The room-temperature electrical conductivity of postannealed polycrystalline films in the CuP 2.0 -CuP 2.2 composition range is between 0.5 S/cm and 1.0 S/cm at room temperature, without a clear dependence on the P/Cu ratio (Fig. ). The conductivity generally increases with increasing annealing temperature, regardless of annealing time (Fig. , Supporting Information). Previously reported conductivites of CuP 2 single crystals range from 0.01 S/cm to 30 S/cm, presumably due to differences in the crystal quality. 6,8-10 Films with severe P losses have significantly higher conductivities (Fig. ), probably due to the presence of the highly conductive Cu 3 P phase. The Seebeck coefficient measured on a freshly annealed CuP 2.0 film is (+390 ± 10) µV/K (Fig. , Supporting Information), indicating native p-type doping. All previously reported CuP 2 single crystals were also p-type with higher Seebeck coefficients in the 690 µV/K-820 µV/K range. The work function, measured with a Kelvin probe in air on a freshly annealed CuP 2.0 film, is (5.0 ± 0.1) eV. CuP 2 is a relatively strong absorber of light. Its absorption coefficient α reaches 10 5 cm -1 at a photon energy hν = E g + 0.6 eV above its band gap E g = (1.5 ± 0.1) eV (Fig. ). This compares favorably even with the most efficient direct gap photovoltaic absorbers such as GaAs, CdTe, and CH 3 NH 3 PbI 3 (MAPI). In fact, the absorption coefficient is as high as in some exciton-enhanced photoabsorbers such as BiI 3 and Cu 2 BaSnS 4 , indicating that CuP 2 may deserve more detailed optoelectronic characterization. We find that α 1/2 is linear in photon energy over a 2 eV spectral range above the band gap (Fig. )), indicating that α ∝ (hν -E g ) 2 . Both the estimated band gap and the spectral dependence of the absorption coefficient are in agreement with previous work on CuP 2 single crystals. Because the α ∝ (hν -E g ) 2 behavior is often associated with an indirect gap in conventional semiconductors, an indirect gap was previously assumed for these CuP 2 crystals. However, there are at least two other factors to consider. (1) The absorption strength of CuP 2 is high even for a direct gap material, so indirect transitions are unlikely to be responsible for it. (2) According to the calculated band structure of CuP 2 , 26 the fundamental gap should be direct and located between the Γ and the Y point of the Brillouin zone. Two indirect gaps with slightly higher energies exist, due to additional valence band pockets at the X point and between the Y and H points. Even though we observe a α ∝ (hν -E g ) 2 behavior, care should be taken when employing the absorption characteristics typical of Group IV and III-V semi-conductors to interpret the nature of the optical transitions of other semiconductors with substantially different band structures. A clear difference between CuP 2 and conventional semiconductors is that the former has many valence-and conduction band pockets at different points of the Brillouin zone. Hence, many different optical transitions can contribute to the overall absorption coefficient. The refractive index of CuP 2 is 3.3-3.4 in the transparent region (Fig. ). Extrapolation of the real part of the dielectric function to zero photon energy (Fig. , Supporting Information) yields a high-frequency permittivity ε ∞ = 10.5 ± 1.0. Interestingly, there seems to be a critical P/Cu ratio close to 1, where the electrical and optical properties shift from being "CuP 2 -like" (semiconducting and IR-transparent) to being "Cu 3 P-like" (metallic and opaque). This transition is manifested by an abrupt change in conductivity (Fig. ) and visual appearance (inset of Fig. ).