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We report measurements of the heat capacity and cross-plane thermalconductivity of 2D (CxH2x+1NH3)2[MAPbBr3]n-1PbBr4 (MA = methylammonium) leadbromide perovskites (2D LHPs) at room temperature as a function of both theoctahedral layer thickness (n = 1,2,3) and the organic spacer chain length(x=4,5,6,7,8) using differential scanning calorimetry (DSC) and frequencydomain thermoreflectance (FDTR) respectively. We observe ultralow thermalconductivities (0.18-0.51 W/m K) for all 2D LHPs studied, but surprisinglyminimal suppression of thermal conductivity with respect to bulk MAPbBr3 (0.5W/m K). Cross-plane thermal conductivity is found to increase monotonically asa function of both the octahedral layer thickness (0.18-0.26 W/m K for n=1-3)and the organic chain length (0.18-0.51 W/m K for x=4-8). Additionally, wemeasure heat capacities that are well described by composite theory, suggestingbulk-like phonon density-of-states within the separate organic and inorganicsubphases of the layered structure. The striking observation of increasingthermal conductivity with increasing organic phase fraction (i.e. increasingorganic chain length) indicates efficient thermal transport along the orderedalkyl chain backbone. Our experimental results agree most closely with apredictive model of ballistic phonon transport with diffuse interfacescattering - rather than normal thermal conduction within each phase. Thisstudy indicates the potential for synthesizing 2D LHPs with thermalconductivity that exceeds the bulk perovskite phase, while also shedding lighton relevant phonon transport pathways in 2D LHPs.

Efficient Thermal Transport across Molecular Chains in Hybrid 2D Lead Bromide Perovskites