Due to the inverse relationship between carrier concentration and thermovoltage, the Seebeck coefficient is highest in CuP 2-y and lowest in Cu 3-z P (Fig. ). Interestingly, the Seebeck coefficient increases linearly with temperature in all three films (Fig. ). This behavior is often a sign of a temperature-independent carrier concentration, a typical feature of materials with non-zero density of states at the Fermi level (i.e., metals and degenerate semiconductors such as Cu 3-z P and Cu 1.61 P). However, a linear increase of the Seebeck coefficient with temperature is not readily explained for a more weakly doped semiconductor like CuP 2-y . In an ideal scenario, we would expect the carrier concentration to increase with temperature due to increasing defect ionization, and the Seebeck coefficient to decrease accordingly. The reason for this discrepancy is unclear. One could invoke the role of film inhomogeneity due to the presence of Cu 3 P secondary phases (Fig. ), or assume that the increase of electrical conductivity with temperature (Fig. ) is due to mobility changes rather than to the hole concentration changes. Yet, a simultaneous increase in hole concentration and Seebeck coefficient with temperature was reported for CuP 2 single crystals, where inhomogeneity effects can be excluded. Multi-band transport could also cause an unusual temperature behavior due to increasing contributions from the additional valence band pockets of CuP 2 with increasing temperature. However, application of the Boltzmann transport equation 39 on the calculated CuP 2 band structure 26 reveals that a significant decrease in the Seebeck coefficient is expected in the 200 K-300 K range assuming a concurrent increase in hole concentration by one order of magnitude (Fig. , Supporting Information).