Thermal Stability of Mixed Cation Metal Halide Perovskites in Air

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5485−5491

Thermal Stability of Mixed Cation Metal Halide Perovskites in Air Wanliang Tan, Andrea R. Bowring, Andrew C. Meng, Michael D. McGehee, and Paul C. McIntyre* Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: We study the thermal stability in air of the mixed cation organic−inorganic lead halide perovskites Cs0.17FA0.83Pb(I0.83Br0.17)3 and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. For the latter compound, containing both MA+ and FA+ ions, thermal decomposition of the perovskite phase was observed to occur in two stages. The first stage of decomposition occurs at a faster rate compared to the second stage and is only observed at relatively low temperatures (T < 150 °C). For the second stage, we find that both decomposition rate and the activation energy have similar values for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 and Cs0.17FA0.83Pb(I0.83Br0.17)3, which suggests that the first stage mainly involves reaction of MA+ and the second stage mainly FA+. KEYWORDS: mixed cation, perovskite, thermal stability, in situ, XRD

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INTRODUCTION Hybrid organic−inorganic perovskite solar cells (PSCs) based on MAPbI3 have attracted tremendous interest in the photovoltaic field. Power conversion efficiencies of PSCs have increased from 31 to 22.1%2 over the past 7 years because of their unique optoelectronic properties3−5 and compositional versatility.6 However, the environmental and thermal instability of MAPbI3 remains a major challenge for this material’s application in commercial solar energy. It is generally known that MAPbI3 is susceptible to decomposition as a result of exposure to moisture, elevated temperature, oxygen, UV light,7−10 and so forth. The degradation process involves the formation of PbI2 accompanied by the creation of volatile species.11−13 To improve the stability of the perovskite layer, various strategies including incorporation of a hydrophobic polymer,14−16 encapsulation of the device,17,18 and use of electrode19−21 materials that are stable with respect to the reaction with the perovskite have been proposed. Recently, improving the stability of perovskites by tuning the composition has received great attention. Replacing the methylammonium (MA+) ion with other cations such as formamidium (FA+) and Cs+ while replacing a portion of the iodide anions with Br− or Cl− has led to significantly improved stability compared to MAPbI3.22,23 McMeekin and co-workers24 found that by replacing both the monovalent cations and a portion of the anions in MAPbI3, Cs0.17FA0.83Pb(I0.83Br0.17)3 achieved superior thermal stability. Later, Saliba et al.25 reported that triple cation perovskites © 2018 American Chemical Society

Csx(MA0.17FA0.83)1−xPb(I0.83Br0.17)3 also show better stability and efficiency, with a stabilized power output of 21.1 and 18% after 250 h under operational conditions for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. Here, we characterize the thermal decomposition behavior of the mixed cation perovskites Cs0.17FA0.83Pb(I0.83Br0.17)3 and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. Decomposition kinetics in air (in the dark) are explored, and effective activation energies are extracted for these two compositions, providing additional insights into the effects of perovskite composition on stability.

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RESULTS AND DISCUSSION A detailed description of the perovskite thin film synthesis procedure is provided in the Supporting Information. Basically, 500 nm thick perovskite films are spun on top of indium tin oxide (ITO) glass substrates and annealed at 100 °C for 1 h. A representative X-ray diffraction (XRD) pattern for Cs0.17FA0.83Pb(I0.83Br0.17)3 is shown in Figure 1. The pattern is labeled with the assignment of crystallographic planes according to ref 26. We see that the diffraction peaks all belong to the α-phase, and no δ-phase peaks appear. To test the performance of these materials, we made a solar cell with the structure of the device shown in Figure 2a. With poly(3,4Received: October 8, 2017 Accepted: January 12, 2018 Published: January 12, 2018 5485

DOI: 10.1021/acsami.7b15263 ACS Appl. Mater. Interfaces 2018, 10, 5485−5491

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ACS Applied Materials & Interfaces

change at the grain boundaries is consistent with previous reports of reduction in conductivity associated with the formation of PbI2 near the grain boundaries of perovskite thin films.27−29 Auger electron spectroscopy (AES) was used to resolve the chemical composition of the grains. As shown in Figure 4, the Pb/(I + Br) ratio on the surface is different for the bright regions compared to the dark regions of the secondary electron image (Figure 4b). The Pb/(I + Br) ratio of the dark region is closer to that of the fresh sample. Also, the carbon elemental fraction on the film surface of the annealed sample is significantly lower than that of the fresh sample. These results suggest a loss of carbon associated with the organic cations and a loss of halide species during annealing. We note that the Pb/ (I + Br) ratio in the fresh sample is higher than the expected 1:3; this could be due to the halide-rich surface of these perovskites because AES is a highly surface-sensitive technique.30,31 To further examine the structural evolution of the perovskite phase during air annealing and its kinetics, we used in situ XRD to study crystalline phase fractions. The perovskites were deposited on top of microscope slides to avoid the peaks from the ITO substrate as shown in Figure 1. Figure 5 shows XRD data collected for scattering angles 2θ = 37.5°−41.5°. The peak at 2θ = 38.6° is consistent with PbI2(003) and that at 2θ = 40.6° is the α-phase perovskite (220) peak. The perovskite peak shifts to a lower angle once the sample is heated, consistent with an increase in the lattice constant. After cooling down, the peak shifts back to its original scattering angles. According to the literature,32 the thermal expansion coefficient of MAPbI3 is 1.57 × 10−4 K−1. Although the thermal expansion coefficient of Cs0.17FA0.83Pb(I0.83Br0.17)3 has not been reported, it is not unreasonable to assume that it is similar to that of MAPbI3. If so, the lattice constant should increase by ∼1.6% when the temperature increases by 100 K. Therefore, a peak at 2θ = 40.6° will shift to 2θ = 39.9°, which is consistent with the observed shift. Figure 6 shows representative in situ XRD data collected during an isothermal annealing experiment at 144 °C. The decrease in intensity of the peak at 2θ = 40.25° [perovskite (220)] is accompanied by a simultaneous increase in the intensity of the peak at 2θ = 38.5° [PbI2(003)], indicating decomposition of the perovskite phase and the reaction product, PbI2. In addition, there are no perovskite peaks that increase in intensity with time in the diffraction pattern,

Figure 1. XRD of the as-prepared Cs0.17FA0.83Pb(I0.83Br0.17)3.

Figure 2. (a) Schematic of the perovskite-based solar cell structure used in this study. (b) J−V characteristics of the ITO/PEDOT/ perovskite/C60/BCP/Ag solar cell.

ethylenedioxythiophene) (PEDOT) as the hole-transporting material and C60/bathocuproine (2,9-dimethyl-4,7-diphenyl1,10-phenanthroline) (BCP) as the electron-transporting layer, the solar cell I−V characteristics (Figure 2b) are consistent with an efficiency of 13.4%. To test the thermal stability of this material, we first used optical microscopy and scanning electron microscopy (SEM) imaging to view the change of the morphology during air annealing at 150 °C. The relative humidity level in air is about 60%. From the optical images of Figure 3, it is evident that the brightness of the images increases with annealing time, indicating decreasing visible absorption, which is also consistent with the UV−visible absorption data in Figure S3. The SEM images indicate grain growth and the formation of high-contrast grain boundaries with increasing annealing time, which is the evidence of increasing inhomogeneity of the film. The contrast

Figure 3. Optical microscopy and SEM characterization of the material after annealing in air at 150 °C. 5486


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Figure 4. AES analysis of the double cation perovskite thin film after annealing for 9 h in air at 150 °C. The red and blue squares in (a) represent the surface scan spot of the respective colored spectra in (b).

A(t) is the area measured at time t. The decomposition kinetics obtained from XRD analysis are shown in Figure 7a. For all the temperatures analyzed, the rate of intensity loss of the perovskite (220) peak is constant, and thus the curves follow zeroth-order kinetics, which are typical of catalyzed reactions. As described in ref 33, zeroth-order kinetics for perovskite phase decomposition are consistent with a reaction, in which water vapor present in the air catalyzes deprotonation of the complex organic cation in the perovskite crystal. If this is the case, the perovskite phase volume fraction in the film, x, will decrease linearly as a function of time

dx = − K (T ) dt

Figure 5. XRD pattern of the Cs0.17FA0.83Pb(I0.83Br0.17)3 thin film.

where K(T) is a kinetic constant that depends on temperature T. The XRD data permit determination of the kinetic constant K(T) for different temperatures. We find that K(T) obeys

(

E

)

Arrhenius behavior: K = K 0 exp − RTa . An activation energy for the decomposition reaction Ea = 0.66 eV is obtained. This value is smaller than that reported for MAPbI3 but close to that reported for FAPbI3 in ref 33. Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 Stability. A representative diffraction pattern measured from a Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 thin film is shown in Figure 8a. Again, we find that the diffraction peaks all belong to the α-phase perovskite structure and no δ-phase perovskite peaks are detected. The solar cell current−voltage data obtained for this absorber are shown in Figure 8b. Optical microscopy and SEM imaging of this perovskite layer were performed before and after annealing at 150 °C in the air. As shown in Figure 9, the increasing brightness of the perovskite film under identical optical imaging conditions suggests that visible light absorption decreases during annealing. From the SEM images, it is apparent that annealing induces grain growth. The grain size increases from ∼400 nm for the fresh sample to ∼1 μm for the sample annealed at 150 °C for 4.5 h. We also measured UV−vis absorption for the freshly prepared perovskite thin films and for samples after annealing. As shown in Figure 10, the onset of absorption shifted to shorter wavelengths as a result of annealing. A more interesting observation is the greater shift after the first 4.5 h of annealing compared to the second 4.5 h. If the decomposition kinetics for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 thin films in air are

Figure 6. Time evolution of XRD spectra of Cs0.17FA0.83Pb(I0.83Br0.17)3 at 144 °C. The labels 1−10 represent the time series of the XRD pattern, 1first scan and 2second scan. The time interval between each measurement is 25 min.

suggesting that the crystallographic texture of the perovskite crystallites is largely maintained during the decomposition reaction. Similarly, we studied the decomposition kinetics of Cs0.17FA0.83Pb(I0.83Br0.17)3 in air at different temperatures. The integrated area of the (220) perovskite peak is used as a representative parameter for the volume fraction of the perovskite remaining as a function of time. The perovskite percentage is obtained by applying the equation: x = [A(t)/ A(0)], where A(0) is the area of the (220) peak at t = 0 and 5487


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Figure 7. (a) Kinetics analysis and fitting of the Cs0.17FA0.83Pb(I0.83Br0.17)3 perovskite phase decomposition curves of samples annealed at six different temperatures in air. (b) Arrhenius plot and extracted activation energy.

Figure 8. (a) XRD pattern of the as-prepared Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. (b) J−V characteristics of the ITO/PEDOT/perovskite/C60/ BCP/Ag solar cell.

Figure 9. Optical microscopy and SEM characterization of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 after annealing in air at 150 °C.

performed. The XRD data were collected for scattering angles 2θ = 37.5°−41.5°, as shown in Figure 12. The key difference between results obtained from the triple cation perovskite and the double cation perovskite (Figure 7) is that the loss in intensity of the perovskite peaks in the triple cation compound occurs at a nonconstant rate for low annealing temperatures (approximately T ≤ 150 °C). As shown in Figure 13b, the decomposition occurs in two stages under such conditions, with the decomposition rate of the first stage being higher than that of the second (later) stage. For the higher temperatures studied, the decomposition rate appears to

similar to the case of Cs0.17FA0.83Pb(I0.83Br0.17)3, that is, zeroth order, then the data in Figure 9 suggest that the former perovskite composition may decompose in two steps with two different rate constants. The results obtained from AES analysis in Figure 11 of the triple cation perovskite films annealed in air for 9 h are similar to those of the double cation perovskite. We observed a loss of carbon and halide anions after annealing. In situ XRD analysis during air annealing of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 thin films was also 5488


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Figure 12. Time-evolved XRD spectra of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 at 127 °C. The labels 1−13 represent the time series of the XRD pattern. The time interval between each measurement is 30 min.

Figure 10. UV−vis absorption spectrum evolution for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 at 150 °C.

be independent of time. However, this may simply indicate that the high-rate initial decomposition process is completed too quickly to be captured in the higher temperature diffraction experiments. Ignoring the temperature dependence of the first stage, because of the limited number of data available, we focus on the slower, second-stage kinetics and extract an activation energy for the triple cation perovskite, as shown in Figure 14. We also studied the thermal decomposition of MAPbI3 in air, and we found that it is much faster than for Cs0.17FA0.83Pb(I0.83Br0.17)3, as shown in Figure S4. On the other hand, we can also compare the initial decomposition kinetics of the Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 composition to that of MAPbI3 at 120 °C. By extracting the first six data points for the integrated (220) perovskite peak intensity of the Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 data and comparing them to the decomposition data for MAPbI3, as shown in Figure S5, it is evident that the data produce very similar slopes. It is reasonable that the decomposition process involves the production of volatile species.34,35 Moreover, the difference between Cs0.17FA0.83Pb(I0.83Br0.17)3 and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 is that the latter composition has two organic cations, MA+ and FA+, whereas the former has only the less reactive FA+ cation. The results in Figure 12, indicating that the decomposition process for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 occurs in two stages, suggests that the first stage involves the water-catalyzed

reaction of the MA+ cation ion and the second stage is dominated by the reaction of the FA+ cation. As the MA+ ions react, producing volatile products and PbI2, the lattice constant of the remaining, increasingly FA+-rich perovskite phase should increase. This is, indeed, observed in the shift to smaller 2θ of the perovskite peak in Figure 12. Both stages of the decomposition reaction produce a PbI2 reaction product while the perovskite phase is decomposing. The evolution of PbI2 peak intensity as a function of air annealing time is included in the Supporting Information. Comparing Figures S1 and S2 in the Supporting Information, we can also conclude that decomposition of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 has two stages, whereas decomposition of Cs0.17FA0.83Pb(I0.83Br0.17)3 has only one stage. From the low-temperature data in Figure 12, it is apparent that the first-stage process is faster than the second stage. Consistent with the greater reactivity of the MA+ cation, the existence of the fast, first stage, decomposition process makes the Cs0.05(MA0.17FA0.83)0.95Pb(I 0.83 Br 0.17 ) 3 composition intrinsically less stable than Cs0.17FA0.83Pb(I0.83Br0.17)3.

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CONCLUSIONS We investigated the thermal decomposition kinetics of the mixed perovskite thin films Cs0.17FA0.83Pb(I0.83Br0.17)3 and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. XRD analysis indicates that the decomposition kinetics are zeroth order and exhibit

Figure 11. AES analysis of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 after 9 h of annealing in air at 150 °C. The red and blue squares in (a) represent the surface scan spot of the respective colored spectra in (b). 5489


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Figure 13. (a) Kinetics of the Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 reaction during annealing at different temperatures in air. (b) Two stage decomposition kinetics.

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ACKNOWLEDGMENTS We acknowledge the Bay Area Photovoltaic Consortium for supporting this research. Also, we acknowledge Arturas Vailionis in the Stanford University X-ray lab for discussions. A.C.M. acknowledges NSF Graduate Research Fellowship Award DGE-114747. A.C.M. acknowledges helpful discussions with Michael Braun.

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ABBREVIATIONS FA formamidium MA methylammonium PEDOT poly(3,4-ethylenedioxythiophene) BCP bathocuproine (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) XRD X-ray diffraction ITO indium tin oxide SEM scanning electron microscopy AES Auger electron spectroscopy

Figure 14. Arrhenius plot and extracted activation energy for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3.

Arrhenius behavior with an activation energy ∼0.66 eV for Cs0.17FA0.83Pb(I0.83Br0.17)3 and 0.76 eV for Cs0.05(MA0.17FA 0.83) 0.95Pb(I0.83Br0.17)3. We find that, for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3, decomposition to form PbI2 and volatile organic products happens in two stages. The first stage is faster than the second stage. The data are consistent with the H2O-catalyzed reaction of organic cations in these perovskite compositions, in which the two-stage decomposition includes a first stage involving the reaction of MA+ and a second stage involving the reaction of FA+.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15263. Device fabrication parameters, experimental procedures, and PbI2 XRD time-evolution (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wanliang Tan: 0000-0003-0820-4027 Andrew C. Meng: 0000-0002-3060-8928 Michael D. McGehee: 0000-0001-9609-9030 Notes

The authors declare no competing financial interest. 5490


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Thermal Stability of Mixed Cation Metal Halide Perovskites in Air

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