Low-temperature photoluminescence spectroscopy of CH3NH3PbBrxCl3-x perovskite single crystals

Article abstract of DOI:10.1016/j.jallcom.2019.04.016

Organic-inorganic halide perovskite (OIHP) has attracted tremendous attention due to its potential applications in optoelectronics such as light emitting device and photodetector. Here, we have grown high quality CH₃NH₃PbBr_x

Full text of DOI:10.1016/j.jallcom.2019.04.016

Accepted Manuscript
Low-temperature photoluminescence spectroscopy of CH NH PbBr Cl
x 3-x
perovskite
3
3
single crystals
Qiang Xu, Wenyi Shao, Xinlei Zhang, Jun Liu, Xiaoping Ouyang, Xiaobin Tang,
Wenbao Jia
PII:
S0925-8388(19)31280-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.04.016
JALCOM 50200
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 18 January 2019
Revised Date: 30 March 2019
Accepted Date: 1 April 2019
Please cite this article as: Q. Xu, W. Shao, X. Zhang, J. Liu, X. Ouyang, X. Tang, W. Jia, Low-
temperature photoluminescence spectroscopy of CH NH PbBr Cl perovskite single crystals, Journal
x 3-x
3
3
of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.04.016.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
Low-temperature photoluminescence spectroscopy of
CH3NH3PbBrxCl3-x perovskite single crystals
a
Qiang Xu a, *, Wenyi Shao a, Xinlei Zhang , Jun Liu b, Xiaoping Ouyang a,b, Xiaobin
Tang a and Wenbao Jia a
a
Department of Nuclear Science and Engineering, Nanjing University of
Aeronautics and Astronautics, Nanjing 211106, China
b
Northwest Institute of Nuclear Technology, Xi’an, 710024, China
ABSTRACT:
Organic-inorganic halide perovskite (OIHP) has attracted tremendous attention due to
its potential applications in optoelectronics such as light emitting device and
photodetector. Here, we have grown high quality CH3NH3PbBrxCl3-x perovskite single
crystals and investigated the luminescence mechanism of these materials.The results
indicate that optical properties of CH3NH3PbBrxCl3-x perovskite materials are strongly
depend on the temperature and the dope concentration. Free exciton recombination
emission has dominant contribution to luminescence spectra of materials at
orthorhombic structure, as bound exciton recombination emission to the spectra of
materials at cubic structure. Based on temperature-dependent photoluminescence, the
exciton binding energy of CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7 are calculated as
about 22.9 meV and 27.1 meV, respectively.
Keywords: Perovskite single crystals; Temperature dependent photoluminescence;
Free exciton emission; Bound exciton emission; Phase transition

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1. Introduction
Organic-inorganic hybrid perovskite (OIHP) CH3NH3PbX3 (X = Cl−, Br−, or I−) has
been considered as the most promising material for energy conversion applications
due to its unique physical and optical properties. Recently, power conversion
efficiency (PCE) of perovskite solar cells reached a record of 23.2% [1]. It has shown
great potential to compete with well-established silicon solar cells in terms of PCE
and cost [2]. In addition, because of its band gap tunability, high color purity, high
photoluminescence (PL) efficiency, and low cost synthesis methods, OIHP has been
gaining extensive attention for use in other optoelectronic applications (LEDs, lasers,
photodetectors, transistors) [3-6]. OIHP is also increasingly being used for ionizing
radiation detection due to its strong stopping power, defect-tolerance, large mobility
lifetime (ꢀτ) [7-10]. Pure CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3 have been
synthesized and fabricated as optoelectronic devices to detect UV-Visible photons or
convert UV-Visible photon energy into electricity. Although single halogen perovksite
crystals have been greatly successful, there are some application limitations. To meet
the requirement of various devices, growing high quality materials with tunable
electrical and optical properties has attracted a lot of attention, and using mixed halide
hybrid perovskite is a key strategy to overcome most application limitations [11-13]. H.
Wei et al [14] reported high quality and dopant compensated CH3NH3PbBr3−xClx
perovskite single crystals that enable to overcome the limitations of device noise and
charge collection. The dopant-compensated CH3NH3PbBr2.94Cl0.06 alloy has over
tenfold improvement on bulk resistivity of 3.6×109ꢁꢂꢁcm, which increases the hole
mobility to 560ꢁcm2ꢁV−1ꢁs−1 and the mobility–lifetime product to 1.8 × 10−2ꢁcm2ꢁV−1.
Therefore, a high-resolution gamma spectrum (137Cs) has been achieved at room
temperature with low electrical field. Additionally, due to the introduction of chlorine,
the synthetic mixed halide perovskite film (CH3NH3PbI3-xClx) has higher carrier
mobility and much longer exciton diffusion length [15]. Wu et al [16] demonstrated a
flexible photodetector array based on CH3NH3PbI3-xClx film with a resolution of 63.5
dpi for photosensing and imaging. The device possessed fine photoresponse

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properties with high on/off current ratio (1.2 × 103) under a light intensity of 38.3 mW
cm−2, high detectivity (9.4 × 1011 Jones) at a light intensity of 0.033 mW cm−2. J.
Hieulle et al [17] investigated the impact of halide mixing on perovskite stability.
Their results indicated that the incorporation of Cl into CH3NH3PbBr3 leads to an
increase of the decomposition energy, which makes the perovskite film more stable
to external stimuli. This increase in stability is related to the stronger bond strength
of Cl-Pb than Br-Pb and I-Pb. In addition, they determined the optimum Cl
incorporation ratio that could provide higher stability without substantially changing
the band gap of the perovskite material, pointing out an important direction for the
fabrication of stable perovskite devices. It is obvious that the performance of OIHP
exhibits a large variation after low ratios Cl doping into either CH3NH3PbBr3 or
CH3NH3PbI3 crystals and films.
During the past several years, intensive efforts have been done on optoelectronics
based on CH3NH3PbCl3 SCs, but the performances of these devices are still far away
from being commercially used. Br-doped CH3NH3PbCl3 has a great potential to
enhance the performances of these devices. However, the fundamental effects of
Br-doped CH3NH3PbCl3 SC on optical properties remain unresolved. Furthermore, the
outstanding properties of solution-growth OIHP have also promotedthe application
developments. Herein, high quality CH3NH3PbBrxCl3-x perovskite SCs materials have
been synthesized by using solution growth method. The structural properties of the
crystals are characterized by X-ray diffraction (XRD), and home-made X-ray
fluorescence (XRF) measurement. Optical properties are obtained by UV-Vis
transmission
spectrometer
and
photoluminescence
measurement.
The
temperature-dependent photoluminescence (PL) properties of CH3NH3PbCl3 and
CH3NH3PbBr0.3Cl2.7 in the range of 10K to 290K have been investigated. The phase
transition is proved by the peak position and full width at half maximum (FWHM). The
exciton binding energy is also calculated from the temperature-dependent PL spectra,
which is a critical factor that affects the optical properties of the perovskite crystals
and can be tuned by halogen doping concentration. Our research helps on

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understanding the temperature-controlled phase transition of photoexcited exciton and
provides a theory for selecting the appropriate halide composition and temperature
conditions for optoelectronics.
2. Experimental section
2.1. Materials
All the chemicals were purchased from commercial company without further
purification. Methylamine (CH3NH2, 33 wt% in absolute ethanol), hydrobromic acid
(HBr, 48 wt% in water), lead bromide crystalline powder (PbBr2, 99.99%) and lead
chloride crystalline powder (PbCl2, 99.99%) were purchased from Aladdin Reagent
Co. Dimethylsulfoxide (DMSO, anhydrous, ≥99.0%); N,N-dimethylformamide (DMF,
anhydrous, ≥99.5%), hydrochloride acid (HCl, 38 wt% in water), diethyl ether
(anhydrous, ≥99.7%) and ethanol (anhydrous, ≥99.7%) were purchased from Nanjing
Chemical Reagent Co.
2.2. Crystals growth
Synthesis of MACl powder. A molar ratio of 1.2:1 methylamine and hydrochloride
acid was reacting in 0°C by using an ice bath. After reacting for 2.5h, the solution was
evaporated at 60 °C in a rotary evaporator, and then white MACl powder was
obtained. In order to improve the MACl purity, it was purified for at least three times
with ethanol and diethyl ether. Finally, high purity white MACl powder was collected
and stored in vacuum atmosphere with a temperature of 60°C for 24h.
Synthesis of CH3NH3PbBrxCl3-x single crystals. High quality CH3NH3PbBrxCl3-x
perovskite SCs were grown with a solution process based on the inverse temperature
crystallization method. Various molar ratios among CH3NH3Cl, PbCl2 and PbBr2
were dissolved into different volume ratios between dimethylsulfoxide and N-N, by
using dimethylformamide as precursor solution. Then the precursor solution was
filtered using PTFE filter with 2 µm pore size and kept in oil bath at temperature of
80°C. After several days, CH3NH3PbBrxCl3-x perovskite SCs were harvested from the
solution.
2.3. Characterization

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The structural properties of the material were investigated by powder X-ray
diffraction (XRD) and X-ray fluorescence (XRF) spectroscopy. XRD patterns were
obtained by an X-ray diffractometer (Rigaku Ultima IV diffractometer) using Cu-Kα
radiation (λ= 0.15406 nm). XRF system consists of an X-ray generator, a silicon drift
detector (AMPTEK Inc. Model X-123SDD, America) with 125 eV resolution at 5.9
keV that was coupled to a multichannel analyzer (AMPTEK Inc. Model DP5,
America) and lase finder. The X-ray tube (Ag anode target) with 7.8 µm beryllium
window was collimated by a 2 mm diameter collimator. [18] The optical transmission
spectrum was recorded on a Shimadzu UV 2550 spectrophotometer at room
temperature. Low temperature PL spectra were measured from 10K to 290K, and
were collected using a spectrometer (Princeton Instruments SP2500i/ SP2300i).
During the measurement, the crystals were loaded into a quartz optical vacuum
chamber and cooled down by a cryocooler (CRYOMECH OptistatAC-V12W CP830).
A wavelength of 325nm helium–Cadmium laser from Kimmon was employed to
excite crystal. The laser power density irradiated on the surface of crystals was about
320mw/cm2.
3. Results and discussion
Fig. 1. XRD pattern of CH3NH3PbBr0.3Cl2.7 and CH3NH3PbCl3 single crystals
powder.
Fig. 1 shows the powder X-ray diffraction (XRD) pattern of CH3NH3PbBr0.3Cl2.7
and CH3NH3PbCl3 powder. The main XRD peaks locating at around 15.44°, 21.84°,

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31.20° and 34.88° are ascribed to (100), (110), (200) and (210), respectively. All these
peaks indicate that CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7 perovskite crystals belong
to cubic system with space group Pm3ꢀ [19]. According to Bragg's Law, interplanar
spacing d could be calculated with the following equation:
2dsinθ=nλ
(1)
where d is the interplanar spacing, θ is the angle between the incident X-ray and the
corresponding crystal plane, λ is the wavelength of the X-ray (Cu Kα λ=0.1542 nm),
and n is the number of reflections. The lattice parameters can be obtained by:
ꢂ
ꢁ = √ꢃ ꢅꢆ ꢅꢇ
(2)
ꢄ
ꢄ
ꢄ
Where h, k, l is the Miller index. The calculated lattice parameter value of
CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7 are 5.743Å and 5.752Å, respectively. These
results are consistent with previous reports [19-22]. The slightly increased lattice
parameter after Br doped into CH3NH3PbCl3 materials is mainly due to the atomic
radius of Br is larger than Cl, which also induced the diffraction peaks shifting to
lower angles. According to Vegard's law, the actual molar ratio between Br to Cl can
be calculated to be about 0.3:2.7 based on the shift of the diffraction peak [14, 23].
Fig. 2. X-ray fluorescence spectra of CH3NH3PbBr0.3Cl2.7 and CH3NH3PbCl3 single
crystals.
An X-ray fluorescence (XRF) system has been employed to measure the actual

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molar ratio of Br to Cl, as shown in Fig. 2. The XRF spectra was excited by X-ray
tube with Mo target that operated at 25kV. The peaks at the energy of 10.55KeV,
12.61KeV, and 14.76 KeV are attributed to the signal from Lα1, Lβ1, Lγ1 of Pb,
respectively. The other two peaks locating at around 2.62 KeV and 11.88 KeV are
ascribed to Kα peak of Cl and Kα2 peak of Br, respectively. The actual molar ratio
between Br and Cl is 0.3:2.7, which is also consistent with the theoretical calculated
value from XRD pattern. The other peaks are come from air or background shielding
materials.
Fig. 3. (a) Optical transmission spectra of CH3NH3PbBr0.3Cl2.7 and CH3NH3PbCl3
single crystals; (b) calculated optical band-gap of CH3NH3PbBr0.3Cl2.7 and
CH3NH3PbCl3 single crystals
Fig. 3a shows transmission spectra of CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7
single crystals in the range from 350nm to 600nm. The crystals exhibit high
transparency in the visible range up to 90% for CH3NH3PbCl3 SCs. The transmission
greatly drops to 40%, while Br doped into the pure CH3NH3PbCl3 SCs. In addition,
the sharp cut-off edge shifts to a longer wavelength with the doped Br. All these
phenomena are ascribed to the optical band-gap change due to doped Br. We
calculated the optical band-gap using Tauc’s equation [24]:
ꢏ
ꢈℎꢉ = ꢊꢋℎꢉ − ꢌꢍꢎꢄ
(3)
where α is the optical absorption coefficient; hv is the energy of the incident photon;
Eg is the optical band gap; and A is an energy-independent constant. The calculated
optical band-gap can be obtained from x-intercept of the line tangent, as shown in Fig.

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3b. The optical band-gap is shifted from 2.93 eV to 3.1 eV, which is ascribed to Br
atom replacing the position of the Cl atom in the crystal structure. These results are
also consistent with previous reports [25-27].
Fig. 4. (a, b) Temperature-dependent photoluminescence spectra, (c) and (d) Positions
and FWHM of the labeled peaks, shown as a curve of temperature.
In order to explore the effect of Br doping into CH3NH3PbCl3 SCs, we have
investigated the temperature-dependent PL spectra of CH3NH3PbCl3 and
CH3NH3PbBr0.3Cl2.7 SCs perovskites, respectively. During the measurement, the
samples were placed in a vacuum chamber and excited by a laser with a wavelength
of 325nm. As shown in Fig. 4a and 4b, temperature-dependent PL spectra in the range
of 350nm to 500nm between 10K and 290K have been obtained. For CH3NH3PbCl3
SCs (Fig. 4a), while the temperature is lower than 120K, three typical distinct peaks
centered at around 387nm (labeled peak 1), 400nm (labeled peak 2) and 415nm
(labeled peak 3) have been observed. As it’s a band-to-band direct semiconductor
material, high energy peak 1 is ascribed to free exciton recombination emission, peak
2 is originated from the near bound exciton emission, the weak peak 3 is caused by
the excitonic transition [28-32]. Both Peak 1 and 3 are disappeared at around 140K to
180K. While for CH3NH3PbBr0.3Cl2.7 perovskite SCs (Fig. 4b), a single peak is
presented. High energy peak 1* is due to free exciton recombination emission, while

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the temperature is below 140K. Low energy peak 2* is ascribed to bound exciton
recombination emission, while temperature is above 140K. It is worth to be noticed
that both high energy peak (1 and 1*) and low energy peak (2 and 2*) are existing at
the temperature around 140K, which is originated from a tetragonal structure [33].
These phenomena indicate that free exciton recombination emission is mainly
contributed to PL spectra at orthorhombic structure, and bound exciton recombination
emission is the dominated emission at cubic structure. The similar phenomenon has
also been observed from solution growth organic or inorganic perovskite materials
[34].
Furthermore, the emission peak energy and the FWHM of the two samples at
different temperatures are plotted in Fig. 4(c, d). The energy between free exciton
recombination emission and bound recombination emission is about 76meV and
130meV for CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7, respectively. It is worth
mentioning that electron-phonon interaction could cause PL emission blue shift, while
at the same time, temperature increasing induced lattice expansion could cause PL
emission red shift [35,36]. This typical phenomenon has been observed in lead
composite semiconductors [28-29,37]. The band-gap changes at various temperatures
can be estimated according to the following equation [36-38]:
ꢄ
ꢔꢕ
ꢐ ꢒ
ꢌꢍ ꢑ = ꢌꢓ − ꢐ
(4)
ꢒ
ꢕꢅꢖ
Where α and β are fitting parameters characteristic of a given material, T is the
temperature. According to this equation, the band-gap exhibits nonlinear decreasing
trend at low temperature. While at relative high temperature, the decreasing trend
tends to be linear. Interestingly, in Fig. 4c, slightly blue shift peak shifts under 140k
have been observed, mainly due to electron-phonon interaction is greater than the
effect of lattice expansion and dominates below 140K in CH3NH3PbCl3. But the
influence of the lattice expansion exceeds the contribution of the electron-phonon
interaction above 140K. Compared to Fig. 4d, peak red shifts in CH3NH3PbBr0.3Cl2.7
is more pronounced than CH3NH3PbCl3 as the temperature increases, which may be
attributed to the reduced bond energy in the lattice structure after Br doping into

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CH3NH3PbCl3 SCs, so the lattice expansion is more sensitive to temperature changes.
As a result, the final PL emission shift is balanced by the lattice expansion and the
electron-phonon interaction, which varies at different temperature [39,40].
Fig. 5. (a,b) Temperature dependent data of integrated intensity of CH3NH3PbCl3 and
CH3NH3PbBr0.3Cl2.7 single crystals, respectively. The solid lines are fittings.
Since the orthorhombic phase is stable between 10-140K for CH3NH3PbCl3 and
CH3NH3PbBr0.3Cl2.7, the exciton binding energy can be calculated from the
temperature-dependent PL spectra, as shown in Figure 5a and 5b. The relationship
between light emission intensity and temperature is as follows [41-43]:
ꢘ
ꢙ
ꢐ ꢒ
ꢗ ꢑ = ꢚꢅꢛꢜ
(5)
ꢝꢞ ∕ꢠ
ꢡ
ꢟ
ꢟ
where I (T) is the PL intensity, T is temperature, I0 is the PL intensity at 0 K, EB the
exciton binding energy, and kB the Boltzmann constant, and A is a fitting parameter.
The fitted exciton binding energy of CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7 crystals
are 22.9 meV and 27.1 meV, respectively. The results indicate that Br doped
CH3NH3PbCl3 materials are expected to increase the value of excition binding energy.
Unlike previously reported, the exciton binding energy value of CH3NH3PbCl3
material is about 40meV [44]. This is because the exciton characteristics of hybrid
perovskites are correlated with the synthesis, composition and structure details [43].

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4. Conclusions
In this article, we have grown high quality CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7
single crystals and investigated their structural and optical properties. XRD and XRF
data demonstrated the crystal structure and the Br dopant concentration of the
materials. Temperature-dependent PL spectra not only reveal the light emission
properties from the orthorhombic-tetragonal-cubic phase transition, but also
demonstrate free exciton recombination emission is mainly contributed to PL spectra
at orthorhombic structure, and bound exciton recombination emission is the
dominated emission at cubic structure. The calculated exciton binding energy of
CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7 crystals are about 22.9 meV and 27.1 meV,
respectively. These results reveal the fundamental mechanism of light emission for
CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7, and promote their future applications in
optoelectronic devices.
Acknowledgements
This work was funded by the National Natural Science Foundation of China (Grants
No. 11705090, 11875166, and 11435010). Prof. Tang acknowledge the supported
by the National Natural Science Foundation of China (Grant No. 11690040,
11690043). The authors acknowledge Dr. Hongying Yang for her expert assistance
for further discussion and language polishing.

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* Low temperature photoluminescence of CH3NH3PbBrxCl3-x single crystals.
* Luminescence mechanism of the CH3NH3PbBrxCl3-x perovskite materials.
* Free exciton recombination emission dominant contribution to luminescence spectra at
orthorhombic structure.
* Bound exciton recombination emission dominant contribution to luminescence spectra
at cubic structure.
* Calculated exciton binding energy of CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7 are 22.9
meV and 27.1 meV, respectively.