Low defects, large area and high stability of all-inorganic lead halide perovskite CsPbBr3 thin films with micron-grains: Via heat-spraying process for self-driven photodetector

Article abstract of DOI:10.1039/c8ra04558e

All-inorganic lead halide perovskite CsPbBr₃ has important applications in photoelectronic devices such as photodetectors, LEDs and photovoltaic devices. However, preparing high-quality CsPbBr₃ thin films has proven to be challenging. In this study, we prepared all-inorganic lead halide perovskite CsPbBr₃ thin films with micron-grains (MG-CsPbBr₃-TF) via a heat-spraying process (HSP) using a CsPbBr₃-saturated solution (CsPbBr₃-SS), and the films exhibited large area, low defects and high stability. The grain size of MG-CsPbBr₃-TF was about 1-5 microns. The micron-sized grains in MG-CsPbBr₃-TF enabled the absorption cutoff edge to be extended from 537 to 545 nm. In addition, the presence of fewer boundaries in MG-CsPbBr₃-TF reduced the defects in MG-CsPbBr₃-TF (the blue shift of luminescence). The response wavelengths of a low-cost and self-driven (zero-biased) photodetector based on MG-CsPbBr₃-TF were from 330 to 600 nm. CsPbBr₃ thin films having a large area (10 cm × 10 cm) and micron-sized grains were also prepared by HSP and exhibited excellent stability (1944 h) in air (T = 298 K, 40% humidity). To the best of our knowledge, this is the first study of high-quality CsPbBr₃ thin films prepared by HSP. The results are of great interest for both fundamental research and practical applications of CsPbBr₃ in photodetectors, LEDs and photovoltaic devices.

Full text of DOI:10.1039/c8ra04558e

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PAPER
Low defects, large area and high stability of all-
inorganic lead halide perovskite CsPbBr3 thin films
with micron-grains via heat-spraying process for
self-driven photodetector†
Cite this: RSC Adv., 2018, 8, 29089
a
*
Huawei Zhou,
Lin Fan,a Guohang He,a Cang Yuan,a Yunying Wang,a
Shaozhen Shi,a Ning Sui,b Baoli Chen,
Jinsheng Zhao, Xianxi Zhang and Jie Yin
Yingtian Zhang,a Qingxia Yao,a
a
a
a
a
*
All-inorganic lead halide perovskite CsPbBr3 has important applications in photoelectronic devices such as
photodetectors, LEDs and photovoltaic devices. However, preparing high-quality CsPbBr3 thin films has
proven to be challenging. In this study, we prepared all-inorganic lead halide perovskite CsPbBr3 thin
films with micron-grains (MG-CsPbBr3-TF) via a heat-spraying process (HSP) using a CsPbBr3-saturated
solution (CsPbBr3-SS), and the films exhibited large area, low defects and high stability. The grain size of
MG-CsPbBr3-TF was about 1–5 microns. The micron-sized grains in MG-CsPbBr3-TF enabled the
absorption cutoff edge to be extended from 537 to 545 nm. In addition, the presence of fewer
boundaries in MG-CsPbBr3-TF reduced the defects in MG-CsPbBr3-TF (the blue shift of luminescence).
The response wavelengths of a low-cost and self-driven (zero-biased) photodetector based on MG-
CsPbBr3-TF were from 330 to 600 nm. CsPbBr3 thin films having a large area (10 cm ꢀ 10 cm) and
micron-sized grains were also prepared by HSP and exhibited excellent stability (1944 h) in air (T ¼ 298
K, 40% humidity). To the best of our knowledge, this is the first study of high-quality CsPbBr3 thin films
prepared by HSP. The results are of great interest for both fundamental research and practical
applications of CsPbBr3 in photodetectors, LEDs and photovoltaic devices.
Received 28th May 2018
Accepted 30th July 2018
DOI: 10.1039/c8ra04558e
rsc.li/rsc-advances
stability of LHP. Compared to organic–inorganic LHP, all-
inorganic LHP materials exhibit superior stability against
1. Introduction
moisture and temperature. An all-inorganic lead bromide
perovskite (CsPbBr3) material functions equally well as the
organic one, in particular for generating high open circuit
voltages, which is an important feature of solar cells.6 In addi-
tion, CsPbBr3 is a direct band gap semiconductor, which meets
most of the requirements for successful high-energy radiation
detection of X-ray and gamma-ray radiation such as high
attenuation, high resistivity, and signicant photoconductivity
response.7 Recently, CsPbBr3 was also applied as the emitting
layer in LEDs8 and as a novel photocatalyst to convert CO2 into
solar fuels in non-aqueous media.9 Therefore, CsPbBr3 is a very
promising optoelectronic material for many applications.10–13
CsPbBr3 nanocrystals were currently synthesized by injecting
the organic cesium precursor Cs-oleate into a ligand solvent
(oleic acid/oleylamine/1-octadecene).14,15 The CsPbBr3 thin lm
with nano-grains (NG-CsPbBr3-TF) can be prepared by depos-
iting CsPbBr3 nanocrystals. However, the CsPbBr3 thin lm with
micron-sized grains (MG-CsPbBr3-TF) has received less atten-
tion; nonetheless, it plays an important role in improving the
performance of optoelectronic devices. Thus, CsPbBr3 thin
lms with micron-sized grains urgently need to be developed. In
Lead halide perovskite (LHP) materials have become a research
hotspot because of their novel physical and chemical proper-
ties.1–4 Recently, organic–inorganic LHP material-based thin
lm solar cells have demonstrated high power conversion effi-
ciencies (PCEs) of more than 22%,5 competing with commer-
cially available silicon solar cells. Thus, organic–inorganic LHP
materials have attracted widespread attention. However, the
stability of organic–inorganic LHP materials is not ideal. One of
the drawbacks of organic–inorganic LHP materials is that the
three-dimensional crystal structure can be easily destroyed by
moisture in the air. In addition, these materials do not exhibit
very good thermal stability above 180 ꢁC. The use of all-
inorganic perovskites is one of the ways to improve the
aSchool of Chemistry and Chemical Engineering, College of Materials Science and
Engineering, Liaocheng University, Shandong Provincial Key Laboratory of Chemical
Energy Storage and Novel Cell Technology, Liaocheng 252000, China. E-mail:
zhouhuaweiopv@163.com; yinjieily@163.com
bCollege of Materials Science and Engineering, Qingdao University of Science and
Technology, Qingdao 266042, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04558e
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this study, CsPbBr3 thin lms with micron-sized grains were studied using an electrochemical workstation system (CHI760,
formed from CsPbBr3-saturated solution (CsPbBr3-SS) by a heat- Chenhua, and Shanghai) equipped with a solar simulator (IV5,
spraying process (HSP).
PV Measurements, Inc., United States). The photocurrent–time
with incident photon (wavelength) and frequency response was
measured by an electrochemical workstation system (CHI760,
Chenhua, and Shanghai) equipped with a quantum efficiency/
spectral response (SR)/incident photon to current conversion
efficiency (IPCE) measurement system (QEX10, PV Measure-
ments, Inc., United States). External quantum efficiencies (EQE)
were also measured by QEX10. The photocurrent–voltage
performances of the devices were measured using a Keithley
digital source meter (Keithley 2400, USA) equipped with a solar
simulator (IV5, PV Measurements, Inc., USA).
2. Experimental
2.1. Fabrication of CsPbBr3 thin lm with micron-sized
grains by heat-spraying method
Patterned FTO-coated glass with sheet resistance of 15 U sqꢂ1
was coated with a TiO2 electronic collection layer (ECL) by spin-
coating a TiO2 organic sol at 3000 rpm for 30 s, followed by
drying at 450 ꢁC for 120 min. Next, a CsPbBr3 thin lm with
micron-sized grains was prepared by ꢁspraying CsPbBr3-satu-
rated solution on the TiO2 lm at 200 C. Finally, carbon elec-
trodes were prepared by doctor-blade coating a conductive
carbon paste on the CsPbBr3 layer using adhesive tapes as
pattern and spaces, followed by drying at 80 ꢁC for 10 min. The
size of the FTO/TiO2/CsPbBr3/carbon electrode device was 1
cm2.
3. Results and discussion
A schematic of the fabrication process of CsPbBr3 lm by HSP is
shown in Fig. 1. The detailed processes can be seen in the
experimental section. In a typical procedure, CsPbBr3 suspen-
sion was obtained by the reaction between CsBr and PbBr2.
Then, the suspension was transferred to one of the branches of
H tube and le for 24 h until CsPbBr3 precipitate and super-
natant (CsPbBr3-saturated solution) were separated. The
CsPbBr3-saturated solution was collected in the other branch of
the H tube by slowly tilting it. Consequently, one side of the H
tube had the CsPbBr3 suspension, whereas the other had
CsPbBr3-saturated solution. There are two advantages to the use
of the H tube. First, the H tube can more effectively separate
precipitate from the saturated solution. Second, CsPbBr3-satu-
rated solution can be re-obtained by adding HBr solution by
dissolution–precipitation equilibrium. The CsPbBr3-saturated
solution can not only produce CsPbBr3 thin lms with micron-
sized grains by a heat-spraying process (HSP) but also grow
single crystals, as shown in Fig. 1.
2.2. Fabrication of CsPbBr3 thin lms with nano-sized
grains by the heat-spraying method
2.2.1. Preparation of Cs-oleate. Cs2CO3 (0.30 g) was added
into 1 mL oleic acid. 1-Octadecene (18 mL) was then added into
the above-mentioned solution. The mixed solution was heated
to 120 ꢁC.
2.2.2. Preparation of CsPbBr3 nanocrystals. PbBr2 (0.10 g),
1 mL oleylamine, 5 mL 1-octadecene and 1 mL oleic acid were
mixed. The mixed solution was heated to 200 ꢁC. Then, 1 mL Cs-
oleate was injected into the above solution. Finally, this solution
was cooled down to room temperature naturally. CsPbBr3
nanocrystals were separated by centrifuging, and they were
washed with butanol. The CsPbBr3 thin lm with nano-grains
was prepared by spraying CsPbBr3 nanocrystals in butanol on
ꢁ
the TiO2 electronic transport layer at 200 C.
Cs2CO3 + 2HBr ¼ 2CsBr + CO2[ + H2O,
CsBr + PbBr2 ¼ CsPbBr3Y
2.3. Growth and characterizations of CsPbBr3 single crystals
In a typical procedure, a 50 mL round-bottom ask was lled
with 0.122 g Cs2CO3 and 1.8 mL 47% HBr aqueous solution;
0.115 g PbBr2 was dissolved in this solution upon heating to
80 ꢁC using an oil bath for 30 min, which resulted in the
formation of a suspension. This suspension was transferred to
one side of an H tube using a plastic dropper. The suspension in
the side of the H tube was le for 24 h until the precipitate and
supernatant were separated. One side of the H tube contained
the suspension, whereas the other side contained a light yellow
solution. The H tube was placed in the dark for two weeks.
Yellow crystal particles were obtained in the side of the H tube
having the pale yellow solution.
CsPbBr3 thin lms can be prepared by spraying CsPbBr3-SS
onto a heated FTO/TiO2-ECL substrate. The color of CsPbBr3
thin lm is affected by the substrate (TiO2-ECL) temperature, as
shown in Fig. 2a. The colour of the thin lm prepared at 200 ꢁC
on TiO2-ECL is orange (Fig. 2a). UV-visible absorption spectra
and diffuse reectance spectra of the lms prepared under
different temperatures are measured. The lm prepared at
200 ꢁC on TiO2-ECL exhibits strong absorption from 250 to
510 nm, as shown in Fig. 2b. The absorbance of the lm
decreases sharply aer 510 nm. The lms prepared at 100 and
300 ꢁC on TiO2-ECL exhibit strong absorptions from 250 to
400 nm. The UV-visible diffuse reectances of the thin lms are
2.4. Characterizations
The nanostructures of our lms were characterized by scanning also measured, as shown in Fig. 2c. The reection intensity of
electron microscopy (SEM, Nova Nano SEM 450). UV-Vis the lm prepared at 200 ꢁC on TiO2-ECL is signicantly weaker
absorption spectra and diffuse reectance spectra were ob- from 250 nm to 510 nm, which corresponds to the strong UV-
tained using a UV/vis/NIR spectrometer (PerkinElmer, lambda, visible absorption from 250 to 510 nm. Meanwhile, the thin
750S). The photocurrent–time performances of the devices were lms prepared at 100 and 300 ꢁC on TiO2-ECL exhibit strong
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Fig. 1 Schematic of CsPbBr3 thin film prepared by heat-spraying process using CsPbBr3-saturated solution.
reectance in the visible range. This corresponds to weak UV- shown in Fig. 2d. A series of diffraction peaks are clearly
visible absorption from 250 to 510 nm. The above results indi- observed. These diffraction peaks are consistent with the Joint
cate that it is impossible to form an orange CsPbBr3 thin lm at Committee on Powder Diffraction Standards Cards (PDF#73-
ꢁ
100 or 300 C on TiO2-ECL. To prove that the orange thin lm 2463 for CsPbBr3), revealing that the thin lm prepared at
prepared at 200 ꢁC on TiO2-ECL is CsPbBr3, the crystal structure 200 ꢁC on TiO2-ECLhas an orthorhombic crystal structure [Pmnb
˚
of the lm is studied. The X-ray diffraction (XRD) pattern is (62) space group, 4.597 ꢀ 9.72 ꢀ 16.81 A, 90.0 ꢀ 90.0 ꢀ 90.0].
Fig. 2 (a) The colour of CsPbBr3 thin film prepared on a substrate at different temperatures. (b) UV-visible absorption spectra of CsPbBr3 thin film
prepared on a substrate at different temperatures. (c) Diffuse reflectance spectra of CsPbBr3 thin film prepared on a substrate at different
temperatures. (d) XRD profiles of CsPbBr3 thin film prepared on a substrate at 200 ꢁC. (e) Transformed Kubelka–Munk spectrum of CsPbBr3 thin
film prepared on a substrate at 200 ꢁC. (f) Large area (10 cm ꢀ 10 cm) of CsPbBr3 film prepared by CsPbBr3-SS-HSP.
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Fig. 3 (a) SEM images of MG-CsPbBr3-TF. (b) The statistical distribution of grain size in MG-CsPbBr3-TF. (c) Cross-sectional SEM images of MG-
CsPbBr3-TF. (d) SEM images of NG-CsPbBr3-TF. (e) The statistical distribution of grain size in NG-CsPbBr3-TF. (f) The photoluminescence (PL)
spectra of MG-CsPbBr3-TF and NG-CsPbBr3-TF thin films. Schematic energy level diagrams of (g) MG-CsPbBr3-TF and (h) NG-CsPbBr3-TF.
The diffraction peaks at 10.6, 14.2, 20.4, 25.6, 29.7, 36.6, and
SEM (Fig. 3a) shows that the grains in CsPbBr3 thin lm are
50.3ꢁ are assigned to the (011), (012), (101), (103), (122), (040), in micron scale. The statistical distribution of grain size indi-
and (145) planes of orthorhombic CsPbBr3, respectively. To cates that the grain size in MG-CsPbBr3-TF is 1–2 microns, as
ꢁ
obtain the bandgap of the CsPbBr3 thin lm sprayed at 200 C shown in Fig. 3b. The micron-sized grains with fewer grain
on TiO2-ECL, its diffuse reectance spectrum is converted into boundaries are conducive to reduce the electron recombination
a Kubelka–Munk spectrum.16,17 According to the Kubelka–Munk and improve photoelectric performance of the device. The large
formula, F(R) ¼ (1 ꢂ R)2/2R, where R represents the percentage coverage of MG-CsPbBr3-TF can reduce the pinhole number and
of reection. The relationship between incident photon energy subsequently the possibility of a short circuit. The cross section
(hn) and Eg can be expressed as the transformed Kubelka–Munk of MG-CsPbBr3-TF is examined by SEM, as shown in Fig. 3c. The
equation [F(R)hn]p ¼ A(hn ꢂ Eg), where Eg is the bandgap energy, thickness of MG-CsPbBr3-TF is about 654 nm. In addition, the
A is the absorption constant, h is the Planck's constant, n is the cross-sectional SEM also shows that the grains of MG-CsPbBr3-
frequency of the light (sꢂ1) and P is the power index related to TF are micrometer-sized. This result is consistent with the top-
the optical absorption process. Theoretically, P is equal to 1/2 or view results of SEM. As reference, CsPbBr3 thin lms with nano-
2 for an indirect or a direct allowed transition, respectively. grains (NG-CsPbBr3-TF) are prepared. The detailed fabrication
According to literature, CsPbBr3 can be regarded as having processes are given in the Experimental section. The SEM image
direct allowed transition. Thus, Eg for the CsPbBr3 thin lm is of NG-CsPbBr3-TF is shown in Fig. 3d. The statistical distribu-
determined to be approximately 2.2 eV from the x-intercept of tion of grain size indicates that the grain size in NG-CsPbBr3-TF
the linear part of [F(R)hn]2 plot, as shown in Fig. 2e. The CsPbBr3 is about 40 nm, as shown in Fig. 3e. The grain size is signi-
lm having a large area of 10 cm ꢀ 10 cm can also be prepared cantly smaller than that of MG-CsPbBr3-TF. The large number
by CsPbBr3-SS-HSP, as shown in Fig. 2f.
of nano-grains in NG-CsPbBr3-TF cause more defects, which
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Fig. 4 (a) External quantum efficiency (EQE) of FTO/TiO2/CsPbBr3/carbon electrode device prepared on TiO2-ECL at different temperatures. (b)
Photocurrent density–time characteristics of the FTO/TiO2/CsPbBr3/carbon electrode device under 540 nm wavelength. (c) Photocurrent
density–time characteristics of the FTO/TiO2/CsPbBr3/carbon electrode device under 600 nm wavelength. (d) UV-Vis absorption spectra of MG-
CsPbBr3-TF stored in the air (T ¼ 298 K, 40% humidity).
results in deleterious photoelectric performance. The UV-Vis measured, as shown in Fig. 4a. The MG-CsPbBr3-TF lm
absorption spectra of MG-CsPbBr3-TF and NG-CsPbBr3-TF are exhibited EQE from 330 nm to 510 nm, which corresponded to
shown in Fig. 3f. The absorption cutoffs of MG-CsPbBr3-TF and the absorption range of the UV absorption spectrum (Fig. 2b).
ꢁ
NG-CsPbBr3-TF are at 545 and 537 nm, respectively. A red shi The device based on the thin lm prepared at 300 C on TiO2-
in the absorption cutoff edge of MG-CsPbBr3-TF is due to ECL only exhibited EQE from 300 nm to 400 nm. The device
micron-sized grains, as shown in Fig. 3a and c. The absorption based on the thin lm prepared at 100 ꢁC on TiO2-ECL exhibited
cutoff edge of MG-CsPbBr3-TF is red shied compared to that zero EQE from 300 to 800 nm. The on–off performance of the
(535 nm) for CsPbBr3 nanocubes18 and is close to that (548–555 devices based on MG-CsPbBr3-TF under different wavelengths
nm) of CsPbBr3 perovskite single crystals.19–21 The photo- of the monochromatic light was investigated. The device
luminescence (PL) spectra of MG-CsPbBr3-TF and NG-CsPbBr3- exhibited signicant on–off photocurrent responses with on–off
TF are obtained, as shown in Fig. 3f. The excitation wavelength operation at 330, 360, 390, 410, 420, 450, 480, 510, 540, 570 and
is set at 510 nm. The emission peaks of MG-CsPbBr3-TF and NG- 600 nm, as shown in Fig. 4b, c and S1.† As is well-known, the
CsPbBr3-TF are observed at 545 nm and 577 nm, respectively. poor stability of organic–inorganic perovskites seriously
The red shi in emission peaks of NG-CsPbBr3-TF is due to the restricts their commercialization. Meanwhile, all-inorganic
numerous grain boundaries, which are caused by the lower perovskites are expected to have improved stability. The
excited levels of NG-CsPbBr3-TF, as shown in Fig. 3g and h.
stability of MG-CsPbBr3-TF stored in the air (T ¼ 298 K, 40%
To investigate the photoelectric properties of the MG- humidity) was tested by obtaining its UV absorption spectrum.
CsPbBr3-TF thin lm, photodetectors based on MG-CsPbBr3-TF No clear change in the absorption intensity was observed aer
were prepared. The preparation procedure of such devices is nearly 2000 h of storage. The device stability data are shown in
shown in Fig. 4. The device structure is FTO/TiO2/CsPbBr3/ Fig. S2.† The device placed in the air for 7 months still exhibited
carbon electrode. External quantum efficiencies (EQE) were clear photoresponse under AM1.5, 100 mW cmꢂ2 simulated
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illumination. These results conrmed that MG-CsPbBr3-TF
possessed good stability. In addition, CsPbBr3 single crystals
were prepared from CsPbBr3/HBr supernatant, as shown in
Fig. S3.† The thermal stability of CsPbBr3 single crystals was
assessed by thermogravimetric analysis (Fig. S4†). The results
indicated that CsPbBr3 single crystal materials begin to
decompose at 500 ꢁC. Compared with organic–inorganic hybrid
CH3NH3PbI3 materials, inorganic CsPbBr3 perovskites indeed
exhibited good thermal stability.
Efficient CsPbBr3 Cells, J. Phys. Chem. Lett., 2015, 6(13),
2452–2456.
7 C. C. Stoumpos, C. D. Malliakas, J. A. Peters, Z. Liu,
M. Sebastian, J. Im, T. C. Chasapis, A. C. Wibowo,
D. Y. Chung, A. J. Freeman, B. W. Wessels and
M. G. Kanatzidis, Crystal Growth of the Perovskite
Semiconductor CsPbBr3: A New Material for High-Energy
Radiation Detection, Cryst. Growth Des., 2013, 13(7), 2722–
2727.
8 X. L. Zhang, B. Xu, J. B. Zhang, Y. Gao, Y. J. Zheng, K. Wang
and X. W. Sun, All-Inorganic Perovskite Nanocrystals for
High-Efficiency Light Emitting Diodes: Dual-Phase
CsPbBr3-CsPb2Br5 Composites, Adv. Funct. Mater., 2016,
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Conflicts of interest
There are no conicts to declare.
9 Y. F. Xu, M. Z. Yang, B. X. Chen, X. D. Wang, H. Y. Chen,
D. B. Kuang and C. Y. Su, A CsPbBr3 Perovskite Quantum
Dot/Graphene Oxide Composite for Photocatalytic CO2
Reduction, J. Am. Chem. Soc., 2017, 139(16), 5660–5663.
Acknowledgements
This work was nancially supported by Shandong Province
Natural Science Foundation (Grant No. ZR2016BQ20;
BS2015NJ013; ZR2014BQ010; ZR2016BQ21), Colleges and 10 J. Z. Song, J. H. Li, X. M. Li, L. M. Xu, Y. H. Dong and
Universities in Shandong Province Science and Technology
projects (Grant No. J16LC05; J17KA097), Science and Tech-
nology Innovation Foundation for the University or College
H. B. Zeng, Quantum Dot Light-Emitting Diodes Based on
Inorganic Perovskite Cesium Lead Halides (CsPbX3), Adv.
Mater., 2015, 27(44), 7162–7167.
students (Grant No. 26312160502), National Science and Tech- 11 G. Nedelcu, L. Protesescu, S. Yakunin, M. I. Bodnarchuk,
nology Innovation Foundation for the University or College
Students (Grant No. 201510447021), Research Fund for the
Doctoral Program of Liaocheng University (Grant No. 31805),
National Natural Science Foundation of China (Grant No.
M. J. Grotevent and M. V. Kovalenko, Fast Anion-Exchange
in Highly Luminescent Nanocrystals of Cesium Lead
Halide Perovskites (CsPbX3, X ¼ Cl, Br, I), Nano Lett., 2015,
15(8), 5635–5640.
21503104; 21601078, 21401095), National Basic Research 12 L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg,
Program of China (Grant No. 2011CBA00701).
R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh and
M. V. Kovalenko, Nanocrystals of Cesium Lead Halide
Perovskites (CsPbX3,
Optoelectronic Materials Showing Bright Emission with
X
¼
Cl, Br, and I): Novel
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