Toward Long-Term Stability: Single-Crystal Alloys of Cesium

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Toward long-term stability: single crystal alloys of cesiumcontaining mixed cations and mixed halides perovskite Liang Chen, Yan-Yan Tan, Zhi-Xin Chen, Tan Wang, Shu Hu, Zi-Ang Nan, Li-Qiang Xie, Yong Hui, Jing-Xin Huang, Chao Zhan, Su-Heng Wang, Jian-Zhang Zhou, Jia-Wei Yan, Bing-Wei Mao, and Zhongqun Tian J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11610 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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Journal of the American Chemical Society

Toward long-term stability: single crystal alloys of cesium-containing mixed cations and mixed halides perovskite Liang Chen,1# Yan-Yan Tan,1# Zhi-Xin Chen,1 Tan Wang,1 Shu Hu,1 Zi-Ang Nan,1 Li-Qiang Xie,1 Yong Hui,1 Jing-Xin Huang,1 Chao Zhan,1 Su-Heng Wang,1 Jian-Zhang Zhou,1 Jia-Wei Yan,1 Bing-Wei Mao1* and Zhong-Qun Tian1* 1

State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of

Chemistry and Chemical Engineering, and Collaborative Innovation Center of Chemistry for Energy Materials, Xiamen University, Xiamen 361005, China.

ABSTRACT: Perovskite solar cells are strong competitor for silicon-based ones, but suffer from poor long-term stability for which the intrinsic stability of perovskite materials is of primary concern. Herein, we prepared a series of well-defined cesium-containing mixed cations and mixed halides perovskite single crystal alloys, which enabled systematic investigations on their structural stabilities against light, heat, water and oxygen. Two potential phase separation processes are evidenced for the alloys as cesium content increases to 10% and/or bromide to 15%. Eventually, a highly stable new composition, (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 emerges with carrier lifetime of 16 μs. It maintains stable during at least 10,000 h water-oxygen and 1,000 h light stability tests, which is very promising for long-term stable devices with high efficiency. The mechanism for the enhanced stability is elucidated through detailed single crystal structure analysis. Our work provides a single crystal-based paradigm for stability investigation,

leading to discovery of stable new perovskite materials.

INTRODUCTION Perovskite solar cells (PSCs) have attracted much attention due to their high photoelectric conversion efficiency (PCE)1-10 and ease of preparation with low cost11-13. Currently, the highest PCE has reached 23.3%14, which exceeds the efficiency of polycrystalline silicon solar cells that are widely used in the market. However, the poor long-term stability of PSCs becomes the biggest bottleneck towards their commercialization compared to the over 20 years’ service life of silicon-based solar cells. The long-term device stability is determined by many complex factors such as the stabilities of perovskite light absorber, hole and electron transport materials, and the interfaces between perovskite and hole/electron transport materials15-19. Among them, the long-term structural stability of the perovskite materials themselves against the light, heat, water and oxygen is of primary concern and needs to be addressed first20-22. For example, the first used MAPbI3 perovskite suffers from poor water-oxygen and thermal stability23. Though the pure FAPbI3 shows better thermal stability, it is structurally unstable and undergoes phase separation with the formation of yellow non-perovskite -FAPbI3 phase at room temperature. Replacing FAPbI3 partially with MAPbBr3 to form mixed cations and mixed halides perovskites, such as the second generation (FAPbI3)0.85(MAPbBr3)0.15, can inhibit the “yellow-phase”, but the light stability of such perovskites is poor because the mixed halides tend to undergo segregation in the case of light illumination24, 25. Fortunately, the incorporation of much smaller inorganic cesium (Cs) ion into the FA-based perovskites can greatly improve 1

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the light stability, forming a promising third generation of perovskite composition25-27. Nevertheless, high content of Cs in the mixed perovskites will reduce the grain size and induce the phase separation of CsPbI3, which degrade the water-oxygen stability and PCE of PSCs28-30. However, the composition and ratio, especially the Cs contents of mixed perovskites for the state-of-art devices reported by different research groups are not consistent. More importantly, the mechanism on how the Cs enhances the stability of FA-based perovskites is less understood. Hence, in-depth investigations of the inherent factors that determine the stability of the mixed perovskite materials is highly desirable, and may be a more effective approach to find new and more stable Cs-containing mixed perovskite compositions for improved long-term stability of PSCs. However, nowadays, most reports about the stability of perovskite materials are based on ill-defined polycrystalline films, which contain a large number of grain boundaries and defects that would complicate the elucidation of the mechanism for long-term stability from structural point of view. Besides, unstable interfaces and interfacial materials also severely restrict the study of the perovskite materials on the long-term stability of devices. Moreover, since the quality of the polycrystalline films is influenced by many external factors involved during preparation, such as spin-coating process, temperature, and even the atmosphere under which the PSCs are manufactured31-34, there is lack of comparability among results reported by different research groups. Therefore, studying the long-term stability of the perovskite materials by employing well-defined single crystal systems can avoid the above-mentioned interference35-38 and is thus a more desirable and reliable way to reveal the intrinsic structural stability that has profound influence on the thermal, light and water-oxygen stability of PSCs. More importantly, compositional studies based on single crystalline perovskites enables detailed structural analysis, which would benefit much to the understanding of the underlying roles of cations and halides in the stabilization of perovskite, and thus can guide the design of new and more stable perovskite compositions. Herein, we prepared a series of Cs-containing mixed cations and mixed halides perovskite single crystals with varying compositions and ratios, namely (FAPbI3)1-x-y(MAPbBr3)y(CsPbBr3)x, to enable systematic investigation on the structures, properties and long-term stabilities against water-oxygen (in more than one year period), light (in 1,000 h period) as well as heat. Throughout this work, we provide not only a single crystal-based paradigm for long-term stability investigation from the intrinsic structural point of view, but also a new perovskite composition with superior stability and long carrier lifetime that is very promising for long-term stable PSCs with high PCE.

EXPERIMENTAL SECTION Materials. CsI (99.999%), CsBr (99.999%), PbBr2 (99.999%) were purchased from sigma Aldrich and PbI2 (99.9985%) from Alfa Aesar. CH3NH3Br (MABr, ≥99.5%), CH3NH3I (MAI, ≥99.5%), NH2CH=NH2Br (FABr, ≥99.5%), and NH2CH=NH2I (FAI, ≥99.5%) were purchased from Xi’an Polymer Light Technology Corp. The solvents including γ-butyrolactone (GBL, 99%), N, N-Dimethylformamide (DMF, 99.8%) were purchased from Sigma Aldrich. All the materials were used as received. Growth of perovskite single crystal alloy. The series of (FAPbI3)1-x-y(MAPbBr3)y(CsPbBr3)x (x + y ≤ 0.15) single crystals were prepared by using the inverse temperature crystallization (ITC) method. In brief, with Cs content

of

5%,

(FAPbI3)0.95(CsPbBr3)0.05,

(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05,

and

(FAPbI3)0.85(MAPbBr3)0.1(CsPbBr3)0.05 can be formed from the 1 M clear GBL solution heated at 95 °C. When the Cs content is increased to 10%, (FAPbI3)0.90(CsPbBr3)0.1 and (FAPbI3)0.85(MAPbBr3)0.05(CsPbBr3)0.1 can be obtained from their 0.7 M and 0.6 M solutions, respectively. For the 15% Cs content, the solubility of (FAPbI3)0.85(CsPbBr3)0.15 is only 0.3 M in GBL so that its single crystal cannot be acquired by ITC method because of its low initial concentration. GBL/DMF mixed solvents with volume ratio of 7:3 were used for the 2


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preparation of this single crystal. For the series of (FAPbI3)1-y(FAPbBr3)y (y = 0.05, 0.1, 0.15), (FAPbI3)1-x-y(FAPbBr3)y(CsPbBr3)x (0.1≤ x + y ≤ 0.15 ) and (FAPbI3)1-x(MAPbBr3)x (x = 0.1, 0.15), all the single crystals

were

prepared

in

GBL

at

95

°C

with

1

M

precursor

solution

except

the

(FAPbI3)0.85(FAPbBr3)0.05(CsPbBr3)0.1 (0.6 M). (FAPbI3)0.90(CsPbI3)0.1 was prepared in GBL with 0.7 M precursor solution. Perovskite single crystal alloys characterization. UV-Vis-NIR absorption spectra were recorded on a Cary 5000 UV-Vis-NIR spectrometer. Powder XRD patterns were measured on an X-ray diffractometer (Rigaku, RINT-2500) with a Cu Kα radiation source. Morphologies together EDS spectra of perovskite single crystal powders were measured via the Hitachi-4800 field-emission scanning electron microscope (SEM). Transient photoluminescence (PL) measurements were carried out on FLS 980-STM (Edinburgh instruments). Steady state PL spectra were measured at different positions on the outside surface of the single crystals on a Renishaw confocal microprobe Raman system with 532 nm laser. Small crystal seeds of two composition received X-ray crystallography characterization, which was conducted on an Oxford Gemini S Ultra system (Mo Kα) at 173 K. Absorption corrections were applied by using the program CrysAlis (multi-scan). The structure was solved by direct methods, and Br, I, Pb atoms were refined anisotropically by least-squares on F2 using the SHELXTL program, and OLEX2 were used as GUI. Long-term Stability tests under different conditions. For long-term stability test under the influence of water and oxygen, samples were stored in a desiccator with humidity of 20% and characterized at intervals during 10,000 h aging time (more than a year). The thermal stability was evaluated by the onset of decomposition temperature acquired by TGA experiments on SDT-Q600 instrument. For short term light stability test, a continuous exposure to 532 nm laser with light power density 430 W/cm2 at humidity of 60% was conducted within one hour. For long-term light stability test, perovskite single crystal powders were stored in a home-made sealing box filled with argon to avoid the influence of water and oxygen. And the transparent sealing device was illuminated by a commercial Sun Full Spectrum Light Source (CLV-S20, OLYMPUS) with light power density 100 mW/cm2 for 1,000 h “day and night” test (the samples were continuously illuminated for 12 h and then stored in the dark for another12 h, then recycled until a total time of 1,000 h was reached).

RESULTS AND DICUSSION Growth and characterization of perovskite single crystal alloys. In our previous work39, the (FAPbI3)1-y(MAPbBr3)y series single crystals were successfully prepared using ITC method40, 41. Here, we partially or totally replaced MAPbBr3 precursor with CsPbBr3 to prepare the single crystals of Cs-containing mixed perovskites with ternary or binary cations, respectively. In pure GBL solvent at 60 °C, the solubility of CsPbBr3 is much less than 0.01 M, which is lower than that of MAPbBr3 ( Table S1). However, in the preparation of perovskite single crystals, more than 0.05 mol CsPbBr3 solids were added into 1 mL GBL solution so that many orange-red solids of CsPbBr3 remained undissolved initially as shown in Figure 1A. Interestingly, after the addition of FAI and PbI2 solids into the solution, the solid part of CsPbBr3 become slowly dissolved, and the solution turned to clear yellowish (Figure 1B), indicating likely the involvement of ion exchange between CsPbBr3 and FAPbI3, such as CsPbBr3 + FAPbI3 = CsPbI3 + FAPbBr3, which promotes the CsPbBr3 dissolution. Note that while pure iodide-based perovskites such as FAPbI3 and MAPbI3 generally show solubility much greater than 1 M in GBL, the solubility of the all-inorganic CsPbI3 perovskite is as small as 0.03 M, which is comparable to that of the MAPbBr3 in pure GBL. These phenomena imply strong interaction between Cs ion and PbX3- anion, favoring the formation of the Cs-containing perovskites. For detailed investigations, we prepared perovskite precursor solutions of (FAPbI3)1-x-y(MAPbBr3)y(CsPbBr3)x with different compositions and ratios, where the total Br content was controlled not to exceed 15%, i.e. x + y ≤ 15% (See experimental details in methods). And finally five 3



kinds of millimeter sized large single crystals were obtained as basis for detailed stability investigation, i.e. (FAPbI3)0.95(CsPbBr3)0.05,



(FAPbI3)0.90(CsPbBr3)0.1 and (FAPbI3)0.85(MAPbBr3)0.05(CsPbBr3)0.1 (Figure 1C, Figure S1). To verify Cs is successfully incorporated into the perovskite single crystals, EDS measurements were performed on the freshly crushed single crystal powders, and elemental Cs signal is detected even for samples containing 5% CsPbBr3 (Figure S2). Then the powders were further used for XRD characterizations. As shown in Figure 1D, incorporation of 5% CsPbBr3 is sufficient to completely inhibit the formation of δ-FAPbI3 so that all the five perovskite compositions show a pure perovskite phase. The XRD peaks shift to higher angles with increasing Cs and Br contents, confirming alloying of Cs and Br in the perovskites with lattice contraction in accordance with the smaller ionic radii of Cs and Br than those of FA /MA and I (Figure 1E). The alloying of the single crystal perovskite is further evidenced by the absorption spectra in Figure S3A that show single and sharp absorption band edges. The surface photoluminescence (PL) spectra of the single crystals also display single and uniform fluorescence peak for each composition, and the tendency of wavelength variation is essentially the same as that of the absorption band edge (Figure S3B). Furthermore, the bandgaps of the five perovskite compositions are estimated according to the Tauc diagrams (Figure 1F), and the bandgap increase is more dominantly by the increase of the Br content than that of the Cs cation. Taken together, these data indicate that we have successfully prepared Cs-containing mixed cations and mixed halides perovskite single crystal alloys with varying compositions and ratios. The influence of cesium content on long-term water-oxygen stability. To investigate the long-term stability of the perovskite single crystal alloys under the conditions of water and oxygen presence, stability tests of up to 10,000 h (more than one year) were conducted on the above-mentioned five compositions. The corresponding single crystal powders were stored in a dryer with humidity of around 20%, and taken out for XRD measurements at about 60% humidity when necessary. Surprisingly, all the compositions containing 5% Cs behave with good structural stability up to 10,000 h (Figure 2A). No any decomposition of the perovskite into PbI2 (12.6 degrees) or δ-FAPbI3 (11.6 degrees) was observed. However, for the two 10% Cs-containing compositions, phase separation into the yellow non-perovskite phase δ-CsPbI3 occurs just after 72 h water-oxygen stability test. Taking (FAPbI3)0.90(CsPbBr3)0.1 as an example, new XRD peaks (Figure 2B), belonging to the δ-CsPbI3 (Figure S4), appear and become enhanced with time. The δ-CsPbI3 peaks were also observed in a non-brominated (FAPbI3)0.90(CsPbI3)0.1 single crystal after in contact with water and oxygen (Figure S5). To further understand the role of Cs in the phase separation of perovskite alloys, we attempted to synthesize (FAPbI3)0.85(CsPbBr3)0.15 single crystal containing even higher Cs content. But yellowish needle-like CsPbI3 crystals precipitated from the beginning, followed by the formation of the black FA-based perovskite crystal blocks (Figure 2C and 2D), leading to failure of synthesis for such a phase. Our experimental results clearly show that the intrinsic structural stability of the single crystal alloys with mixed FA and Cs cations depends crucially on the exact content of Cs: The perovskite single crystal alloys with 10% Cs become unstable and the one with 15% Cs cannot be even formed, while the ones with 5% Cs are excellent in long-term water-oxygen stability. The influence of bromide content on long-term water-oxygen stability. Noteworthy is that the black pure FA-based perovskite phase solids formed in the attempt of synthesizing the high Br-content and high Cs-content (FAPbI3)0.85(CsPbBr3)0.15 single crystals experienced a follow-up phase transition into red powders after exposure to water and oxygen (Figure 3A) as is confirmed by new XRD peaks at 12.1 degree and 13.0 degree (Figure 3B). The process is probably associated with the contents of the FAPbBr3 component formed during the synthesis through ion exchange. To distinguish the influence of Br from Cs, 4


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another

two

series

of

single

crystals

containing

FAPbBr3

with

and

without

Cs,

namely

(FAPbI3)1-y(FAPbBr3)y and (FAPbI3)1-x-y(FAPbBr3)y(CsPbBr3)x, were synthesized for comparison. For the Cs-free series, as can be seen from the remarkably different XRD patterns in Figure 3C, the δ-FAPbI3 still appears with 5% of FAPbBr3; but pure perovskite phase may be obtained if FAPbBr3 is increased to 10%, indicating the effect of Br on stabilizing the FA-based cubic perovskite phase (Figure S5)39. However, with further increase of FAPbBr3 content to 15%, the red substance is formed from the inside of perovskite, as shown by the XRD peaks at 12.1 and 13 degrees (Figure S6)42, measured from a freshly-cut perovskite single crystal. Therefore, the content of FAPbBr3 does play an important role in stabilizing the corresponding perovskites. For the Cs-containing series, however, the XRD results (Figure 3D) show that good phase stability is retained provided that 5% Cs content is introduced. Even for the (FAPbI3)0.85(FAPbBr3)0.1(CsPbBr3)0.05 where Br content is 15%, there is no direct phase separation, unlike the

Cs-free

(FAPbI3)0.85(FAPbBr3)0.15.

However,

when

the

Cs

content

reaches

10%,

the

(FAPbI3)0.85(FAPbBr3)0.05(CsPbBr3)0.1 become instable and turns red after 72 h under the humidity of 20%. The characteristic XRD peaks of both the red substances and δ-CsPbI3 appear (Figure S7). This implies that the cations and the halides synergistically influence the structural stability, and appropriate combinations of the Cs and Br contents are indeed necessary to achieve stable mixed cations and mixed halides perovskite alloys. Optimized highly stable new composition. The Br and Cs content-dependent stability of the perovskite alloys may be attributed to the large radii difference between FA (2.53 Å) and Cs ions (1.88 Å) and between iodide (2.20 Å) and bromide ions (1.96 Å), which results in the greatly distinguishable lattice constants and tolerance factors of different compositions (Table S2) 43-45. As a result, the Cs-rich (≥10%) and/or the Br-rich (≥15%) alloys are expected to introduce significant lattice stress or defects due to the large ion radii differences, thus favoring the undesirable potential phase separation of the alloys. Especially, the action of water and oxygen would generate more defects in the alloys, and thus speed up the phase separation processes. However, small amount of Cs (e.g. 5%) in the alloy can adapt the 15% Br-induced lattice contraction and stabilize the perovskite structure such as in the case of (FAPbI3)0.85(FAPbBr3)0.1(CsPbBr3)0.05, On the other hand, if Cs content reaches 10%, the cation-induced structural instability becomes dominant, which in turn promotes the phase transition caused by 15% Br, such as in the case of (FAPbI3)0.85(FAPbBr3)0.05(CsPbBr3)0.1. Besides, compared with the (FAPbI3)0.85(FAPbBr3)0.05(CsPbBr3)0.1, the red

substance

phase

transition

induced

by

Br

does

not

occur

for

MA-incorporated

(FAPbI3)0.85(MAPbBr3)0.05(CsPbBr3)0.1 (Figure S8A). The latter only shows δ-CsPbI3 peaks, implying that the MA, whose radius (2.17 Å) is slightly smaller than that of FA ions, can inhibit to some extent the phase transition occurring when Br content is 15%. It is worthy emphasizing that incorporation of MA cation alone cannot eliminate the potential phase separation tendency caused by 15% Br. The widely used (FAPbI3)0.85(MAPbBr3)0.15 perovskite material undergoes the phase transition with formation of the red substances after 6,000 h of water-oxygen stability test, compared with the (FAPbI3)0.9(MAPbBr3)0.1 that shows no phase changes even after one year (Figure S8B). More remarkably, with 5% CsPbBr3 replacement of MAPbBr3 to form the ternary cation perovskite composition of (FAPbI3)0.85(MAPbBr3)0.1(CsPbBr3)0.05, the stability against water and oxygen can be significantly improved to 10,000 h (Figure 2A). Nevertheless, taking into account the potential phase separation tendency with high Br content in general, we suggest adopt 10% Br and 5% Cs contents to avoid the potential phase separation, and 5% MA to adapt the Br induced lattice contraction and to balance the radius difference between FA and Cs. In particular, the (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05, an optimized perovskite composition among the ones being studied in the present work, not only exhibits superior long-term water-oxygen stability (Table S3), but also 5



Page 6 of 15

possesses a narrow bandgap (1.52 eV) comparable to the ones of widely used MAPbI3 and (FAPbI3)0.85(MAPbBr3)0.15 perovskites. Long-term light stability. To assess the light stability of the optimized composition, short-term light stability test was conducted first under a strong laser power density (430 W/cm2) in the atmospheric condition (60% humidity), and Cs-free (FAPbI3)0.9(MAPbBr3)0.1 was chosen as a control to verify the impact of Cs on light stability. As shown in Figure 4A, the surface PL peak wavelength of the Cs-free composition gradually becomes red-shifted with illumination time, and the PL peak intensity grows sharply, which is an indication that iodide ions gradually migrate to the surface to form iodide-rich phase46, 47. In contrast, the Cs-containing composition exhibits no change under the same condition (Figure 4B), manifesting that the introduction of Cs can effectively inhibit the migration of iodide. Then the long-term light stability test of the (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 was conducted under a "day and night" testing mode up to 1,000 h under one standard sun (See the methods). To gain intrinsic light stability, the single crystal powders were placed in a home-made sealed chamber filled with argon atmosphere to eliminate the interference of water-oxygen. Nevertheless, no ultraviolet light filtering or cooling protection was adopted and therefore the influence of ultraviolet light and high temperature (about 60 °C) on this perovskite material cannot be excluded. After 1,000 h "day and night" light stability test, this optimized composition still maintains good structural stability with no phase change or decomposition, as confirmed by the similar absorption spectra and XRD patterns (Figure S9). More importantly, the almost identical transient PL spectra before and after illumination suggest that long-term illumination (including UV light and heat) does not lead to increase of defects (Figure 4C). To verify the enhanced stability as a benefit from Cs, TGA and transient PL measurements were conducted. As shown in Figure 4D, Figure S10 and Figure S11, the Cs-containing perovskite materials show increased onset decomposition temperature compared to the Cs-free ones, which is in accordance with the small solubility of the Cs-based perovskite species and supports the strong interaction of Cs with the lead iodide (bromide)

octahedral

framework.

In

Figure

4E,

the

average

carrier

lifetime

of

(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 is up to 16.0 μs, much longer than that of (FAPbI3)0.9(MAPbBr3)0.1 (3.6 μs), meaning that defects in the crystal of the former are indeed reduced and thus iodide migration relieved. The reduced defects and enhanced interaction between cations and anions explain the greatly improved light stability as well as thermal and water-oxygen stabilities of the optimized (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05. Single crystal structure analysis. To further understand the underlying mechanism for the improved light stability of the optimized (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 perovskite composition from the crystal structure

level,

we

performed

single

crystal

X-ray

diffraction

measurements

on

(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 and (FAPbI3)0.9(MAPbBr3)0.1, and the latter was chosen as a control. Both compositions show a distorted inorganic octahedral framework in a cubic symmetry as exemplified by Figure 4F and Figure S12. In these Br-I mixed halide alloys, the Pb-Br bond length is elongated while the Pb-I bond length shortened, compared with those of pure Br-based and I-based perovskites, respectively (Table S2). Therefore, the Pb-Br-Pb bond takes a linear configuration with 180 degree of bond angle while the shortened Pb-I-Pb bonds are allowed to adopt a distorted configuration with bond angle less than 180 degree, and each I atom has the possibility at four equivalent positions in lattice, which helps release the lattice stress caused by the Br-induced lattice contraction (Figure S13). Interestingly, the Cs-containing (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 composition show significant lattice distortion with Pb-I-Pb bond angle of 164.2 degree compared to the Cs-free (FAPbI3)0.9(MAPbBr3)0.1 composition with bond angle of 166.9 degree, leaving an elongated Pb-I bond length with further releasing the lattice stress (Table 1). In 6


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other words, the elongated Pb-I bond is actually stabilized through the bond angle distortion caused by the stronger interaction between the cations and the inorganic octahedral framework, which reduce the formation of iodide vacancies and enhance the long-term stability as confirmed by the above mentioned TGA and transient PL results.

CONCLUSIONS In summary, based on single crystal-based approach we have systematically studied the intrinsic structural stability and the stabilities against environmental, heat and light exposures of mixed perovskite materials. The appropriate contents and ratios of Cs and Br play vitally important roles in the control of the stability and crystallinity of perovskite single crystal alloys, while excess Cs and Br would lead to phase separation due

to

large

lattice

stress.

Finally,

we

have

reached

a

new

perovskite

composition-(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 having an appropriate bandgap of 1.52 eV and a long carrier lifetime of up to 16 s, which exhibits at least 10,000 h water-oxygen stability, 1,000 h light stability and excellent thermal stability. The single crystal structural analysis discloses an important information about the possible underlying mechanism for the enhanced long-term stability. Such a new composition provides a basis for the preparation of promising solar cells with high efficiency as well as long-term stability. The present work provides a single crystal-based paradigm for investigations of the intrinsic long-term stability of perovskite materials, and combining with theoretical simulation using single crystal structure data would further guide precise design and discovery of new and more stable perovskite compositions for highly efficient long-term stable PSCs.

ASSOCIATED CONTENT Supporting Information. The Supporting Information including additional characterizations and two crystal structure files are available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * [email protected];

[email protected]

Author Contributions #

L. Chen and Y. Y. Tan contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by MOST (Grant 2016YFA0200703) and NSFC (21621091, 21473147). L. Chen especially thanks Mr. Guo-Cheng Deng, and Dr. Zi-Er Yan for single crystal structure measurement and analysis. Profs. Hai-Ping Xia, Su-Yuan Xie, Bin Ren, and Nan-Feng Zheng at Xiamen University and Prof. Quan-Ming Wang at Tsinghua University are gratefully acknowledged for their generous supports.

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Figure 1. Perovskite crystal growth process and alloy property characterizations. A) CsPbBr3 solids in GBL solution; B) CsPbBr3 solids dissolve into a clear solution after adding FAI and PbI2; C) The (FAPbI3)0.95(CsPbBr3)0.05 single crystal formed by heating the solution in (B); D) XRD patterns of (FAPbI3)1-x-y(MAPbBr3)y(CsPbBr3)x single crystals; For convenience, it is simplified as FA1-x-yMAyCsx, and FA1-x-y represents (FAPbI3)1-x-y, MAy and Csx represent (MAPbBr3)y and (CsPbBr3)x, respectively. E) Enlarged XRD patterns of (002) plane and F) Tauc plots of the five compositions; the black curves represent for FA0.95Cs0.05 with band gap 1.48 eV, the red for FA0.9MA0.05Cs0.05 with band gap 1.52 eV, the pink for FA0.9Cs0.1 with band gap 1.53 eV, the blue for FA0.85MA0.1Cs0.05 with band gap 1.55 eV, and the green for FA0.85MA0.05Cs0.1 with band gap 1.57 eV.

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Figure 2. Long-term water-oxygen stability and the phase separation induced by Cs. A) XRD patterns of three compositions containing 5% CsPbBr3 after 10,000 h water-oxygen stability aging with humidity 20%; B) XRD pattern changes of (FAPbI3)0.90(CsPbBr3)0.1 after 360 h aging with humidity 20%; C) Yellow needle-like crystals and black block crystals precipitating from the (FAPbI3)0.85(CsPbBr3)0.15 solution; D) XRD patterns of the two phase-separated substances shown in (C).

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Figure 3. Long-term water-oxygen stability and the phase separation induced by Br. A) Photographs showing the phase transitions from black solids to red solids. B) XRD patterns of the black block and red crystals separated from (FAPbI3)0.85(CsPbBr3)0.15 after 72 h aging with humidity 20%; C) and D) XRD patterns of (FAPbI3)1-y(FAPbBr3)y (short for FA1-yFAy) and (FAPbI3)1-x-y(FAPbBr3)y(CsPbBr3)x (short for FA1-x-yFAyCsx) series single crystals after 72 h aging with humidity 20%.

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Figure 4. Cs enhances the light stability and single crystal structure. Tests with strong lasing power density

under

the

atmosphere

(60%

(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05.

C)

humidity)

for

Transient

A)

(FAPbI3)0.90(MAPbBr3)0.1

photoluminescence

and

comparisons

B) of

(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 before and after 1,000 h “day and night” light stability test. D) Thermal gravity analysis and onset decomposition temperature of the above two compositions. E) Transient photoluminescence spectra and average carrier lifetimes of the two compositions. F) Crystal structure of (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05. The green balls represent for Pb atoms, the red balls for Br atoms, and the pink ones for I atoms, the yellow ones for Cs atoms, and the gray and blues ones for C and N atoms respectively.

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Table 1. The crystal structure parameters of different compositions. The crystals were characterized at 173 K and show distorted inorganic octahedral framework with cubic system.

Table of Contents (TOC)

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Toward Long-Term Stability: Single-Crystal Alloys of Cesium

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