The room-temperature thermal conductivity of CuP 2-y is 1.1 W/Km (Fig. ). This value is lower than in CuP 2 single crystals (3.6 W/Km-4.7 W/Km depending on lattice direction) as may be expected for a polycrystalline sample. Our measured conductivity is, however, in excellent agreement with the calculated 1.12 W/Km amorphous limit for bulk CuP 2 . The increasing thermal conductivity with increasing temperature is unlike the ∝ 1/T behavior typical of crystalline semiconductors in this temperature range. Instead, it is often observed in amorphous or highly disordered materials, consistently with the observation that our measured conductivity is very close to the amorphous limit. Based on these results, we assume that the phonon mean free path in the CuP 2-y film is low due to disorder and/or phonon boundary scattering. The latter is likely enhanced by the small grains, low thickness, and porous morphology of the film. The electronic contribution to the thermal conductivity is negligible due to the low hole concentration of CuP 2-y (Fig. ). The thermal conductivities of Cu 3-z P and Cu 1.61 P are only slightly higher and their temperature dependences are similar to the case of CuP 2-y . Thus, we conclude that the thermal conductivity is phononmediated and strongly limited by film morphology in all three films. In fact, scattering of charge carriers (holes) is also morphology-limited. The hole mobility of the present Cu 3-z P film (0.27 cm 2 /Vs) is two orders of magnitude lower than in a continuous Cu 3 P film on glass with about the same carrier concentration (28.8 cm 2 /Vs, see Fig. ). CuP 2 has recently been proposed as a potential thermoelectric material. Our measurements on a CuP 2-y film confirm that the lattice contribution to its thermal conductivity is indeed sufficiently low for thermoelectric applications. However, the thermoelectric figure of merit zT at room temperature is still low for all investigated compositions (Fig )) due to low power factors (Fig S8, Supporting Information). In the vicinity of the Cu 3 P stoichiometry, the main issue is a low Seebeck coefficient. In the vicinity of the CuP 2 stoichiometry, the main issue is low electrical conductivity. Even taking the more favorable properties of our CuP 2 films on glass (Table ) or of previously reported CuP 2 single crystals 8 the zT value at room temperature would only be 0.004 and 0.05, respectively. It might be possible to optimize the hole con- centration of CuP 2 by extrinsic doping to obtain higher zT values. Nevertheless, phosphorus losses at moderate temperatures and potential stability issues under ambient conditions are likely to limit its practical applicability in thermoelectric devices. Similar issues might exist in other phosphorus-rich phosphides.