A facile, environmentally friendly synthesis of strong photo-emissive methylammonium lead bromide perovskite nanocrystals enabled by ionic liquids

Article abstract of DOI:10.1039/d0gc01081b

Herein we demonstrate a new method for the synthesis of methylammonium lead bromide perovskite nanocrystals (MAPbBr₃) by using environmentally friendly ionic liquids. The effects of different carboxylate anions in ionic liquids on crystal growt

Full text of DOI:10.1039/d0gc01081b

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Wang, Green Chem., 2020, DOI: 10.1039/D0GC01081B.
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DOI: 10.1039/D0GC01081B
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A facile, environmental-friendly synthesis of strong photo-
emissive methylammonium lead bromide perovskite nanocrystals
enabled by ionic liquids
Received 00th January 20xx,
Accepted 00th January 20xx
DOI:
a
a
a
a
a
a
Minh Tam Hoang, Ngoc Duy Pham, Yang Yang, Vincent Tiing Tiong, Chao Zhang, Ke Gui, Hong
b
b
b
a
a,c
a
Chen, Jin Chang, Jianpu Wang, Dmitri Golberg, John Bell, and Hongxia Wang*
Herein we demonstrate a new method for synthesis of Perovskite nanocrystals (PeNCs) in the form of nanowires or
methylammonium lead bromide perovskite nanocrystals nanoplatelets have demonstrated promising performance for
(
MAPbBr
effects of different carboxylate anions in the ionic liquids on optoelectronic devices
crystal growth and properties of the MAPbBr were have exhibited exceptional progress with external quantum
investigated. This work paves a new avenue for sustainable efficiency reached to 21.3%
synthesis of perovskite materials by using ionic liquid based In all these applications, synthesis of the perovskite materials
3
) by using environmental-friendly ionic liquids. The applications such as photodetectors, lasers and other
8
-10
. LEDs devices made from PeNCs
3
1
1-16
.
green solvent.
normally involves extensive use of hazardous solvent.
Traditionally thin film perovskite materials are synthesized by
In the last decade organo-metallic perovskite materials have wet chemical solution process and PeNCs or QDs can be made
attracted enormous attention in the communities of material by ligand-assisted reprecipitation or hot injection methods
science and photovoltaics (PV) due to their unprecedented which involves the use of noxious solvents like N,N-
achievement in power conversion efficiency with solar cells, Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO) or N-
which has increased from 3.2 to 25.2% in ten years with the methyl-2-pyrrolidone in order to dissolve the lead halide salt
1
-3
17-21
perovskite material made by solution processing method . precursor
. These material synthesis methods are not only
The outstanding performance of perovskite materials in PV complex and difficult to control material properties, use of the
devices is ascribed to their unique optical and electrical hazardous solvents are also highly unfavourable in industrial
properties including composition dependent optimal bandgap, fabrication due to their harmful impact on human health and
2
2
strong light absorption, high defects tolerance, efficient environment . Furthermore, the highly coordinating solvents
4
separation of electron-hole pair . Besides PVs, the perovskite (e.g., DMF, DMSO) used in PeNCs synthesis could also lead to
has also demonstrated potential in other optoelectronic degradation of the perovskite NCs by the residual DMF, DMSO
2
3
devices such as light emitting diodes (LEDs), photodetectors solvent in the solution . To address the issues, efforts have
5
,
and transistors, owing to their strong photo-emissive nature
.
been made to develop synthetic process with lower
6
Compared to thin films based bulk materials, nanocrystal is environmentally harmful impact for organo-metal perovskite
another important form of a perovskite material owing to the NCs. For example, less toxic solvent such as acetonitrile (ACN)
advantageous feature of small particles such as stronger was used to dissolve PbI powder to synthesis
2
photoemission, size -dependent tuneable band gaps which methylammonium lead triiodide perovskite nanocrystals.
offer efficient light absorption and luminescence in a broader However not only the material synthesis method is complex,
7
spectral wavelength ranging from violet to near-infrared . the low solubility of the lead salt in ACN and the high vapour
pressure of flammable ACN make it challenge for application in
2
4
large scale material production .
Ionic liquids (ILs) are a type of material composed of cations
and anions with a melting temperature of less than 100 C. The
unique properties of low melting temperature and decent
solubility to certain materials have enabled ILs a good choice
as a green solvent in material synthesis . The attempts of
using ILs based on methylammonium formate (MAF) for
synthesis of perovskite thin films were reported. Nevertheless,
aSchool of Chemistry, Physics and Mechanical Engineering, Science and Engineering
Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia.
E-mail: hx.wang@qut.edu.au
o
b
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing
2
11800, China.
c
25
Division of Research and Innovation, University of Southern Queensland, PO Box
4
393, Raceview, Qld 4305 Australia.
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
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2
Figure 1: a) Reaction phenomenon between PbBr and MAP; b) UV-vis absorption and photoluminescence spectrum of the
product formed by PbBr
of the product from reaction between PbBr
perovskite phase. Blue and red bars are standard XRD peak for PbBr
2
and MAP (inset: dispersion of the sample in toluene under UV-365 nm excitation; c) XRD patterns
and MAP after different reaction times. The indexed peaks correspond to the
and MAPbBr
2
2
3
.
2
6,
the quality of the synthesized perovskite film was very poor.
at room temperature. Figure 1a shows a representative
Most recently methylammonium acetate was used for the example of a sample formed through the reaction between
production of perovskite films for solar cells, which showed MAP and PbBr . The white PbBr powder changes to yellow
2
7
2
2
2
8
decent performance . To the best of our knowledge, there is colour immediately in the presence of MAP, which emits
no report of synthesis of perovskite nanocrystals using ionic strong green light under photoexcitation by a UV lamp (365
liquids. Herein, we demonstrated the first facile synthesis of nm). The instantaneous change in crystal structure of PbBr
highly luminescent MAPbBr perovskite nanocrystals by using powder was also revealed by in-situ transmission electron
ionic liquids based on methylammonium carboxylate with microscopy (TEM) measurement. From the selected area
different alkyl chains of anions. Highly crystalline MAPbBr diffraction pattern of PbBr powder before and after contact
2
3
3
2
nanocubes with sizes of 10-20 nm and nanorods with diameter with MAP, we observed the appearance of a new diffraction
of 80-90 nm were made by controlling carbon chain of anions pattern which can be assigned as the fingerprint of the
of the ILs. The synthesized NCs exhibited bright green MAPbBr
photoluminescence with narrow full width at half maximum of the reaction can be easily dispersed uniformly into nonpolar
8 nm and short radiative lifetime of around 6 ns. This novel solvent like toluene or hexane for further characterisation
method offers a facile, environmental-friendly synthesis (Inset in Figure 1b).
3
lattice (Figure S3). The yellow powder collected after
2
pathway for MAPbBr
various applications.
3
NCs with controlled properties for The UV-visible absorption spectrum of the dispersion solution
(Figure 1b) shows that the material has an absorption peak at
A series of methylammonium carboxylate protic ionic liquids ~526 nm corresponding to a band gap of 2.26 eV. A single
with different alkyl chain length were synthesized via reactions strong photoemission peak at ~530 nm with narrow FWHM of
of methylamine (Brønsted base) with formic acid, propionic ~28 nm is observed in photoluminescence (PL) spectrum of the
acid and butyric acid (Brønsted acid) to form material (Figure 1b). The optical property of the material is
2
0, 29
methylammonium
propionate (MAP) and methylammonium butyrate (MAB), In order to understand the formation mechanism of the
respectively. All the materials are liquid at room temperature perovskite formed by MAP IL with PbBr , we monitored the
formate
(MAF),
methylammonium consistent with that of MAPbBr
3
NCs reported previously
.
2
and are thermally stable with no weight loss until the evolution of crystal structure of the yellow product at different
o
temperature is more than 90 C (Figure S1). we have reaction times by X-ray diffraction (XRD) (Figure 1c). As can be
conducted nuclear magnetic resonance (NMR) measurement seen, after keeping the PbBr
for the as-synthesized ionic liquids MAF, MAP and MAP. The H new diffraction peaks belonging to cubic MAPbBr
NMR spectra of the ionic liquid are shown in figure S2. As can appear. The intensity of the MAPbBr
2
powder in MAP liquid for 5 min,
perovskite
peaks keeps increasing
be seen, all the detected protons can be assigned to while intensity of diffraction peaks of PbBr decreases with the
corresponding chemical shift in anion and cation of ionic reaction time. After 2 hours of reaction time, only MAPbBr
liquids. The result indicates the successful synthesis of the ILs. phase was detected with the primary diffraction peaks at
1
3
3
2
3
o
o
o
When PbBr
2
powder is added to the ionic liquid, we found that 14.93 , 30.12 and 33.79 corresponding to (100), (200) and
instantaneously (210) planes of cubic MAPbBr perovskite. We fitted the XRD
all the above ionic liquids reacted with PbBr
2
3
2
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Figure 2 a) Illustration depicting the reaction of PbBr
under UV-365 nm excitation; b) Comparison in PL spectrum of MAPbBr
c-e) SEM images of MAPbBr (MAF), MAPbBr (MAP) and MAPbBr
g) MAPbBr (MAB) nanorods (Insets in f and g: high resolution TEM image showing the lattice images of corresponding
nanocrystals).
2
with MAF, MAP and MAB. The product powder shows green emission
3
(MAP) and MAPbBr
3
(MAB) NCs dispersed in toluene;
3
3
3 3
(MAB) powder; f) TEM images of MAPbBr (MAP) NCs and
3
patterns of the material collected at different reaction (MAAc) with PbI
duration by means of the Rietveld refinement method. We MAAc ILs solvent . Furthermore Seo et al. confirmed
2 3
during crystallization of MAPbI thin film in
2
8
-
2+
used standard MAPbBr
02-0259) phases as reference in the fitting. The results show additives in the perovskite precursor solution . The
that the percentage of MAPbBr phase in the material was intermediate phases did not affect the crystal structure and
1.9%, 79.0%, 83.3%, 93.8% and 100% at the reaction duration phase purity of the perovskite film, which is similar to our case.
of 5 min, 15 min, 30 min, 60 min and 120 min, respectively. However, it can have significant effect on the nucleation and
3 2
(PDF-01-084-9476) and PbBr (PDF-04- formation of complex HCOO -Pb when using MAF as
3
1
0
3
7
Interestingly, during the phase transformation, an XRD peak at crystallization of the perovskite material. For example,
o
2
6.01 (labelled as unknown peak in Figure 1c) was detected the intermediate phase between PbI
2
and MAAc ILs effectively
3
2
for the perovskite formed by MAP with a reaction time up to hampers the reaction between PbI
2
and MAI in the solution .
6
0 min. After this, the unknown peak gradually decreases and Thus, we believe the anions of formate, propionate and
completely disappeared after 2 hrs. This peak is probably butyrate are involved in the intermediate phases, which in
associated with formation of an intermediate phase or turn affects the crystallization kinetics and ultimately the
complex mainly related with cations of the IL during the morphology of the synthesized NCs. For clarity, the perovskites
formation of perovskite crystals, which is often observed in formed with the different ionic liquids of MAF, MAP and MAB
3
0
perovskite phase evolution . Similar phase transformation are named as MAPbBr
towards formation of MAPbBr perovskite was also found with MAPbBr (MAB) respectively in the following to differentiate
MAF and MAB based ionic liquid in the presence of PbBr the different samples.
Figure S4). The peak position of the intermediate is located at Since optical property and morphology are critical features of
3 3
(MAF), MAPbBr (MAP) and
3
3
2
(
7.3 for MAF-based reaction and 26.4o for MAB-based the perovskite materials, we investigated the effect of the ILs
o
2
reaction, respectively. This implies that the intermediate phase with different anions on optical properties and morphology of
formed from PbBr and the ILs is also affected by the anions of MAPbBr formed by MAF, MAP and MAB. It is worth to note
the ILs. We suppose that the phase involves (PbBr that carbon chain length of the three ILs is MAF<MAP< MAB.
octahedron which is caged and isolated by carboxylate As can be seen in Figure 2a, the brightness of green emission
bridges, forming MAPbBr3-x (F/P/B) intermediate. Chao et al. of MAPbBr products made from the three PILs increases in the
2
3
4
-
6
)
x
3
reported a similar phenomenon where an intermediate phase sequence MAF <MAP <MAB, which is the same trend with the
was formed by self-assembly of methylammonium acetate increased length of the carbon chain. The different brightness
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in photoemission of MAP and MAB products is also clearly reaction times. As shown in Figure S7, the NCsVfieowrAmrtieclde Oinnlinae
seen in the PL measurement (Figure 2b), where short reaction time of 1 min exhibit veryDtOhIi:n10la.1y0e3r9e/Dd0sGtCru0c10tu8r1Be
MAPbBr
MAPbBr
3
(MAB) shows a much stronger PL peak than the with rod or rectangle shape with length of around 400 nm and
(MAP) when the absorbances of the samples were width of 200-250 nm. Nanorods shape embedded inside the
3
adjusted to almost the same value. The stronger PL peak is thin layered structure with diameter of 40 nm and length of
assigned to the higher radiative recombination. It is worth to 300 nm can also be clearly seen. It suggests anisotropic growth
point out that we were not able to measure PL spectrum of of MAPbBr
the MAPbBr (MAF) sample because the MAPbBr (MAF) solid Theoretically the stable cubic structure of MAPbBr
cannot be dispersed in toluene due to strong agglomeration of materials prefers to grow in all direction uniformly. Practically
the material (more details are shown below). Furthermore, the it is a challenge to make MAPbBr nanorods. Petrov et al.
different carbon chain of anion also affects the stability of the reported synthesis of MAPbBr nanowires through anion-
product. We have found that the MAPbBr (MAF) is slowly exchange with MAPbI
nanowires, which is tedious 34. The
dissolved in residual MAF at room temperature due to the anisotropic growth of the MAPbBr (MAB) nanocrystals in this
3
(MAB) at the beginning of the chemical reaction.
3
3
3
means the
3
3
3
3
3
presence of N-methylformamide (DMF) and water as a work is probably related with the butyrate anion which acted
consequence of condensation reaction of methylammonium as structure-direct ligands by anchoring on the surface of as-
2
6
cation with the formate anion of MAF . The highly synthesized MAPbBr
3
as confirmed by the FTIR measurement
coordinating DMF from the condensation reaction can easily below. Since the PL peak of the nanocrystals is similar to that
dissolve perovskite materials. In contrast, MAPbBr
and MAB appears to be much more stable.
3
from MAP of bulk MAPbBr
nanocrystals does not possess quantum confinement
3
powder made characteristics, which requires the particle size with at least
3
, implying that the lateral dimension of the
The morphology of as-synthesized MAPbBr
from the different ionic liquids was analysed by a scanning one dimension less than the Bohn radius of MAPbBr
electron microscope (Figure 2c-e). The MAPbBr (MAF) shows a Interestingly, as reaction time increases, the MAPbBr
sticky, compact morphology, while the MAPbBr (MAP) shows material collected after 2 hrs of reaction time predominantly
morphology consisting of lots of egg-like small particles. In has the nanorod morphology with a width of 80-90 nm and a
contrast, the MAPbBr (MAB) shows distinct morphology few hundred nanometres long. When the reaction time is
composed of small cubic crystals and nanorods. It is known further increased, the nanorods gradually changes to cubic
that smaller particles normally benefit the radiative structures, which are similar to MAPbBr (MAP) but having a
recombination in perovskites NCs . This explains the different larger size of roughly 40-50 nm. The formation of cubic shape
brightness of the green photoemission of the MAPbBr nanocrystal with increased reaction time could be assigned to
materials formed by the three ILs where weak luminescence the exfoliation of the pre-formed MAPbBr nanorods due to
was observed with MAPbBr (MAF) and the strongest intercalation of ionic molecules, which have been witnessed in
luminescence with MAPbBr (MAB).
We further investigated the morphology of the MAPbBr
3
5
3
(3 nm) .
(MAB)
3
3
3
3
3
3
3
3
3
3
3
6
3
the formation of MAPbBr
3
nanoplatelets in a previous report .
3
After that, no significant morphology changes were observed
materials nanocrystals by TEM measurement of dispersion of with increase of reaction time. We believe the morphology
the as-synthesized solid in toluene. As mentioned before, due change is related to the anions of MAB. We also investigated
to the agglomerate compact material structure, the the morphology evolution of MAPbBr
MAPbBr (MAF) could not be dispersed in solvents for MAPbBr (MAP) materials collected at 1 min reaction time
characterisation by TEM and PL. Therefore, we only obtained possesses nanospheres shape with size of less than 10 nm,
the TEM images of the MAPbBr (MAP) and MAPbBr (MAB) which gradually grow to cubic structures of 10-20 nm in size
materials which could be easily dispersed into nonpolar (Figure S8). It suggests MAPbBr (MAP) adopted an isotropic
solvent like toluene or hexane. Figure 2f-g shows the TEM growth mechanism in the material nucleation and growth
images of MAPbBr (MAP) and MAPbBr (MAB). Apparently, the process.
MAPbBr (MAP) nanocrystals possess a cubic shape with a size The different crystal formation mechanism clearly indicates
3
(MAP). We found the
3
3
3
3
3
3
3
3
ranging from 10 to 20 nm. High-resolution TEM reveals a the influence of the anions of the ILs on the crystal shape.
highly ordered crystalline lattice with clear lattice fringes Fundamentally it is a challenge to understand the formation
(
Figure 2f inset and Figure S5) of 0.29 and 0.41 nm, mechanism of perovskite nanocrystals because of their fast
3
7
corresponding to (200) and (110) planes of cubic MAPbBr
3
nucleation and growth rate . We believe the formation of
3
perovskite, respectively. Whilst the MAPbBr (MAB) exhibits a perovskite NCs with different morphology in this work is
collection of different shapes of nanocrystals with nanorods as related to (1) the intercalation ability of carboxylate ligands;
the most dominant shape. The nanorods have length up to (2) different diffusion dynamic of the ILs; and (3) the different
1
m and a diameter of 80-90 nm. Besides, some small cubic density of the carboxylate ligands on the surface of the as-
nanocrystals with similar shape but larger size compared to formed perovskite. The carboxyl group with different alkyl
the materials of MAPbBr (MAP) (Figure S6), were also chain have a very different viscosity. The viscosity of MAP is
observed in the MAPbBr (MAB). 40% smaller than MAB and MAF’s viscosity is only 10% that of
In order to understand the formation mechanism for different MAB (Figure S9). A faster diffusion dynamic of MAP is thus
morphologies of MAPbBr (MAB) nanocrystals, we measured expected, which means the nucleation of the MAPbBr (MAP) is
TEM of MAPbBr (MAB) that were collected at different faster and the anions ligand can quickly leave the surface
3
3
3
3
3
4
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3
Figure 3: a) FTIR spectra of MAPbBr NCs prepared while using different ionic liquids; b) illustration of reaction scheme
between ionic liquid MAX with tetragonal PbBr
surface ligands.
2 3
to form MAPbBr nanocubes and nanorods crystals where X is acting as
4
5
compared to MAB, which thus favours an isotropic growth. hydrogen-bonded O–H group of carboxylic acid . These
When high concentration of large anion such as butyrate observations imply that the carbon chain in the carbonyl group
3
anchors on the as-formed MAPbBr nanocrystals, the material is indeed attached onto the surface of the perovskite NCs after
3
8
can follow anisotropic growth . In additional, ionicity is an the fast reaction.
important factor of ILs, which can influence the PeNCs Based on the FTIR spectrum of these three samples, we have
synthesis. Ionicity can be quantified as the effective fraction of also found that the intensity of the vibration peaks of the
3
9
ions available to participate in conduction . To compare the surfactants in the N–H, C–H bending and C–H stretching region
ionicity of the MAF, MAP and MAB, considering that having follows the order of MAPbBr (MAB)> MAPbBr (MAP)>
shorter alkyl chain can lead to lower viscosity and higher MAPbBr (MAF). The distinct vibration peak of the surfactant is
ionicity , and the odds number of carbon chain tends to have the strongest in MAPbBr (MAB), while these fingerprint peaks
a higher ionicity than expected comparing to the even number almost disappear in MAPbBr (MAF), suggesting different
3
3
3
4
0
3
3
2
5
of carbon presented , we expected that the ionicity follows density of the ligands on the material surface. This low
MAF > MAP > MAB. It is possible that the higher ionicity of surfactant density can result in agglomeration of the
MAP offers a more rapid crystallization compared to MAB. synthesized NCs and increased surface traps on the NCs, which
Furthermore the ionicity of IL can also affect the surface ultimately leads to poor optical properties of the nanocrystal
4
6
stabilization and aggregation of nanoparticles through material . This explains the different dispersion ability of the
influence the layer of anionic species at the surface of particles perovskite in solvent and their different photoemission
4
1-43
or formation of ionic aggregates close to the surface
.
property. Furthermore, the highest density of long chain
Ionicity have strong correlation with conventional solvent butyrate anion of MAB means some facet of the perovskite
parameters like polarity, conductivity, viscosity and surface nanocrystals was densely protected by the anion ligand from
tension, thus, the effect of ionicity can also described through growth, leading to anisotropic grow of the nanocrystals and
those conventional parameters. For example, ionicity can be formation of nanorods. While the mild density and shorter
evaluated from polarity, which is greatly contributed from chain of propionate ligand of MAP did not provide enough
4
4
dipole moment of dissociated anions in ILs . On the other impact to induce anisotropic growth of the materials. Clearly
hand, the dipole moment indicates the affinity of anion toward the surface ligands in this work act as several beneficial roles
2
+
reactants such as Pb
transformation. Overall, ionicity can be a useful indicator to structure-direct agent for growth of the MAPbBr
characterize the ability of ILs in the PeNCs synthesis and the passivation of surface defects of the NCs, just similar to the
ion, which trigger the phase including (1) prevention of aggregation of the NCs; (2) act as
3
and (3)
4
7
effect of which will require further study.
role of Oleic acid which is widely used in PeNCs synthesis .
In order to confirm the existence of different density of However, compared to the long chain oleate (18 carbon chain)
carboxyl ligands on the surface of the perovskite nanocrystals that is extensively used in PeNCs synthesis, our work shows
made from the ILs with different anions, we recorded Fourier- that shorter chain carboxylate (three (MAP) or four carbon
transform infrared spectra (FTIR) of the synthesized chain (MAB)) can also work as an effective surface ligand for
perovskite. As shown in Figure 3a, the observed vibration growth of perovskite nanocrystals. An additional advantage of
-1
-1
-1
peaks of O–H (939 cm ) and C=O (1714 cm , 1550 cm ) the shorter chain carboxyl ligands is that they are more
indicate existence of carboxylate groups on the surface of all desirable for charge transfer in semiconductor materials
three perovskite materials. At a higher wavenumber region, a compared to insulating nature of long chain ligands. 4
6, 48
-1
broad peak in the range of 2500 – 3300 cm matches the
characteristic C–H stretching, which is characteristic of the
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Figure 4: a) Printing pattern made from MAPbBr
excitation-intensity-dependent photoluminescence quantum efficiencies of thin films fabricated from MAPbBr
MAPbBr (MAB) ink; c) TR-PL spectrum MAPbBr (MAP) and MAPbBr (MAB).
3
(MAB) NCs showing strong green emission under UV-365 nm excitation; b)
3
(MAP) and
3
3
3
3 3
In order to demonstrate the universal of this synthesis MAPbBr (MAB) and ~40% for MAPbBr (MAP) which is
method, we have also used ILs containing longer alkyl chains comparable to the materials made by using other solvents
made from valeric acid or from nonlinear chain such as reported in literature (Table S1). The excitation-intensity-
aromatic ring like benzoic acid. The methylammonium valerate induced increase of PLQE is mainly attributed to the defect
(MAV) made from valeric acid is liquid while methylammonium filling process under illumination. The time-resolved
benzoate (MABe) made from benzoic acid is solid at room photoluminescence (TR-PL) spectrum of the thin film, fitted
o
temperature. However, MABe becomes liquid at around 60 C. with a biexponential function, shows a very short lifetime of
Similar to MAP and MAB, we found both MAV and MABe can the NCs. The short-lived (휏
) and long-lived (휏
) components of
1
2
react with PbBr immediately to form MAPbBr NCs, which the luminescence decay are 6.02 ns and 42.29 ns for
2
3
also showed strong photoemission. It should be noted that the MAPbBr
3
(MAB), 6.04 ns and 73.38 ns for MAPbBr
3
(MAP)
o
reaction with PbBr
properties of MAPbBr
figure S10a, b. The TEM measurement also reveals cubic recombination, which indicates that both materials have
perovskite NCs with size up to 30nm for MAPbBr (MAV) while effective radiative recombination compared to PeNCs made by
(MABe) have much smaller sizes (10 using other methods24, 50. The longer PL lifetime (휏
nanocrystals of MAPbBr ) is
nm) with irregular shape (figure S10c, d). FTIR measurement associated with longer charge trap/detrapping process. The
also was conducted to confirm the presence of carboxylate longer 휏
of MAPbBr (MAP) compared to MAPbBr (MAB) is
ligand on surfaces of the NCs (Figure S11).The synthesized NCs consistent with its higher surface defects due to lower density
is similar to MAPbBr NCs synthesized by using benzylamine of surface ligands according to the FTIR results and the PLQE
and benzoic acid as capping ligands as reported previously
These results indicate the universal of this ionic liquid based There was no observable degradation after the material was
green solvent method for synthesis of MAPbBr NCs. They also immersed in water for one hour (Figure S13). This is probably
2
was conducted at 60 C. The optical respectively. The detail fitting data are listed in table S2. The
3 3
(MAV) and MAPbBr (MABe) is shown in short-lifetime is originated from exciton radiative
3
3
2
2
3
3
3
4
9
.
measurement. Interestingly, the film was even stable in water.
3
clearly show the effect of different anions on tuning the size due to the hydrophobic nature of the alkyl chains on the
and shape of the perovskite nanocrystals. The underlying surface of PeNCs.
mechanism controlling the formation and growth of perovskite Furthermore, the ionic liquid based green synthesis method
NCs by MAV and MABe will be investigated in the future.
To test the performance of the MAPbBr nanocrystals formed fabrication of MAPbBr
by MAP and MAB, we made perovskite thin films using an ink MAPbBr NCs method by using 1g of PbBr
incorporating the MAPbBr (MAB) NCs perovskite NCs by MAB (MAP), which produced 425.5 mg (348.6 mg) of MAPbBr
has good reproducibility and is suitable for large scale
NCs. We scaled-up the synthesis of
precursor with
3
3
3
2
3
3
doctor blading (Figure 4a, Figure S12 a, b). SEM shows NCs in one batch. The obtained NCs exhibit high purity and
compact, uniform morphology of the film (Figure S12c). As good optical properties to the material made at small scale as
shown in Figure 4a, a patterned perovskite film made from the shown in Figure S14).
as-synthesized
3
MAPbBr (MAB)
NCs
exhibits
strong
luminescence under UV excitation (365 nm). It is notable that,
the photoluminescence quantum efficiency (PLQE) of
Conclusions
MAPbBr
suggesting that MAPbBr
which is probably related with the smaller crystal sizes and the
higher density of the carboxyl ligands on the materials surface.
At high light intensity of ~100 mW cm , the PLQE is ~50% for
3
(MAB) is much higher than that of MAPbBr
3
(MAP),
In conclusion, we have demonstrated a benign method to
3
(MAB) possesses less trap density,
3
synthesize green emissive MAPbBr PeNCs by using an
environmentally friendly solvent based on ionic liquids. The
perovskite NCs with controlled sizes and shapes (nanocubes,
nanorods) and subsequently high photoluminescence have
-2
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been obtained by controlling the alkyl chain length of the 14.
carboxylate group and reaction time. Compared to other
synthesis method of PeNCs, our method has the advantages of
D. Liang, Y. Peng, Y. Fu, M. J. Shearer, J. ZhaVinewg,AJrt.icZlehOani,linYe.
Zhang, R. J. Hamers, T. L. AndrewDOaIn: 1d0.S10. 3J9in/D, 0AGCCS0n10a8n1oB,
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016, 10, 6897-6904.
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Y. Liu, J. Cui, K. Du, H. Tian, Z. He, Q. Zhou, Z. Yang, Y.
facile processing and is environmentally friendly. These two
merits are highly desirable and important for the large-scale
production of perovskites. Therefore, the demonstrated facile,
green material synthesis method paves a new way towards
sustainable industrial production of perovskite materials for
variable applications.
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The authors declare no conflict of interest.
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Acknowledgements
This work was supported by Australian Research Council
Discovery Project (DP190102252) and Centre for Materials
Science at Queensland University of Technology (QUT). M.T. H.
QUT postgraduate scholarship. The data of SEM, TEM,
fluorescence reported in this paper were obtained at the
Central Analytical Research facility (CARF), QUT. Access to
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