Collective and individual impacts of the cascade doping of alkali cations in perovskite single crystals

Article abstract of DOI:10.1039/d0tc03085f

Alkali ion (Cs+, Rb+ and K+) incorporated hybrid perovskite polycrystalline films have shown great potential for obtaining highly efficient and stable perovskite solar cells. A study on the intrinsic influence of different alkali ions on perovskites is urgently needed to gain insights into the optoelectronic performance improvement. Single crystals with fewer defects can be ideal candidates for the study of intrinsic properties. Herein, for the first time, we prepared a series of perovskite (FA0.85MA0.15PbI2.55Br0.45) single crystals with Cs+, Rb+, and K+ cascade doping using the inverse temperature crystallization method and systematically characterized their structure and optoelectronic properties, as well as phase, humidity and thermal stability. Our work highlights the structure-property relationship, that is, diversified A-site occupation (especially the FAMACsRb system) can significantly promote the carrier transport behavior of perovskites, as well as achieving better thermal and humidity stability, while K+ doping degrades the intrinsic properties of perovskites, which is probably due to the interstitial occupation of K+. These findings deepen the understanding of the influence of alkali ion doping on the intrinsic properties of perovskite materials, which will provide an exemplary paradigm for further exploration. This journal is

Full text of DOI:10.1039/d0tc03085f

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DOI: 10.1039/D0TC03085F
The collective and individual impacts in perovskite single crystals by cascade doping of alkali
cations
Yu Zhanga, Yuan Huanga, Xiaoge Wangb, Junliang Sunb, Rui Sic, Huanping Zhou*a
a. Y. Zhang, Dr. Y. Huang, Prof. H. Zhou
Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Key
Laboratory of Polymer Chemistry and Physics of Ministry of Education, BIC-ESAT, Department
of Materials Science and Engineering, College of Engineering, Peking University, Beijing
100871, P. R. China.
b. X. Wang, Prof. J. Sun
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R.
China.
c. Prof. R. Si
Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese
Academy of Sciences, Shanghai 201204, P. R. China
*Corresponding author: happy_zhou@pku.edu.cn

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DOI: 10.1039/D0TC03085F
Abstract
The alkali ions (Cs+, Rb+ and K+) incorporated hybrid perovskite polycrystalline films have shown
great potential in obtaining more efficient and stable perovskite solar cells. The intrinsic influence
of different alkali ions on perovskites is urgently needed to gain insight into the optoelectronic
performance improvement. Single crystals with less defects can be an ideal candidate for the
intrinsic property study. Herein, for the first time, we prepared a series of perovskite
(FA0.85MA0.15PbI2.55Br0.45) single crystals with Cs+, Rb+, K+ cascade doping through “inverse
temperature crystallization” method, and systematically characterized their structure and
optoelectronic properties, as well as phase, humidity and thermal stability. Our work highlights the
structure-property relationship, that is, the diversified A-site occupation (especially FAMACsRb
system) can significantly promote the carrier transport behavior of perovskites, as well as obtaining
better thermal and humidity stability, while K+ doping weakens the intrinsic properties of
perovskites which probably due to the interstitial occupation of K+. These findings deepen the
understanding of alkali ions doping on the intrinsic properties influence of perovskite materials,
which providing an exemplary paradigm for further explorations.
Introduction
The power conversion efficiency (PCE) of organic-inorganic perovskite (ABX3, where A is a
monovalent cation, such as methylammonium (MA+), formamidinium (FA+) or Cs+; B is divalent
cation normally Pb2+ or Sn2+; X is a halide ion, e.g. Cl, Br, I) solar cells (PSCs) has achieved
25.2%, which mainly originated from the perovskite component optimization, process engineering,
device structure adjustment and additive engineering, etc.1-5 Particularly, alkali metal ions (Cs, Rb,
K, etc.) doping has shown great potential in improving the efficiency and stability of PSCs, through
either acting as the perovskite composition or serving as an additive. Previous studies have shown
that the doping of alkali ions mainly brings the following effects:
1) To interfere carrier behavior in the material. For example, Saliba et al. doped Rb into (FA, MA,
Cs)Pb(I, Br)3 perovskites and proved that the density of defect states was greatly reduced and
non-radiative recombination was suppressed.6 Hu et al. studied the effect of Cs, Rb doping on
the carrier transport and defects in the polycrystalline perovskite films, which had less defects,
large carrier mobility, and thus the high stable output power of the devices.7 With regard to K
ion doping, Park et al. speculated that the formation energy of interstitial K was small due to its

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DOI: 10.1039/D0TC03085F

smaller ion radius, which inhibited the formation of Frankel I defects, and reduced the
hysteresis.8 Abdi-Jalebi et al. demonstrated substantial mitigation of both non-radiative losses
and photoinduced ion migration in perovskite films and interfaces by decorating the
((Cs0.06FA0.79MA0.15)Pb(I0.85Br0.15)3 surfaces and grain boundaries with passivating KI layers.9
2) To improve materials stability. On one hand, Cs and Rb can partially replace the A-site ions to
participate in the lattice, which can improve the phase stability of the formamidinium (FA)
based perovskite. On the other hand, appropriate amount of K and Na, as interstitial ions, can
improve the stability by blocking the ion migration channel and reduce the device hysteresis.8,
10, 11 For instance, Lee et al. firstly studied the stability of (FA, Cs)PbI3, and found that the phase
stability of perovskite film against illumination and moisture were significantly improved when
10% of FA was replaced by Cs, which was attributed to the enhanced interaction between FA
and iodide due to contraction of cubo-octahedral volume.10 Meanwhile, Zhou et al. introduced
Cs into FA-based perovskite in a two-step method and obtained largely improved phase stability
of polycrystalline perovskite films.12 Further, MA-free (FA, Cs, Rb)PbI3 perovskite system
developed by Turrencruz et al. exhibited excellent stability due to improved crystal quality and
the stability of the composition itself.13
3) To adjust film quality and crystal orientation. Many works have shown that alkali ions will
participate in and affect the crystallization kinetics of perovskite films. For instance, Chang et
al. found the Na or K doping could increase the grain size of the perovskite film and thus
improved the charge carrier lifetime.14 Zheng et al. reported a controllable approach to
manipulate the facet orientation within the ABX3 hybrid perovskites polycrystalline films by
cation cascade doping at A-site, which was found to facilitate photocarrier transport across the
absorber and pertaining interface in the resultant device.15
Despite the above-mentioned progress, most studies were conducted on perovskite polycrystalline
films to improve the stability and efficiency of the resulting devices. However, the defects in
polycrystalline films are difficult to be decoupled to understand the intrinsic properties of alkali
ions-doped perovskites. Compared to polycrystalline films, single crystals are regarded as the best
platform to reflect the intrinsic materials properties for fundamental understanding due to its low
defects density and simple structure. Up to now, it lacks the understanding on the effect of the alkali
ions doping on perovskite single crystals, and it is worthy to establish a correlation between doping

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DOI: 10.1039/D0TC03085F
ions and intrinsic properties of perovskite materials.
In this work, we synthesized a series of FA0.85MA0.15Pb(I0.85Br0.15)3 single crystals with series of
alkali ions cascade doping, and systematically characterized the effect of these ions on the
optoelectronic and stability properties of perovskites. First of all, we determined the exact content
of alkali metal ions in each single crystals. Further, we found that all single crystals belonged to the
simple cubic phase in the temperature range from 180 K to 360 K, except FAMACsRbK perovskite
which changed to a body-centered cubic phase at 180 K. However, their carrier dynamics varied
dramatically. Compared to FA0.85MA0.15Pb(I0.85Br0.15)3 (FAMA) single crystals, the alkali ions
doped ones exhibit fewer defects, improved carrier lifetime and mobility. Interestingly, CsRb and
Cs doping led to single crystals outperformed that of the CsRbK doping. Moreover, the CsRb doped
single crystals possessed the best moisture and thermal stability. Deep study revealed that the alkali
ions occupied different sites of the perovskite lattice, which mainly contributed to the properties
variation. Simply, the richer occupant diversity at A-sites (CsRb), the more improvement in
materials properties, but the interstitial occupants of alkali cations (K) will decrease materials
performance. The mechanism of K doping effects is discussed in detail. The understanding about
the alkali doping on the intrinsic materials properties shed light on the application of perovskite in
the various optoelectronic devices, such as solar cells, light-emitting diode, lasers and
photodetectors.
Results and Discussion
The Cs, Rb, K cascade doped FA0.85MA0.15Pb(I0.85Br0.15)3 single crystals in this work were simply
marked as FAMACs, FAMACsRb, FAMACsRbK, respectively. Due to retrograde solubility of
perovskite in γ-butyrolactone solvent, we adopted “inverse temperature crystallization” method to
synthesize the single crystals as showed in Figure 1a, and introduced Cs, Rb, K cations through the
addition of KI, RbI and CsI compounds to keep the fixed ratio of I/Br in the precursors solutions.16,
17 After forming the single crystal seeds by continuously heating the solutions, we lifted the solution
temperature by 1 C at interval of 1 h. Since the nucleation consumed solutes then reduced the
supersaturation of solution and the growth speed of crystals, increasing the temperature during the
growth process could provide enough supersaturation and supply the solutes timely, which
accelerated the growth process and improved the quality of single crystals. More growth details can
be found in experimental section.

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Figure 1. (a) Schematic diagram of perovskite single crystal growth. The photos of single crystals:
(b) FAMA, (c) FAMACs, (d) FAMACsRb, and (e) FAMACsRbK. The (0kl) diffraction patterns of
single crystals at different temperature: (f) FAMA, (g) FAMACs, (h) FAMACsRb, (i)
FAMACsRbK single crystals at room temperature; (j) FAMA, (k) FAMACs, (l) FAMACsRb, (m)
FAMACsRbK single crystals at 180 K. (n) FAMA, (o) FAMACs, (p) FAMACsRb, (q)
FAMACsRbK single crystals at 360 K.

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The photos of various single crystals (Figure 1b-e) showed that they had an average size of ~3mm
with uniform size distribution. All crystals were black with regular rhombic dodecahedron shapes,
indicating the availability of the inverse temperature crystallization method. Most importantly, this
phenomenon suggested that the involvement of alkali ions had no dramatically impact on the
nucleation and growth process of FAMA single crystals.
We first performed a detailed characterization of its actual alkali metal ions content through
inductively coupled plasma-atomic emission spectrometry (ICP-AES). The ratio of each alkali ion
to Pb determined by ICP-AES was shown in Table 1. In order to eliminate the influence of surface
residues, the newly prepared crystals were washed three times with γ-butyrolactone before the test.
Table 1. The actual ratio of each alkali ion to Pb in the perovskite single crystal.
Cs/Pb
4.1%
4.2%
Rb/Pb
0
K/Pb
FAMACs
0
0
FAMACsRb
FAMACsRbK
0.11%
4.1%
0.07%
0.2%
The actual content of Cs was about 4.1%, which was close to the feed ratio in the precursor. Due to
the mismatched ion radius, the actual doping content of Rb and K was very low (~0.1%), but not
zero, which indicated that the cascade doping method could introduce a small amount of Rb or K
into the bulk perovskite. In contrast, Rb could hardly be detected in FAMARb samples (only Rb
doped, see the experimental section for details). However, the actual positions occupied by different
alkali metal ions in the perovskite lattice may be different, which will be discussed later.
To further demonstrate the phase evolution of single crystals, we performed single crystal X-ray
diffraction (XRD) characterization under different temperature. The (0kl) diffraction patterns of
single crystals at room temperature (~300 K) were listed in the Figure 1 f-i, and the corresponding
patterns at low (180 K) and high (360 K) temperature were listed in Figure 1 j-m and Figure 1 n-
q, respectively. The diffraction patterns of all single crystals were clear, indicating the high quality
of single crystals. Previous studies have shown that the FAxMA1-xPbI3 perovskite single crystal
would change from a cubic phase to a tetragonal phase when the temperature reduced from room
temperature to 255-285 K.18 However, our results indicated that the FAMA single crystals always
maintained a pure cubic phase at varied temperatures (Figure 1, f, j, n). This may be related to the
15% bromine substitution in the composition, which brought into stabilizing effect on the cubic

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phase due to the reduced defect density.19 In addition, both FAMACs and FAMACsRb single
crystals maintained the pure cubic phase at all temperature points, further proving that Cs and CsRb
doped perovskites obtained good phase and thermal cycling stability. However, FAMACsRbK
perovskite maintained a cubic phase at 300K and 360K, while they changed to a body-centered
cubic phase at 180K, indicating the phase instability associated with K ions doping. We speculated
that K ions actually occupied interstitial instead of the A-site due to the unmatched ionic radius of
K ion to accommodate the A-site occupation, which led to phase instability. The effect of interstitial
K ions will be discussed further below.
DOI: 10.1039/D0TC03085F
In order to observe the effect of Cs, Rb, K cascade doping on the perovskite lattice constant, we
grinded single crystals and performed the powder X-ray diffraction (PXRD) measurement. The
normalized PXRD data were shown in the Figure 2, and the diffraction peak at 14.1, 20.0 , 24.6
, 28.4 , 31.87 , 35.0  (Figure 2a) was assigned to the (001), (011), (111), (002), (012), (112)
crystal plane20 of the cubic phase perovskites, respectively. We then selected the peak of (012)
which centered at 31.8-32  to analyze the peak position shift in detail. (Figure 2b) The positions
of the (012) peak corresponding to FAMA, FAMACs, FAMACsRb, and FAMACsRbK were 31.85
, 31.89 , 31.91 , and 31.90 , respectively, which clearly indicated that the decrease of lattice
parameter in cascade doped single crystals followed the sequence from FAMA, FAMACs, to
FAMACsRb.
It was also observed that the peak shift from the FAMACs to FAMACsRb is smaller than that from
FAMA to FAMACs sample, which indicated that Cs could easily incorporate into the lattice of the
FAMA sample to significantly reduce the perovskite lattice constant, while Rb could only partially
enter the lattice and occupy the A-site so that the peak shift tendency upon Rb doping was decreased.
Further analyzing FAMACsRbK, we found that the corresponding lattice constant did not decrease
further, which indicated that K cannot substitute the A-site within perovskite lattice due to its too
small size. We speculated that K may be incorporated into the lattice in the form of interstitial ions
and then affected the carrier behavior of perovskites as shown in following characterizations.8
Figure 2c showed the possible occupation of different alkali metal ions in the FAMA based crystal
lattice.

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Figure 2. (a) The powder X-ray diffraction (PXRD) data of single crystals: FAMA, FAMACs,
FAMACsRb,and FAMACsRbK; (b) The magnified PXRD at the range of 31.4  to 32.3 . And (c)
Schematic diagram of lattice with speculated alkali ions occupation.
We characterized the optoelectronic properties of single crystals by measuring their UV-Vis
absorption, photoluminescence (PL), and the time-resolved photoluminescence (TRPL) spectra. It
can be seen from Figure 3a that the absorption band edge was located at 820 nm, which was about
50 nm red-shifted compared to the PL peak position. This anti-Stokes shift was always observed in
perovskite crystals with a certain thickness, which was caused by below-bandgap absorption.21 Thus,
the absorption spectrum of the single crystals cannot reflect its optical band gap well. At the same
time, PL always originated from the direct band gap. Therefore, for perovskite single crystals, PL
was a more reliable method to determine the optical band gap due to the existence of large anti-
Stokes shift. The PL emission peaks of FAMA, FAMACs, FAMACsRb, FAMACsRbK single
crystals were located at 762 nm, 765 nm, 767 nm, and 768 nm, and the corresponding band gaps
were 1.627, 1.621, 1.617 and 1.615 eV, respectively. This red-shift along with the Cs, Rb, K cascade

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doping indicated the reduced bandgap of corresponding single crystals. It is generally recognized
that for hybrid ABX3 perovskite, the conduction band minimum (CBM) and valance band maximum
(VBM) are mainly composed of Pb 6p orbitals, hybrid orbitals of Pb 6s and X 5p, respectively.22
However, the size of A-site ions still can affect the orbitals overlapping of Pb and X, then resulting
in the band gap variations of perovskites. Therefore, we speculated that the small amount of Cs, Rb,
K cascade doping induced the shrinking of crystal lattice (Figure 2b), thus increasing the overlap
of Pb and X and then decreasing the band gap.23
The TRPL spectra of single crystals are shown in Figure 3b, and carrier lifetime extracted from the
bi-exponential fitted TRPL spectra are listed in the Table 2. The short lifetime (τ1) corresponds to
the surface recombination process, where the surface traps assisted the carrier recombination.24 The
long lifetime (τ2) resulted from the bulk recombination process, in which the electron from CBM
recombined with the hole from VBM.25 The long τ1 and τ2 indicated low surface defects and high
quality of single crystals. Compared to FAMA single crystals with τ1=16.30 ns and τ2=971.67 ns,
both τ1 and τ2 greatly improved in FAMACsRb and FAMACs single crystals (FAMACs: τ1=24.13
ns, τ2=1832.52 ns; FAMACsRb: τ1=29.14 ns, τ2=1739.20 ns), while the FAMACsRbK single
crystals exhibited shorter τ1=10.55 ns and longer τ2=1537.31 ns. In addition, the proportions of τ1
and τ2 components in FAMA were 77% and 23%, respectively, indicating strong surface non-
radiative recombination. The proportion of the τ1 component decreased from 77% to 74%/70% after
the Cs/CsRb doping, while that of FAMACsRbK rose to 80%. It was concluded that Cs, Rb cascade
doping could improve the bulk quality of single crystals, among them, the FAMACsRb and
FAMACs single crystals exhibited the highest carrier lifetime, which was in line with previous work
that doped alkali metal ions into polycrystalline perovskite thin films.7 Meanwhile, longer τ1 and
lower proportion of τ1 in FAMACsRb and FAMACs single crystals suggested that the surface
defects recombination was effectively inhibited in these samples, which may be due to the better
defects passivation of Cs and Rb. Compared with Cs and CsRb, the cascade doping of K ions
weakened both τ1 and τ2, which fully proved that interstitial doping will have a negative impact on
carrier lifetime.

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Figure 3. (a) The photoluminescence (PL) emission / UV-Vis absorption spectra, and (b) the time-
resolved photoluminescence spectra (TRPL) of single crystals: FAMA, FAMACs, FAMACsRb,
FAMACsRbK single crystals. (c) X-ray absorption near edge structure (XANES) and (d) extended X-
ray absorption fine structure (EXAFS) of Pb(FAMACs) and Pb(FAMACsRb).
Table 2. The carrier lifetime extracted from bi-exponential fitted TRPL in Figure 3b.
Lifetime
τ1 / ns
FAMA
16.30
FAMACs
24.13
FAMACsRb
29.14
FAMACsRbK
10.55
τ2 / ns
971.67
1832.52
1739.20
1537. 31
In addition, in order to understand the influence of the alkali metal ions cascade doping on the
microstructure of perovskites, we performed X-ray absorption near edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS) characterization on the FAMACs and
FAMACsRb single crystals. The experimental details were shown in the characterization section.
XANES is useful in identifying the chemical state of the absorbing element and hence inferring its
chemical species. For FAMACs and FAMACsRb, the Pb L3 edge energy positions (E0 = 13035.0
eV) were not changed as showed in Figure 3c, which indicated there were no detectable changes in

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the oxidation state of Pb(II) in these two samples. Further, the fine structure from the X-ray
absorption spectra in the high energy range (beyond the ionization potential) is a powerful tool to
determine the chemical environment such as interatomic distances.26, 27 Thus based on the R space
EXAFS data (Figure 3d), we could conclude that FAMACs and FAMACsRb have the same local
coordination structure around Pb sites. Therefore, the microstructure of alkali metal ions doped
perovskites showed no visible difference, which excluded the possible interaction between alkali
metal ions and Pb ions.
Figure 4. The dark current-voltage curve of ITO/NiOx/perovskite/Au devices according to the
SCLC model: (a) FAMA, (b) FAMACs, (c) FAMACsRb, (d) FAMACsRbK.
To explore the effect of Cs, Rb, K cascade doping on the carrier mobility of perovskite single
crystals, we performed space-charge-limited current (SCLC) measurement. The dark J-V curves
were obtained based on the ITO/NiOx/perovskite single crystal/Au structure. The traps density (ntrap
)
and mobility (μ) was estimated according to the equation JD = 90μV2/8L3 and VTFL = entL2/20,
respectively. Figure 4 showed the measured and calculated results for all single crystals. It can be
seen that the mobility of FAMA single crystals approached 21.69 cm2V1s1, which was consistent

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28
with our previous reports , while the mobility of Cs, CsRb, CsRbK doped perovskite single crystals
were 41.69 cm2V-1s-1, 46.43 cm2V-1s-1, 36.21 cm2V-1s-1, respectively. The enhanced mobility and
reduced traps density for Cs, Rb, K cascade doped perovskites implied the higher quality of
corresponding single crystals, which was in accordance with our TRPL results.
Based on these results, we speculated that FAMACsRb and FAMACs single crystals had higher
quality than the FAMA, and we contribute this to the contraction of cubo-octahedral volume upon
Cs and Rb doping, which in turn enhancing the interaction between A-site ions and iodide, and
greatly reducing the trap density. However, although FAMACsRb single crystals exhibited the
highest carrier mobility which was consistent with the results of the CsRb doped polycrystalline
films,7 we still argued that there are some difference between the published results and our work:
the doped Rb ions mainly located at the grain boundaries (GBs) of the polycrystalline film, however,
the doping of Rb in single crystals (without GBs) still improved the corresponding carrier mobility,
indicating that Rb could also play a positive role in the perovskite bulk through partial A-site
substitution. The stable cubic phase and large carrier mobility of CsRb doped perovskite system
highlight the potential application in fabricating high-efficiency PSCs. Unexpectedly, K cascade
doping significantly reduced carrier mobility compared to FAMACs and FAMACsRb, which was
not consistent with the trend in polycrystalline films of previous report9, 14. The results based on
single crystal platform fully indicated that interstitial doping will decrease the carrier mobility of
perovskite, but K ions may had a passivating effect on the interface and grain boundaries of the
polycrystalline film, so their overall effect is positive.
For further application of perovskite materials toward commercialization, the highly-efficient PSCs
should also have the competitively intrinsic stability of absorber. To characterize the humidity and
thermal stability of alkali metal ions doped perovskites, we measured the PXRD after heating the
single crystals powder at 300 C under 85% relative humidity (RH) for 5 min (Figure 5a). The
intensity of perovskite (001) diffraction peak were normalized to compare the relative content of
secondary phase (PbI2). For freshly prepared single crystals, the signal of PbI2 was absent (Fig. 2a),
however, PbI2 emerged in all aged single crystals at 300 C under 85% RH for 5 min, indicating the
partial decomposition of perovskites after humidity and thermal stressors.

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Figure 5. (a) PXRD of FAMA, FAMACs, FAMACsRb, FAMACsRbK single crystals powder after
heating at 300 C and 85% relative humidity for 5 min. (b) the ratio of the (001) peak intensity of PbI2
at 12.7 compared to the (001) peak intensity of each perovskite at 14.1. (c) Proposed mechanism
diagram of alkali ions doping improves the humidity and thermal stability of perovskite.
Further, we used the ratio of the (001) peak intensity of PbI2 at 12.7 compared to the (001) peak
intensity of perovskite at 14.1 to quantify the decomposition degree of single crystal (Figure 5b).
The FAMA single crystal exhibited the most severe decomposition, with the ratio of PbI2/perovskite
reaching 12%. Fortunately, all alkali metal ions doped single crystals could effectively suppress the
decomposition of perovskites. Among them, FAMACsRb showed the best resistance to humidity
and heat, and the PbI2/perovskite ratio was only 3%. The other two kind of perovskite single crystals,
FAMACsRbK and FAMACs had relatively poor resistance to heat and humidity, with the
PbI2/perovskite ratio keeping at 5% and 7%, respectively. Specifically, the doped Cs and Rb could
occupy the A-site and reduce the intrinsic defects (most of which are vacancy defects) density of
single crystals, which was proved by our SCLC measurement.29, 30 Combining with the reported

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results31, 32, we reasonably speculated that vacancy defects are the important channels for water
molecules invading on perovskite single crystals, so FAMACs and FAMACsRb with fewer vacancy
defects and tighter lattices exhibited significantly improved moisture and heat resistance stability.
Proposed mechanism diagram was showed in Figure 5c. On the contrary, the cascade doping of
CsRbK reduced the resistance to moisture and heat. According to previous studies, we attributed
this phenomenon to the microstrain increasing caused by a small amount of K ions which occupied
interstitial sites of perovskite crystal.15 Overall, the FAMACsRb perovskite exhibits excellent
humidity and thermal stability, as well as the best carrier performance, which ensure its greater
prospect of wide application.
DOI: 10.1039/D0TC03085F
Conclusion
We systematically synthesized FA0.85MA0.15PbI2.55Br0.45 single crystals doped with Cs+, Rb+, K+ in
a cascade manner to analyze their collective and individual impacts on the intrinsic properties of
perovskite. The results revealed that the cascade doping of alkali ions with smaller size could reduce
the lattice constant and shrink the energy band gap. With various characterizations, it was found
that the optoelectronic and carrier transport properties were improved profoundly upon CsRb and
Cs cation doping, while the effect is reduced upon CsRbK doping. Among all perovskite single
crystals, the FAMACsRb perovskite exhibited the best stability in courses of humidity and thermal
stressors. We explain these differences in the structure-property relationship. Diversified occupants
at A-site will improve the intrinsic properties of perovskite, while additional interstitials such as K-
interstitial will decrease the materials properties. It thus suggests to introduce A-site doping with
rational materials design to avoid introducing extra interstitial defects. This understanding about the
intrinsic properties of perovskite doped by alkali ions may shed light on further applications of
hybrid halide perovskite with alkali doping.
Experimental
Chemicals
CH3NH2 aqueous solution (40% w/w), HI aqueous solution (57% w/w, stabilized with 1.5% H3PO2),
HBr aqueous solution (48% w/w), HN=CHNH2·CH3COOH (FAAC, 99%, Aladdin), γ-
butyrolactone (GBL, 99%, Aladdin), PbI2 (99.9%, Aladdin), PbBr2 (99.999%, sigma), CsI (99.98%,
sigma), RbI (99.9%, sigma), KI (99.9%, sigma).

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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC03085F
Synthesis of FAI33
FAI was synthesized by stirring 14 g FAAC and 32.3 Ml HI in the 250 mL three-neck round bottom flask
at 0 °C ice bath under the N2 atmosphere. After 2 h, the solvent was removed by Rotary evaporation at
50 C, and the remained yellow solids were washed by ether. Then the solids was dissolved in ethanol at
80 C to form saturation solution. Decreasing the temperature of solutions could precipitate white solids.
Dissolving and recrystallization process were repeated three times to obtain the pure products. After
heating at 30 Cfor 24 h in the vaccum oven for 24 h, the final products were stored in the N2 glove box.
Synthesis of MABr34
MABr was synthesized similar to FAI, except by stirring 30 mL CH3NH2 and 36 mL HBr.
Synthesis of Cs, Rb, K cascade doped FA0.85MA0.15PbI2.55Br0.45 single crystals
FA0.85MA0.15PbI2.55Br0.45 single crystals
0.5 mL GBL and the molar ratios of FAI, PbI2, MABr, PbBr2 were inputted in the 5 mL vial to form
1.2 M precursor solutions. After heating the solutions at 78 C for 2 h, there were few small crystal
seeds in the vial. Increasing the temperature at the speed of 1 C/h could obtain millimeter sized
single crystals, which were then heated at 100 C for 15 min in the glove box to remove the remained
solvent on the crystal surface The products were marked as FAMA single crystals.
Cs0.05(FA0.85MA0.15 0.95PbI2.55Br0.45 single crystals
)
Except the different molar ratios of CsI, FAI, PbI2, MABr, PbBr2 and the different initial heating
temperature (80 C), the other processes were the same as FAMA single crystals. The products were
marked as FAMACs single crystals.
Rb0.05(Cs0.05(FA0.85MA0.15 0.95 0.95PbI2.55Br0.45 single crystals
)
)
Except the different molar ratios of RbI, CsI, FAI, PbI2, MABr, PbBr2 and the different initial
heating temperature (84 C), the other processes were the same as FAMA single crystals. The
products were marked as FAMACsRb single crystals.
K0.05(Rb0.05(Cs0.05(FA0.85MA0.15 0.95 0.95)PbI2.55Br0.45 single crystals
)
)
Except the different molar ratios of KI, RbI, CsI, FAI, PbI2, MABr, PbBr2 and the different initial
heating temperature (87 C), the other processes were the same as FAMA single crystals. The
products were marked as FAMACsRbK single crystals.
Characterization
The test samples of inductively coupled plasma-atomic emission spectrometry (ICP-AES, Prodigy

Journal of Materials Chemistry C
Page 16 of 18
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DOI: 10.1039/D0TC03085F
7-Leeman) were obtained by dissolving a certain amount of perovskite single crystals in 5% nitric
acid solutions. The actual concentration measured was shown in the table below.
Sample
Pb
Cs
Cs/Pb
(mole
ratio)
Rb
Rb/Pb
K
K/Pb
(ug/ml) (ug/ml)
(ug/ml) (mole
ratio)
(ug/ml) (mole
ratio)
FAMACs
17.7219 0.4711
17.7363 0.4804
4.1%
4.2%
4.1%
0.0009
0.0962
FAMACsRb
0.0086 0.11%
0.0056 0.07%
0.0014 ≈ 0
0.1043
FAMACsRbK 19.5133 0.5187
FAMARb 18.9004 <0.0001
0.1182 ~0.2%
0.1007
In order to compare with cascade doping, we also tested the actual content of Rb in samples doped
with Rb (5%) only. Compared with FAMACsRb samples, Rb could hardly be detected in FAMARb
samples. The results confirmed the effectiveness of cascade doping.
Single crystal X-ray Diffraction patterns were obtained by measuring the single crystal particle at
different temperature (180 K, 300 K, 360 K) through single crystal X-ray diffractometer with Mo
Kα ray (λ= 0.071073 nm). Powder X-ray Diffraction (PXRD) data of single crystals were obtained
on the Rigaku D/MAX-2400 diffractometer. Absorption of perovskite single crystal particles were
measured by UV-Visible/NIR Spectrophotometer UH4150. Photoluminescence (PL) spectrum of
single crystal particles were characterized by FLS980 (Edinburgh Instruments Ltd), and Time-
Resolved Photoluminescence (TRPL) spectrum was obtained by 470 nm excitation wavelength. We
measured the dark current-voltage of Au/perovskite/Au devices through Keithley 4200 system to
calculate the mobility of single crystals according to the Space-Charge-Limited Current (SCLC)
model. Both sides of the single crystals sheets cleaved from big particles was deposited 80 nm Au
to form the hole-only device. The X-ray absorption fine structure (XAFS) spectra at Pb L3 (E0 =
13035.0 eV) edge was performed at BL14W1 beamline of Shanghai Synchrotron Radiation
Facility(SSRF) operated at 3.5 GeV under “top-up” mode with a constant current of 200 mA. The
XAFS data were recorded under transmission mode with Oxford ion chambers. The energy was
calibrated accordingly to the absorption edge of pure Pb foil. Athena and Artemis codes were used
to extract the data and fit the profiles.
Acknowledgments
This work was supported by National Natural Science Foundation of China (51722201), (51672008),

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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC03085F
(51972004), (91733301), National Key Research and Development Program of China Grant No.
2017YFA0206701, Natural Science Foundation of Beijing, China (Grant No. 4182026).
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