Pseudohalogen-Based 2D Perovskite: A More Complex Thermal

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Pseudohalogen-Based 2D Perovskite: A More Complex Thermal Degradation Mechanism Than 3D Perovskite Yanping Lv,† Duanhui Si,† Xuedan Song,† Kai Wang,† Shi Wang,† Zhengyan Zhao,† Ce Hao,*,†,‡ Lijuan Wei,‡ and Yantao Shi*,† †

State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian, 116024, China Dalian University of Technology, Panjin Campus, Panjin, 124221, P. R. China

‡

S Supporting Information *

ABSTRACT: (MA)2Pb(SCN)2I2, a new pseudohalogen-based 2D perovskite material, was reported as a very stable and promising photo-absorber in PSCs previously. However, the later researchers found that MA2Pb(SCN)2I2 was not as stable as claimed. Thus, it is very critical to clarify the controversy and reveal the degradation mechanism of MA2Pb(SCN)2I2. On the other hand, a large number of studies have indicated that adding a small amount of SCN− improves surface topography and crystallinity. However, whether SCN− ions can be incorporated into a 3D perovskite film remains debatable. In this work, the thermal degradation pathway of (MA)2Pb(SCN)2I2 is revealed by thermal gravimetric and differential thermal analysis coupled with quadrupole mass spectrometry and density functional theory calculations. The decomposition of (MA)2Pb(SCN)2I2 has been proved experimentally to be more complex than that of MAPbI3, involving four stages and multi-reactions from room temperature to above 500 °C. By combining the experimental results and theoretical calculations, it is found that 2D (MA)2Pb(SCN)2I2 actually is unstable when serving as photo-absorber in PSCs. Moreover, the role of SCN− in improving the crystallinity of 3D perovskite has also been discussed in detail.

1. INTRODUCTION Organic−inorganic hybrid halide perovskites are known as a class of superior photoelectric materials with many desirable attributes, such as strong light absorption, small exciton binding energy, ambipolar charge transport,1 and ultralong diffusion of carriers.2 The application of such materials in photovoltaics is successful because perovskite solar cells (PSCs) have achieved impressive progress in power conversion efficiency (PCE) from 9.7%3 to over 22.1%4 within a few years (2012−2016). However, despite this increased efficiency, the long-term stability of PSCs remains unsolved and is one of the main issues that limit the future commercialization and application of PSCs. Understanding the factors that affect the stability of PSCs usually requires probing the intrinsic degradation mechanism of perovskite materials. Intensive studies have been conducted to determine the degradation pathways of common 3D perovskites, such as CH3NH3PbI3, under various conditions (e.g., thermal degradation and that occurring in moisture or other specific atmospheres).5−9 The advancement of PSCs has been accompanied by the introduction of new perovskites, whose intrinsic properties, including thermal degradation mechanisms, require comprehensive investigations. The chemical characteristics of pseudohalogen ions, such as CN−, OCN−, SCN−, and SeCN−, are similar to those of their normal halide ion counterparts. Therefore, the properties of halide perovskites can be modified via substitution by pseudohalogen ions. (CH3NH3)2Pb(SCN)2I2 (MAPSI), a new 2D perovskite, has been synthesized and received considerable attention because of its crystal structure, stability, © XXXX American Chemical Society

and photoelectric properties. Initially, this material was regarded as a 3D perovskite defined as CH3NH3Pb(SCN)2I by Jiang et al., who claimed that this new material is superior to the conventional CH3NH3PbI3 in terms of stability against humidity.10 Daub and Hillebrecht determined that the single crystal of this material is a 2D K2NiF4-type structure.11 Improved stability is one of the advantages of MAPSI, but it is an issue in dispute at present. Ganose et al. studied the thermal stability of perovskites by referring to hybrid density functional theory.12 By calculating reaction enthalpy (ΔH), they claimed that MAPSI is more stable than CH3NH3PbI3 because the two possible decomposition pathways, (CH3NH3)2Pb(SCN)2I2 → CH3NH3I + Pb(SCN)2 and (CH3NH3)2Pb(SCN)2I2 → 2CH3NH3SCN + PbI2, have positive ΔH, which indicates a nonspontaneous process. However, other research groups pointed out that MAPSI is not as stable as others have claimed. For example, the phenomenon of red-to-black piezochromism in a mechanical compress test was reported by Umeyama et al., who also observed that MAPSI decomposes into a black solid rapidly in ambient humidity.13 With regard to the synthesis of 3D perovskites, such as CH3NH3PbI3, a large number of studies14−17 have indicated that adding a small amount of SCN− improves surface topography and crystallinity. However, whether SCN− ions can be incorporated into a 3D perovskite film remains debatable. A systematic study must be conducted Received: November 20, 2017

A

DOI: 10.1021/acs.inorgchem.7b02949 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

around 2050 cm−1, which can be assigned to the C-N stretching vibration. Several research groups12,18,19 that conducted theoretical studies are optimistic about the future PV application of this material. We assembled the MAPSI film into a solar cell with an architecture of FTO/compact TiO2/ MAPSI/Spiro-OMeTAD/Ag. However, the device could not operate as a PV because PCE was close to zero. We also observed that the MAPSI film was not as chemically stable as expected when removed from the glovebox and exposed to ambient atmosphere. The moisture stimulated degradation of MAPSI has been reported by Umeyama et al.13 Chemical stability is an important issue, but we focused on the thermal stability of this 2D material. Although MAPSI has been theoretically predicted to possess good thermal stability, only a few relevant experimental investigations have been reported so far. Considering that the thermal degradation of MAPSI is likely to be accompanied by mass loss, gas release, and endothermic/ exothermic phase transitions, thermal gravimetric and differential thermal analyses coupled with mass spectrometry (TGDTA-MS) can be used as a suitable and powerful analytical method to thoroughly examine the degradation pathway. Juarez-Perez et al. applied TG-DTA-MS and discovered the thermal degradation pathways of CH3NH3PbI3 and CH3NH3I with NH3 and CH3I are degradation products.20 In our experiment, TG-DTA-MS measurement of the MAPSI powder sample was conducted in an inert atmosphere (He). As shown in Figure 2, the thermal degradation of MAPSI underwent four

on the thermal degradation mechanism of MAPSI to clarify these arguments. Only a few studies have examined such issues. The current work aims to reveal the thermal degradation pathway of MAPSI through thermal gravimetric and differential thermal analyses coupled with quadrupole mass spectrometry (TG-DTA-MS). We found that the decomposition of MAPSI is more sophisticated than that of CH3NH3PbI3 and involves four stages and multi-reactions from room temperature to above 500 °C. In the first stage below 100 °C, a reversible process of solid−solid phase separation without weight loss occurred. Thermal decomposition of solid compounds with gas evolution occurred in the second stage between 100 and 200 °C as six major gaseous products were produced, leading to ∼20.5% weight loss. Degradations occurred extensively in the third and fourth stages, which corresponded to the decomposition of CH3NH3PbI3 and sublimation of two lead salts of PbI2 and Pb(SCN)2, respectively. On the basis of experimental results and theoretical calculations, the degradation pathways in each stage were revealed. We conclude that 2D MAPSI is unstable when used as a photo-absorber in PSCs.

2. RESULTS AND DISCUSSION Our film and powder samples were prepared by spin-coating and solid reaction, respectively, using Pb(SCN)2 and CH3NH3I as precursors. The details of the synthesis are provided in the Experimental Section. Our two samples were identified to be 2D MAPSI by X-ray diffraction (XRD), as shown in Figure 1a, and the main diffraction peaks can be assigned according to the previous report.18 The FTIR spectra illustrated in Figure 1b show the characteristic peaks of the film and powder samples at

Figure 2. TG-DTA traces (upper) and m/z peaks (below) registered simultaneously during the thermal degradation (heating rate of 5 °C min−1) of MAPSI.

stages. The first stage occurred at about 70−80 °C, accompanied by an endothermic effect but without weight loss. This process was further examined through differential scanning calorimetry (DSC) measurement below 80 °C. As demonstrated in Figure 3a, several endothermic/exothermic peaks were observed in the heating and cooling processes. Accordingly, a reversible color change between red and dark

Figure 1. (a) X-ray diffraction (XRD) patterns of (CH3NH3)2Pb(SCN)2I2 (MAPSI) film and powder samples; the inset two figures are their photos. (b) Corresponding FTIR spectra of the two samples. B

DOI: 10.1021/acs.inorgchem.7b02949 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Energy Profiles for the Transformation of CH3NH3NCS and CH3NH3SCNa

a

relativistic hybrid density functional theory.12 In their study, several possible routes in the form of phase separations were proposed to describe the thermal decomposition of MAPSI analogues. Calculations suggested that the enthalpies (ΔH) of all decomposition routes are positive. Thus, the researchers concluded that MAPSI analogues are more stable than CH3NH3PbI3. The theoretical prediction in this previous study contradicts our experimental finding, which presents the indisputable fact that MAPSI decomposes easily even when subjected to heating below 100 °C. Therefore, we posit that the interaction between Pb2+ and pseudohalide anions may not be as strong as that between Pb2+ and halogen ions. Nevertheless, we acknowledge that another possible driving force could lead to MAPSI decomposition. With the further increase in heating temperature, many gaseous products were detected by MS in the two subsequent stages (stage II, 110−200 °C; stage III, 200−310 °C). Among the three possible products in stage I, Pb(SCN)2 was thermally stable below 200 °C (Figure S1). CH3NH3PbI3 could not have caused weight loss because only solid−solid phase separation occurred (from CH3NH3PbI3 to CH3NH3I and PbI2).21 Thus, we conjecture that the gaseous products in stage II were released from the thermal decomposition of CH3NH3NCS. The complete original MS results are provided in the Supporting Information (Figures S2−S10). The mass-to-charge ratio (m/z) peaks were assigned carefully according to the database of the National Institute of Standards and Technology (NIST) Chemistry WebBook. The detailed results are summarized in Table 1. Six major products were generally produced in stage II: NH3, CH3NH2, CH3SCN (or CH3NCS), CH3NHCH3, (CH3)3N, and (NH2)2CS. We proposed a mechanism by which CH3NH3NCS thermally decomposed following three possible pathways by means of DFT calculation, as shown in Scheme 2. Although the pathway resulting in product 1 (eq 2-1) has a relatively small Ea (13.2 kcal mol−1) and is kinetically favorable, we cannot simply preclude the occurrence of the two other pathways depicted as eqs 2-2 and 2-3 by which NH3 and CH3SCN (or CH3NCS) are produced and recorded by MS. HSCN, as one of the two products in eq 2-1, was not detected probably because this compound is unstable, and no relevant MS information was found in NIST. This result encouraged us to speculate that HSCN is consumed promptly by another consecutive reaction, as depicted by eq 24, where HSCN reacted with NH3 to form (NH2)2CS (sulfocarbamide). Meanwhile, CH3NHCH3 and (CH3)3N are

Figure 3. (a) Differential scanning calorimetry (DSC) curve for MAPSI during a heating and cooling cycle, photos showing the color of MAPSI film at 25 and 80 °C, respectively. (b) XRD patterns of MAPSI at different temperatures.

gray was clearly observed (inset photos in Figure 3a). Thus, we infer that stage I corresponds to a process of solid−solid phase separation. The XRD patterns in Figure 3b indicate that, when the film was heated to 80 °C, the characteristic peaks of MAPSI vanished. The emerging peaks can be assigned to CH3NH3PbI3 and Pb(SCN)2. When the film was cooled to 25 °C, the characteristic peaks of MAPSI reappeared, further verifying that this process is reversible to some degree. On the basis of this analysis, we described the chemical process of stage I as eq 1. 3(CH3NH3)2 Pb(SCN)2 I 2 ⇔ 2CH3NH3PbI3 + 4CH3NH3NCS + Pb(SCN)2 −

White color sphere: H; yellow: S; grey: C; cyan: N.

(1)

−

Thiocyanate (SCN ) and isothiocyanate (NCS ) possess an identical chemical composition but different bonding geometric structures. CH3NH3NCS, rather than CH3NH3SCN presented in eq 1, is supported by DFT calculation. The energy profile in the transformation of CH3NH3NCS and CH3NH3SCN is shown in Scheme 1. The energy barrier (or Ea) for the forward reaction starting from CH3NH3NCS was much larger than that for the reverse process. Furthermore, the ΔG for the reaction CH3NH3NCS → CH3NH3SCN was theoretically determined to be 14.9 kcal mol−1, indicating that CH3NH3NCS was more thermodynamically stable than CH3NH3SCN. In other words, CH3NH3SCN can be transformed into CH3NH3NCS spontaneously at room temperature. Ganose et al. recently conducted investigations on the electronic and defect properties of MAPSI analogues using C

DOI: 10.1021/acs.inorgchem.7b02949 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Detected m/z Signals and Their Corresponding Gases According to NIST in Stage II and Stage III during the Thermal Degradation of MA2Pb(SCN)2I2 m/z 15, 17 28, 29, 30 15, 26, 27, 44, 45, 46, 58, 70, 71, 72, 73, 74, 75 33, 34, 42, 43, 48, 59, 60, 76, 77, 78 26, 27, 29, 30, 42, 43, 58 28, 42, 44, 45 15, 16, 17 26, 27, 29, 30, 42, 43, 58 127, 139, 140, 141,142

CH3NH 2 + CH3NCS → CH3NHCH3 + H+ + NCS− (2-6)

CH3NHCH3 + CH3NCS → (CH3)3 N + H+ + NCS−

assignments NH3 CH3NH2 CH3SCN or CH3NCS

(2-7)

stage II temperature: 110−200 °C time: 2000−2300 s

As reported by several groups, in the synthesis of 3D perovskites (e.g., CH3NH3PbI3), the effects of SCN− as an additive for tuning crystalline morphology and enhancing photovoltaic performance and moisture tolerance are impressive. With a very small amount of Pb(SCN)2 addition (e.g., 5% vs PbI2), for example, grain size can be enlarged remarkably from the nanometer to the micrometer scale.15 An argument exists as to whether SCN− ions acting as additives can be incorporated into the resulting 3D perovskite film. Several researchers found SCN− or a sulfur element in the final films,14,16,17 whereas others reported the opposite.15 On the basis of our results, we emphasize that SCN− can hardly be incorporated into the crystal network of 3D perovskite and can only exist as thiocyanate salts (e.g., Pb(SCN)2) in the grain boundaries of 3D perovskite. After the thorough decomposition of CH3NH3NCS in stage II, three solid residuals are assumed to exist: CH3NH3I, Pb(SCN)2, and PbI2. Among them, CH3NH3I is the one that decomposes into gaseous products and eventually results in a weight loss in stage III. A dispute existed about the decomposition products of CH3NH3I until Qi et al. conducted a schematic study through TG-DTA-MS. 20 Currently, CH3NH3I is believed to decompose into CH3I and NH3 as the temperature increases to over 240 °C. The m/z peaks recorded in stage III can be assigned to three chemicals listed in Table 1. (CH3)3N was possibly produced by the mechanism proposed in the above-mentioned literature: the previously formed NH3 reacts with the remaining CH3NH3+ trapped in the PbI3− octahedral network to form CH3NH2, which then reacts with the excess CH3I to form CH3NHCH3 (secondary amine), finally ending until the formation of (CH3)3N (tertiary amine) via the Menshutkin reaction. Notably, the weight loss in stage III (12.3%) was smaller than the theoretical value (16.6%) probably because some CH3NH3+ ions were consumed in advance by the as-formed NH3 (in stage II). This condition can explain why the weight loss in stage II (20.5%) was higher than the theoretical value (18.7%). The residuals after stage III were Pb(SCN)2 and PbI2, which were further identified by the XRD patterns shown in Figure S11. The sublimation of the two compounds occurred in the last stage (heated up to 520 °C).

(NH2)2CS (CH3)3N CH3NHCH3 NH3 (CH3)3N

stage III temperature: 200−320 °C

CH3I

time: 2600−3300 s

Scheme 2. Left Panel: Activation Energy Illustrating the Decomposition Process of CH3NH3NCS for the Chemical Reaction Pathways Producing CH3NH2 and HSCN (Black), CH3SCN and NH3 (Blue), and CH3NCS and NH3 (Red). Right Panel: Optimized Molecular Geometries and Structures for Reactant (R1), Intermediate Product (IM1), and Product (P1, P2, P3)a

a

White color sphere: H; yellow: S; grey: C; cyan: N.

produced possibly by a mechanism similar to that reported by Juarez-Perez et al.,20 as depicted by equations 2-5, 2-6, and 2-7. The previously formed NH3 reacts with the remaining CH3NH3+ to form CH3NH2, which then reacts with the excess of CH3SCN (or CH3NCS) to form CH3NHCH3 (secondary amine), finally ending until the formation of (CH3)3N (tertiary amine) via the Menshutkin reaction.

3. CONCLUSIONS In conclusion, the thermal degradation pathway of MSPAI, a known 2D perovskite, was studied in detail by TG-DTA-MS in combination with other characterizations. Unlike that of common 3D perovskites, such as CH3NH3PbI3, the decomposition of MAPSI, which involves multi-step reactions from 70 to 520 °C, is more sophisticated. Contrary to previous theoretical prediction, our experimental finding suggests that MAPSI decomposes easily via a reversible process of solid− solid phase separation even when subjected to heating below 100 °C. The thermal decomposition of solid compounds with gas evolution occurs in the second stage between 100 and 200 °C as six major gaseous products take form, leading to ∼20.5% weight loss. Degradations occur extensively in the third and fourth stages, which correspond to the decomposition of CH3NH3PbI3 and sublimation of two lead salts of PbI2 and

CH3NH3NCS → TS1 → CH3NH3SCN → TS2 → HSCN + CH3NH 2

(2-1)

CH3NH3NCS → TS1 → CH3NH3SCN → TS4 → NH3 + CH3SCN

(2-2)

CH3NH3NCS → TS3 → NH3 + CH3NCS

(2-3)

NH3 + HSCN → (NH 2)2 CS

(2-4)

CH3NH3NCS + NH3 → CH3NH 2 + NH4 + + NCS− (2-5) D

DOI: 10.1021/acs.inorgchem.7b02949 Inorg. Chem. XXXX, XXX, XXX−XXX

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Pb(SCN)2, respectively. On the basis of the experimental results and theoretical calculations, the degradation pathways in each stage were revealed in detail. We posit that 2D MAPSI is unsuitable for use as a photo-absorber in PSCs.

Synthesis of (MA)2Pb(SCN)2I2 Powder and Film Samples. Pb(SCN)2 and CH3NH3I were purchased from Sigma-Aldrich and Xi’an Polymer Light Technology Corp, respectively. N,N-Dimethylformamide (99.8%, superdry) was purchased from J&K Scientific Ltd. 203 mg of Pb(SCN)2 and 200 mg of MAI were ground in a nitrogenfilled glovebox; then a dark red powder was yielded. For fabrication of the film sample, the precursor solution was prepared by dissolving 203 mg of Pb(SCN)2 and 200 mg of CH3NH3I in 0.6 mL of anhydrous N,N-dimethylformamide (DMF) under stirring at 70 °C for 3 h. Then, 100 μL of precursor solution was spin-coated onto a glass substrate at 2000 rpm for 60 s; subsequently, a red film formed. Characterization. X-ray diffraction (XRD) measurements of all samples were performed by SmartLab 9 (Rigaku Corporation), and the heating attachment was XRK900 (Anton Paar Corporation). FTIR spectra of both powder and film samples were recorded with a reflective infrared spectrometer (6700, Thermo Fisher). Simultaneous thermal gravimetric (TG) and differential thermal analysis (DTA) (TGA/DSC1, METTLER TOLEDO) data of powder samples were recorded at a heating rate of 5 °C min−1 in a temperature range from 30 to 700 °C using He as carrier gas. Released gases from the TGDTA apparatus were fed directly into the mass spectrometer (MS, Pfeiffer Vacuum) for analysis from m/z = 1 to m/z = 150. Calculation Method. All calculations were carried out at the B3LYP/6-311++G(d,p) level22 of DFT using Gaussian 09 software.23 Transition states (TS) were confirmed by the sole virtual frequency of the structure. Connectivity of each TS was acquired by the intrinsic reaction coordinate (IRC) approach. Gibbs free energies were depicted by the relative free energies (298.15 K, 1 atm) calculated in gas phase. Zero-point energy correction has been taken into consideration in all calculations.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02949. XRD spectra, and gases evolution curves recorded simultaneously in the MS equipment (PDF)

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REFERENCES

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4. EXPERIMENTAL SECTION

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S.). *E-mail: [email protected] (C.H.). ORCID

Ce Hao: 0000-0002-4379-0474 Yantao Shi: 0000-0002-7318-2963 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51402036 and 51773025), and the International Science & Technology Cooperation Program of China (Grant No. 2013DFA51000). E

DOI: 10.1021/acs.inorgchem.7b02949 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Perovskite: A Combined Experimental and Density Functional Theory Study. J. Phys. Chem. Lett. 2016, 7, 1213−1218. (19) Xiao, Z.; Meng, W.; Wang, J.; Yan, Y. Defect properties of the two-dimensional (CH3NH3)2Pb(SCN)2I2 perovskite: a density-functional theory study. Phys. Chem. Chem. Phys. 2016, 18, 25786−25790. (20) Juarez-Perez, E. J.; Hawash, Z.; Raga, S. R.; Ono, L. K.; Qi, Y. Thermal degradation of CH3NH3PbI3 perovskite into NH3 and CH3I gases observed by coupled thermogravimetry-mass spectrometry analysis. Energy Environ. Sci. 2016, 9, 3406−3410. (21) Supasai, T.; Rujisamphan, N.; Ullrich, K.; Chemseddine, A.; Dittrich, T. Formation of a passivating CH3NH3PbI3/PbI2 interface during moderate heating of CH3NH3PbI3 layers. Appl. Phys. Lett. 2013, 103, 183906. (22) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Selfconsistent molecular orbital methods. XX. A basis set for correlated wave functions. J. Chem. Phys. 1980, 72, 650−654. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A. 01; Gaussian Inc.: Wallingford, CT, 2009.

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DOI: 10.1021/acs.inorgchem.7b02949 Inorg. Chem. XXXX, XXX, XXX−XXX