Charge Transport between Coupling Colloidal Perovskite Quantum Dots Assisted by Functional Conjugated Ligands

Article abstract of DOI:10.1002/anie.201801780

Long alkyl-chain capping ligands are indispensable for preparing stable colloidal quantum dots. However, its insulating feature blocks efficient carrier transport among QDs, leading to inferior performance in light-emitting diodes (LEDs). The trade-off between conductivity and colloidal stability of QDs has now been overcome. Methylamine lead bromide (MAPbBr₃) QDs with a conjugated alkyl-amine, 3-phenyl-2-propen-1-amine (PPA), as ligands were prepared. Owing to electron cloud overlapping and the delocalization effect of conjugated molecules, the conductivity and carrier mobility of PPA-QDs films increased almost 22 times over that of OA-QD films without compromising colloidal stability and photoluminescence. PPA-QDs LEDs exhibit a maximum current efficiency of 9.08 cd A^?1, which is 8 times of that of OA-QDs LEDs (1.14 cd A^?1). This work provides critical solution for the poor conductivity of QDs in applications of energy-related devices.

Full text of DOI:10.1002/anie.201801780

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Functional Conjugated Ligands Assisted Charge Transport
between Coupling Colloidal Perovskite Quantum Dots
Authors: Jinfei Dai, Jun Xi, Lu Li, Jingfeng Zhao, Yifei Shi, Wenwen
Zhang, Chenxin Ran, Bo Jiao, Xun Hou, Xinhua Duan, and
Zhaoxin Wu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801780
Angew. Chem. 10.1002/ange.201801780
Link to VoR: http://dx.doi.org/10.1002/anie.201801780
http://dx.doi.org/10.1002/ange.201801780

10.1002/anie.201801780
Angewandte Chemie International Edition
COMMUNICATION
Functional Conjugated Ligands Assisted Charge Transport
between Coupling Colloidal Perovskite Quantum Dots
Jinfei Dai, Jun Xi, Lu Li ,JingFeng Zhao, Yifei Shi, Wenwen Zhang, Chenxin Ran, Bo Jiao, Xun Hou,
Xinhua Duan, and Zhaoxin Wu*
Abstract: Long alkyl-chain capping ligands, such as n-octylamine
However, there is a dilemma in applications of QDs such as
LEDs, solar cell, field-effect transistor and others optoelectronic
(OA), are always indispensable for preparing stable colloidal
quantum dots. However, its insulating feature blocks efficient carrier
devices. For one thing, in order to prepare stable colloidal QDs
transport among QDs, leading to inferior performance of light-
emitting diode (LED).To date, several methods are adopted to
increase the film conductivity of QDs, but still didn’t break the
dilemma of the trade-off between the conductivity and colloidal
stability of QDs. For the first time, we overcome this dilemma,
successfully prepare methylamine lead bromide (MAPbBr3) QDs
with a conjugated alkyl-amine, 3-Phenyl-2-propen-1-amine (PPA), as
ligands. Owing to the electron cloud overlapping and delocalization
effect of conjugated molecules, the conductivity, carrier mobility, of
PPA-QDs films is nearly increased 22 times of that of OA-QDs films
without the compromise on its colloidal stability and
photoluminescence. As the application in LEDs, the PPA-QDs LEDs
exhibit a maximum current efficiency of 9.08 cd A-1, which is 8 times
of that of OA-QDs LEDs (1.14 cd A-1). This work provides critical
solution for the poor conductivity of QDs in applications of energy-
related devices.
with low surface defect density, capping ligands are
indispensable. However, the previously reported long chain
hydrocarbon such as n-octylamine (OA), oleylamine (OLA) or
distinctive qusi-spherical shaped capping ligand are always
insulator and block efficient carrier transport between QDs. [20-24]
For another, all of these QDs’ applications require efficient
carrier injection and transport in QDs films. But the insulating
ligands capped on QDs deteriorate the carriers transport of QDs
films, and then lead to inferior performance of devices [1,25] .This
is one of the biggest problems that limit the application of QDs-
based devices.
Learning from previous studies on traditional II-VI and III-V
group inorganic QDs, ligands exchange method is appreciated
to enhance carrier transport by replacing long chain ligand with
[25]
smaller molecules after QDs synthesized.
For HOIP QDs,
which is different with II-VI and III-V semiconductor by their ionic
property, ligand exchange methods are hardly used to improve
the carrier transport in HOIP QDs solid films. [26] Considering the
Semiconductor quantum dots (QDs) with specific small size
structure and customization of energy bandgap exhibit ultrahigh
photoluminescence quantum yield (PLQY), high color purity and
finely tunable emission spectrum[1] owing to quantum
confinement effect.[2] With such advantages, for decades, II-VI
and III-V group inorganic QDs have been popularly studied and
applied for different functional devices, particularly for light-
emitting diodes (LEDs).[3-5] Recently, hybrid organic-inorganic
perovskites (HOIP) QDs have attracted more attentions because
trade-off between the stability and carriers transport of HOIP
[27]
QDs films, ligand density control
length
and reducing ligand chain
[28]
have been adopted to improve carrier injection and
transport in HOIP QDs film. However, owing to insulating ligands
being not discarded, the dilemma of the trade-off between
conductivity of QDs films and colloidal stability of QDs colloidal
solution still did not fundamentally break.
As we know, conjugated molecules or polymers can exhibit
higher conductivity owing to the delocalization of the molecular
electric orbitals. In this work, for the first time, we introduce an
organic conjugated amine (containing C=C group) molecule, 3-
Phenyl-2-propen-1-amine (PPA) and use it as capping ligand to
prepare methylamine lead bromide (MAPbBr3) QDs. Taking OA
ligand as reference, we found the above dilemma was perfectly
solved, that is, effectively improve the conductivity of MAPbBr3
QDs films without compromising its optical property and colloidal
stability by using PPA as ligands. The conjugated PPA ligands
enable a 22 times increase in carrier mobility of the MAPbBr3
QDs films compared with OA-capped MAPbBr3 QDs (OA-QDs)
films. Thus, it significantly boosted the performance of the
electroluminescence device. As an example of the application in
LEDs, devices based on PPA-capped MAPbBr3 QDs (PPA-QDs)
exhibited a maximum luminance (L) of 9052 cd m-2 and a
maximum current efficiency (CE) of 9.08 cd A-1, while OA and
BA (n-butylamine) QDs-based LEDs only showed a maximum
luminance (L) of 457.5 cd m-2, 3522 cd m-2 and a maximum CE
of 1.14 cd A-1 and 2.60 cd A-1. This work provides a fundamental
solution for the conductivity problem of QDs, and paves the way
for the applications of QDs in energy-related devices.
of not only the more stable and ultra-high PLQY[6-7] but also their
[8]
lower expense and more facile processing progresses
and
have been dramatically exploited in many different research
[9-15]
fields
on account of their superior semiconductor properties
[16-19], including tunable bandgap, long carrier transport distance,
high absorption coefficient and so on.
[a]
J. Dai, Dr. J. Xi, L. Li, Y. Shi, Dr. C. Ran, Dr. B.Jiao, Prof. X.
Hou and Prof. Z. Wu
Key Laboratory for Physical Electronics and Devices of the
Ministry of Education & Shaanxi Key Lab of Information
Photonic Technique, School of Electronic and Information
Engineering, Xi’an Jiaotong University, No.28, Xianning West
Road, Xi’an, 710049, China
E-mail: zhaoxinwu@mail.xjtu.edu.cn
[b]
[c]
[d]
Prof. Z. Wu
Collaborative Innovation Center of Extreme Optics, Shanxi
University, Taiyuan 030006, China
Dr.J.Xi
Global Frontier Center for Multiscale Energy Systems, Seoul
National University, Seoul 08826, Korea
Prof. X.Duan, Dr,J.Zhao
The detailed synthesis process of PPA and its structure
characteristic are shown in the experiment section and Figure
S1, S2 of Supporting Information. Colloidal QDs in this work
were prepared by modified ligands-assisted reprecipitation
(LARP) technique[20] as illustrated in Figure 1a. The optimal
dosage of PPA 30 μ l and OA 20 μ l for quantum dots
preparation was determined by a series experiments, (Figure S3
illustrates the QD solution PL changing with amine dosage).
Figure 1b is a typical structural representation of PAA-QDs.
Figure 1c shows representable steady-state PL spectra of PPA-
QDs and OA-QDs colloidal solution with optimal amine dosage
Department of Chemistry, School of science and MOE Key
Laboratory for Nonequilibrium Synthesis and Modulation of
Condensed Matter, Xi’an Jiaotong University, Xi’an 710049,
China.
[e]
Dr. W. Zhang
School of Electronic Engineering, Xi’an University of Post &
Telecommunication, Xi’an 710121, China
Supporting information for this article is given via a link at the
end of the document.
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10.1002/anie.201801780
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COMMUNICATION
which is conforming to previous study. [20] A little red-shift of PL
peak was found for PPA-QDs colloidal solution caused by the
weaker quantum confinement effect with the larger diameter.
The insert is the corresponding representable photograph of the
colloidal solutions under 365 nm UV light. PLQY results of PPA-
QDs and OA-QDs is 63.10 % and 68.35 % respectively. Figure
1d shows the time-resolved PL (TRPL) spectra of the two
colloidal solutions under a 404 nm pulsed laser excitation. The
curves can be fitted well with a typical bi-exponential decay
function,
0.42 nm deviation. This slight differences in size might be
resulted from different ligand molecular structures.[31] The
inserted High Resolution Transmission Electron Microscopy
(HRTEM) images in Figure 2a and 2b both show well-
crystallized PPA-QDs and OA-QDs. The inter-planner distance
illustrated in the inserted of Figure 2a and Figure 2b both
corresponded to the (210) crystal face of cubic MAPbBr3. Figure
2c shows FTIR spectra of OA-QDs and PPA-QDs. The peaks
positions in FTIR spectra of PPA-MAPbBr3 QDs and OA-
MAPbBr3 QDs revealed that for any QDs, PPA or OA molecules
was successfully anchored to the surfaces of MAPbBr3 QDs.
L=A1exp(-t/τ1)+A2exp(-t/τ2)
1
Such a result was also proved by H NMR results of PPA-QDs
and OA-QDs (Figure S5, S6).
According to previous study of surface-related emission in high
luminescent CdSe QDs,[29,30] the more percentage of long-lived
life-time implies the more superior PLQY. Hence, as our
experimental result shown in Figure 1d, OA-QDs with a long-
lived lifetime τ2 with 72.08 % percentage would account for the
slight higher PLQY of OA-QDs compared to PPA-QDs whose
percentage of τ2 is 71.18 %. In order to illustrate that PPA can
be a common ligand for HOIP QDs preparation, a series of
color-tunable MAPbX3 (X is Cl, Br, I or their mixture) QDs
colloidal solution was synthesized. Figure 1e displays
photographs of color-tunable MAPbX3 QDs colloidal solution
under 365 nm UV light and their steady state PL spectra.
Figure 2. TEM and the HRTEM images of the as-prepared (a) PPA-QDs
and (b) OA-QDs (scale bar: 200 nm, inset scale bar: 3 nm). (c) FTIR
spectra of PPA-QDs and OA-QDs. (d) XRD patterns of PAA-QDs and OA-
QDs films. (e) PL intensity change of PPA-QDs and OA-QDs colloidal
solution varying with time.
We investigated the XRD patterns of both QDs as-formed
films to further analyze the phase structure of different ligands
assisted MAPbBr3 QDs, The XRD pattern of PPA-QDs film and
OA-QDs film in Figure 2d both indicated (100), (110), (200) and
(210) crystal plane of cubic phase MAPbBr3 respectively.[20]
Additionally, the atomic ratios between Pb and Br were analyzed
by EDS. The atom ratio between Br and Pb in PPA-QDs and
OA-QDs were estimated to 3.11 and 3.3, respectively, as shown
in Figure S7. Finally, we comparatively studied the stability of
PPA-QDs and OA-QDs colloidal by testing their PL intensity
varying with time, as shown in Figure 2e. Figure S8 shows
absolute PL intensity change of PPA-QDs and OA-QDs colloidal
solution varying with time. This result illustrated the stability of
PPA-QDs colloidal solution is analogous with that of OA-QDs.
All above characteristic analyses indicate that MAPbBr3 QDs are
well controllably formed with desired crystal structure by PPA
capping ligands, whose properties including the stability and
dispersity are almost similar with OA-QDs, irrespective the
subtle molecular structure of ligands.
Figure 1. (a) Schematic illustration of the MAPbBr3 QDs processing
progress. (b) The structural representation of PAA-QD in which PAA
instead of OA acts as capping ligands. (c) Steady-state PL spectra and
representative photograph and PLQY values of PPA-QDs and OA-QDs
colloidal solution under 365 nm UV light. (d) TRPL spectra of PPA-QDs
and OA-QDs colloidal solution. (e) color-tunable MAPbX3 QDs with PPA
as capping ligand.
To further confirm whether PPA ligands on QDs lead to
superior structures advantages, we comparatively studied the
structure characteristics of PPA-QDs and OA-QDs by
transmission electron microscopy (TEM), Fourier Transform
Infrared (FTIR), nuclear magnetic resonance (NMR), X-ray
diffraction (XRD) and energy dispersive spectrums (EDS). The
typical TEM images of PPA-QDs and OA-QDs in Figure 2a, 2b
exhibits a monodisperse character and narrow size distribution.
The corresponding size distribution of PPA-QDs and OA-QDs
were counted (see Figure S4). The statistic results show that the
PPA-QDs had an average diameter of 3.72 nm with a 0.67 nm
deviation, and OA-QDs had an average diameter of 2.93 nm and
To explore the fundamental impacts of PPA capping ligands
on carriers mobility property of MAPbBr3 QDs, we firstly
compared current densities of single carrier devices, they were
hole-only and electron-only devices. Figure 3a, 3b show the
current density results of electron-only and hole-only devices,
whose configurations are shown in the insert of corresponding
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images. For both devices, the current density of PPA-QDs
based devices significantly suppressed that of OA-QDs based
devices, revealing that compared to the inferior charge transport
within long chain ligands assisted HOIP QDs, the conjugated
or OA-QDs devices show the current density of electron-only
devices is suppressed compared to that of hole-only devices,
agreeing well with general reports. Sheet Hall Effect of bulk
MAPbBr3 film, PPA-QDs film and OA-QDs films were
investigated to confirm the conjugated capping ligand is
favorable to carriers transport. The schematic illustration of
Sheet Hall effect measurement is shown in Figure 3c. The Hall
Effect results displayed in Figure 3d revealed that carrier
mobility (μH) of bulk MAPbBr3 film, PPA-QDs film, OA-QDs films
are 16.4±3.7 cm2 V-1 s-1, 4.3±0.55 cm2 V-1 s-1 and 0.19±0.27 cm2
V-1 s-1 respectively, which substantially implied that conjugated
capping ligands could dramatically enhance the carrier mobility
of HOIP QDs film by 22 times compared to non-conjugate OA
ligands.
PPA-QDs films with higher carrier transport provide the
opportunity to improve the performance of QDs-based LEDs. To
demonstrate their performance in QDs-based LEDs, a group of
devices with a configuration of indium tin oxide (ITO) / PEDOT：
Figure 3. Current density-voltage (J-V) characteristics of (a) electron-only
and (b) hole-only HOIP quantum dots device with OA-QDs and PPA-QDs
as active layer. (c) Hall Effect schematic diagram and (d) corresponding
measured results of different MAPbBr3 films.
PSS (30 nm)
/ MAPbBr3 QDs / 1,3,5-Tris(1-phenyl-1H-
benzimidazol-2-yl) benzene (TPBI) (40 nm) / LiF (1 nm) / Al (100
nm) were fabricated. Figure 4a shows the energy diagram of the
QDs-based LEDs and Figure 4b shows cross sectional SEM
images. The measured thickness of MAPbBr3 QDs film from
both devices SEM cross sectional images were 90±5 nm,
indicating similar thickness of the two QDs film. The in-plan SEM
images and main element distribution images, indicating similar
morphology and element distribution of the two QDs films
(Figure 4c). (AFM image results also illustrated the similar
capping ligands assembled on the surfaces of QDs could
improve charge injection and transport in QDs formed films in
principle. Generally, MAPbBr3 is regarded as
a P type
semiconductor, in which holes rather than electrons are the
dominant carriers. Our measurement results in either PPA-QDs
Figure 4. (a) Energy-level diagram of LED devices with OA-MAPbBr3 or PPA-MAPbBr3 as emitting layer. (b) SEM image of responding device cross
section (scale bar: 250 nm). (c) SEM images of PPA-QDs film and OA-QDs film and the corresponding element distribution (Pb and Br) in each film
(scale bar: 500 nm). (d) Luminous intensity (L), (e) current efficiency (CE), (f) current density (J) as a function of driving voltage in PAA-MAPbBr3 and
OA-MAPbBr3 QDs film, respectively. (g), (h) schematically illustration of carrier transport in difference QDs films (PPA-QDs and OA-QDs) .
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10.1002/anie.201801780
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morphology of the two kinds of QD films, see Figure S9). The
light-emitting properties of PPA-QDs and OA-QDs based LEDs
were thoroughly investigated. It was found that, with PPA-QDs
as active emitting layer, devices performance had been greatly
improved in luminance, current efficiency and current density,
which can be observed in Figure 4d, 4e, 4f (and Figure S10,
photograph of devices under working state). PPA-QDs LEDs
exhibited the maximum luminance about 9052 cd m-2 and current
efficiency about 9.08 cd A-1, which suppressed the result of OA-
QDs LEDs (457.5 cd m-2 and 1.14 cd A-1). These results were
consistent with the demonstrated carrier transport character of
PPA-QDs film, proving that the conjugated capping ligands not
only can be an effective ligand for HOIP colloidal QDs
preparation but also can enhance the carrier transport in HOIP
QDs films, so as to the high performance of LED device. Figure
4g, h schematically illustrates carriers transport in MAPbBr3
PPA-QDs and OA-QDs films. QDs films with the conjugated
PPA ligands can exhibit the higher conductivity owing to the
delocalization of the molecular electric orbitals and π-π stacking.
In addition, BA-QDs with shorter hydrocarbon chains were also
employed as the emitting materials in LEDs, that is, BA-QDs
LEDs with the same structured as PPA-QDs and OA-QDs LEDs,
shown in Figure S11. It is easily found that BA-QDs LEDs exhibit
luminance and current efficiency is 3522 cd m-2 and 2.60 cd A-1,
which is higher than those of OA-QDs LEDs, but quite lower
than those of PPA-QDs LEDs. This unveiled that although BA
with the shorter hydrocarbon chains could improve current
efficiency of QDs-LEDs compared with OA-QDs LEDs, its
performance is still limited by the insulating property of BA.
of PLQY and lifetime testing. Finally, we appreciated for the
meaningful discussing with M. Zhang and F. Yuan.
Keywords: HOIP QDs • conjugate ligand • PPA• carrier mobility
•QD-based LED
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10.1002/anie.201801780
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Jinfei Dai, Jun Xi, Lu Li , Jinfeng
Zhao, Yifei Shi, Wenwen Zhang,
Chenxin Ran, Bo Jiao, Xun Hou,
Xinhua Duan, and Zhaoxin Wu*
Page No. – Page No.
PPA is proved to be a common
ligand for stable HOIP QDs’
preparation, and it in favour of
carriers transport in QDs film.
As an example of LED device,
PPA-QD based LED exhibits
much higher performance than
that of OA-QD.
Functional Conjugated
Ligands Assisted Charge
Transport between Coupling
Colloidal Perovskite Quantum
Dots
This article is protected by copyright. All rights reserved.