Certain phosphorus-containing III-V semiconductors (GaP, InP and related alloys) are among the best-performing PV absorbers. Yet, there is hardly any other phosphide material that has received extensive attention for applications in PV or optoelectronics in general. Exciting progress has been reported within the family of Zn-based phosphide compounds (Zn₃P₂, ZnGeP₂, ZnSnP₂ etc.), but high PV efficiencies are yet to be demonstrated. In this talk, I will discuss two radically different classes of semiconductors that harness the unique ability of phosphorus to exist in a broad range of oxidation states.

The first class is “P-rich phosphides”. In stark contrast to almost any other compound semiconductor previously investigated for PV applications, P-rich phosphides contain bonds between nonmetallic atoms (in their specific case, phosphorus-phosphorus bonds). I will present the first successful thin-film synthesis [1] of any polycrystalline P-rich phosphide. The synthesized material is CuP₂, a 1.5 eV band gap semiconductor with strong optical absorption and native p-type doping in an attractive range for thin-film heterojunction solar cells.

A second intriguing family of materials can be obtained by combining phosphorus with a more electropositive and a more electronegative species. Of particular interest are “phosphosulfides”, where sulfur is the more electronegative species. Many phosphosulfides are predicted to be stable semiconductors with direct band gaps in the visible and disperse band edges. Yet, there are less than five reports of phosphosulfides in thin-film form and hardly any optoelectronic characterization [2].

Backed by a unique suite of combinatorial thin-film deposition setups with access to S and P sources, we have explored three ternary phosphosulfide phase diagrams by high-throughput experiments. In this process, we have synthesized several semiconducting compounds that were previously unknown or that had only been synthesized in bulk form. We will show that the photoluminescence decay time of some of these phosphosulfides is already above 100 ns, demonstrating that phosphosulfides also deserve close attention by the PV research community.

To understand how good (or bad) these early-stage PV materials are at their current development stage, I will discuss a recently proposed figure of merit to assess the quality of a generic PV absorber [3], [4].