Shedding (x-ray) light on As-local structures and defects in CdTe solar cells*

Arsenic (As) is being used as a state-of-the-art p-type dopant for polycrystalline cadmium telluride (CdTe) thin film solar cells as it leads to significant increases in acceptor concentration, greater than 10¹⁶ cm⁻³ compared to 10¹⁴ cm⁻³ of the conventional copper p-type doping. However, among As atoms that were incorporated into the CdTe devices, only ~1 % of As atoms are activated, serving as p-type doping and contributing to acceptor concentration. The reason for the low dopant activation and the structures and defects that account for the remaining percentage of the As atoms present is still unknown.

Herein, we use nano x-ray absorption near edge spectroscopy (XANES)—a powerful technique for unraveling atomistic structures—to probe As local structures across the depth of two CdTe devices, which have the same As atomic concentration but different acceptor concentrations. To resolve the As-local structures from the experimental collected XANES spectra, we compare them with the XANES spectra simulated from the As-related structural models including As point defects and various As-related phases (As-O, As-Te, As-Cd, As-Cl, As-O-Cl, and As-Cd-Cl).

We will show for the first time the existence of As_Te defects and some Cl complexes in samples with high activation levels. For the low activation counterpart, we will present fittings suggesting the presence of a chlorinated ternary phase containing As. Finally, we will present our experimental evaluation of the controversial AX center, which has been hypothesized to exist in As-doped CdTe samples.  We believe these findings could guide future fabrication processes to prevent undesirable chemical reactions and phase transformations, thus increasing the acceptor concentration and the conversion efficiency.