As another hypothesis, one could assume that CuP 2-y is highly compensated by donor defects at low temperatures, but its p-type character becomes more dominant at higher temperatures due to activation of a deeper acceptor. If this hypothesis is correct, one would expect both the electrical conductivity and the Seebeck coefficient to increase with temperature as we experimentally observe -the former due to an increase in the concentration of ionized acceptors, the latter due to a decreasing contribution from the (negative) n-type Seebeck coefficient. Previous work also suggested the possibility of charge compensation in CuP 2 single crystals based on the temperature dependence of their carrier mobility. The position of the acceptor level in our CuP 2-y film can be estimated as (121 ± 3) meV above the valence band from an Arrhenius plot of the electrical conductivity in the 230 K-300 K temperature range (Fig. , Supporting Information).