Bi-inorganic-ligand coordinated colloidal quantum dot ink

Article abstract of DOI:10.1039/c9cc04157e

Quantum dot light emitting diodes (QLEDs) are rising as a promising light emitting technology. However, the widely used insulating organic ligands hamper carrier injection. Herein, we developed a bi-inorganic-ligand strategy to replace organic ligands and dispersed QDs in a benign solvent butylamine. The all-inorganic QD film shows enhanced luminescence intensity and superior thermal stability and conductivity. In the end, we exploited the first prototype all inorganic QLED.

Full text of DOI:10.1039/c9cc04157e

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Bi-Inorganic-ligands Coordinated Colloidal Quantum Dot Ink
Xianyuan Jiang,a‡ Hansheng Li,a‡ Yuequn Shang,a Fei Wang,a Hao Chen,a Kaimin Xu,a Ming Yin,a Hefei
Received 00th January 20xx,
Accepted 00th January 20xx
Liu,b Wenjia Zhou,a and Zhijun Ning*a
DOI: 10.1039/x0xx00000x
Quantum dot light emitting diode (QLED) is rising as a promising However, to the best of our knowledge, there are no reports of using
light emitting technology. However, the widely used insulative QD ink based inorganic ligands for QLEDs. Firstly, inorganic ligands
organic ligands hamper carriers injection. Herein, we developed a coordinated QDs generally are dispersed in high polarity solvent with
bi-inorganic-ligands strategy to replace organic ligands and high boiling temperature, which makes it difficult to fabricate QD
dispersed QDs in a benign solvent butylamine. The all-inorganic QD films. Secondly, the strong interaction between solvent and ligands
film shows enhanced luminescent intensity, superior thermal typically brings ligand loss during film fabrication and gives rise to the
stability and conductivity. In the end, we exploited the first low PL QY in the solid state.
prototype all inorganic QLED.
Herein, we explored hydroxide and sulfide bi-ligands to coordinated
QD surface, which enabled QDs to be dispersed in low polarity
The narrow emission peak, high photoluminescence quantum yield
solvent. This significantly reduced the interaction force between
(PL QY), size-dependent bandgap tunability, and solution
solvent and ligands and alleviated ligand loss during film fabrication,
processability of colloidal quantum dots (QDs) make them ideal
giving rise to much-enhanced luminescence intensity. The
materials for light emitting diodes and lasers.1, 2 The performance of
conductivity of the inorganic ligand coordinated QD film is one order
QD-based light emitting diodes (QLED) has been significantly
of magnitude improved and it shows superior thermal stability. In the
improved in recent years via QD structure manipulation,3, 4 surface
end, by using ZnO and NiO as the electron and hole transporting
ligand engineering,5, 6 and device structure manipulation.7
layers respectively, we constructed, for the first time, a prototype all
Almost all QLEDs are based on QDs with organic ligands such as alkyl inorganic QLED.
molecules with acid or thiol groups. These QD films show high
luminescent quantum yield. However, the insulating molecules could
hamper carriers injection, bringing low current density and
luminescence intensity. Especially, this has been a bottleneck that
limited the development of electrical pumped laser based on QDs.
Furthermore, the weakly coordinated organic ligands can desorb
under electrical field boiling, leading to the formation of dangling
bond and deterioration of device performance.
In comparison to organic ligands, inorganic ligands can improve
conductivity and stability of the QD films.8-12 Inorganic ligands were
used for QD solar cells and greatly enhanced the efficiency and
stability of this kind of solar cells. 8-13 Very recently, inorganic ligand
Figure 1. Photographs of QD before (a) and after (b) ligand exchange under
UV light. (c) Photograph of QD dissolved in BA under UV light. (d) Absorption
spectra of QDs. (e) Schematic illustration of ligand exchange and dissolution
was explored for QLEDs base on solid-state ligand exchange.6
in BA with the addition of (NH4)2S.
a. School of Physical Science and Technology, ShanghaiTech University, Shanghai
201210, China.
b. Ming Hsieh Department of Electrical Engineering, University of Southern
California, Los Angeles, CA 90089, USA.
Solution phase ligand exchange was used to prepare the inorganic
ligands coordinated QD ink. Chemical-composition-gradient
CdSe@ZnS core-shell QDs were synthesized by a typical hot injection
method.14 QDs capped with oleic acid ligands (QD-OA) dispersed in
*.Corresponding author.
‡.These authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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hexane were mixed with N-methylformamide (NMF) containing KOH (Figure 1d), indicating that the size of QDs is maintained.
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DOI: 10.1039/C9CC04157E
(Figure 1a). After a few minutes, the QDs were transferred into NMF Transmission electron microscopy (TEM) images show that the
phase (Figure 1b), indicating ligand exchange of the alkyl chain with distance between QDs is reduced when OH ligands are used,
OH, and we name it QD-OH.8 The ligand-exchanged QDs were then ascribing to the smaller size of the inorganic ligands relative to the
separated via the addition of acetonitrile as an anti-solvent and organic ligands (Figure 2d, 2e). The aggregation of QD-OH could be
subsequently dispersed in solvent for film fabrication (Figure 1c).
caused by the burning of unremoved high boiling temperature
solvent NMF on QD surface. Boundaries of QDs are clearly observed
for QD-OS (Figure 2f), indicating that the shapes of QDs are
maintained well.
The use of solvent with low boiling temperature and polarity for QD
ink facilitates film fabrication. Here we utilized butylamine (BA) as
the solvent for film fabrication.15 The exchanged QDs can’t be
dispersed in BA directly, while the addition of (NH4)2S significantly We then analysed the composition of QD surface in the films
increases the solubility, enabling the formation of a non-scattering fabricated by spin coating using BA or NMF as solvents respectively.
solution (Figure S1c, d). A possible mechanism is proposed in Figure The residual organic ligands of the QD films are analysed by Fourier
1e. Due to their strong bonding strength of cadmium with sulfide transform infrared spectroscopy (FTIR). The disappearance of the C-
anions, (NH4)2S can replace a fraction of OH groups on the QD H stretching frequency (2800−3000 cm−1) for the film prepared with
surface, the ligand exchanged QDs were named QD-OS. The bivalent QD-OH confirms that the organic ligands are substituted by OH
sulfide ions offer a negative charge to attract ammonium cations, as ligands (Figure 2b).17 Furthermore, thermogravimetric analysis (TGA)
evidenced by the existence of S2- by X-ray photoelectron shows a 10% total weight loss for QD-OH, much lower than the 24%
spectroscopy (XPS) measurement (Figure 5b, blue line). Meanwhile, weight loss for QD-OA (Figure 2c). The weight loss of QD-OH can be
the proton exchange between NMF and BA enables the formation of ascribed to the remaining NMF solvent in the film. For the film made
butylammonium, as indicated by 1H nuclear magnetic resonance (1H with QD-OS, transmission peak at 2800-3000 cm-1 in the FTIR
NMR) spectra (Figure S2).16 Butylammonium can be attracted by spectrum is weak as well, and only a 5% (orange line) weight loss is
anions on QD surface. Therefore, an electric double layer, consists of observed, illustrating only a small amount of BA ligands remains in
S2-, NH4+ and short chain butylammonium is formed, which repels the the film.
aggregation of QDs. Furthermore, butylammonium on QD surface
brings a steric hindrance to prevent approach of QDs.
Figure 3. High-resolution Cd 3d (a) and S 2s (b) XPS spectra of the QD-OH
with 200 ℃ annealing, EXP is the original data. (c) Scheme of the ligand-
solvent interaction, ligand loss and QDs oxidized.
The use of NMF as solvent causes defects on QD surface. For the film
prepared by QD-OH, a shoulder peak at 228.2 eV (Figure 3b)
corresponding to the S 2s peak from SOX2- is observed, indicating the
oxidation of QDs (Figure 3c).18 Most probably the oxidation occurs
at QD surface, since the steric hindrance for the atoms inside
2-
prevent the reaction. SOX groups are known to result in the
Figure 2. (a) PL spectra and PL QY of CdSe@ZnS QDs. (b) FTIR spectra of QDs.
(c) Thermogravimetric analyses of QDs. (d-f) The TEM image of QD-OA, QD-
OH, and QD-OS.
formation of deep energy level trap states, bringing the decrease of
2-
emission intensity. In contrast, the peak belonging to SOX is not
found for the film that fabricated from QD-OS. Furthermore, the
atomic ratio of OH to Zn is 62.5% for the film made by QD-OH, while
it is 92% for QD-OS (Table S1, Figure S3), indicating that the amount
of OH ligand for the film prepared by QD-OH is much less than that
by QD-OS. The loss of ligands contributes to the formation of defects
on QD surface. Non-radiative recombination at defects as well as the
We then tested the photophysical properties of QDs. The QD-OA
exhibits a high PL QY of 89% in hexane, while the PL QY decreases to
31% for QD-OH and 22% for QD-OS (Figure 2a). No obvious shift of
the exciton peak position is observed in the absorption (Abs) spectra
2 | J. Name., 2012, 00, 1-3
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energy transfer between QDs and ‘bad’ dots with numerous trap
states bring the decrease of PL QY.19 The PL QY of the QD-OS film is
19%, much higher than that of QD-OH (9%) (Figure S4).
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DOI: 10.1039/C9CC04157E
We then analysed the possible mechanism for the loss of ligands on
QD surface. When two polar molecules close to each other, the
negative ends will attract the positive ends of the other molecule and
this is so called dipole-dipole force,20, 21 a kind of van der Waals force.
Owing to its large dipole moment, NMF strongly attracts OH ligands.
Hence, when we removing solvent in film fabrication, it will cause the
loss of ligands on QD surface simultaneously (Figure 3c). The use of
low polarity solvent BA alleviated intermolecular forces between
QDs, thereby suppressed ligand loss and surface oxidation.
Figure 5. High-resolution Cd 3d, N 1s (a) and S 2s (b) XPS spectra of QD-OA
with annealing at 60 °C, QD-OS with annealing at 60 ℃, and QD-OS with
annealing at 200 ℃. EXP is the original data.
We fabricated QLED with chalcogenide-ligand-capped QDs. An
inverted device structure was utilized employing NiO and ZnO as the
hole and electron transport layer respectively, and an ultrathin
passivating Al2O3 layer between NiO and QDs.24 The band diagram of
this structure is shown in Figure 6a using the work function
reported.25
Figure 4. PL study of films fabricated from QD-OA (a) and QD-OS (b) with
different annealing temperatures. (c) Schematic of the surface evolution via
annealing.
The inorganic ligands capped QDs film shows excellent thermal
stability. For the QD-OA film, the PL intensity decreased as
increasing the annealing temperature (Figure 4a, S5, Table S2),
most probably due to ligand desorption and the formation of
dangling bonds on the QD surface. In contrast, PL intensity of the
QD-OS film increased upon increasing annealing temperature
(Figure 4b, S5, Table S2). However, it needs to be noted that the PL
QY of the film after annealing is still lower than that with oleic acid.
XPS measurement was performed to analyse the mechanism for the
enhanced thermal stability and PL intensity. The N 1s peaks at 402.1
+
eV and 400.1 eV are ascribed to NH4 and BA groups, respectively
(Figure 5a).22 As the annealing temperature increased to 200 ℃, the
N 1s peaks in XPS spectra disappeared, indicating that organic ligands
on the QDs surface were removed. The S 2s peaks at 225.9 eV and
227.9 eV are ascribed to ZnS (Figure 5b, red line) and S2- (blue line),
respectively. After annealing, S 2s of S2- and Se 3s peaks were
disappeared, which can be attributed to the formation of CdS and
ZnS shells,23 since it removed the charges of S2- and blocked
photoelectron emission from the inner CdSe in XPS measurement
(Figure 4c). The formation of CdS and ZnS shells passivates the
Figure 6. (a) Energy level diagram of the QLED. (b) EL spectrum of the
corresponding device; inset: a photo of the operating device. (c) J-V-L plot
and (d) EQE plot of QLED based on QD-OS, QD-OA and QD-OT.
surface dangling bonds and brings slight redshift of PL peak. The PL The QLED based on QD-OS shows an electroluminescence (EL) peak
intensity increase of QD-OS might because of the formation of well- at 541 nm with a full width half maximum value of approximately 45
passivated shell structures and the corresponding lower defect nm (Figure 6b). The device shows one order of magnitude higher
density.
current density compared to OA and 1-Octanethiol (OT) capped
control device (Figure 6c), which indicates its better conductivity. The
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J. Name., 2013, 00, 1-3 | 3
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9.
Z. Ning, X. Gong, R. Comin, G. Walters, F. Fan, O. Voznyy,
all-inorganic QLED exhibits a saturated green colour with luminance
of 1149 cd m-2, higher than those of the devices based on QD-OA and
QD-OT (Figure 6c). Moreover, the organic-ligands-capped device
break-down in high current region, while all-inorganic device shows
good tolerance of high current (Figure S6). However, due to the
much enhanced current density, the external quantum efficiency
(EQE) of all-inorganic devices (0.07%) is lower than the organic one
(0.22% of OA, 0.32% of OT) (Figure 6d).
E. Yassitepe, A. Buin, S. Hoogland and E. H. SVaierwgeAnrttic,le Online
DOI: 10.1039/C9CC04157E
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This work was supported by the Shanghai International
Cooperation Project (16520720700), Shanghai Key Research
Program (16JC1402100), ShanghaiTech Start-up Funding, 1000
Young Talent Program, National Natural Science Foundation of
China (U1632118, 21571129), and Centre for High-resolution
Electron Microscopy (CħEM), SPST, ShanghaiTech University
under contract No. EM02161943. The authors also thank the
Test Center of Shanghai Tech University and Dr. Peihong Cheng.
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