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Fabrication of efficient CsPbBr3 perovskite solar cells by single-source thermal evaporation
Journal Pre-proof Fabrication of efficient CsPbBr3 perovskite solar cells by single-source thermal evaporation Juan Li, Rongrong Gao, Fei Gao, Jie Lei, Haoxu Wang, Xin Wu, Jianbo Li, Hao Liu, Xiaodong Hua, Shengzhong (Frank) Liu PII:
S0925-8388(19)34149-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.152903
Reference:
JALCOM 152903
To appear in:
Journal of Alloys and Compounds
Received Date: 28 August 2019 Revised Date:
31 October 2019
Accepted Date: 2 November 2019
Please cite this article as: J. Li, R. Gao, F. Gao, J. Lei, H. Wang, X. Wu, J. Li, H. Liu, X. Hua, S.(F.) Liu, Fabrication of efficient CsPbBr3 perovskite solar cells by single-source thermal evaporation, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152903. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Fabrication of efficient CsPbBr3 perovskite solar cells by single-source thermal
2
evaporation
3 4
Juan Lia, Rongrong Gaoa, Fei Gao a*, Jie Leia, Haoxu Wanga, Xin Wua, Jianbo Lia,
5
Hao Liua, Xiaodong Huaa, and Shengzhong (Frank) Liua,b*
6 7
a
8
Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi
9
Engineering Lab for Advanced Energy Technology, School of Materials Science and
Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of
10
Engineering, Shaanxi Normal University, Xi’an 710119, China.
11
b
12
Physics, Chinese Academy of Sciences, Dalian 116023, China
Dalian National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical
13 14
*Corresponding authors:
15
E-mail: [email protected] (F. Gao), [email protected] (S. F. Liu).
16 17 18 19 20 21
1
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Abstract
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It is promising to improve the stability of organic–inorganic hybrid halide perovskite
24
solar cells by using all-inorganic perovskite materials. Herein, a facile one-crucible
25
single-source vacuum thermal evaporation (VTE) approach is developed, which is used
26
to evaporate two different melting points materials CsBr (630 °C) and PbBr2 (370.6 °C)
27
to deposit high-quality inorganic CsPbBr3 perovskite films. Molar ratio of PbBr2 to
28
CsBr in their mixture in the crucible is a key factor influencing the stoichiometry,
29
structure, photoelectrical and photovoltaic properties of the CsPbBr3 films. The other
30
important factor is the thickness of the CsPbBr3 films. High-quality CsPbBr3 films with
31
good uniformity and compact and large grains are prepared. Planar CsPbBr3 perovskite
32
solar cells are fabricated giving a high power conversion efficiency of 8.65%. The
33
fabricated CsPbBr3 solar cells exhibit a good stability in air without encapsulation. This
34
study opens up the possibility to deposit multi-element compound thin films by facile
35
single-source VTE of different melting points materials.
36 37
Keywords: CsPbBr3 films, Single-source evaporation, Molar ratio, Thickness, Solar
38
cells
39 40 41
2
42
1.
Introduction
43
It is promising to improve the stability of organic–inorganic hybrid halide
44
perovskite solar cells by using all-inorganic perovskite materials. In these years, the
45
solar cells based on inorganic halide perovskites CsMX3 (M = Pb or Sn; X = I, Br, Cl
46
or mixed halides) materials [1,2], such as
47
[11,12], CsPbIBr2 [13,14], CsPbBr3 [15-33], Cs3Bi2I9 [34], RbPbI3 [35],
48
CsPb0.9Sn0.1IBr2 [36，37], Cs0.925K0.075PbI2Br [38], and CsSn0.5Ge0.5I3 [39] have been
49
fabricated and better device performance have been obtained. Besides, Bulbak et al.
50
research indicates that all-inorganic lead bromide perovskite materials exhibit as well
51
as the organic one [16]. Compared with other inorganic halide perovskites, the
52
CsPbBr3 is more stable [10,12,17,22,23,40].
CsSnI3 [3-6], CsPbI3 [7-10], CsPbI2Br
53
An appropriate approach to prepare CsPbBr3 films is important in order to
54
achieve good film quality and excellent device properties. Usually, chemical solution
55
method was used to fabricate CsPbBr3 films/solar cells [15-33]. Solution method does
56
not need expensive equipments, however, it is challenging to fabricate uniform and
57
large-area films by this method. Vacuum thermal evaporation (VTE) is widely used to
58
prepare various thin films with large areas and good uniformity. In comparison with
59
solution methods, there are relatively few studies about the fabrication of perovskite
60
solar cells by VTE. Liu et al. first used dual-source VTE of CH3NH3I and PbCl2 to
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fabricate CH3NH3PbI3-xClx solar cells with high efficiency of 15% [41]. They found
62
that the films deposited by VTE are very uniform, while the films prepared by
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solution method coat the substrate only partially: there are voids in the
3
64
solution-processed films, which extend to the compact TiO2 layer on the FTO glass
65
substrate. Due to high vapor pressure and low density, CH3NH3I easily diffuses in the
66
vacuum chamber. It is very difficult to monitor/control the deposition rate of
67
CH3NH3I during VTE. In order to solve this problem, the methods of
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sequential/alternating evaporation [42,43], separating evaporation [44], and hybrid
69
evaporation [45]
70
two-source VTE to fabricate high performance planar CsPbBr3 solar cells with
71
efficiencies of 7.78% and 6.95, respectively [19,46]. Dual-source VTE was also used
72
to fabricate CsPbI3 [9], CsPbI2Br [12], CsPbIBr2 [14], and MA1-xCsxPbI3 [47]
73
perovskite solar cells. Lidón et al. used 4 sources of MAI, CsBr, FAI, and PbI2 to
74
deposit triple-cation Cs0.5FA0.4MA0.1Pb(I0.83Br0.17)3 perovskite films and fabricated a
75
16% efficiency solar cell [48].
has been developed. Recently, Chen et al. and we also used the
76
In multi-source thermal evaporation, the ratio of the evaporation rates of source
77
materials is a key factor affecting the stoichiometry of the deposited film, and the
78
efficiency of the solar cells. There are several factors, such as the pressure of the
79
vacuum chamber, heating power of the crucibles, and the quantity and distribution of
80
evaporation materials in the crucibles, that influence the ratio of evaporation rates.
81
During the lengthy deposition process (usually 2-3 h), it is difficult to maintain a
82
correct ratio of evaporation rates of source materials throughout the process because
83
the experimental conditions fluctuate. Adjustment and control of the ratio of
84
evaporation rates of source materials is difficult and time-consuming.
4
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In this study, we report a facile and effective one crucible single-source VTE
86
approach to deposit high-quality CsPbBr3 films. We mix CsBr and PbBr2 powders as
87
the evaporation materials, which are then pressed into tablets and put into a quartz
88
crucible, and the stoichiometry of the deposited CsPbBr3 films can be determined by
89
the molar ratio of PbBr2 to CsBr in their mixture. We studied the effects of the molar
90
ratio of CsBr to PbBr2 and the thickness of the CsPbBr3 films on the film quality and
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corresponding solar cell performance. Through optimization, high-quality CsPbBr3
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films were prepared, and stable planar CsPbBr3 solar cell with high efficiency of
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8.65% was fabricated.
94 95
2.
Experimental
96
Planar CsPbBr3 inorganic perovskite solar cells were fabricated on FTO-coated
97
glass: FTO/c-TiO2/CsPbBr3/Spiro-MeOTAD/Au. The detailed processes of FTO glass
98
substrate cleaning and the preparation of compact TiO2 layer can be found in our
99
previous report [46]. The CsPbBr3 light absorption layer was deposited on the TiO2
100
layer by thermally evaporating the mixture of CsBr and PbBr2 in vacuum, as shown in
101
Fig. 1a. The mixture of CsBr and PbBr2 powders was used as the evaporation materials,
102
which was pressed into tablets (with the pressure of ~ 10 MPa) and put into a quartz
103
crucible, and the stoichiometry of the deposited CsPbBr3 films can be determined by
104
the molar ratio of PbBr2 to CsBr in their mixture. The substrate was kept at 300 °C and
105
rotated to obtain uniform coating during the deposition process. The evaporation rate
106
(deposition speed, ～0.06 nm/s) was monitored by a quartz crystal monitor near to the
5
107
substrate, which was calibrated by the ratio of actual film thickness to deposition time.
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The temperature of the quartz crucible is about 450 °C. For hole-transport layer
109
deposition, 90 mg mL−1 spiro-OMeTAD solution with 36 µL 4-tert-butylpyridine and
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22 µL lithium bisimide (trifluoromethylsulfonyl) of 520 mg mL−1 in acetonitrile was
111
spin-coated on CsPbBr3 films at 4000 rpm for 30 s. A 70 nm thick gold electrode was
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thermally evaporated on the spiro-MeOTAD film and the area of the solar cells is 3 × 3
113
mm2.
114
XRD spectra were measured by DCIP (XRD, DX-2700) using Cu Kα radiation.
115
SEM images were obtained by field emission SEM (FESEM, SU-8020, Hitachi).
116
EDX analysis was performed by X-ray energy dispersive spectrometer (EX-270,
117
Horiba). UV-Vis spectra were measured by a spectrophotometer (Lambda 950;
118
Perkin-Elmer). Time-resolved PL spectra were measured using an Edinburgh
119
Instruments (FLS920) fluorescence spectrometer. J-V curves of CsPbBr3 solar cells
120
were measured under 100 mWcm-2 using Keithley 2400 source at 300 K in air. The
121
light power output of AM 1.5G solar simulator was calibrated using an
122
NREL-KG5-filtered silicon cell. EQE measurements were performed on a QTest
123
Station 500TI system (Crowntech. Inc., USA). Electrochemical impedance
124
spectroscopy (EIS) measurements were performed by a ZAHNER ENNIUM (PP211)
125
device in the frequency range of 10-10×106 Hz with an AC perturbation signal of 20
126
mV.
127 128
6
129
3.
Results and discussion
130
As shown in Fig. 1b and Supplementary Materials Fig. S1, the as-deposited
131
CsPbBr3 films appear yellow and uniform. The stoichiometry and crystallinity of the
132
CsPbBr3 films are crucial for their photovoltaic properties. The stoichiometry of the
133
deposited CsPbBr3 films are determined by the molar ratios of PbBr2 to CsBr in their
134
mixtures.
135
First, we studied the effect of the molar ratio on the crystallinity and stoichiometry
136
of the deposited CsPbBr3 films (with an optimal substrate temperature of 300°C).
137
Figure 2 shows X-ray diffraction (XRD) patterns of the CsPbBr3 films (350 nm thick)
138
deposited using different molar ratios of PbBr2 to CsBr. The peaks from CsPbBr3
139
located at 2θ = 16.26 º, 21.6 º, 26.34 º, 30.37 º, 37.74 º, and 44.15 º correspond to the
140
(100), (110), (111), (002), (211), and (220) crystal planes diffraction, respectively
141
(JCPDs no. 18-0364). The three main peaks corresponding to (110), (002), and (211)
142
become stronger with increasing molar ratio (0.8:1-1.1:1), indicating better
143
crystallinity of the deposited CsPbBr3 films. The diffraction peaks located at 2θ = 12.4
144
º, 19.9 º, 25.3 º, 28.5 º, and 30.1 º are from Cs4PbBr6 (JCPDs no. 73-2478) [49]. It is
145
found that as the molar ratio increases, the diffraction peaks of Cs4PbBr6 weaken and
146
almost disappear, reflecting a purer CsPbBr3 phase obtained at higher molar ratio
147
(0.9:1-1.1:1). The atomic percentages of Cs, Pb, and Br elements in the deposited
148
CsPbBr3 films were measured by energy dispersive X-ray spectroscopy (EDX)
149
(shown in Fig. S2), and the influence of the molar ratio on the atomic percentages is
150
given in Fig. S3. It can be seen that the deposited CsPbBr3 films with the molar ratios
7
151
of 0.9:1.0 is closer to the correct stoichiometry of CsPbBr3. It should be noted that the
152
crucible heating temperature of 450°C and the substrate temperature of 300°C are
153
optimized to obtain a good quality film and high performance cell (as shown in Fig.
154
S4 and S5).
155
In Fig. 3a, the deposited CsPbBr3 films exhibit an absorbance onset at ～530 nm
156
in their UV−Vis absorbance spectra. The optical gap of the CsPbBr3 films was
157
determined to be 2.33-2.34 eV (near to the reported value of 2.3 eV [15,16,50]) by the
158
Tauc plots, as shown in Fig. 3b. Photoluminescence (PL) and time-resolved PL decay
159
measurements (TRPL) are shown in Fig. 3c and d, respectively. In Fig. 3c, the PL
160
peaks from the CsPbBr3 films are at 549-551 nm (corresponding to 2.26-2.25 eV),
161
which is close to their optical band gap. The slight difference of peak positions might
162
be due to the difference of the stoichiometry of the CsPbBr3 films. The PL intensity
163
increases as the molar ratio of PbBr2 to CsBr increases from 0.6:1 to 0.9:1 and then
164
drops as the molar ratio further increases to 1.1:1. The CsPbBr3 film deposited with
165
the molar ratio of 0.9:1 has the strongest PL intensity.
166
Figure 3d shows the PL decay of the CsPbBr3 films deposited with different molar
167
ratios of PbBr2 to CsBr. Using a biexponential decay function (equation 1), the PL
168
decay curves were fitted to determine the decay times of the fast (τ1) and slow (τ2)
169
components. The PL decay is attributed to recombination at the defects in the films
170
[51,52]. The average carrier lifetimes were calculated (equation 2) to be 1.2, 2.1, 2.4,
171
5.06, 3.55, and 3.21 ns for the CsPbBr3 films deposited with different molar ratios
172
(PbBr2 to CsBr) of 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, and 1.1:1, respectively [53,54].
8
173
CsPbBr3 film deposited with a 0.9:1 molar ratio has the longest carrier lifetime. The
174
PL intensity and lifetime results demonstrate that the CsPbBr3 film has the best
175
optoelectronic properties at the molar ratio of 0.9:1, i.e. this film shows the highest
176
efficiency of photo-generating carrier and less defects [55-58].
f (t ) = ∑ Ai exp( −t / τ i ) + B
177 178
(1)
i
179 180
τ ave
181
∑ Aiτ i = ∑ Aiτ i
2
(2)
182 183
The deposited CsPbBr3 films (350 nm thick) were used as the light absorption
184
layer to fabricate planar CsPbBr3 perovskite solar cells with the structure
185
FTO/c-TiO2/CsPbBr3/spiro-MeOTAD/Au. We first studied the influence of the molar
186
ratio of CsBr to PbBr2 on the photovoltaic performance of the CsPbBr3 solar cells.
187
The current density-voltage (J-V) curves and the power conversion efficiencies (PCE)
188
of the CsPbBr3 cells are given in Fig. 4a and b, respectively. Compared with its effect
189
on the short-circuit current density (Jsc), the molar ratio has a larger influence on
190
open-circuit voltage (Voc). As the molar ratio increases from 0.6:1 to 0.9:1, the PCE of
191
the best devices increases from 5.66% to 7.14%, and then drops to 3.57% with further
192
increase of the molar ratio to 1.1:1 (the photovoltaic parameters are also given in
193
Table S1). The average PCE (for 20 cells) has a similar variation trend compared to
194
the maximum PCE. The 0.9:1 molar ratio is optimal for obtaining the best cell
195
performance.
196
The melting point of CsBr is 630 °C, and that of PbBr2 is 370.6 °C. Usually, two
197
different melting point materials cannot be thermally evaporated in one crucible or 9
198
boat.
199
materials to deposit CsPbBr3 films. To reveal the evaporation mechanism, we
200
performed XRD measurements of CsBr powder, PbBr2 powder, the heated mixture of
201
CsBr and PbBr2 powder in the crucible (the heating temperature of the quartz crucible
202
in our experiment is around 450°C), and the deposited CsPbBr3 film without substrate
203
heating, using the optimal 0.9:1 molar ratio, as shown in Fig. 5a. It can be seen that
204
the heated mixture of CsBr and PbBr2 powders has changed into CsPbBr3 and
205
Cs4PbBr6 (and less PbBr2 and some intermediate phase), while the as-deposited film
206
on the non-heated substrate contains mainly the CsPbBr3 phase. Also, since the
207
heating temperature of the crucible (450°C) is far below the melting point of CsBr, in
208
principle, CsBr should not be evaporated in our case. This was further proven by our
209
single CsBr evaporation experiment, in which the evaporation rate of CsBr is 0
210
detected by a quartz crystal monitor. This demonstrates that there is no CsBr gas in
211
the vacuum deposition chamber, and during evaporation, the CsBr and PbBr2 react
212
and the product (if CsPbBr3) then congruently evaporates, or at least decomposes (if
213
Cs4PbBr6) to give CsPbBr3. So, we suggest the mechanism of the one-crucible VTE is
214
(as shown in Fig. 5b): First, the CsBr and PbBr2 powder chemically react upon
215
heating to produce CsPbBr3 and Cs4PbBr6:
216
However, we realized the single-crucible thermal evaporation of these two
CsBr + PbBr2 → CsPbBr3 (Cs4PbBr6)
(3)
217
Then, the solid CsPbBr3 (Cs4PbBr6) is heated to produce mainly CsPbBr3 and
218
Cs4PbBr6 gas. These gases arrive at the heated substrate surface (300 °C) and
219
crystallizes to form CsPbBr3 grains and CsPbBr3 film. A more detailed mechanism of
10
220
the chemical reaction and evaporation of the mixture need further study.
221
Since the evaporation precursor is a mixture of PbBr2 and CsBr powders, which
222
was pressed to tablets, the chemical reaction between PbBr2 and CsBr powder is not
223
complete due to the solid phase reaction, and there still are some PbBr2 and CsBr in
224
the mixture. Besides, the melting point of PbBr2 is less than the thermal evaporation
225
temperature (450°C, heating temperature of the crucible), if there is redundant PbBr2
226
in the mixture, it will evaporate and join into the deposited CsPbBr3 film, resulting
227
inferior film quality and properties. While the redundant CsBr in the mixture will not
228
evaporate due to its higher melting point compared with the thermal evaporation
229
temperature, it has no effect on the deposited CsPbBr3 film. Therefore, evaporating
230
the precursor mixture with slightly low PbBr2 content (optimal 0.9:1 molar ratio of
231
PbBr2 to CsBr) results in a purer phase CsPbBr3 film.
232
The surface morphology of the CsPbBr3 films with different thicknesses were
233
investigated by scanning electron microscopy (SEM), as shown in Fig. 6. The
234
CsPbBr3 films are composed of dense CsPbBr3 grains and there is full or almost full
235
surface coverage, and good homogeneity. The CsPbBr3 grain size increases as the
236
thickness of the CsPbBr3 film increases from 350 to 650 nm, and it decreases with
237
further increase of the thickness to 850 nm. This reflects two different film growth
238
mechanisms: When the thickness is less than 650 nm, the film growth is from the
239
growth and fusion of existing CsPbBr3 grains. When the thickness exceeds 650 nm,
240
small grains grow on the large grains and their boundaries. This could be explained by
241
a relatively high density of nucleation centers which might suggest a high (structural)
11
242
defect density on the larger grain in the 650 nm film [59,60],
seen also in the
243
cross-sectional SEM images of 650 and 850 nm thick CsPbBr3 films (Fig. S6).
244
Figure 7a shows a cross-sectional SEM image of a CsPbBr3 solar cell fabricated
245
with a 650 nm thick CsPbBr3 film. The CsPbBr3 grains are very large (0.6-2 µm) and
246
span the entire film thickness. The grains are intimately connected with fused grain
247
boundaries. These features are beneficial for photogenerated carrier transport, which
248
is desired for photovoltaic devices.
249
The thickness of the light-absorbing layer plays an important role in determining
250
the performance of a thin-film solar cell [61]. An appropriate thickness of the
251
absorption layer is a balance between the light absorption and photogenerated carrier
252
collection. The effect of CsPbBr3 film thickness on the photovoltaic performance of
253
CsPbBr3 solar cells was studied. The measured current density-voltage (J-V) curves
254
and photovoltaic parameters of the best-performing devices with different CsPbBr3
255
layer thicknesses are shown in Fig. 7b and Table 1, respectively. The Voc, Jsc, FF, and
256
PCE statistics (20 cells for each thickness) are plotted as a function of CsPbBr3 film
257
thickness in Fig. S7. The thickness affects the open-circuit voltage Voc, Jsc, FF, and,
258
therefore, the PCE of the cells. The optimal thickness of the CsPbBr3 film is 650 nm,
259
for which the cell has a Voc of 1.37 V, Jsc of 7.79 mA/cm2, FF of 0.81, and high PCE
260
of 8.65%, the highest efficiency reported for planar CsPbBr3 cells without interface
261
modification or doping [15-33]. The preparation methods, structures, and PCEs of
262
CsPbBr3 solar cells fabricated by us and other researchers are listed in Table 2. This
263
method also demonstrates a good repeatability, as shown in Fig. S8.
12
264
Although thicker CsPbBr3 films increase the light absorption, the series resistance
265
and carrier recombination also increase, resulting in lower PCE. This is further
266
demonstrated by EIS measurements of the CsPbBr3 cells (in the dark at a bias close to
267
Voc). The Nyquist plots of the cells are given in Fig. 8, which also includes the fitting
268
curves using an equivalent circuit model (fitted parameters are listed in Table S2)
269
[62,63]. The series resistance RS and recombination resistance Rrec of the cells with
270
CsPbBr3 film thickness of 350, 650, and 850 nm are 11.8/565.6, 15.6/976.5,
271
22.9/173.4 Ω, respectively. The increase of RS is mainly from the increase of the
272
thickness of the CsPbBr3 films. The cell with the 850 nm thick CsPbBr3 film has the
273
minimum Rrec, indicating serious carrier recombination.
274
Figure 7c gives a comparison of the J-V curves for the best CsPbBr3 cell measured
275
in the reverse and forward scan directions. There is only a small difference between
276
the J-V curves (～3.5% difference in efficiency), reflecting a good equilibrium
277
between electron and hole transfer in the cell. In Fig. 7d, the best cell has a high
278
external quantum efficiency (EQE) of ～90% across the 370-520 nm wavelength
279
range. Integrating the overlap of the IPCE spectra with the AM 1.5G solar photon flux
280
yields photocurrents of 7.56 mA/cm2, which is in agreement with the measured Jsc
281
values from the J–V measurements. The CsPbBr3 solar cell also shows good
282
long-term storage stability: its PSC exhibited nearly no degradation (maintained
283
approximately 96% of its initial efficiency) even after storage for two months under
284
ambient conditions (30% relative humidity) without encapsulation, as shown in Fig.
285
7e.
13
286
We also studied the fabrication of CsPbBr3 solar cells by two-crucible
287
double-source VTE. The highest PCE we obtained was 6.95% (under optimal
288
structure and experimental conditions) [46]. Moreover, the as-deposited CsPbBr3 film
289
requires a 500 °C high temperature annealing process compared to the 300 °C
290
substrate temperature of the single source sample. This, together with the higher
291
efficiency of the single source cells indicate the superiority of the latter technique.
292 293 294
4.
Conclusions
295
In conclusion, we have developed a facile one-crucible single-source VTE method
296
to evaporate two different melting points materials CsBr and PbBr2 to deposit
297
high-quality, compact CsPbBr3 perovskite films with good uniformity and large grains.
298
The molar ratio of PbBr2 to CsBr in the mixture in the crucible is a critical factor
299
influencing the stoichiometry, structure, photoelectrical, and photovoltaic properties
300
of the CsPbBr3 films. Planar CsPbBr3 perovskite solar cells were fabricated giving a
301
PCE as high as 8.65%. The CsPbBr3 cell has good long-term stability in air without
302
encapsulation. Furthermore, the performance of CsPbBr3 cell fabricated by
303
one-crucible VTE is better than that of two-crucible VTE. The one-crucible VTE
304
method simplifies the process of traditional multi-crucible VTE and gives better films.
305
Our study opens up the possibility to deposit multi-element compound films by
306
one-crucible VTE of materials with different melting points.
307 308 14
309
Appendix A. Supporting information
310 311
Acknowledgments
312
This work was supported by The National Key Research and Development
313
Program of China (No. 2016YFA0202403); National University Research Fund of
314
China (GK261001009); Chinese National 1000-talent-plan program; and National
315
Natural Science Foundation of China (61604091and 91733301). We also thank Prof.
316
Gary from Weizmann Institute of Science (Isreal) for his beneficial discussion and
317
revision.
318 319
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533 534
24
535
Figure caption
536
Fig. 1. (a) Illustration of the single-source vacuum thermal evaporation deposition
537
setup for CsPbBr3 films. (b) Photo of a typical deposited CsPbBr3 film sample.
538
Fig. 2. XRD patterns of the deposited CsPbBr3 films with different molar ratios of
539
PbBr2 to CsBr in their mixture.
540
Fig. 3. (a) UV−Vis absorption spectra, (b) Tauc plots: (Ahν)2 vs hν curves, (c)
541
Photoluminescence (PL), (d) Time-resolved PL decay of the CsPbBr3 films deposited
542
with various molar ratios of PbBr2 to CsBr.
543
Fig. 4. (a) J-V curves and (b) the PCE of the CsPbBr3 cells fabricated with different
544
molar ratios of CsBr to PbBr2.
545
Fig. 5. (a) XRD pattern of CsBr powder, PbBr2 powder, a heated mixture of CsBr and
546
PbBr2 powder, and the deposited CsPbBr3 film for 0.9:1 molar ratio without substrate
547
heating. (b) Schematic diagram of the mechanism of single-source thermal
548
evaporation of CsBr and PbBr3 to deposit CsPbBr3 film.
549
Fig. 6. Surface SEM images of the deposited CsPbBr3 films with different thicknesses
550
from 350 to 850 nm.
551
Fig. 7.
552
cell: FTO/c-TiO2/CsPbBr3/spiro-MeOTAD/Au. (b) J-V curves of the planar CsPbBr3
553
perovskite solar cells with different CsPbBr3 film thicknesses. (c) J-V curves of the
554
best cell (for 650 nm thick CsPbBr3 film) measured in the reverse and forward scan
555
directions. (d) EQE spectra and the integrated photocurrent of the best cell. (e)
556
Stability (in air at 30% relative humidity without encapsulation) of the fabricated
(a) Cross-sectional SEM image of a typical planar CsPbBr3 perovskite solar
25
557
CsPbBr3 solar cell.
558
Fig. 8. Nyquist plots of CsPbBr3 solar cells with different CsPbBr3 film thicknesses
559
measured in dark and at a bias close to the open-circuit voltage.
560 561
26
562
Figure 1
563 564
(a)
565
(b)
566 567 568 569 570 571
572
573 574
Figure 2
575 576 577 578
579
580
581
582
583
27
584
Figure 3
585 586
(a)
(b)
587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605
(c)
(d)
606 607 608
28
609 610 611 612 613 614 615 616 617 618 619
Figure 4 (a)
(b)
620 621 622 623 624
Figure 5
625 626
(a)
(b)
627 628 629 630 631 632 633 634 635 636 637 638 639 640 641
29
642
Figure 6
643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663
30
664
Figure 7
665 666
(a)
667 668 669 670 671 672 673 674
(b)
(c)
675 676 677 678 679 680 681 682 683 684 685
(e)
(d)
686
31
687
Table 1. Photovoltaic performance of CsPbBr3 solar cells with different CsPbBr3 film
688
thicknesses.
689 690 691
Thickness [nm]
Jsc [mAcm-2]
Voc [V]
FF
η [%]
350
6.46
1.40
0.79
7.14
550
7.39
1.25
0.79
7.28
600
7.87
1.37
0.78
8.38
650
7.79
1.37
0.81
8.65
700
7.87
1.29
0.70
7.15
850
7.60
1.25
0.70
6.67
692 693 694 695 696 697 698
Table 2. The performance of CsPbBr3 perovskite solar cells with different structures
699
fabricated by different methods.
700
Fabrication method
Structure
PCE [%]
Reference
Evaporation
Planar TiO2
8.65
This work
Evaporation
Planar TiO2
6.95
[46]
703
Evaporation
Planar ZnO
7.78
[19]
704
Laser Deposition
Meso-TiO2
6.3
[33]
Solution
Meso-TiO2
5.95
[14]
Solution
Meso-TiO2
6.7
[15]
Solution
Meso-TiO2, Modification
9.72
[19]
707
Solution
Meso-TiO2, Spectra
10.26
[20]
708
Solution
Meso-TiO2, Doping
10.14
[21]
Solution
Meso-TiO2, Modification
10.6
[22]
Solution
Meso-TiO2, Modification
10.03
[25]
Solution
Meso-TiO2, Doping
9.63
[26]
701 702
705 706
709 710
32
711
Figure 8
712 713 714 715 716 717 718 719
33
Highlights High-quality CsPbBr3 films are deposited by single-source evaporation. Molar ratio of PbBr2 to CsBr influences stoichiometry and photovoltaic properties. Thickness of CsPbBr3 films affects its photovoltaic properties. Planar CsPbBr3 solar cells with high efficiency of 8.65% are fabricated.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34149-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.152903
Reference:
JALCOM 152903
To appear in:
Journal of Alloys and Compounds
Received Date: 28 August 2019 Revised Date:
31 October 2019
Accepted Date: 2 November 2019
Please cite this article as: J. Li, R. Gao, F. Gao, J. Lei, H. Wang, X. Wu, J. Li, H. Liu, X. Hua, S.(F.) Liu, Fabrication of efficient CsPbBr3 perovskite solar cells by single-source thermal evaporation, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152903. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Fabrication of efficient CsPbBr3 perovskite solar cells by single-source thermal
2
evaporation
3 4
Juan Lia, Rongrong Gaoa, Fei Gao a*, Jie Leia, Haoxu Wanga, Xin Wua, Jianbo Lia,
5
Hao Liua, Xiaodong Huaa, and Shengzhong (Frank) Liua,b*
6 7
a
8
Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi
9
Engineering Lab for Advanced Energy Technology, School of Materials Science and
Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of
10
Engineering, Shaanxi Normal University, Xi’an 710119, China.
11
b
12
Physics, Chinese Academy of Sciences, Dalian 116023, China
Dalian National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical
13 14
*Corresponding authors:
15
E-mail: [email protected] (F. Gao), [email protected] (S. F. Liu).
16 17 18 19 20 21
1
22
Abstract
23
It is promising to improve the stability of organic–inorganic hybrid halide perovskite
24
solar cells by using all-inorganic perovskite materials. Herein, a facile one-crucible
25
single-source vacuum thermal evaporation (VTE) approach is developed, which is used
26
to evaporate two different melting points materials CsBr (630 °C) and PbBr2 (370.6 °C)
27
to deposit high-quality inorganic CsPbBr3 perovskite films. Molar ratio of PbBr2 to
28
CsBr in their mixture in the crucible is a key factor influencing the stoichiometry,
29
structure, photoelectrical and photovoltaic properties of the CsPbBr3 films. The other
30
important factor is the thickness of the CsPbBr3 films. High-quality CsPbBr3 films with
31
good uniformity and compact and large grains are prepared. Planar CsPbBr3 perovskite
32
solar cells are fabricated giving a high power conversion efficiency of 8.65%. The
33
fabricated CsPbBr3 solar cells exhibit a good stability in air without encapsulation. This
34
study opens up the possibility to deposit multi-element compound thin films by facile
35
single-source VTE of different melting points materials.
36 37
Keywords: CsPbBr3 films, Single-source evaporation, Molar ratio, Thickness, Solar
38
cells
39 40 41
2
42
1.
Introduction
43
It is promising to improve the stability of organic–inorganic hybrid halide
44
perovskite solar cells by using all-inorganic perovskite materials. In these years, the
45
solar cells based on inorganic halide perovskites CsMX3 (M = Pb or Sn; X = I, Br, Cl
46
or mixed halides) materials [1,2], such as
47
[11,12], CsPbIBr2 [13,14], CsPbBr3 [15-33], Cs3Bi2I9 [34], RbPbI3 [35],
48
CsPb0.9Sn0.1IBr2 [36，37], Cs0.925K0.075PbI2Br [38], and CsSn0.5Ge0.5I3 [39] have been
49
fabricated and better device performance have been obtained. Besides, Bulbak et al.
50
research indicates that all-inorganic lead bromide perovskite materials exhibit as well
51
as the organic one [16]. Compared with other inorganic halide perovskites, the
52
CsPbBr3 is more stable [10,12,17,22,23,40].
CsSnI3 [3-6], CsPbI3 [7-10], CsPbI2Br
53
An appropriate approach to prepare CsPbBr3 films is important in order to
54
achieve good film quality and excellent device properties. Usually, chemical solution
55
method was used to fabricate CsPbBr3 films/solar cells [15-33]. Solution method does
56
not need expensive equipments, however, it is challenging to fabricate uniform and
57
large-area films by this method. Vacuum thermal evaporation (VTE) is widely used to
58
prepare various thin films with large areas and good uniformity. In comparison with
59
solution methods, there are relatively few studies about the fabrication of perovskite
60
solar cells by VTE. Liu et al. first used dual-source VTE of CH3NH3I and PbCl2 to
61
fabricate CH3NH3PbI3-xClx solar cells with high efficiency of 15% [41]. They found
62
that the films deposited by VTE are very uniform, while the films prepared by
63
solution method coat the substrate only partially: there are voids in the
3
64
solution-processed films, which extend to the compact TiO2 layer on the FTO glass
65
substrate. Due to high vapor pressure and low density, CH3NH3I easily diffuses in the
66
vacuum chamber. It is very difficult to monitor/control the deposition rate of
67
CH3NH3I during VTE. In order to solve this problem, the methods of
68
sequential/alternating evaporation [42,43], separating evaporation [44], and hybrid
69
evaporation [45]
70
two-source VTE to fabricate high performance planar CsPbBr3 solar cells with
71
efficiencies of 7.78% and 6.95, respectively [19,46]. Dual-source VTE was also used
72
to fabricate CsPbI3 [9], CsPbI2Br [12], CsPbIBr2 [14], and MA1-xCsxPbI3 [47]
73
perovskite solar cells. Lidón et al. used 4 sources of MAI, CsBr, FAI, and PbI2 to
74
deposit triple-cation Cs0.5FA0.4MA0.1Pb(I0.83Br0.17)3 perovskite films and fabricated a
75
16% efficiency solar cell [48].
has been developed. Recently, Chen et al. and we also used the
76
In multi-source thermal evaporation, the ratio of the evaporation rates of source
77
materials is a key factor affecting the stoichiometry of the deposited film, and the
78
efficiency of the solar cells. There are several factors, such as the pressure of the
79
vacuum chamber, heating power of the crucibles, and the quantity and distribution of
80
evaporation materials in the crucibles, that influence the ratio of evaporation rates.
81
During the lengthy deposition process (usually 2-3 h), it is difficult to maintain a
82
correct ratio of evaporation rates of source materials throughout the process because
83
the experimental conditions fluctuate. Adjustment and control of the ratio of
84
evaporation rates of source materials is difficult and time-consuming.
4
85
In this study, we report a facile and effective one crucible single-source VTE
86
approach to deposit high-quality CsPbBr3 films. We mix CsBr and PbBr2 powders as
87
the evaporation materials, which are then pressed into tablets and put into a quartz
88
crucible, and the stoichiometry of the deposited CsPbBr3 films can be determined by
89
the molar ratio of PbBr2 to CsBr in their mixture. We studied the effects of the molar
90
ratio of CsBr to PbBr2 and the thickness of the CsPbBr3 films on the film quality and
91
corresponding solar cell performance. Through optimization, high-quality CsPbBr3
92
films were prepared, and stable planar CsPbBr3 solar cell with high efficiency of
93
8.65% was fabricated.
94 95
2.
Experimental
96
Planar CsPbBr3 inorganic perovskite solar cells were fabricated on FTO-coated
97
glass: FTO/c-TiO2/CsPbBr3/Spiro-MeOTAD/Au. The detailed processes of FTO glass
98
substrate cleaning and the preparation of compact TiO2 layer can be found in our
99
previous report [46]. The CsPbBr3 light absorption layer was deposited on the TiO2
100
layer by thermally evaporating the mixture of CsBr and PbBr2 in vacuum, as shown in
101
Fig. 1a. The mixture of CsBr and PbBr2 powders was used as the evaporation materials,
102
which was pressed into tablets (with the pressure of ~ 10 MPa) and put into a quartz
103
crucible, and the stoichiometry of the deposited CsPbBr3 films can be determined by
104
the molar ratio of PbBr2 to CsBr in their mixture. The substrate was kept at 300 °C and
105
rotated to obtain uniform coating during the deposition process. The evaporation rate
106
(deposition speed, ～0.06 nm/s) was monitored by a quartz crystal monitor near to the
5
107
substrate, which was calibrated by the ratio of actual film thickness to deposition time.
108
The temperature of the quartz crucible is about 450 °C. For hole-transport layer
109
deposition, 90 mg mL−1 spiro-OMeTAD solution with 36 µL 4-tert-butylpyridine and
110
22 µL lithium bisimide (trifluoromethylsulfonyl) of 520 mg mL−1 in acetonitrile was
111
spin-coated on CsPbBr3 films at 4000 rpm for 30 s. A 70 nm thick gold electrode was
112
thermally evaporated on the spiro-MeOTAD film and the area of the solar cells is 3 × 3
113
mm2.
114
XRD spectra were measured by DCIP (XRD, DX-2700) using Cu Kα radiation.
115
SEM images were obtained by field emission SEM (FESEM, SU-8020, Hitachi).
116
EDX analysis was performed by X-ray energy dispersive spectrometer (EX-270,
117
Horiba). UV-Vis spectra were measured by a spectrophotometer (Lambda 950;
118
Perkin-Elmer). Time-resolved PL spectra were measured using an Edinburgh
119
Instruments (FLS920) fluorescence spectrometer. J-V curves of CsPbBr3 solar cells
120
were measured under 100 mWcm-2 using Keithley 2400 source at 300 K in air. The
121
light power output of AM 1.5G solar simulator was calibrated using an
122
NREL-KG5-filtered silicon cell. EQE measurements were performed on a QTest
123
Station 500TI system (Crowntech. Inc., USA). Electrochemical impedance
124
spectroscopy (EIS) measurements were performed by a ZAHNER ENNIUM (PP211)
125
device in the frequency range of 10-10×106 Hz with an AC perturbation signal of 20
126
mV.
127 128
6
129
3.
Results and discussion
130
As shown in Fig. 1b and Supplementary Materials Fig. S1, the as-deposited
131
CsPbBr3 films appear yellow and uniform. The stoichiometry and crystallinity of the
132
CsPbBr3 films are crucial for their photovoltaic properties. The stoichiometry of the
133
deposited CsPbBr3 films are determined by the molar ratios of PbBr2 to CsBr in their
134
mixtures.
135
First, we studied the effect of the molar ratio on the crystallinity and stoichiometry
136
of the deposited CsPbBr3 films (with an optimal substrate temperature of 300°C).
137
Figure 2 shows X-ray diffraction (XRD) patterns of the CsPbBr3 films (350 nm thick)
138
deposited using different molar ratios of PbBr2 to CsBr. The peaks from CsPbBr3
139
located at 2θ = 16.26 º, 21.6 º, 26.34 º, 30.37 º, 37.74 º, and 44.15 º correspond to the
140
(100), (110), (111), (002), (211), and (220) crystal planes diffraction, respectively
141
(JCPDs no. 18-0364). The three main peaks corresponding to (110), (002), and (211)
142
become stronger with increasing molar ratio (0.8:1-1.1:1), indicating better
143
crystallinity of the deposited CsPbBr3 films. The diffraction peaks located at 2θ = 12.4
144
º, 19.9 º, 25.3 º, 28.5 º, and 30.1 º are from Cs4PbBr6 (JCPDs no. 73-2478) [49]. It is
145
found that as the molar ratio increases, the diffraction peaks of Cs4PbBr6 weaken and
146
almost disappear, reflecting a purer CsPbBr3 phase obtained at higher molar ratio
147
(0.9:1-1.1:1). The atomic percentages of Cs, Pb, and Br elements in the deposited
148
CsPbBr3 films were measured by energy dispersive X-ray spectroscopy (EDX)
149
(shown in Fig. S2), and the influence of the molar ratio on the atomic percentages is
150
given in Fig. S3. It can be seen that the deposited CsPbBr3 films with the molar ratios
7
151
of 0.9:1.0 is closer to the correct stoichiometry of CsPbBr3. It should be noted that the
152
crucible heating temperature of 450°C and the substrate temperature of 300°C are
153
optimized to obtain a good quality film and high performance cell (as shown in Fig.
154
S4 and S5).
155
In Fig. 3a, the deposited CsPbBr3 films exhibit an absorbance onset at ～530 nm
156
in their UV−Vis absorbance spectra. The optical gap of the CsPbBr3 films was
157
determined to be 2.33-2.34 eV (near to the reported value of 2.3 eV [15,16,50]) by the
158
Tauc plots, as shown in Fig. 3b. Photoluminescence (PL) and time-resolved PL decay
159
measurements (TRPL) are shown in Fig. 3c and d, respectively. In Fig. 3c, the PL
160
peaks from the CsPbBr3 films are at 549-551 nm (corresponding to 2.26-2.25 eV),
161
which is close to their optical band gap. The slight difference of peak positions might
162
be due to the difference of the stoichiometry of the CsPbBr3 films. The PL intensity
163
increases as the molar ratio of PbBr2 to CsBr increases from 0.6:1 to 0.9:1 and then
164
drops as the molar ratio further increases to 1.1:1. The CsPbBr3 film deposited with
165
the molar ratio of 0.9:1 has the strongest PL intensity.
166
Figure 3d shows the PL decay of the CsPbBr3 films deposited with different molar
167
ratios of PbBr2 to CsBr. Using a biexponential decay function (equation 1), the PL
168
decay curves were fitted to determine the decay times of the fast (τ1) and slow (τ2)
169
components. The PL decay is attributed to recombination at the defects in the films
170
[51,52]. The average carrier lifetimes were calculated (equation 2) to be 1.2, 2.1, 2.4,
171
5.06, 3.55, and 3.21 ns for the CsPbBr3 films deposited with different molar ratios
172
(PbBr2 to CsBr) of 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, and 1.1:1, respectively [53,54].
8
173
CsPbBr3 film deposited with a 0.9:1 molar ratio has the longest carrier lifetime. The
174
PL intensity and lifetime results demonstrate that the CsPbBr3 film has the best
175
optoelectronic properties at the molar ratio of 0.9:1, i.e. this film shows the highest
176
efficiency of photo-generating carrier and less defects [55-58].
f (t ) = ∑ Ai exp( −t / τ i ) + B
177 178
(1)
i
179 180
τ ave
181
∑ Aiτ i = ∑ Aiτ i
2
(2)
182 183
The deposited CsPbBr3 films (350 nm thick) were used as the light absorption
184
layer to fabricate planar CsPbBr3 perovskite solar cells with the structure
185
FTO/c-TiO2/CsPbBr3/spiro-MeOTAD/Au. We first studied the influence of the molar
186
ratio of CsBr to PbBr2 on the photovoltaic performance of the CsPbBr3 solar cells.
187
The current density-voltage (J-V) curves and the power conversion efficiencies (PCE)
188
of the CsPbBr3 cells are given in Fig. 4a and b, respectively. Compared with its effect
189
on the short-circuit current density (Jsc), the molar ratio has a larger influence on
190
open-circuit voltage (Voc). As the molar ratio increases from 0.6:1 to 0.9:1, the PCE of
191
the best devices increases from 5.66% to 7.14%, and then drops to 3.57% with further
192
increase of the molar ratio to 1.1:1 (the photovoltaic parameters are also given in
193
Table S1). The average PCE (for 20 cells) has a similar variation trend compared to
194
the maximum PCE. The 0.9:1 molar ratio is optimal for obtaining the best cell
195
performance.
196
The melting point of CsBr is 630 °C, and that of PbBr2 is 370.6 °C. Usually, two
197
different melting point materials cannot be thermally evaporated in one crucible or 9
198
boat.
199
materials to deposit CsPbBr3 films. To reveal the evaporation mechanism, we
200
performed XRD measurements of CsBr powder, PbBr2 powder, the heated mixture of
201
CsBr and PbBr2 powder in the crucible (the heating temperature of the quartz crucible
202
in our experiment is around 450°C), and the deposited CsPbBr3 film without substrate
203
heating, using the optimal 0.9:1 molar ratio, as shown in Fig. 5a. It can be seen that
204
the heated mixture of CsBr and PbBr2 powders has changed into CsPbBr3 and
205
Cs4PbBr6 (and less PbBr2 and some intermediate phase), while the as-deposited film
206
on the non-heated substrate contains mainly the CsPbBr3 phase. Also, since the
207
heating temperature of the crucible (450°C) is far below the melting point of CsBr, in
208
principle, CsBr should not be evaporated in our case. This was further proven by our
209
single CsBr evaporation experiment, in which the evaporation rate of CsBr is 0
210
detected by a quartz crystal monitor. This demonstrates that there is no CsBr gas in
211
the vacuum deposition chamber, and during evaporation, the CsBr and PbBr2 react
212
and the product (if CsPbBr3) then congruently evaporates, or at least decomposes (if
213
Cs4PbBr6) to give CsPbBr3. So, we suggest the mechanism of the one-crucible VTE is
214
(as shown in Fig. 5b): First, the CsBr and PbBr2 powder chemically react upon
215
heating to produce CsPbBr3 and Cs4PbBr6:
216
However, we realized the single-crucible thermal evaporation of these two
CsBr + PbBr2 → CsPbBr3 (Cs4PbBr6)
(3)
217
Then, the solid CsPbBr3 (Cs4PbBr6) is heated to produce mainly CsPbBr3 and
218
Cs4PbBr6 gas. These gases arrive at the heated substrate surface (300 °C) and
219
crystallizes to form CsPbBr3 grains and CsPbBr3 film. A more detailed mechanism of
10
220
the chemical reaction and evaporation of the mixture need further study.
221
Since the evaporation precursor is a mixture of PbBr2 and CsBr powders, which
222
was pressed to tablets, the chemical reaction between PbBr2 and CsBr powder is not
223
complete due to the solid phase reaction, and there still are some PbBr2 and CsBr in
224
the mixture. Besides, the melting point of PbBr2 is less than the thermal evaporation
225
temperature (450°C, heating temperature of the crucible), if there is redundant PbBr2
226
in the mixture, it will evaporate and join into the deposited CsPbBr3 film, resulting
227
inferior film quality and properties. While the redundant CsBr in the mixture will not
228
evaporate due to its higher melting point compared with the thermal evaporation
229
temperature, it has no effect on the deposited CsPbBr3 film. Therefore, evaporating
230
the precursor mixture with slightly low PbBr2 content (optimal 0.9:1 molar ratio of
231
PbBr2 to CsBr) results in a purer phase CsPbBr3 film.
232
The surface morphology of the CsPbBr3 films with different thicknesses were
233
investigated by scanning electron microscopy (SEM), as shown in Fig. 6. The
234
CsPbBr3 films are composed of dense CsPbBr3 grains and there is full or almost full
235
surface coverage, and good homogeneity. The CsPbBr3 grain size increases as the
236
thickness of the CsPbBr3 film increases from 350 to 650 nm, and it decreases with
237
further increase of the thickness to 850 nm. This reflects two different film growth
238
mechanisms: When the thickness is less than 650 nm, the film growth is from the
239
growth and fusion of existing CsPbBr3 grains. When the thickness exceeds 650 nm,
240
small grains grow on the large grains and their boundaries. This could be explained by
241
a relatively high density of nucleation centers which might suggest a high (structural)
11
242
defect density on the larger grain in the 650 nm film [59,60],
seen also in the
243
cross-sectional SEM images of 650 and 850 nm thick CsPbBr3 films (Fig. S6).
244
Figure 7a shows a cross-sectional SEM image of a CsPbBr3 solar cell fabricated
245
with a 650 nm thick CsPbBr3 film. The CsPbBr3 grains are very large (0.6-2 µm) and
246
span the entire film thickness. The grains are intimately connected with fused grain
247
boundaries. These features are beneficial for photogenerated carrier transport, which
248
is desired for photovoltaic devices.
249
The thickness of the light-absorbing layer plays an important role in determining
250
the performance of a thin-film solar cell [61]. An appropriate thickness of the
251
absorption layer is a balance between the light absorption and photogenerated carrier
252
collection. The effect of CsPbBr3 film thickness on the photovoltaic performance of
253
CsPbBr3 solar cells was studied. The measured current density-voltage (J-V) curves
254
and photovoltaic parameters of the best-performing devices with different CsPbBr3
255
layer thicknesses are shown in Fig. 7b and Table 1, respectively. The Voc, Jsc, FF, and
256
PCE statistics (20 cells for each thickness) are plotted as a function of CsPbBr3 film
257
thickness in Fig. S7. The thickness affects the open-circuit voltage Voc, Jsc, FF, and,
258
therefore, the PCE of the cells. The optimal thickness of the CsPbBr3 film is 650 nm,
259
for which the cell has a Voc of 1.37 V, Jsc of 7.79 mA/cm2, FF of 0.81, and high PCE
260
of 8.65%, the highest efficiency reported for planar CsPbBr3 cells without interface
261
modification or doping [15-33]. The preparation methods, structures, and PCEs of
262
CsPbBr3 solar cells fabricated by us and other researchers are listed in Table 2. This
263
method also demonstrates a good repeatability, as shown in Fig. S8.
12
264
Although thicker CsPbBr3 films increase the light absorption, the series resistance
265
and carrier recombination also increase, resulting in lower PCE. This is further
266
demonstrated by EIS measurements of the CsPbBr3 cells (in the dark at a bias close to
267
Voc). The Nyquist plots of the cells are given in Fig. 8, which also includes the fitting
268
curves using an equivalent circuit model (fitted parameters are listed in Table S2)
269
[62,63]. The series resistance RS and recombination resistance Rrec of the cells with
270
CsPbBr3 film thickness of 350, 650, and 850 nm are 11.8/565.6, 15.6/976.5,
271
22.9/173.4 Ω, respectively. The increase of RS is mainly from the increase of the
272
thickness of the CsPbBr3 films. The cell with the 850 nm thick CsPbBr3 film has the
273
minimum Rrec, indicating serious carrier recombination.
274
Figure 7c gives a comparison of the J-V curves for the best CsPbBr3 cell measured
275
in the reverse and forward scan directions. There is only a small difference between
276
the J-V curves (～3.5% difference in efficiency), reflecting a good equilibrium
277
between electron and hole transfer in the cell. In Fig. 7d, the best cell has a high
278
external quantum efficiency (EQE) of ～90% across the 370-520 nm wavelength
279
range. Integrating the overlap of the IPCE spectra with the AM 1.5G solar photon flux
280
yields photocurrents of 7.56 mA/cm2, which is in agreement with the measured Jsc
281
values from the J–V measurements. The CsPbBr3 solar cell also shows good
282
long-term storage stability: its PSC exhibited nearly no degradation (maintained
283
approximately 96% of its initial efficiency) even after storage for two months under
284
ambient conditions (30% relative humidity) without encapsulation, as shown in Fig.
285
7e.
13
286
We also studied the fabrication of CsPbBr3 solar cells by two-crucible
287
double-source VTE. The highest PCE we obtained was 6.95% (under optimal
288
structure and experimental conditions) [46]. Moreover, the as-deposited CsPbBr3 film
289
requires a 500 °C high temperature annealing process compared to the 300 °C
290
substrate temperature of the single source sample. This, together with the higher
291
efficiency of the single source cells indicate the superiority of the latter technique.
292 293 294
4.
Conclusions
295
In conclusion, we have developed a facile one-crucible single-source VTE method
296
to evaporate two different melting points materials CsBr and PbBr2 to deposit
297
high-quality, compact CsPbBr3 perovskite films with good uniformity and large grains.
298
The molar ratio of PbBr2 to CsBr in the mixture in the crucible is a critical factor
299
influencing the stoichiometry, structure, photoelectrical, and photovoltaic properties
300
of the CsPbBr3 films. Planar CsPbBr3 perovskite solar cells were fabricated giving a
301
PCE as high as 8.65%. The CsPbBr3 cell has good long-term stability in air without
302
encapsulation. Furthermore, the performance of CsPbBr3 cell fabricated by
303
one-crucible VTE is better than that of two-crucible VTE. The one-crucible VTE
304
method simplifies the process of traditional multi-crucible VTE and gives better films.
305
Our study opens up the possibility to deposit multi-element compound films by
306
one-crucible VTE of materials with different melting points.
307 308 14
309
Appendix A. Supporting information
310 311
Acknowledgments
312
This work was supported by The National Key Research and Development
313
Program of China (No. 2016YFA0202403); National University Research Fund of
314
China (GK261001009); Chinese National 1000-talent-plan program; and National
315
Natural Science Foundation of China (61604091and 91733301). We also thank Prof.
316
Gary from Weizmann Institute of Science (Isreal) for his beneficial discussion and
317
revision.
318 319
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533 534
24
535
Figure caption
536
Fig. 1. (a) Illustration of the single-source vacuum thermal evaporation deposition
537
setup for CsPbBr3 films. (b) Photo of a typical deposited CsPbBr3 film sample.
538
Fig. 2. XRD patterns of the deposited CsPbBr3 films with different molar ratios of
539
PbBr2 to CsBr in their mixture.
540
Fig. 3. (a) UV−Vis absorption spectra, (b) Tauc plots: (Ahν)2 vs hν curves, (c)
541
Photoluminescence (PL), (d) Time-resolved PL decay of the CsPbBr3 films deposited
542
with various molar ratios of PbBr2 to CsBr.
543
Fig. 4. (a) J-V curves and (b) the PCE of the CsPbBr3 cells fabricated with different
544
molar ratios of CsBr to PbBr2.
545
Fig. 5. (a) XRD pattern of CsBr powder, PbBr2 powder, a heated mixture of CsBr and
546
PbBr2 powder, and the deposited CsPbBr3 film for 0.9:1 molar ratio without substrate
547
heating. (b) Schematic diagram of the mechanism of single-source thermal
548
evaporation of CsBr and PbBr3 to deposit CsPbBr3 film.
549
Fig. 6. Surface SEM images of the deposited CsPbBr3 films with different thicknesses
550
from 350 to 850 nm.
551
Fig. 7.
552
cell: FTO/c-TiO2/CsPbBr3/spiro-MeOTAD/Au. (b) J-V curves of the planar CsPbBr3
553
perovskite solar cells with different CsPbBr3 film thicknesses. (c) J-V curves of the
554
best cell (for 650 nm thick CsPbBr3 film) measured in the reverse and forward scan
555
directions. (d) EQE spectra and the integrated photocurrent of the best cell. (e)
556
Stability (in air at 30% relative humidity without encapsulation) of the fabricated
(a) Cross-sectional SEM image of a typical planar CsPbBr3 perovskite solar
25
557
CsPbBr3 solar cell.
558
Fig. 8. Nyquist plots of CsPbBr3 solar cells with different CsPbBr3 film thicknesses
559
measured in dark and at a bias close to the open-circuit voltage.
560 561
26
562
Figure 1
563 564
(a)
565
(b)
566 567 568 569 570 571
572
573 574
Figure 2
575 576 577 578
579
580
581
582
583
27
584
Figure 3
585 586
(a)
(b)
587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605
(c)
(d)
606 607 608
28
609 610 611 612 613 614 615 616 617 618 619
Figure 4 (a)
(b)
620 621 622 623 624
Figure 5
625 626
(a)
(b)
627 628 629 630 631 632 633 634 635 636 637 638 639 640 641
29
642
Figure 6
643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663
30
664
Figure 7
665 666
(a)
667 668 669 670 671 672 673 674
(b)
(c)
675 676 677 678 679 680 681 682 683 684 685
(e)
(d)
686
31
687
Table 1. Photovoltaic performance of CsPbBr3 solar cells with different CsPbBr3 film
688
thicknesses.
689 690 691
Thickness [nm]
Jsc [mAcm-2]
Voc [V]
FF
η [%]
350
6.46
1.40
0.79
7.14
550
7.39
1.25
0.79
7.28
600
7.87
1.37
0.78
8.38
650
7.79
1.37
0.81
8.65
700
7.87
1.29
0.70
7.15
850
7.60
1.25
0.70
6.67
692 693 694 695 696 697 698
Table 2. The performance of CsPbBr3 perovskite solar cells with different structures
699
fabricated by different methods.
700
Fabrication method
Structure
PCE [%]
Reference
Evaporation
Planar TiO2
8.65
This work
Evaporation
Planar TiO2
6.95
[46]
703
Evaporation
Planar ZnO
7.78
[19]
704
Laser Deposition
Meso-TiO2
6.3
[33]
Solution
Meso-TiO2
5.95
[14]
Solution
Meso-TiO2
6.7
[15]
Solution
Meso-TiO2, Modification
9.72
[19]
707
Solution
Meso-TiO2, Spectra
10.26
[20]
708
Solution
Meso-TiO2, Doping
10.14
[21]
Solution
Meso-TiO2, Modification
10.6
[22]
Solution
Meso-TiO2, Modification
10.03
[25]
Solution
Meso-TiO2, Doping
9.63
[26]
701 702
705 706
709 710
32
711
Figure 8
712 713 714 715 716 717 718 719
33
Highlights High-quality CsPbBr3 films are deposited by single-source evaporation. Molar ratio of PbBr2 to CsBr influences stoichiometry and photovoltaic properties. Thickness of CsPbBr3 films affects its photovoltaic properties. Planar CsPbBr3 solar cells with high efficiency of 8.65% are fabricated.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: