Molecular Disorder Explains the Ultralow Thermal Conductivity of Hybrid Perovskites

The ultralow thermal conductivity of the hybrid perovskite methylammonium lead iodide (MAPbI3) arises from the disorder generated by the motion of its organic molecules, according to a combined experimental and computational study. By measuring single crystals with frequency-domain thermoreflectance and simulating heat transport with equilibrium molecular dynamics, researchers directly quantified the link between molecular disorder and heat flow, rather than inferring it indirectly from crystal structure. The simulations show that when the disorder introduced by the organic molecules is excluded, the vibrational thermal conductivity of the material follows the classical inverse-temperature dependence seen in conventional crystals. When that disorder is included, the conductivity becomes nearly temperature-independent. At 150 kelvin, the vibrational thermal conductivity increases by roughly a factor of four when the disorder induced by the organic molecules is removed.

Hybrid perovskite photovoltaics are limited in part by the very property the study examined: a room-temperature thermal conductivity usually below about 1 watt per meter-kelvin, which strongly hinders heat dissipation during device operation. Raising the thermal conductivity requires a comprehensive understanding of how heat travels through these crystals, yet the detailed transport mechanism and the dynamics of the heat carriers have remained debated. Previous experiments and calculations place the thermal conductivity of MAPbI3 at 0.3 to 0.6 watts per meter-kelvin. Some researchers attributed the low value to weak chemical bonds, which reduce elastic stiffness and therefore sound velocity. Others ascribed it to the organic methylammonium molecules, invoking vibrational resonant scattering from their confined diffusion or from their rotation, and it has even been proposed that molecular rotation provides an additional thermal transport channel. The temperature dependence of the conductivity also deviates from the classical inverse-temperature relation followed by conventional crystals such as silicon and diamond.

To resolve the question, the team synthesized single-crystalline MAPbI3 and characterized it with high-resolution transmission electron microscopy, fast Fourier transform analysis, and powder X-ray diffraction, which matched the reference card JCPDS No. 96-451-8044 and previous reports. Thermal conductivity was then measured from 140 to 380 kelvin using frequency-domain thermoreflectance, a non-contact optical pump-probe technique in which a roughly 100-nanometer gold transducer layer is heated by a pump laser and the phase lag between pump and probe beams is fitted to a multilayer heat diffusion model. Low temperatures were maintained with a sealed, evacuated liquid-nitrogen chamber to exclude moisture, and high temperatures with a controlled stage. The reported experimental conductivity decreases from 0.66 plus or minus 0.084 watts per meter-kelvin at 140 kelvin to 0.32 plus or minus 0.13 watts per meter-kelvin at 380 kelvin, based on the mean of six samples with at least three sweeps per sample.

On the computational side, the researchers calculated thermal conductivity from the Green-Kubo linear response formula, running molecular dynamics with the LAMMPS package and the MYP interatomic potential on a 12 by 12 by 12 supercell over 80 to 500 kelvin. The kinetic contribution to the heat flux proved negligible, so the total conductivity could be treated as the potential heat flux contribution and decomposed into parts from the inorganic framework and the organic molecules. The calculated room-temperature conductivity of MAPbI3 was 0.38 plus or minus 0.025 watts per meter-kelvin with weak temperature dependence, agreeing with the measured values. The framework contribution itself displayed glass-like, nearly temperature-independent behavior.

The decisive test came from comparing three idealized states of the organic molecules: random, randomly fixed, and orderly fixed. With all molecules randomly fixed, the framework conductivity was 0.26 plus or minus 0.018 watts per meter-kelvin at room temperature and stayed essentially constant across temperatures. With all molecules orderly fixed, the framework conductivity rose to 0.62 plus or minus 0.042 watts per meter-kelvin and strictly followed the classical inverse-temperature relationship. To quantify this, the team defined a degree of disorder based on the angular deviation of each carbon-nitrogen vector from its ground-state orientation. The degree of disorder for the orderly fixed system is always zero, while for the random or randomly fixed systems it is already about 0.8 at 50 kelvin and converges to unity by 200 kelvin, meaning the molecules are fully disordered. The glass-like framework conductivity therefore stems from the disorder the molecules introduce, not from the molecules’ presence alone.

Independent experimental observations support this picture. Quasi-elastic neutron scattering and backscattering neutron spectroscopy have resolved two reorientation modes of the organic molecules, the rotation of the methyl and ammonium groups about the carbon-nitrogen axis and the tumbling of that axis itself. The residence time of the first mode decreases from 8 picoseconds at 140 kelvin to 2.6 picoseconds at 370 kelvin, showing that molecular disorder grows with temperature. Raman spectra reinforce the trend: the modes at 315 inverse centimeters, assigned to carbon-nitrogen torsion, and 968 inverse centimeters, assigned to carbon-nitrogen stretch, disappear as temperature rises from 100 to 300 kelvin. In the simulations, the autocorrelation function of the carbon-nitrogen bond rotation drops sharply above 200 kelvin and stays near unity below it, consistent with the defined disorder metric. The researchers also note that organic molecules can be locked into their lowest-energy orientations by strong dipole-dipole interactions and cage-deformation energy, which suggests the disorder, and with it the thermal conductivity, may be tunable.

The study also clarified the role of the organic molecules as a heat pathway. The molecular contribution to thermal conductivity decreases with temperature, and when the interaction between the framework and the molecules was reduced to half its original value, or the framework was frozen, that contribution barely changed, indicating it arises from interactions among the organic molecules rather than between molecules and the cage. Although the molecules diffuse substantially inside the octahedral cage, that diffusion is confined: the mean square displacement converges above 350 kelvin, and the closest approach between the trajectories of two neighboring molecules is 2.5 angstroms, nearly beyond the interaction range of the interatomic potential. Thermal energy carried by the molecules therefore travels through their vibrations alone. A separate calculation using an Onsager framework, which separates a mass-transport heat flux from the conductive flux, returned nearly identical conductivities for the conductive, virial, and total heat fluxes, confirming that a collision-based transport channel between molecules is negligible.

Spectral analysis sharpened the microscopic picture. Spectral energy densities showed that the linewidths for systems with random and randomly fixed molecules are similar, indicating comparable scattering strength among vibrations, while the orderly fixed system shows clear dispersions with small linewidths and only moderate scattering. The mean vibrational relaxation time, extracted from the decay of the heat flux autocorrelation, is nearly temperature-independent for the random and randomly fixed systems, evidence that the disordered molecules scatter the vibrations strongly. In the orderly fixed system the relaxation time decreases with temperature, as expected when Umklapp processes dominate, which is why the conductivity then recovers the classical inverse-temperature law. Across all three cases, vibrations below 6 terahertz carry most of the heat, and when the molecules rotate randomly the acoustic vibrations below 2 terahertz contribute more than half of the total. Non-equilibrium simulations gave accumulative conductivities of 0.32, 0.21, and 0.51 watts per meter-kelvin for the randomly rotated, randomly fixed, and orderly fixed systems respectively; the authors attribute differences from the equilibrium values to size effects in the non-equilibrium model and emphasize that its purpose was frequency-domain decomposition rather than absolute conductivity.

X-ray and neutron scattering calculations tied the transport findings to structural signatures. X-ray spectra at 80, 350, and 500 kelvin show sharp Bragg peaks and diffuse rods whose intensity generally falls as temperature rises, reflecting the more ordered framework at low temperatures, but X-rays, being sensitive to atomic mass, barely register the light organic molecules. Neutron scattering, which interacts with nuclei, was therefore computed for the organic molecules alone at 80 kelvin. When the molecules are orderly fixed, sharp Bragg peaks appear; when they are random, both Bragg peaks and diffuse rods appear; and when they are randomly fixed, both vanish into a featureless diffuse background, because random orientations suppress any periodic or correlated modulation of the coherent scattering. The diffuse rods are thus caused by both octahedral tilting and molecular disorder, and the background intensity, which grows with temperature, reflects inelastic scattering from the disordered molecules. Building on earlier work that reconstructed the correlated structure of the dynamic octahedral sublattice and the organic cations, the authors conclude that the disorder induced by the organic molecules, rather than the tilting of the octahedral framework itself, is the origin of the ultralow and weakly temperature-dependent thermal conductivity of hybrid perovskites, a microscopic picture they expect to guide the design of higher-performance hybrid perovskite materials.

Subject of Research: Thermal transport mechanism in hybrid perovskite MAPbI3

Article Title: Unveiling the Ultralow Thermal Conductivity of Hybrid Perovskites

Article References: He, Q., Chen, Y., Li, Z., & Zhou, Y. (2026). Unveiling the Ultralow Thermal Conductivity of Hybrid Perovskites. Advanced Science, 13(59), Article e76841. https://doi.org/10.1002/advs.76841

Image Credits: AI Generated

DOI: 10.1002/advs.76841

Keywords: hybrid perovskites, MAPbI3, thermal conductivity, molecular disorder, methylammonium, phonon scattering, molecular dynamics, FDTR, neutron scattering, spectral energy density, photovoltaics, heat dissipation