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

Abstract

Se alloying has enabled significantly higher carrier lifetimes and photocurrents in CdTe solar cells, but these benefits can be highly dependent on CdSexTe1‐x processing. This work evaluates the optoelectronic, chemical, and electronic properties of thick (3 µm) undoped CdSexTe1‐x of uniform composition and varied processing conditions (CdSexTe1‐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 CdSexTe1‐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., VSe, VTe) and ClTe generation.