Investigation of Sub-Bandgap Emission and Unexpected n-Type Behavior in Undoped Polycrystalline CdSe_xTe_1-x

Se alloying has enabled significantly higher carrier lifetimes and photocurrents in CdTe solar cells, but these benefits can be highly dependent on CdSe_xTe_1-x processing. This work evaluates the optoelectronic, chemical, and electronic properties of thick (3 µm) undoped CdSe_xTe_1-x of uniform composition and varied processing conditions (CdSe_xTe_1-x evaporation rate, CdCl₂ anneal, Se content) chosen to reflect various standard device processing conditions. Sub-bandgap defect emission is observed, which increased as Se content increased and with "GrV-optimized CdCl₂" (i.e., CdCl₂ anneal conditions used for group-V-doped devices). Low carrier lifetime is found for GrV-optimized CdCl₂, slow CdSe_xTe_1-x deposition, and low-Se films. Interestingly, all films (including CdTe control) exhibited n-type behavior, where electron density increased with Se up to an estimated ≈10¹⁷ cm^-3. This behavior appears to originate during the CdCl₂ anneal, possibly from Se diffusion leading to anion vacancy (e.g., V_Se, V_Te) and Cl_Te generation.