Single-stage co-evaporation and surface characterization of (Ag,Cu)(In,Ga)Se2 semiconductors for solar cell application

Abstract

Substituting Ag for some of the Cu in (Ag,Cu)(Ga,In)Se2 solar cells may improve their performance. We have used photoelectron spectroscopy to study how this substitution changes the surface electronic structure of these compounds. The research is motivated by two factors: 1) decreasing the amount of Cu leads to less Se p- metal d hybridization, thereby lowering the valence band maximum (VBM), and 2) since the Ag4d level is deeper than the Cu3d level, the hybridization of Ag4d-Se4p results in a different VBM, further below the Fermi level than that in Cu-In-Ga-Se systems. ☐ The VBM of CuInSe2, Cu(In,Ga)Se2, and AgInSe2 single crystals were verified by angle-resolved photoemission spectroscopy. We show that AgInSe2 has a VBM roughly 0.72eV below the VBM of Cu(In,Ga)Se2 film and 0.66eV below CuInSe2, with a total shift of 1.25eV below the Fermi level. Polycrystalline films of (Ag,Cu)(In,Ga)Se2 were then fabricated by co-evaporation, and the valence band measurements confirmed the shift in the VBM of Ag polycrystalline alloys. X-ray photoemission spectroscopy and energy dispersive spectroscopy successfully identified the phase segregation between (Ag,Cu)(In,Ga)3Se5 (ordered vacancy compound - OVC) and (Ag,Cu)(In,Ga)Se. The phase segregation, as determined by composition, is apparent at all ratios Ag/(Ag+Cu) (AAC). Concomitant changes in electronic structure were verified by soft x-ray photoemission spectroscopy. Furthermore, increasing the Ga content, while keeping the Ag and Cu content constant in these alloys, also causes a decrease in the apparent presence of OVCs. In addition, we have evidence to support that these new surface (bulk-like) valence bands are most likely CuGaSe2 and not CuInSe2 related compounds.