDs fabricated using 469 nm perovskite nanocrystals and either NiOx (orange) or
TFB/PFI (blue) as the HTL.

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have emission wavelengths at 469, 481, 488, and 511 nm, with
narrow FWHM. The current–voltage–luminescence (J–V–L)
characteristics are similar for all devices, with relatively low
dark currents and turn-on voltages that increase with increased
bandgap. The EQEs for these devices are plotted in Figure 4c.
There is a substantial increase in efficiency as the emission
wavelength redshifts; indeed, a small 7 nm emission wave-
length difference between the 481 and 488 nm device results in
a threefold difference in maximum EQE.
As the energetics are shifted toward the green, we see less
gain in quantum efficiency relative to literature. While the
469 nm device shows a strong efficiency enhancement with the
HTL change, the bromide device (511 nm) peaks at 2.37% EQE
and 3423 cd m−2, on the order of other CsPbBr3 devices recently
published.[11] We hypothesize that the energetic requirements
for lower bandgap nanocrystals are much less restrictive, and
thus the benefits of the TFB/PFI structure fade as the emission
peak redshifts and other structures provide an equally favorable
environment. The full set of device parameters can be found in
Table 1.
Though an important step forward, significant work remains
to be done before these materials are commercially relevant.
The main drawback of the blue nanocrystals is their low PLQY
in thin films, which we measure at 9% for the 469 nm emission
batch. Future work must focus on increasing this lumines-
cence yield while maintaining the spectrally narrow emission,
as well as continual improvements to the surrounding archi-
tecture of the device. In addition, these materials suffer from
degradation and ion segregation[28] under continuous bias in
air, see Figure S7 in the Supporting Information, that must be
addressed before commercialization.
In this work, we have demonstrated and overcome one of
the key efficiency barriers in blue perovskite nanocrystal LEDs:
the architecture itself. We have shown that the HTL can induce
nonradiative recombination of the emissive state, fundamen-
tally limiting device performance. The introduction of a new
HTL, TFB/PFI, that does not greatly influence the nanocrys-
tals, provides a strong overall boost in efficiency, with values
reaching as high as 0.50% EQE for 469 nm emitting devices.
We further demonstrate that this structure provides strong ben-
efits across the blue-green portion of the spectrum, opening the
door for a variety of new applications.
Experimental Section
Perovskite Nanocrystal Synthesis: All synthetic materials were
purchased from Sigma Aldrich and used as received unless otherwise
Adv. Mater. 2018, 30, 1706226
Figure 4. Devices across the blue-green spectrum. a) Electroluminescence, b) J–V–L, c) EQE, and d) pictures of devices with electroluminescence peaks
at 469, 481, 488, and 511 nm, demonstrating the universality of the TFB/PFI device structure. Tabulated device parameters can be found in Table 1.
Table 1. Device parameters of cesium lead halide blue-green devices.
Peak Emission
[nm]
Maximum EQE
[%]
Maximum brightness,
[cd m−2]
Maximum Lit. EQEa)
[%]
469 0.50% 111 0.07%[10,20]
481 0.44% 212 0.07%[10,20]
488 1.41% 830 1.9%[12]
511 2.37% 3423 8.73%[13]
a)Approximately at the Emission Wavelength or Bluer.

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noted. Cs2CO3 (0.814 g, purity 99.9%), octadecene (40 mL, purity 90%),
and oleic acid (2.5 mL, purity 90%) were loaded into a 100 mL flask,
dried under vacuum at 120 °C for 1 h, and then heated to 150 °C under
stirring and N2 protection, yielding a clear Cs-oleate precursor solution.
The solution was cooled to room temperature for storage, and reheated
to 100 °C under vacuum before use.
PbBr2 (179 mg, 0.488 mmol, purity 98%), PbCl2 (73.2 mg,
0.2632 mmol, purity 98%), octadecene (20 mL), oleylamine (2 mL,
purity 98%), oleic acid (2 mL), and trioctylphosphine (2 mL, purity 97%)
were loaded into 100 mL three-neck flask, dried under vacuum at 130 °C
for 45 min, and then heated to 150 °C under stirring. The yielded clear
solution was heated to 165 °C under N2 protection. Then, 1.75 mL of
preheated Cs-oleate precursors were swiftly injected into the above
solution which turned to a yellow color immediately. After reacting for
10 s, the product was cooled to room temperature in an ice/water
bath. The bromide to chloride ratio in nanocrystals was tuned by
postexchanging by a solution of PbBr2/oleic acid/oleylamine in
octadecene. For the 511 nm emission device, only the PbBr2 precursor
was used to synthesize CsPbBr3.
To purify the nanocrystals, an equal volume of anhydrous ethyl
acetate (purity 99.8%) was added to the above crude product to
precipitate the nanocrystals. After centrifuging at 4000 rpm for 5 min,
the precipitation was dissolved in 10 mL of anhydrous hexane (purity
95%). The nanocrystals were washed again by anhydrous ethyl acetate
(volume ratio of ethyl acetate:hexane 3:1), centrifuging at 7000 rpm for
5 min, and redispersed in 8 mL of octane or hexane. The solution was
filtered by a PTFE filter (0.2 µm) before use.
LED Fabrication: Ni(NO3)2•6H2O, Nafion perfluorinated resin
solution 5 wt% in lower aliphatic alcohols and water (PFI), LiF
(evaporation grade), and aluminum were purchased from Sigma Aldrich
and used as received. ITO substrates, TPBi, and TFB were purchased
from Luminescence Technology, Inc and used as received. PEDOT:PSS
was purchased from Heraeus (Clevios P VP AI 4083) and used as
received.
For the NiOx precursor, 1.5 m of each Ni(NO3)2•6H2O and ethylene
diamine were dissolved in ethylene glycol to obtain a blue colored
complex. After stirring for 10 min, the solution was filtered by a 0.4 µm
PVDF filter.
15 Ω ITO patterned glass was cleaned by sequential sonicating in
Micron-90 detergent, 2× water, 2× acetone and then soaking in boiling
isopropanol for 10 min. The films were dried under blowing air and
treated with O2 plasma at 200 W using 0.5 Torr O2 gas for 5 min. On
these clean ITO substrates, a thin layer of PEDOT:PSS (Clevios PVP AI
4083, filtered using 0.4 µm PVDF filter) was spun at 4000 rpm for 45 s
(ramp = 2500 rpm s−1), and annealed at 140 °C for 30 min in a nitrogen
glovebox. After cooling, a TFB (4 mg mL−1 in chlorobenzene) layer was
spin coated at 3000 rpm for 45 s (2000 rpm s−1 ramp), and annealed
at 125 °C for 15 min. A thin layer of PFI (0.05 wt% in isopropanol) was
then coated at 3000 rpm for 45 s and dried at 145 °C for 10 min. For
NiOx thin films, the precursor solution was spun on plasma cleaned
ITO at 2000 rpm for 90 s (2000 rpm s−1), and annealed at 300 °C for
1 h in air. On top of these layers, perovskite nanocrystals in octane
were coated using different spin conditions (see the Supporting
Information for individual conditions) to achieve a uniform layer.
These films were then taken in the evaporation chamber, where 40 nm
TPBi, 1.1 nm LiF, and 60 nm Al were deposited at 6 × 10−6 mbar at
2 Å s−1, 0.2 Å, 3 Å s−1, respectively. Devices were unpackaged and
measured in air.
Materials Characterization: Morphologies of perovskite nanocrystals
were characterized by TEM (JEOL 2100, 120 kV). UV–vis absorption
spectra were recorded by a PerkinElmer Lambda 25, and the
photoluminescence spectra were collected by an Ocean Optics
spectrometer (QE Pro) pumped by a 365 nm LED source. The TFB
mobility reported is from ref. [25], measured between 40 and 160 kV cm−1.
Streak Camera: Samples were excited by a 379 nm laser from
Hamamatsu (C10196) with an 81 ps pulse width. OD filters were used
to reduce the excitation intensity. The excitation was incident at a 45°
angle to the glass face. The PL was collected with a 25.4 mm focal length
lens normal to the glass face. It was focused through a 400 nm longpass
filter into an SP2150i spectrograph coupled to a Hamamatsu C10627
streak unit and C9300 digital camera. Data were integrated across the
nanocrystal emission wavelength from 440 to 500 nm to obtain the
curves presented in Figure 2.
Device Characterization: EL spectra were taken with an Ocean Optics
QE Pro with 100 ms integration time with 1 mA sourced to the device
from a Keithley 2400. Current–voltage and EQE characteristics were
measured with an HP 4145A with a calibrated half inch ThorLabs
photodetector physically pressed to the face of the device, removing
the need for a geometric correction. The device (1 mm radius) was
much smaller than the photodetector (9.7 mm). The photodetector
was smaller than the glass slide (12.2 mm), which, combined with the
black material construction of the EQE holder, blocks the collection of
waveguided light, preventing overestimation of the EQE.[29] Luminance
was calculated from the J–V–L curves and the spectra of the device. The
J–V–L curves were measured from low to high voltage. The reduction in
performance at high current densities is likely a reflection of both roll-off
and degradation while under test (see Figure S7 in the Supporting
Information).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
M.G. and S.H. contributed equally to this work. S.H. synthesized the
perovskite nanocrystals and performed materials characterization. M.G.
fabricated and characterized the LEDs. D.N.C. performed materials
characterization. All authors designed the experiments and contributed
to the manuscript. The authors acknowledge the support of the Rowland
Fellowship at the Rowland Institute at Harvard University.
Conflict of Interest
Harvard University is filing a patent based on this work.
Keywords
blue LEDs, device architecture, perovskite LEDs
Received: October 26, 2017
Revised: January 30, 2018
Published online: March 25, 2018
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