Structural Phase- and Degradation-Dependent Thermal Conductivity of CH₃NH₃PbI₃ Perovskite Thin Films

Organic–inorganic lead halide perovskites have shown great promise in photovoltaics and optoelectronics. In these applications, device performance and reliability can be strongly influenced by thermal transport in the materials. Through laser pump–probe experiments, different microstructures of CH₃NH₃PbI₃ perovskite thin films are found to give rise to different phonon scattering mechanism. The thermal conductivity in CH₃NH₃PbI₃ neat film decreases with temperature. Even though this agrees with the behavior of its bulk crystalline counterparts, an apparent thermal conductivity change near the structural phase transition temperature of this perovskite (orthorhombic vs tetragonal) has only been observed in the spin-coated films. Analyses suggest that this may be attributed to either an energy landscape change related to organic cation disorder or the thickness change of ferroelectric domain walls formed in the neat perovskite films that affects the phonon scattering at the domain boundaries. In contrast, no thermal conductivity discontinuity has been observed in the CH₃NH₃PbI₃/Al₂O₃ mesostructured films, where the thermal conductivity first shows an increasing trend at low temperature (<80 K) and then stays nearly constant. Such a trend is typical in amorphous materials and nanostructured composites where phonon scatterings are due to morphological disorder and internal interfaces play key roles in the thermal transport. When exposed to the ambient environment, humidity induced degradation is found to have a significant impact on the overall thermal conductivity of the spin-coated CH₃NH₃PbI₃ neat film