Nanometer-Scale Imaging on Electrical Potential in Absorber of As-doped CdSeTe Solar Cells

Chun-Sheng Jiang, Eric Colegrove, Steve P. Harvey, Joel N. Duenow, Matthew O. Reese National Renewable Energy Laboratory, Golden, CO, United States

CdTe technology has been largely improved by adding Se in region of CdTe absorber near the front interface. Replacing the low Cu-doping by group-V elements has become a major focus for further improving Voc. Indeed, average carrier concentrations have reached ~1016/cm3. However, the best devices remain significantly Voc deficient. Group-V doping can induce defects and affect the front interface in different ways from Cu-doping. In this contribution, we report on a nm-scale electrical potential imaging throughout the As-doped CdSeTe absorber using Kelvin probe force microscopy (KPFM). The potential imaging was conducted both laterally and vertically on beveled films using ion-milling in a small glancing angle. KPFM images electrical potential on the uniformly beveled surface and assesses defect-charging in the subsurface region within a screening length from the beveled surface. We found that grain boundaries are positively charged and there are significant potential fluctuations in both grain boundary versus grain interior and intragrain. We further found that these potential fluctuations decrease significantly toward the front interface. Time of flight secondary ion mass spectrometry imaging shows Se content increases toward the front interface, consistent with Se-passivation of defects. The potential fluctuation was induced by defect-charging, with positive charges by donor defect levels above the Fermi level and with negative charges by acceptor defect level below the Fermi level. Furthermore, the results elucidate different details of defect configurations and grain structures of the films with different CdCl2 treatment temperatures. The defect configurations in the region near the front interface can be a main factor contributing to the device performance difference. Our potential imaging provides insights about the defects throughout the absorber films, and shows that the potential fluctuation has a direct correlation to the Voc deficit.