Thermal Transport Properties of Hybrid Semiconductors Investigated by Vibrational-Pump Visible-Probe Spectroscopy

Understanding thermal transport at the micro- to nanoscale is crucially important for a wide range of technologies ranging from device thermal management and protection systems to thermal-energy regulation and harvesting. In the past decades, optical methods such as time-domain and frequency-domain thermoreflectance (TDTR and FDTR) have grown as extremely powerful and versatile thermal metrological techniques for the measurement of material thermal conductivities. Here, we report the measurement of thermal conductivity of thin films of CH₃NH₃PbI₃ (MAPbI₃), a prototypical metal-halide perovskite, by developing a time-resolved optical technique called vibrational-pump visible-probe (VPVP) spectroscopy. The VPVP technique relies on the direct thermal excitation of MAPbI₃ by femtosecond (fs) mid-infrared (MIR) optical pump pulse that is wavelength-tuned to a vibrational mode of the material, after which the time dependent optical transmittance across the visible range is probed in the ns to µs time window using a broadband pulsed laser. Using the VPVP method, we determine the thermal conductivities of MAPbI₃ thin films deposited on different substrates. Leveraging the transducer-free VPVP method reported here we further performed transient thermal imaging experiments, which permits us to reveal buried interfaces between MAPbI₃ and the underlying flexible substrates. We expect our reported experimental scheme will permit spectrally resolving and spatiotemporally imaging the dynamic lattice temperature variation in organic, polymeric, and hybrid organic-inorganic semiconductors.