Letter pubs.acs.org/JPCL
Quantitative Phase-Change Thermodynamics and Metastability of Perovskite-Phase Cesium Lead Iodide Subham Dastidar,† Christopher J. Hawley,‡ Andrew D. Dillon,† Alejandro D. Gutierrez-Perez,‡ Jonathan E. Spanier,‡ and Aaron T. Fafarman*,† †
Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States ‡ Department of Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: The perovskite phase of cesium lead iodide (α-CsPbI3 or “black” phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional “yellow” phase (δ-CsPbI3). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state. Using differential scanning calorimetry, it is shown herein that the metastable state is maintained in the absence of moisture, up to a temperature of 100 °C, and a reversible phase-change enthalpy of 14.2 (±0.5) kJ/mol is observed. The presence of atmospheric moisture hastens the black-to-yellow conversion kinetics without significantly changing the enthalpy of the transition, indicating a catalytic effect, rather than a change in equilibrium due to water adduct formation. These results delineate the conditions for trapping the desired phase and highlight the significant magnitude of the entropic stabilization of this phase. apid advances in the field of organic−inorganic hybrid perovskites (OHPs) have significantly altered the photovoltaic research landscape. These materials boast exciting photophysical properties and robust, inexpensive fabrication techniques and reported power conversion efficiencies up to 22%.1 However, OHPs suffer from chemical instability due to the volatile2 and hygroscopic3,4 nature of the organic component. Recently, perovskite-phase cesium lead iodide (CsPbI3) has emerged as an all-inorganic alternative to the OHPs with strikingly similar optoelectronic properties.5−7 Cesium has also been shown to play a critical role as a substitutional dopant, imparting enhanced stability to the recently discovered mixed cation perovskites that form the basis of the highest performing OHP solar cells to date.8−13 According to the Goldschmidt tolerance factor, an empirical index of the structural stability of perovskites, cesium is almost too small to form a stable perovskite with lead iodide,10,14 despite being the largest nonradioactive elemental cation. Nonetheless, at elevated temperature (>305 °C) CsPbI3 adopts a cubic perovskite structure with a band gap 1.73 eV,5,6,15,16 known as the “black” phase or α-CsPbI3 (Figure 1a). The equilibrium phase of CsPbI3 at room temperature is a highbandgap, nonperovskite “yellow” phase (δ-CsPbI3) that is essentially nonfunctional as a photovoltaic material.17 It follows simply that the yellow δ-phase is enthalpically favored, while the functional, black α-CsPbI3 is entropically favored, however, the magnitudes of these competing contributions have been heretofore unknown. For practical applications, several strategies are being explored to form the α-CsPbI3 phase and preserve it in a
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© 2017 American Chemical Society
metastable state for use as a photovoltaic absorber. Our group recently demonstrated that chloride doping of CsPbI3, by codeposition of a mixture of CsPbI3 and CsPbCl3 nanocrystals and subsequent chemical sintering, improves the phase stability of the resulting polycrystalline α-CsPbI3.18 Deliberate formation of small crystal grains was associated with improved metastability in α-CsPbI3 thin films.5 This effect may also be at play in the demonstration that α-CsPbI3 quantum dots are stable under ambient conditions.6,19,20 Also, CsPbI3−xBrx mixed crystals have been shown to be more stable than the pure-phase CsPbI3.21,22 Developing a quantitative understanding of the phase-change thermodynamics and kinetics of α-CsPbI3 will be critical to these ongoing efforts to improve its stability, particularly in the presence of atmospheric moisture. Herein, the thermodynamics of the reversible phase transition between the black and yellow phases of CsPbI3 has been examined, and the transition enthalpies are reported using differential scanning calorimetry (DSC). The DSC results show a narrow endothermic peak around 320 °C and a much wider exothermic feature around 270 °C, signifying the yellow-toblack phase conversion and reappearance of the yellow phase, respectively. The position and breadth of the peaks provide insights into the kinetics of the phase transition in the regime in which the perovskite phase is metastable. Both indicators are measured herein as a function of the rate of heat flow, demonstrating significant thermal hysteresis, which is, in turn, Received: January 17, 2017 Accepted: March 3, 2017 Published: March 3, 2017 1278
DOI: 10.1021/acs.jpclett.7b00134 J. Phys. Chem. Lett. 2017, 8, 1278−1282
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Figure 1. (a) Crystal structure of black (α) and yellow (δ) phases of CsPbI3 at their thermodynamically stable temperatures. (b) XRD of CsPbI3 drop-cast films in black and yellow phase at room temperature. (c) UV−vis transmittance spectra of a thin film of CsPbI3, displaying complete transition from black to yellow phase within a period of 75 min after exposing to relative humidity (RH) of 33% at 23 °C. (d) Microscopic images of a thin film of CsPbI3, showing the generation of a yellow nucleation site, subsequent growth, and complete conversion within 15 min. The images were taken at 2, 5, 10, and 15 min after the exposure, respectively, at RH 50% and 23 °C.
metastable “parent” black phase (Figure 1d). Subsequently, the yellow phase continues to grow and eventually converts the entire film. The same general phenomena are observed if metastable films are instead subjected to elevated temperature while maintaining an inert nitrogen environment, as shown in Figure 2. In this experiment, transmittance data are recorded as the
critical to the formation of a trapped state. In the literature, thermal hysteresis is observed in OHPs as well. 23,24 Interestingly, in the formamidinium lead iodide system, it has been associated with conformational entropy of the A-site molecular cation,24 whereas there can be no such contribution from cesium. In the current work, additional calorimetric studies with and without moisture make it possible to establish the parameters necessary to kinetically trap the black phase of CsPbI3 in its metastable form and clarify the deleterious role played by moisture. Films and powders are fabricated according to a modified literature procedure5,25 in which solvent evaporation of a solution of CsI and PbI2 leads to the formation of the nonfunctional orthorhombic or “yellow” phase (δ-CsPbI3). Similar to bulk CsPbI3 crystals16 the polycrystalline samples undergo phase change above 305 °C and convert into a photoactive black phase (α-CsPbI3). To maintain the desired αCsPbI3 phase the films are cooled rapidly (“quenched”) to capture the metastable black phase at room temperature. These steps are performed in the inert atmosphere of a nitrogen glovebox because otherwise, in the ambient atmosphere, αCsPbI3 transforms into δ-CsPbI3 in minutes. For powder samples, a drop-cast film is scraped off the substrate using a spatula and then ground to a powder. (Details can be found in the Supporting Information (SI).) X-ray diffraction measurements are performed for the dropcast films, indicating that quenched samples, encapsulated in nitrogen and black to the eye, are indeed the metastable, cubic α-phase (Figure 1b, black curve). Samples at equilibrium with ambient atmosphere and yellow to the eye are the orthorhombic, nonperovskite δ-phase (Figure 1b, red curve). The moisture dependence of phase transition is shown in Figure 1c, where transmittance spectra (shown as the negative logarithm of transmittance) are collected over a period of 75 min for a thin film of CsPbI3 exposed to a 33% relative humidity (RH) nitrogen atmosphere at 23 °C. At the start, the spectrum (black curve) represents black α-CsPbI3 with a bandgap ∼1.73 eV. In contrast, as the film transforms to yellow δ-phase, a strong absorption feature is observed at 420 nm (orange curve). The phase transition proceeds via the formation of a nucleation site of yellow δ-phase in the
Figure 2. Contour plot of transmittance spectra as a function of temperature, taken for a thin film of metastable, black-phase α-CsPbI3. Spectra are taken in 5 °C increments over the course of several hours, achieving an effective scan rate of 5kBT per formula unit at room temperature, where kB is the Boltzmann constant); second, it highlights the fact that the entropic favorability of the black phase must be quite substantial to reverse the relative stability of the two phases at the moderate phase-transition temperature of ∼300 °C. To draw further insight from calorimetry, we note that although the phase transition is reversible, the exothermic and endothermic peak centers are not aligned and there is a considerable amount of thermal hysteresis. To understand how the rate of temperature change and atmospheric moisture affect the phase change kinetics and particularly the kinetic trapping of desired black phase, DSC analysis is performed at a higher temperature ramp rate (10 °C/min). At the faster scan rate, we see qualitatively similar behavior to that shown in Figure 4 but with substantial differences in the transition peak locations, peak width, and onset temperature of the phase change (Figure S2; see also Table 1). The exothermic peak corresponding to the destabilization of the quenched black-phase sample now
significantly higher than, for example, MAPbI3, in which the initial decomposition is governed by the release of HI and CH3NH2 and begins by ∼300 °C.26 For CsPbI3 the initial mass loss begins in approximately the same temperature range as PbI2 sublimation (Figure 3, red curve). The two distinct massloss features in the TGA curve for CsPbI 3 coincide approximately with literature values for the temperatures at which pure PbI2 and CsI reach an appreciable vapor pressure (arbitrarily defined as 200 Pa), at 493 and 762 °C, respectively.27 This analysis allows us to assign the spectral changes above 450 °C in Figure 2 to the formation of residual CsI, coinciding with the loss of PbI2. We speculate that the broadly absorbing material observed in the temperature range between 420 to 450 °C is due to a PbI2-depleted intermediate that forms transiently. As a consequence of the hightemperature threshold for sublimation, it is possible to directly measure the reversible enthalpies of phase changes using DSC. To quantify the free-energy landscape of the CsPbI3 crystal phases we employed DSC. Aluminum DSC crucibles are filled with powder samples, and except where otherwise noted, crimped inside a N2-filled glovebox to control the composition of the head space of the crucible. The yellow sample is obtained by exposing a black-phase powder sample to moist ambient (22−23 °C, RH 20−40%) and subsequently transferring it back into the glovebox prior to sealing the crucible. The heating and cooling curves are obtained sequentially by heating the sample from 20 to 350 °C and then cooling it back to the starting point. Figure 4 represents the DSC thermograms for both black and yellow samples at a 2 °C/min scan rate. While the yellow sample, being the stable phase, shows no sign of thermal activity at temperatures below 200 °C, the quenched black sample undergoes an exothermic change (i.e., positive heat flow out of the sample) between 90 and 150 °C, magnified in the inset to Figure 4. The negative enthalpy of change indicates that the rise in temperature provides enough thermal energy to overcome the energy barrier and allows the metastable black phase to convert into a stable yellow phase. We speculate that the relatively greater range of metastability (>200 °C) observed in Figure 2 is due to the formation of smaller grain sizes5 in spincast films in comparison with powders made by dropcasting. As temperature continues to rise, at 321 °C both samples shown in Figure 4 exhibit a strong narrow endothermic 1280
DOI: 10.1021/acs.jpclett.7b00134 J. Phys. Chem. Lett. 2017, 8, 1278−1282
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The Journal of Physical Chemistry Letters Table 1. Phase-Transition Enthalpies of CsPbI3 Powders under Varied Conditions exothermic
endothermic
phase/atmosphere
rate (°C/min)
peak (°C)
fwhm (°C)a
|ΔH| (kJ/mol)
|ΔS| (J/K·mol)b
peak (°C)
fwhm (°C)
|ΔH| (kJ/mol)
|ΔS| (J/K ·mol)b
black − N2
10 2 10 2 10 2
256.7 278.7 259.9 275.1 267.6 281.8
14.6 6.6 19.4 6.5 8.9 4.5
15.1 14.4 15.0 14.6 14.6 14.2
29 27 28 27 27 26
324.5 321.7 325.3 321.8 325.8 323.1
3.8 2.3 3.6 2.2 3.7 2.0
13.8 14.0 13.8 13.9 14.0 13.7
23 24 23 23 23 23
yellow − N2 yellow − air
fwhm: full width at half-maximum. bApproximate entropy values were calculated from ΔG = ΔH − TΔS using the temperature at the transition peak. a
increased sharpness of the peaks at either scan rate. At 2 °C/ min, in the presence of water molecules, the reappearance of the yellow phase from the black phase (Figure S3b) appears 4 to 5 °C before the transition temperatures in a dry atmosphere. Most importantly, however, the enthalpy of the phase change is the same with or without water molecules present, and no new distinct enthalpic features are observed anywhere else in the thermogram. By contrast, in OHPs, irreversible enthalpic features in the thermogram have been observed and hypothesized to originate from H2O desorption.23 In computational studies of moisture-induced instability of OHPs, the emphasis has been on the energetics of complex formation between the perovskite and H2O.28,29 For the present results, no such H2O adducts are evidenced, and the observed effect can be explained simply as a transitory stabilization of the transition state, with no effect on the equilibrium enthalpies of the two phases. The enthalpy and the temperature of the reversible phase transition between the functional perovskite α-phase of CsPbI3 and the nonfunctional yellow δ-phase was measured as a function of atmosphere, heating rate, and sample history. The black α-phase is intrinsically unstable at room temperature, even in a moisture-free atmosphere. Only above 321 °C is it spontaneously formed from the δ-phase. In the absence of moisture, the α-phase can be rapidly cooled or “quenched” to form a metastable functional material at room temperature, which remains metastable for significant periods of time, up to ∼100 °C. Moisture, which was previously known to destabilize the α-phase, is shown here to play a primarily catalytic role, lowering the kinetic barrier to the phase transition without impacting the enthalpy significantly. CsPbI3 shows promise as an all-inorganic alternative to the hybrid halide perovskite photovoltaic materials; however, to realize this potential, methods such as those demonstrated herein will be critical to understand the phase stability of α-CsPbI3 and related halide perovskites.
extends over a temperature range of 110−185 °C. To facilitate the comparison for different scanning rates, the ordinate (y axis) is transformed to heat capacity rather than heat flow. Figure 5 exhibits a significant shift toward lower onset
Figure 5. Effect of cooling rate and moisture on phase transition. The exothermic transition for yellow phase δ-CsPbI3 powders, sealed either under N2 atmosphere (red curves) or in moist atmosphere (blue curves) at cooling rate of 2 °C/min (solid lines) and 10 °C/min (dashed lines), respectively. Arrow indicates temperature scanning direction.
temperature and increased breadth of the transition feature with faster cooling. At all temperatures below the phasetransition temperature, the yellow phase is thermodynamically favored, yet the system is kinetically trapped in the black phase over the temperature interval defined by the magnitude of the hysteresis, which itself depends on the cooling rate. At the limit of the extremely fast cooling (>200 °C/min) used to quench the samples in the metastable black phase, the system is depleted of the thermal energy it would require to overcome the activation barrier on the time scale of observation. Thus the system becomes frozen in a metastable state. It is interesting to note here that only small hysteresis values (1 to 2 °C) have been previously reported for MAPbI3 for the tetragonal to cubic phase transition, and even those have been ascribed to presence of moisture.23 It is known that moisture has the effect of destabilizing the metastable black phase of CsPbI3; however, to identify the mechanistic role of moisture in the phase transition, DSC analysis is repeated in the presence of moisture. Figure 5 depicts the exothermic transition during the cooling cycle for yellow phase δ-CsPbI3 with and without a humid atmosphere. To achieve a humid environment, a yellow phase CsPbI3 powder sample is sealed in a DSC crucible in open atmosphere (22 to 23 °C, RH = 40%). The striking differences between the N2- (red curve) and air- (blue curve) filled samples are the earlier transition onsets (i.e., at higher temperature) and the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00134. Experimental procedures, additional DSC plots, and method for baseline correction and Gaussian fitting for transition peaks. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 1281
DOI: 10.1021/acs.jpclett.7b00134 J. Phys. Chem. Lett. 2017, 8, 1278−1282
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Subham Dastidar: 0000-0002-3025-4787 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.D. and A.T.F. acknowledge funding from CBET-1604293. A.D.D. acknowledges funding from CMMI-1463412. C.J.H., A.D.G.-P., and J.E.S. acknowledge support from ONR under N00014-14-1-0761. The TGA was performed in Prof. Giuseppe R. Palmese’s lab. We thank Weichun Huang from Prof. Christopher Li’s group for helping with the DSC measurement.
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