Elsevier

Thermochimica Acta

Volume 695, January 2021, 178813
Thermochimica Acta

Thermodynamics of cesium lead halide (CsPbX3, x= I, Br, Cl) perovskites

https://doi.org/10.1016/j.tca.2020.178813Get rights and content

Highlights

  • Thermodynamic stability of CsPbX3 perovskite increases in the order CsPbI3, CsPbBr3, CsPbCl3.

  • Thermodynamic differences between the high and low temperature phases decreases in the order CsPbI3, CsPbBr3 and CsPbCl3.

  • CsPbI3 perovskite is barely stable under ambient conditions, and thus may not be suitable for long term applications.

Abstract

Inorganic cesium lead halide (CsPbX3, X = I, Br, Cl) perovskites have sparked broad interest for photovoltaic applications, which, however, may be plagued by chemical and thermodynamic instabilities. This work reports calorimetric studies on the formation enthalpies of CsPbX3 perovskites, confirming decreasing thermodynamic stability in the order CsPbCl3, CsPbBr3 and CsPbI3. Cesium lead iodide (CsPbI3), one of the most studied inorganic lead halide perovskites, is shown to be only barely stable from a thermodynamic perspective. This order of increasing instability is consistent with the variation of Goldschmidt tolerance factor. The perovskites are high temperature entropy-stabilized phases with respect to low temperature polymorphs, with the thermodynamics of phase transitions reflecting the above thermodynamic trends.

Introduction

Inorganic cesium lead halide (CsPbX3, X = I, Br, Cl) perovskites are an emerging class of materials that find various applications, especially in perovskite solar cells (PSCs). Its generic formula CsPbX3 renders this materials class superior thermal and moisture stabilities compared to hybrid organic-inorganic perovskites (HOIPs, such as CH3NH3PbI3 and HC(CH2)2PbI3), owing to the replacement of intrinsically volatile organic cations (mostly CH3NH3+ and HC(CH2)2+) with their robust inorganic counterpart (Cs+) [[1], [2], [3]]. Such favorable stabilities open avenues for possible large-scale commercialization of inorganic PSCs, in which CsPbX3 materials function as alternatives to traditional light absorbing HOIPs.

Despite tremendous progress in improving the power conversion efficiencies of PSCs, [4] understanding of thermodynamic properties to explain their intrinsic stabilities (or lack thereof) remains scarce. Moreover, previous studies of CsPbX3 based PSCs centered on CsPbI3 and CsPbBr3, whereas CsPbCl3 was claimed to be less of an option for light harvesting, which was largely attributed to its unsuitably large band gap [[5], [6], [7]]. Yet, CsPbCl3 perovskite, especially in the form of quantum dots (QDs), plays a role in optoelectronic applications such as lasing, light-emitting diodes and even photovoltaics. [[8], [9], [10]] It is thus important to understand the thermodynamic properties of CsPbCl3 perovskite, and further, to evaluate the energetics - structure correlations underlying the entire set of CsPbX3 (Xdouble bondI, Br, Cl) perovskites.

Using room temperature solution calorimetry, we measured the thermodynamic properties of CsPbCl3 perovskite. We then evaluated the CsPbX3 stability trend with X moving down the halogen (VIIA) group, correlating their stabilities to structural variations. To complement the CsPbX3 data, we compared the thermodynamic behavior of relevant perovskites, including HOIPs and titanate (oxide) perovskites.

Section snippets

Materials and synthesis

CsPbCl3 was prepared via solution synthesis: a 1:1 precursor solution of equimolar CsCl and PbCl2 was prepared by dissolving 1.2 mmol cesium chloride (CsCl, Alfa Aesar, 99.9 %) and 1.2 mmol lead chloride (PbCl2, Sigma Aldrich, 99.999 %) into 2 ml anhydrous dimethyl sulfoxide (99.9 % DMSO, Sigma Aldrich) at room temperature (25 °C). Magnetic stirring was applied to aid the dissolution of CsCl and PbCl2 until an even precursor solution was obtained. The precursor solution was evenly applied to a

Results and discussion

The X-ray diffraction (XRD) pattern of CsPbCl3 is shown in Fig. 1, which confirms pure CsPbCl3 in its cubic phase. A schematic crystal structure of cubic CsPbCl3 is also depicted.

The measured solution enthalpies in DMSO of CsPbCl3 and constituent halides are given in Table 1. Earlier findings on CsPbI3 and CsPbBr3 [11,12], as well as the thermochemical cycle to calculate the formation enthalpy of CsPbCl3 are also included. A table with individual experiments for each dissolution enthalpy (used

Conclusions

We conclude that thermodynamic stability of these halide perovskites increases as the Goldschmidt tolerance factor increases towards unity, in the order CsPbI3, CsPbBr3 and CsPbCl3, along with a diminution of thermodynamic difference between the high temperature and low temperature polymorphs. Cesium lead iodide (CsPbI3), the inorganic lead halide perovskite with the most suitable bandgap for solar cell applications among CsPbX3 perovskites, is barely stable under ambient conditions, and thus

CRediT authorship contribution statement

Bin Wang: Conceptualization, Methodology, Investigation, Writing - original draft. Alexandra Navrotsky: Conceptualization, Project administration, Writing - review & editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgment

This work was supported by the U.S. Department of Energy Office of Basic Energy Science, grant DE-FG02-03ER46053.

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