Colossal barocaloric effects with ultralow hysteresis in two-dimensional metal–halide perovskites

Abstract

Pressure-induced thermal changes in solids—barocaloric effects—can be used to drive cooling cycles that offer a promising alternative to traditional vapor-compression technologies. Efficient barocaloric cooling requires materials that undergo reversible phase transitions with large entropy changes, high sensitivity to hydrostatic pressure, and minimal hysteresis, the combination of which has been challenging to achieve in existing barocaloric materials. Here, we report a new mechanism for achieving colossal barocaloric effects that leverages the large volume and conformational entropy changes of hydrocarbon order–disorder transitions within the organic bilayers of select two-dimensional metal–halide perovskites. Significantly, we show how the confined nature of these order–disorder phase transitions and the synthetic tunability of layered perovskites can be leveraged to reduce phase transition hysteresis through careful control over the inorganic–organic interface. The combination of ultralow hysteresis and high pressure sensitivity leads to colossal reversible isothermal entropy changes (>200 J kg−1 K−1) at record-low pressures (<300 bar).

Abnormal In-Plane Thermomechanical Behavior of Two-Dimensional Hybrid Organic–Inorganic Perovskites

“…We start the experiment at RT, slowly cool the sample to 5 °C, and then heat it back to 25 °C. Due to the high mismatch between the TECs of C4n3 (∼1 × 10 –4 /K) ,, and SiO 2 /Si (∼3 × 10 –6 /K), , lowering the temperature will result in higher tension in the membrane. However, within this temperature range, the maximum thermal strain in the membrane is <0.2%, which is comparable to the strain from the pretension due to transferring in many similar 2D material studies ,,,, and is much less than the elastic strain imposed by the tip loading here. , Thus, the thermal strain will only slightly change the pretension in the membrane and should not affect the measured mechanical property.…”

“…To better reveal the temperature-dependent change in E ∥ , we normalized the E ∥ values measured at different temperatures to the value at RT (Figure ) below, if not specified. HOIPs are widely reported with high TECs due to their strong anharmonicity and soft lattices. ,,, Hence, as temperature goes down, the bond length should shrink and become stiffer. Because E ∥ correlates positively to the metal–halide bond strength in the PbI 6 4– inorganic framework, ,,, this anharmonic picture suggests a thermal softening behavior, i.e.…”

“…HOIPs are widely reported with high TECs due to their strong anharmonicity and soft lattices. 6,7,43,50 Hence, as temperature goes down, the bond length should shrink and become stiffer. Because E ∥ correlates positively to the metal−halide bond strength in the PbI 6 4− inorganic framework, 3,23,38,39 this anharmonic picture suggests a thermal softening behavior, i.e., E ∥ should get higher at lower temperature.…”

“…2D HOIPs will ubiquitously encounter mechanical strain during device fabrications and operations, which can arise from thermal stress (owing to the high thermal expansion coefficients (TECs) of 2D HOIPs , ), piezo-/ferro-electricity, , photostriction, photothermal effects, , or flexible substrate deformation . These mechanical strains can alter the bond angle/length, trigger ion migration, and induce phase transition, which can affect the local and overall functional properties of the materials in a device configuration.…”

“…2,5 Furthermore, the diversity of the structure and chemistry of both the organic spacer molecules and the inorganic metal−halide framework in 2D HOIPs have granted a tremendous parameter space to tailor the functional properties of the materials for various application needs. 2 2D HOIPs will ubiquitously encounter mechanical strain during device fabrications and operations, 3 which can arise from thermal stress (owing to the high thermal expansion coefficients (TECs) of 2D HOIPs 6,7 ), piezo-/ferro-electricity, 8,9 photostriction, 10 photothermal effects, 11,12 or flexible substrate deformation. 13 These mechanical strains can alter the bond angle/length, 14 trigger ion migration, 15 and induce phase transition, 16 which can affect the local and overall functional properties of the materials in a device configuration.…”