Utilizing Time-of-Flight SIMS to Investigate Group V Dopant Distribution in CdTe
Steven P. Harvey1, Eric Colegrove1, Brian McCandless2, Dave Albin1, Mowafak Al-Jassim1, Wyatt Metzger1
1NREL, Golden, CO, United States
/2university of delaware, newark, DE, United States

Enabling continued development of polycrystalline CdTe solar cells with open-circuit voltages approaching 1 V requires new dopants and processes to increase acceptor concentrations and minority-carrier lifetimes. We have investigated multiple group V dopants in single- and polycrystalline CdTe materials as well as Cl, using numerous dopant incorporation and activation strategies. The diffusion kinetics and spatial distribution of the dopants is revealed with a combination of 1-D standard depth-profiling and high-resolution 3-D tomography with time-of-flight secondary-ion mass spectrometry (TOF-SIMS). For all cases where the dopant is incorporated during growth, including vapor transport deposition, close spaced sublimation, and molecular beam epitaxy, the dopant concentration is not enhanced at grain boundaries. When the dopant is incorporated through a post-growth incorporation process, fast grain-boundary diffusion leads to an enhanced concentration of the dopant at grain boundaries.  While both methods are applicable, in-situ incorporation offers control of dopants and devices independent of diffusion kinetics, grain structure, and grain boundary chemistry.

Area: Sub-Area 5.3: Characterization of Polycrystalline or Amorphous Thin Film PV