Surface stoichiometry of pulsed ultraviolet laser treated polycrystalline CdTe

Abstract

Dependence of film texture on substrate and growth conditions for CdTe films deposited by close-spaced sublimation J.Alternative procedure for the fabrication of close-spaced sublimated CdTe solar cellsThe effects of nanosecond pulsed ultraviolet laser annealing on the surface stoichiometry of closespace sublimated polycrystalline thin films are investigated using angle-resolved x-ray photoemission spectroscopy (XPS). The raw data suggest the formation of a Cd-rich surface layer, but this is counter to the expectation based on Cd and Te vapor pressures above CdTe that predicts a Te-rich layer and to direct observation of elemental Te at the surface. In order to explain this apparent discrepancy, we analyze our XPS data in the context of prior reports of lateral segregation of Cd and Te at the surface after pulsed laser treatments with a simple model of angular dependent XPS in the presence of surface roughness. This analysis reveals that a uniform Te layer cannot explain our results. Instead, our analysis suggests that Te enrichment occurs near grain boundaries and that a sub-monolayer Cd layer exists elsewhere. These complex yet repeatable results underscore the challenges in measuring surface stoichiometry to high precision on films relevant for polycrystalline CdTe devices. It also suggests that the Cd and Te vapor pressures above grain boundaries may differ from those above grain interiors and that ohmic contact may be made preferentially at the grain boundaries after pulsed laser annealing. V C 2014 AIP Publishing LLC.

Laser annealing in thin-film chalcogenide photovoltaics

“…annealing to demonstrate a dry back-contact treatment for CdTe solar cells. 4,6,10,11 CdTe has a high work function (5.9eV) and typically forms Schottky contacts to most metals. Devices treated with 100 pulses of a 25ns excimer laser (248nm) at 50mJ/cm 2 at the back contact did not show a rollover effect (reduced current density on increasing voltage) in the current density vs. voltage curve as generally observed in devices with a non-ohmic back contact.…”

Cadmium Selective Etching in CdTe Solar Cells Produces Detrimental Narrow-Gap Te in Grain Boundaries

“…GBs are generally classified in terms of their coincident site lattice character. While this description is sufficient to distinguish individual grains, it is insufficient to describe their interfacial atomic arrangement and thus their electronic structure. ,,− Our past work on laser annealing of the CdTe back surface and our work on the investigation of GB chemistry near the back contact have demonstrated chemical inhomogeneity within a GB. , These investigations have opened an avenue of exploration in TFPV on the variation in GB properties with position along their surface and its implications on device physics. Especially given historical low-doping in CdTe grains and the propensity of surfaces and interfaces to form intragap states, the formation of Ohmic contacts has been challenging and led to efficiency losses.…”