Zulkifl Hussain¹, Hongling Lott², Adam B Phillips¹, Eric Colegrove², Randy J. Ellingson¹, Matt Reese², Michael J. Heben^1
1University of Toledo, Toledo, OH
²National Laboratory of the Rockies, Golden, CO
Arsenic (As) doping of CdTe has emerged as a promising approach for achieving stable p-type conductivity in thin-film photovoltaics, addressing long-standing challenges with copper-based back contacts. However, arsenic activation in CdTe remains problematic due to the deep acceptor level of As and its tendency to occupy both Cd and Te sites. While these defects, intrinsic to the material, serve as the primary sources of self-compensation that limit net carrier concentration, the landscape becomes increasingly complex when extrinsic processing elements are introduced. Extrinsic defects such as grain boundaries can act as potential epicenters for defect perpetuation in polycrystalline CdTe, due to As segregation around grain boundaries and formation of complexes of As-Cl-O, which can be detrimental to dopant activation. Here we identify potential sources of As deactivation in polycrystalline CdTe by studying As-doped single crystal CdTe grown in a customized thermal evaporation system with an integrated As cracker cell for in-situ doping. This system enables in-situ independent and reproducible control of the flux of As species (As₂, As₄, As₈), CdSe, CdTe, etc., needed for absorber layer fabrication of CdTe solar cell. We have grown a 110” FWHM single crystal, with quality rivaling MBE grown crystals and exhibiting 100% dopant activation, as verified by SIMS and Hall Effect Measurements. These results highlight the possible role of grain boundary density in dopant activation/deactivation in CdTe. The study narrows down the probable sources of problems, to yield a highly active and > 25% efficient CdTe solar device.