  We report measurements of the heat capacity and cross-plane thermal
conductivity of 2D (CxH2x+1NH3)2[MAPbBr3]n-1PbBr4 (MA = methylammonium) lead
bromide perovskites (2D LHPs) at room temperature as a function of both the
octahedral layer thickness (n = 1,2,3) and the organic spacer chain length
(x=4,5,6,7,8) using differential scanning calorimetry (DSC) and frequency
domain thermoreflectance (FDTR) respectively. We observe ultralow thermal
conductivities (0.18-0.51 W/m K) for all 2D LHPs studied, but surprisingly
minimal suppression of thermal conductivity with respect to bulk MAPbBr3 (0.5
W/m K). Cross-plane thermal conductivity is found to increase monotonically as
a 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, we
measure heat capacities that are well described by composite theory, suggesting
bulk-like phonon density-of-states within the separate organic and inorganic
subphases of the layered structure. The striking observation of increasing
thermal conductivity with increasing organic phase fraction (i.e. increasing
organic chain length) indicates efficient thermal transport along the ordered
alkyl chain backbone. Our experimental results agree most closely with a
predictive model of ballistic phonon transport with diffuse interface
scattering - rather than normal thermal conduction within each phase. This
study indicates the potential for synthesizing 2D LHPs with thermal
conductivity that exceeds the bulk perovskite phase, while also shedding light
on relevant phonon transport pathways in 2D LHPs.
