Solution-Processed MAPbBr3 and CsPbBr3 Single-Crystal Detectors with Improved X-Ray Sensitivity via Interfacial Engineering

Article abstract of DOI:10.1002/pssa.202000104

Organic–inorganic hybrid perovskites have been recognized as prospective materials for use in radiation detection application due to their high atomic number, ease of crystal processing, and excellent electronic properties. Herein, solution-synthesized MAPbBr₃ and CsPbBr₃ detectors with different structures are constructed. Charge diffusion length and trap density analysis reveal that CsPbBr₃ possesses better thermal stability but worse charge-transport properties than MAPbBr₃. To improve the signal current under X-ray radiation, MoO₃ is applied as a hole-extraction layer to increase hole-carrier collection. The sensitivity of X-ray detectors with MoO₃ layer can reach up to 2552 μC Gy_air ^?1 cm^?2 at an electric field of 45 V cm^?1, which is higher than that without the MoO₃ layer. It is shown that charge-transport capability significantly affects the response of the signal current to X-ray radiation; it also reveals the importance of interface engineering as a means to realize high-sensitivity X-ray detectors.

Full text of DOI:10.1002/pssa.202000104

ORIGINAL PAPER
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Solution-Processed MAPbBr3 and CsPbBr3 Single-Crystal
Detectors with Improved X-Ray Sensitivity via Interfacial
Engineering
Zhengfang Fan, Jiang Liu,* Wentao Zuo, Guoqiang Liu, Xulin He, Kun Luo,
Qinyan Ye, and Cheng Liao*
sensitive semiconductor.[11,12,16] The other
Organic–inorganic hybrid perovskites have been recognized as prospective
materials for use in radiation detection application due to their high atomic
number, ease of crystal processing, and excellent electronic properties. Herein,
solution-synthesized MAPbBr3 and CsPbBr3 detectors with different structures
are constructed. Charge diffusion length and trap density analysis reveal that
CsPbBr3 possesses better thermal stability but worse charge-transport properties
than MAPbBr3. To improve the signal current under X-ray radiation, MoO3 is
applied as a hole-extraction layer to increase hole-carrier collection. The sensi-
tivity of X-ray detectors with MoO3 layer can reach up to 2552 μC Gyairꢁ1 cmꢁ2 at
an electric field of 45 V cmꢁ1, which is higher than that without the MoO3 layer.
It is shown that charge-transport capability significantly affects the response of
the signal current to X-ray radiation; it also reveals the importance of interface
engineering as a means to realize high-sensitivity X-ray detectors.
type is indirect conversion, which involves
a scintillator, which first converts X-rays
into visible light, and a photodiode, which
subsequently then converts light into
charge signal.[17,18] The former can achieve
a higher resolution because it is not limited
by the process of additional scattering in
the scintillator; it is also easier to realize
because the system is less complex.
Some high-quality semiconductor materi-
als such as crystalline Si, α-Se, high-purity
Ge, and CdZnTe have been successfully
developed as X-ray detectors,[11,19–21] but
there are still concerns regarding cost,
up-scaling, and sensitivity. For example,
crystalline Si, which is the most commonly
used photodiode semiconductor, has a low
X-ray attenuation coefficient due to its low
atomic number.[21] High-purity Ge could exhibit good resolution,
but it needs to operate at a low-temperature environment cooled
by liquid nitrogen due to its low bandgap. In recent decades, the
trend has been to use CdZnTe-based detectors; however, prob-
lems such as high cost and charge-carrier trapping are still
present.[20] Organic–inorganic hybrid perovskites have been
reported to have outstanding properties involving a large X-ray
attenuation coefficient, large μτ product, high resistivity at
room temperature, and low trap density, making them ideal
candidates for use as attenuation materials in radiation
detectors.[10,11,13,19,22–24] Using Si-integrated MAPbBr3 single-
crystal (SC)-based detectors, Wei et al. obtained an impressive
X-ray sensitivity of 2.1 ꢀ 104 μC Gyairꢁ1 cmꢁ2 at 8 keV X-ray
energy, significantly outperforming present commercial α-Se
X-ray detectors.[12] Kim et al. demonstrated the printable
polycrystalline perovskite detector with the sensitivities of up
to 1.1 ꢀ 104 μC Gyairꢁ1 cmꢁ2 at 100 keV X-ray irradiation.[13] In
addition, X-ray detectors based on MAPbI3, Cs2AgBiBr6, and
MAPbBrxCl(3ꢁx) SCs have also been demonstrated.[10,11,25,26]
Inorganic halide perovskite CsPbX3 could be more thermally
stable compared with the organic halide counterpart.[27–30]
Furthermore, Cs-based compounds may have a higher average
atomic number and material density. Therefore, exploring the
implementation of inorganic halide perovskites in X-ray detec-
tors is very meaningful. In addition, the impact of device types
and some components on the sensitivity of X-ray detectors at
low-dose X-ray detection still requires further exploration.
1. Introduction
Organic–inorganic hybrid perovskites have emerged as a good
candidate material for optoelectronic devices including photode-
tectors, light-emitting diodes, and solar cells,[1,2] because of their
excellent characteristics in terms of high absorption,[3] good
charge-transport capability,[4,5] and
a
tunable bandgap.[6]
Single-crystalline perovskite, in particular, has unique advan-
tages over its thin-film counterpart, as it is associated with higher
hole/electron mobility, a longer diffusion length, and a higher
tolerance toward ambient corrosion.[7–9] In addition, due to
the high atomic number of composition elements, this type of
perovskite also has potential as a radiation attenuation material
for X-ray detectors, which are widely applied in medical imaging,
nondestructive inspection of luggage and industrial products,
and scientific research.[10–15] There are usually two types of radi-
ation detection. One type is direct conversion, which entails the
direct conversion of X-rays into charge signals using an X-ray
Z. Fan, Dr. J. Liu, W. Zuo, G. Liu, X. He, K. Luo, Q. Ye, Dr. C. Liao
Chengdu Development Center of Science and Technology
China Academy of Engineering Physics
Chengdu 610200, China
E-mail: 546jiang@163.com; cliao@pku.edu.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssa.202000104.
DOI: 10.1002/pssa.202000104
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In this work, we synthesized organic–inorganic hybrid perov-
skite MAPbBr3 and the fully inorganic perovskite CsPbBr3
crystals based on solution methods.[7,30] The electronic properties
of MAPbBr3 and CsPbBr3 SCs were comprehensively investi-
gated by analyzing their absorption spectra, time-resolved photo-
luminescence (TRPL) spectra, and space charge limit current
(SCLC). Lateral SC-based devices with interdigitated Au electro-
des were prepared to research the photon responsivities of
MAPbBr3 and CsPbBr3 at 520 nm wavelength laser illumination.
The MAPbBr3-based devices exhibited a superior photoelectronic
performance because of their longer carrier diffusion length and
superior ability to absorb light. Furthermore, vertical SC-based
devices with symmetrical Au electrodes and vertical diode
structure devices with the MoO3 barrier layer were fabricated to
study the sensitivities of MAPbBr3 and CsPbBr3 SC under X-ray
irradiation. We found that the vertical devices with the MoO3
extraction layer exhibited a higher sensitivity. The introduction
of the MoO3 layer allowed us to obtain a significant performance
gain, with sensitivity of up to 2552 and 619 μC Gyairꢁ1 cmꢁ2 for
MAPbBr3- and CsPbBr3-based devices, respectively. This perfor-
mance enhancement is mainly attributable to the band bending
that occurred at the interface, increasing the rate of charge
extraction. Based on these results, a comparison of the optoelec-
tronic properties, responsivity, and sensitivity of MAPbBr3-
and CsPbBr3-based photodetectors and X-ray detectors was
conducted.
with approximate dimensions of around 4 mm ꢀ 14 mm ꢀ 2 mm
are shown in Figure 1a,b. The energy-dispersive X-ray spectros-
copy (EDS) spectra of one CsPbBr3 SC confirmed the composi-
tion ratio of Cs:Pb:Br to be approximately about 1:1:3.2
(Figure S1, Supporting Information), which is nearly equal to
that of its precursor solution. The X-ray diffraction patterns for
these crystals fabricated in this study are shown in Figure 1.
The results show the main peaks are located at 15.01ꢂ and 30.21ꢂ
for MAPbBr3, which can be assigned to (100) and (200) planes
of the cubic MAPbBr3 phase; for CsPbBr3, the main peaks were
found to occur at 15.54ꢂ and 31.03ꢂ, corresponding to the (101)
and (202) planes of the orthorhombic phase.
We estimated the trap density and carrier mobility (hole
only) from the dark current–voltage curve using the SCLC
technique[3,11] (Figure S1, Supporting Information). The I–V
curves were found to follow Mott–Gurney’s law. SCs were
typically sandwiched (Au/SC/Au) to create hole-only devices.
According to the results shown in Figure 2a,b, the VTFL values
of MAPbBr3 and CsPbBr3 SCs are 10.1 and 31.2 V, respectively.
Therefore, the hole trap density could be calculated to be
2.26 ꢀ 109 cmꢁ3 for MAPbBr3 and 3.87 ꢀ 1010 cmꢁ3 for
CsPbBr3. Note that the calculated trap densities of MAPbBr3
and CsPbBr3 SCs are consistent with those provided in previous
reports[7,32,33] and are also about three orders of magnitude lower
than that of some common semiconductors, such as Si, CdTe,
and CIGS.[20,34] The Child’s law region was used to estimate
the carrier mobility. MAPbBr3 SC exhibits a hole mobility of
89.8 cm2 Vꢁ1 sꢁ1, which is eight times higher than that of
CsPbBr3 SC (10.7 cm2 Vꢁ1 sꢁ1). The high carrier mobility and
low trap density of MAPbBr3 SC may imply that MAPbBr3 is
better suited for photodetection and X-ray detection.
2. Results and Discussion
MAPbBr3 SCs were obtained from the precursor solution using
the inverse temperature crystallization method,[7] whereas
CsPbBr3 SCs were harvested using the antisolvent method.[30,31]
The photographs of one MAPbBr3 SC with approximate dimen-
sions of around 12 mm ꢀ 13 mm ꢀ 3 mm and one CsPbBr3 SC
TRPL was used to investigate the carrier lifetime of MAPbBr3
and CsPbBr3 SCs, as shown in Figure 2c. The decay was fitted
to
a
biexponential function, giving fast components of
(a)
(b)
(c)
(d)
MAPbBr3
CsPbBr3
(202)
(200)
(101)
(020)
(100)
(040)
10
15
20
25
30
35
40
10
15
20
25
30
35
40
2
(Degree)
2 (Degree)
Figure 1. Photographs of a) MAPbBr3 and b) CsPbBr3 SCs. X-ray diffraction patterns of our c) MAPbBr3 and d) CsPbBr3 crystals.
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(a)
(b)
1E-4
1E-6
I~Vn
n=1
I~Vn
n=1
CsPbBr3
MAPbBr3
1E-6
n>3
n>3
n=2
n=2
1E-7
1E-8
d=3.85mm
d=1.5mm
1E-8
1E-9
VTFL
100
VTFL
1E-10
1E-10
0.1
1
10
Voltage (V)
100
0.1
1
10
Voltage (V)
(c)
(d)
100
10-1
10-2
10-3
100
80
60
40
20
0
MAPbBr3
CsPbBr3
CsPbBr3
MAPbBr3
0
200
400
600
800
100 200 300 400 500 600 700 800
Temperature (ºC)
Time delays (ns)
Figure 2. Current–voltage curves of a) MAPbBr3 and b) CsPbBr3 devices with the sandwich structure of Au/SC/Au. c) TRPL of MAPbBr3 and CsPbBr3 SCs.
d) TGA curves of MAPbBr3 and CsPbBr3.
58.25 ꢃ 1.06 ns and 3.07 ꢃ 0.24 ns, together with slow compo-
nents of 501.06 ꢃ 3.1 and 38.28 ꢃ 5.8 ns for MAPbBr3 and
CsPbBr3 SC, respectively. The short lifetime (fast component)
and the long lifetime (slow component) are usually attributed
to surface and bulk recombination process.[4] The MAPbBr3
SC also exhibits significantly a higher average carrier lifetime
(τAV ¼ 422.85 ꢃ 1.06 ns), compared with that of CsPbBr3 SC
(τAV ¼ 6.25 ꢃ 1.06 ns). Using the carrier mobility μ obtained
via the SCLC method, and the average lifetime τ derived from
the TRPL results, we can calculate the carrier diffusion length
(Ld)[35] by solving the equation Ld ¼ (kBTμτ/e)0.5. The calculated
long diffusion lengths reached 10.66 ꢃ 0.84 and 1.26 ꢃ 0.17 μm
for MAPbBr3 and CsPbBr3 SC based on their bulk carrier
lifetime, whereas the short diffusion lengths were 2.95 ꢃ 0.49
and 0.29 ꢃ 0.08 μm for MAPbBr3 and CsPbBr3 SC, respectively.
To find out the thermal stability of our SCs, the thermal
analysis wasꢁc1onducted from 30 to 800 ꢂC at the heating rate
decomposition of MAPbBr3 started at around 268 ꢂC and
required two steps for completion, whereas CsPbBr3 began to
decompose at ꢄ511 ꢂC. These results indicate that CsPbBr3
has better thermal stability than MAPbBr3.
We further investigated the optical properties of MAPbBr3 and
CsPbBr3 SCs. From the absorption and photoluminescence (PL)
spectra (Figure 3a,b), sharp band edges at 575 and 553 nm for the
MAPbBr3 and CsPbBr3 SCs are observed. The PL peaks for
MAPbBr3 and CsPbBr3 SC were located at 578 and 562 nm,
respectively. The bandgaps obtained according to Tauc’s law[36]
(αhv ¼ A(hv ꢁ Eg)0.5, where A is a constant, hv is photon energy,
and Eg is optical bandgap energy) were calculated to be 2.16
and 2.25 eV for the MAPbBr3 and CsPbBr3 SCs, respectively
(Figure 3c).
The lateral SC-based devices with five pairs of interdigitated
Au electrodes, which have a width of 0.1 mm, a length of 1.5 mm,
and a thickness of 50 nm, were used for photodetection (Figure 4a).
The active area of devices is 3 mm2. The current–voltage traces
of 5 ꢂC min
(Figure 2d). The results showed that the
(a)
(b)
(c)
3.0x103
CsPbBr3
MAPbBr3
MAPbBr3
CsPbBr3
absorption
PL
absorption
PL
0.0
2.1
500
600
500
600
2.2
2.3
Wavelength (nm)
Wavelength (nm)
h
(eV)
Figure 3. Normalized absorption and PL spectra of a) MAPbBr3 and b) CsPbBr3 SC. c) Tauc plot extrapolation from the absorption analysis of MAPbBr3
and CsPbBr3 SCs.
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(b)
(c)
(a)
MAPbBr3
CsPbBr3
10-6
10-8
10-5
10-7
22.70 mW cm-2
1.67 mW cm-2
43 W cm-2
5.65 mW cm-2
0.50 mW cm-2
dark
22.70 mW cm-2
1.67 mW cm-2
43 W cm-2
5.65 mW cm-2
0.50 mW cm-2
dark
10-9
10-10
10-11
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage (V)
(d)
(e)
(f)
101
100
MAPbBr3
10-4
MAPbBr3
CsPbBr3
MAPbBr3
CsPbBr3
CsPbBr3
1013
1012
1011
10-5
10-1
10-2
10-6
10-7
0.1
1
10
0.1
1
mW cm-2
)
10
0.1
1
10
P ( )
mW cm-2
mW cm-2
)
P
(
P
(
Figure 4. a) Optical photograph of lateral-structure photodetector with interdigital electrodes. The current–voltage traces for the b) MAPbBr3- and
c) CsPbBr3-based photodetectors measured in the dark and under different illumination intensities. The light intensity-dependent d) photocurrent,
e) responsivity, and f) detectivity of MAPbBr3- and CsPbBr3-based photodetectors.
of MAPbBr3- and CsPbBr3-based devices were measured in dark
conditions and under 520 nm wavelength laser illumination, as
shown in Figure 4b,c. The light intensity, as measured by a
power meter (Newport, 843-R), ranged from 43 μW cmꢁ2 to
22.7 mW cmꢁ2. Both devices show very low dark currents. For
example, at a reverse bias voltage of 5 V, the dark current of
MAPbBr3- and CsPbBr3-based devices is found to be less than
10ꢁ7 and 10ꢁ8 A, respectively. Low leakage current is an important
indicator of high-quality photodetectors. After 22.7 mW cmꢁ2
illumination, the photocurrent rises significantly, three orders of
magnitude higher than the dark current. As the applied voltage
increases, the current gain also gradually increases.
The parameters used to determine the level of performance of
the detectors (i.e., responsivity and detectivity) were calculated
and discussed in detail. Without considering the reflection loss,
responsivity (R) is described as the ratio of photocurrent to inci-
dent light intensity, according to[9,37] the following equation.
detectivity is shown in Figure S4, Supporting Information.
The responsivity and detectivity are found to decrease with
increasing incident light intensity, possibly due to the strong
charge recombination induced by high light intensity. At a light
intensity of 43 μW cmꢁ2, a responsivity of 5.7 A Wꢁ1 and detec-
tivity of 2.6 ꢀ 1013 jones were recorded for the MAPbBr3-based
photodetector, operating with an 8 V bias. Under the same
conditions, the CsPbBr3-based device exhibits a responsivity of
0.08 A Wꢁ1 and detectivity of 1.0 ꢀ 1012 jones.
To further investigate the response of each of these two mate-
rials under X-ray irradiation, we fabricated two vertical SC-based
devices for X-ray detection, as shown in Figure 5a. One type is a
symmetrical device with a Au/SC/Au structure, and the other
type is an asymmetric diode device a with Ag/SC/MoO3/Au
structure. A commercial X-ray source with a maximum 50 kV
tungsten anode is used. Using the mass energy attenuation coef-
ficient from the photo cross-section database, we are able to
obtain the absorption spectra and X-ray attenuation efficiency
as a function of X-ray energy for the compound MAPbBr3 and
CsPbBr3 materials (Figure S5, Supporting Information). Some
wrinkles can be observed in the attenuation coefficient curves,
because the absorption coefficient decreases in the short energy
range when the energy of the incident photon drops below the
ionization potential of some inner (K&L) shell electrons. In this
study, 2.6 mm-thick MAPbBr3 and 2 mm-thick CsPbBr3 SCs
were selected as X-ray detector materials to ensure that nearly
all 50 kV X-ray photons could be absorbed. Generally, when
X-ray photons with energies below 50 kV permeate the
MAPbBr3 and CsPbBr3 SCs, the photoelectric effect process
occurs dominantly because of the high atomic number of the
materials and low-energy X-ray photons.[16] More specifically, a
higher atomic number of the absorbing material increases the
strength of the photoelectric effect. During this process, tightly
Jph ꢁ Jd
Plight
R ¼
(1)
Detectivity can be expressed as[37]
R
*
pffiffiffiffiffiffiffiffi
D
¼
(2)
2eJd
where Jph is photocurrent density, Plight is incident light inten-
sity, Jd is dark current density, and e is the elementary charge.
The photocurrent, responsivity, and detectivity results are,
respectively, shown in Figure 4d–f as functions of the intensity
of the incident light (range: 43 μW cmꢁ2–22.7 mW cmꢁ2) applied
in the conditions of 520 nm wavelength illumination at 8 V bias.
Detailed information on the photocurrent, responsivity, and
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(c)
(a)
(b)
MAPbBr3 vertical symmetrical structure
80
60
40
20
0
20.9 Gyair s-1
37.6 Gyair s-1
0
20
40
60
80
Electric field (V cm-1)
(d)
(e)
(f)
50
40
30
20
10
0
MAPbBr3 vertical diode structure
CsPbBr3 vertical diode structure
20.9 Gyair s-1
37.6 Gyair s-1
CsPbBr3 vertical symmetrical structure
37.6 Gyair s-1
15
10
5
250
200
150
100
50
1.2 Gyair s-1
20.9 Gyair s-1
37.6 Gyair s-1
0
0
20
40
60
80
100
0
20
40
60
80
0
20
40
60
80
100
Electric field (V cm-1)
Electric field (V cm-1)
Electric field (V cm-1)
Figure 5. a) Schematic illustration of the vertically symmetrical and vertical diode devices. b) Energy-level diagram for the vertical diode structure devices.
Current density as a function of applied electric field with different X-ray dose rates for the c,d) vertical symmetrical and e,f) diode structure detectors
based on c,e) MAPbBr3 and d,f) CsPbBr3 crystals.
bound electrons inside the material are excited upon X-ray
illumination and the excited atoms subsequently generate more
electron–hole pairs through energy loss, thereby improving the
conductivity of the material. By measuring the output current of
the photoconductive detector operating at a certain bias voltage in
response to X-ray illumination, the amount of charge generated
by the perovskite crystals can be obtained.
The photoelectric process induced by X-ray photons is affected
by the X-ray absorption coefficient, the applied electric field,
charge transport capability of the material, as well as the device
structure. Figure 5c,d shows the output current density of our
symmetrical devices as a function of the applied electric field.
The obtained current is found to increase as the applied electric
field and X-ray dose rate increase. No obvious signal current
structure forms. Figure S7, Supporting Information, shows an
SEM cross-section image of a vertical diode structure device.
The energy-level diagram for vertical structure devices is shown
in Figure 5b. The MAPbBr3 and CsPbBr3 materials have a rela-
tively large bandgap of ꢄ2.3 eV and thus a low valence band max-
imum. The MoO3 layer has a very high work function and is
often used to efficiently increase the rate of hole extraction in
solar cells.[38,39] Incorporating MoO3 allows the charge to be first
extracted to the MoO3 layer prior to being collected by the electro-
des. Figure 5e,f shows the current densities of the MAPbBr3- and
CsPbBr3-based devices with vertical diode structures at the X-ray
ꢁ1
dose rate of 1.2, 20.9, and 37.6 μGy
s
. The current density of
air
the MAPbBr3-based device with a vertically diode structure is up
to 95.9 nA cmꢁ2, which is approximately three times larger than
that of the device with a vertically symmetrical structure
(28.6 nA cmꢁ2) at the same applied electric field of 30 V cmꢁ1
and an X-ray dose rate of 37.6 μGyair sꢁ1. A higher photocurrent
was also observed for the CsPbBr3-based diode device. After cal-
culating the signal-to-noise ratio of the CsPbBr3-based diode
could be detected for the CsPbBr3-based symmetrical device at
ꢁ1
a dose rate of 1.2 and 20.9 μGy
s
; for the MAPbBr3-based
air
symmetrical device, the current density is ꢄ8.5 nA cmꢁ2 in
the conditions of an applied electric field of 15 V cmꢁ1 and dose
ꢁ1
rate of 20.9 μGy
s
. Regarding the shorter diffusion length of
air
CsPbBr3 SCs, the poor charge transport capability of CsPbBr3
may negatively affect the detection limit even though its X-ray
absorption is higher. In addition, the MAPbBr3-based symmet-
rical device achieves a larger current gain than the CsPbBr3-
based device under X-ray radiation. Therefore, optimizing the
charge transport capability of the detectors is expected to signifi-
cantly improve X-ray detection performance.
When bias is applied, the X-ray-induced electron carriers are
collected by the anode as the hole carriers pass through the whole
SC and arrive at the cathode. To improve hole transport, MoO3
with a very deep-level conduction band (ꢄ6.7 eV) is implemented
as an interfacial layer to extract the hole carriers generated by
X-ray photons. With Ag and Au as anode and cathode, a diode
device (Figure S8, Supporting Information), we determined that
ꢁ1
an X-ray dose rate of 20.9 μGy
s
is the limit for reliable detec-
air
tion. Thus, we can conclude that the MoO3 layer enhances X-ray
detection ability by increasing the hole transport. A linear rela-
tionship between the output current and X-ray dose rate is shown
in Figure S9, Supporting Information; the sensitivity can be
obtained by calculating the slope of fitted lines (Table 1).
Figure 6a,b shows the response of the MAPbBr3- and
CsPbBr3-based devices to X-ray photons by switching X-ray sour-
ces on and off. The results demonstrate that X-ray photons gen-
erate a strong signal current. The increase in the current passing
through the vertical diode devices is observed to be larger than
that passing through the symmetrical devices immediately after
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Table 1. Sensitivity summary for the different types of X-ray detectors
fabricated in this article.
also conducted for the CsPbBr3-based devices. An increase from
0.07 to 0.29 is observed in the conditions of an applied electric
field of 15 V cmꢁ1 and X-ray dose rate of 37.6 μGyair sꢁ1. A com-
parison between vertically symmetrical and diode devices reveals
that the MoO3 layer facilitates hole collection in the condition of
X-ray radiation.
Devices
Sensitivity [μC Gyairꢁ1 cmꢁ2
]
15 V cmꢁ1
30 V cmꢁ1
45 V cmꢁ1
Au/MAPbBr3/Au
276
786
93
638
1647
146
1121
2552
200
Au/MoO3/MAPbBr3/Ag
Au/CsPbBr3/Au
3. Conclusions
In summary, we obtained high-quality MAPbBr3 and CsPbBr3
crystals by solution methods and fabricated corresponding detec-
tors with three types of structures for photon and X-ray detection.
A lateral MAPbBr3-based photodetector exhibited a high respon-
sivity of 5.7 A Wꢁ1 and a detectivity of 2.6 ꢀ 1013 jones in the
conditions of 520 nm wavelength illumination and 8 V bias;
these values are significantly higher than those achieved by the
CsPbBr3-based device because of the superior electrical perfor-
mance of MAPbBr3 crystals. The charge-transport capability of
the comprising materials significantly affects the device perfor-
mance in X-ray detection. The MAPbBr3- and CsPbBr3-based
devices with a MoO3 extraction layer achieved sensitivities of
up to 2552 and 619 μC Gyairꢁ1 cmꢁ2, respectively, in the condi-
tion of an applied electric field of 45 V cmꢁ1. The addition of the
MoO3 layer resulted in all devices, achieving higher sensitivity
levels, regardless of the type of active perovskite material. The
introduction of the MoO3 layer with a high work function is
found to facilitate charge transfer and boost carrier collection.
Our work illustrates the importance of 1) hole extraction in a
device purposed for X-ray detection and 2) customizing carrier
collection to realize high-sensitivity detection.
Au/MoO3/CsPbBr3/Ag
221
509
619
the X-ray source is turned on. We also calculate the signal-to-
noise ratio of both the symmetrical and diode devices. With
the assist of the MoO3 layer, the value is found to increase from
29 to 107 for the MAPbBr3-based devices and from 5 to 18 for
the CsPbBr3-based devices. This increase is due to the deep-level
conduction band of the MoO3 layer, which increases the rate of
carrier extraction and thus improves the stability of the output
current.
We use gain factor to analyze the roles of MoO3 layer in the
vertical diode device. The gain factor is defined as Jout/Jeh, where
Jout is the output current density, Jeh is the current density
from electron–hole pairs generated by X-ray radiation,
Jeh ¼ (e ꢀ F ꢀ D ꢀ Em)/Δ, F is fluence per air kerma, D is the
dose rate, Em is the mean energy, and Δ is the electron–hole
pair creation energy, given by the empirical model as
Δ ¼ 1.43 þ 2Eg.[11] The mean energy of the X-ray beam and
fluence per air kerma can be obtained using the software
SPEKTR3.0, which are determined to be 31.09 keV and
127 937 photonꢁs1mGyꢁ1 mmꢁ2, respectively, in the conditions
4. Experimental Section
of 37.6 μGyair s
and 50 kV X-ray energy. Using the same
method, we can determine the mean energy of the X-ray beam
and fluence per air kerma to be 22.31 keV and 63 399 photons
mGyꢁ1 mmꢁ2 for 20.9 μGyair sꢁ1 at 30 kV X-ray energy.
Further details are available in Figure S3, Supporting
Information. The gain factor of the MAPbBr3-based device
with a vertically symmetrical structure is, respectively, found to
be 0.91 and 0.27ꢁi1n the conditions of X-ray dose rates of 20.9
MAPbBr3 SC: MAPbBr3 SCs were synthesized by the inverse tempera-
ture crystallization growth method. MABr and PbBr2 precursors were dis-
solved at a 1:1 molar ratio in the N,N-dimethylformamide solvent. To
assure full dissolution, the solution was maintained at 25 ꢂC under mag-
netic stirring for 12 h and then subjected to filtration. After nucleation in
precursors was observed, MAPbBr3 crystals were obtained slowly by heat-
ing the solution from 40 to 80 ꢂC with a ramp rate of 1 ꢂC hꢁ1. The
obtained MAPbBr3 SC was washed three times with diethyl ether.
CsPbBr3 SCs: CsPbBr3 SCs were prepared by the antisolvent-assisted
crystallization method. Specifically, CsPbBr3 solution with a slight excess
and 37.6 μGyair s and an applied electric field of 15 V cmꢁ1
.
After the introduction of MoO3 layers, the gain factors increase
to 3.36 and 0.95, respectively. An estimation of the gain factor is
of Pb over Cs was first obtained by dissolving CsBr (0.4 M) and PbBr2
(a)
(b)
vertical diode structure
MAPbBr3
CsPbBr3
vertical diode structure
vertical symmetrical structure
vertical symmetrical structure
10
20
30
40
50
10
20
30
40
50
Time (s)
Time (s)
Figure 6. Responses of a) MAPbBr3- and b) CsPbBr3-based devices to intermittent X-ray irradiation.
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Received: February 28, 2020
Revised: March 10, 2020
Published online:
(0.5 M) in dimethyl sulfoxide. The resulting solution, in an open vial, was
then placed in a larger closed vessel containing a mixture of ethanol and
H2O with a 1:6 volume ratio (antisolvent). Crystals grew slowly as the anti-
solvent vapor gradually diffused into the CsPbBr3 solution.
Detector Fabrication: Three kinds of detectors were fabricated for photo
and X-ray detection. For lateral-structure photodetectors, 50 nm-thick
interdigitated Au electrodes (Figure 4a) were deposited on the crystal
surface via thermal evaporation. The effective illuminated area for each
photodetector was 3 mm2. For X-ray detectors with a vertical symmetrical
structure, Au electrodes with a thickness of 80 nm were deposited by evap-
oration on the top and bottom sides of the crystals to form the Au/SC/Au
structure. For X-ray detectors with a vertical diode structure, Ag/SC/
MoO3/Au structure was used. An 80 nm-thick Ag electrode was thermally
deposited on one side of the MAPbBr3 and CsPbBr3 crystals, whereas
30 nm MoO3 and 50 nm Au were successively deposited on the other side.
Characterizations: The compositions in the SCs were determined using
an energy dispersive X-ray fluorescence analyzer (XRF). X-ray diffraction
patterns were obtained by DX-2700B (Dandong Haoyue Instrument
Corporation), using Cu Kα radiation at a scan rate of 6ꢂ minꢁ1. The absorp-
tion spectra were measured by a spectrophotometer (Shanghai spectrum,
SP-756) in the transmittance mode from 400 to 900 nm wavelength at
room temperature. Thermogravimetric analysis (TGA) was conducted
on a NETZSCH STA 449 C. The sample was placed in an Al2O3 crucible
and heated from 30 to 800 ꢂC at a ramp rate of 5 ꢂC minꢁ1 with an argon
flow rate of 50 mL minꢁ1. Approximately 520 mg of crystals were used for
each experiment. Steady PL and TRPL measurements were carried out with
a spectrofluorometer (Fluoroma-4, Horiba Scientific).
Photon Response Measurement: A 520 nm wavelength laser diode with
adjustable power which was monitored by a power meter (Newport,
843-R) was used as the light source. The dark and photon currents of
MAPbBr3 and CsPbBr3-based detectors were measured using a source
meter (Keithley 2400).
X-Ray Response Measurements: An X-ray source (Fischerscope X-ray
XDV-SDD) with a tungsten anode was used. The X-ray tube operated
at a voltage of 10, 30, and 50 kV. A collimator with a diameter of 3 mm
was used to limit the size of the X-ray beam. The X-ray dose rate depending
on the operating voltage of the X-ray tube was calibrated by a dosimeter.
The current induced by X-ray photons was recorded by a source meter
(Keithley 2400).
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Acknowledgements
This work was supported by Sichuan Science and Technology Program
(grant no. 2018GZ0480), National Natural Science Foundation of China
(grant nos. 11704425 and 61674174), National Key Research and
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