Simultaneous Mapping of Thermal Conductivity, Thermal Diffusivity, and Volumetric Heat Capacity of Halide Perovskite Thin Films: A Novel Nanoscopic Thermal Measurement Technique

Abstract

Local thermal conductivity, thermal diffusivity, and volumetric heat capacity of all-inorganic halide perovskite thin films are mapped simultaneously and with highest spatial resolution for the first time. These various thermal properties are detected by a scanning near-field thermal microscope operated at two different frequencies simultaneously. We apply this technique to analyze the thermal properties of halide perovskites on the nanoscale. In addition to an ultralow thermal conductivity of 0.43 ± 0.03 and 0.33 ± 0.02 W/(m•K), a low thermal diffusivity of 0.3 ± 0.1 mm 2 /s and a small heat capacity of 0.29 ± 0.9 and 0.18 ± 0.6 J/(g•K) are obtained for CsPbBr 3 and CsPb 2 Br 5 films, respectively. The findings of our thermal microscopy are of great general importance for the thermal design of thin-film devices based on halide perovskites, while the measurement technique itself is generally applicable for other thin-film optoelectronic materials.

Thermal properties of CsPbCl3 thin films across phase transitions

“…An initial study by backscattered scanning electron microscopy (SEM) revealed a notable contrast ( figure 1(b)). Note, a similar contrast has previously been evidenced by electron backscatter diffraction (EBSD) for Cs x Pb y Br z perovskite thin films that had undergone a similar procedure [2,32], where the contrast could be clearly assigned to CsPbBr 3 (3D) and CsPb 2 Br 5 (2D) grains in the film. In the present case, the coexistence of 3D phase CsPbCl 3 and the 0D phase Cs 4 PbCl 6 has been confirmed by XRD ( figure 1(c)).…”

“…The thermal measurement methodology is described in detail in [32]. Briefly summarized, the resistive thermal probe of the SThM (VITA-SThM probe from Bruker) is powered by a mixed signal of an adder at two AC currents I 0 1 sin(ω 1 t) and I 0 2 sin(ω 2 t) with angular frequencies ω 1 and ω 2 .…”

“…Individual layers of 10 nm CsCl and 5 nm PbCl 2 were deposited alternately to produce a 200 nm thick pristine layer on glass substrates. Subsequently, a planar hot pressing process (PHP) was carried out with a pressing time of 30 min at 100°C and a pressure of 100 bar to re-crystallize the film and to form the perovskite as reported earlier [2,32]. This re-crystallization step affords thin films consisting of large crystals with a low surface roughness (∼1 nm rms) [34,35].…”

“…In this work, we analyze the topography, the local thermal conductivity, the local thermal diffusivity, and the local volumetric heat capacity of Cs x Pb y Cl z perovskite thin films with high spatial resolution by using the SThM operated at two different frequencies simultaneously [32]. The Cs x Pb y Cl z thin films studied in this work were obtained by re-crystallization of thermally evaporated PbCl 2 /CsCl multilayers using a thermal imprint process (see experimental and figure 1(a)).…”

“…On the other hand, we have recently shown that scanning near-field thermal microscopy (SThM) [31], which is a technique based on atomic force microscopy (AFM), is particularly suited to assess simultaneously thermal properties of lead-halide perovskite thin films. The interaction volume to determine thermal properties is ultra-small due to the width of the heater/thermometer probe w=2 r contact (r contact : contact radius) being on the order of 100 nm [32]. Using this technique we were able to determine the ultra-low thermal conductivity, thermal diffusivity, and small heat capacity of (3D) CsPbBr 3 and (2D) CsPb 2 Br 5 thin films, at room temperature.…”