Measuring the Cross-Plane Thermal Conductivity of 2D Hybrid Perovskite with Vibrational-Pump-Visible-Probe Spectroscopy

2D organic-inorganic hybrid metal halide perovskites (MHPs) are an essential subset of the MHP family, valued for their favorable optoelectronic properties and diverse applications in photovoltaics, LEDs, photodetectors, and thermoelectrics. The broad range of available organic spacers and tunability of quantum-well thickness offer a versatile platform for optimizing material properties and device performance by modulating crystal structures, interlayer distances, and octahedral connectivity. However, the inherently low thermal conductivity of 2D MHPs poses significant challenges for thermal management, making the study of their thermal transport properties as crucial as understanding their electrical transport. While cross-plane thermal transport in 2D MHPs is more complex than simple thermal resistance stacking, current thermal metrology techniques are better suited for polycrystalline powders or thin films, where abundant grain boundaries obscure the evaluation of intrinsic material properties. Additionally, common methods like TDTR and FDTR require the deposition of metallic transducers onto the sample surface, complicating the sample preparation and may lead to potential sample damage.

To address these issues for measuring the thermal conductivity of 2D hybrid perovskites, we developed a vibrational-pump-visible-probe (VPVP) technique for easy and accurate measurement of the thermal conductivities in high-quality single-crystal 2D hybrid MHPs, with minimal sample preparation. VPVP utilizes a mid-infrared (MIR) pump to excite vibrational modes the organic molecules within the materials, inducing temperature excursions, while a visible probe monitors the refectivity change at the exciton resonance, where 2D MHPs exhibit strong thermoreflectance effect. With a time resolution of 1 ns and a measurement window of up to 1 ms, VPVP provides precise temporal monitoring of thermal transport. The thermal conductivities of 2D MHPs can thus be extracted from the temperature decay after excitation. Since the 2D MHP itself acts as the transducer, the intrinsic properties of high-quality single crystals can be measured without interference from grain boundaries, defects, or possible contaminations from sample preparation.