Paper TF-FrM8
New Insights on the Structure and Chemistry of the Tin Oxide-emitter Interface in CdTe Solar Cells as revealed by Thermomechanical Cleavage and Electron Spectroscopy

Friday, November 11, 2016, 10:40 am, Room 105A

CdTe solar cells having superstrate architectures have a poorly understood, complex front surface formed via interdiffusion of the CdS-based emitter and the CdTe absorber. Interfaces in this region of the cell are difficult to probe by standard surface analytical methods because they are bound by glass on one side and microns of CdTe on the other. Post-growth processing with CdCl₂ and for back contacting is likely to further change these buried interfaces, making the traditional scheme of interface analysis – interleaved depositions and analyses – impractical. Yet these front interfaces are important. Recent modeling shows that recombination at the cell front will be increasingly critical to cell efficiency as doping levels are improved from ~10¹⁴/cm³ to 10¹⁶/cm³. In this study we make use of a LN2-based thermomechanical cleavage technique and a surface analysis cluster tool to probe in detail the tin oxide-emitter interface in completed CdTe solar cells. We show that this thermomechanical cleavage occurs within a few angstroms of the SnO₂-emitter interface. An unexpectedly high concentration of chlorine, ~20%, was determined from a calculation that assumed a uniform chlorine distribution. Angle-resolved X-ray photoelectron spectroscopy was used to further probe the structure of the chlorine containing layer, revealing that both sides of the cleave location are covered by a single unit cell of CdCl₂. Exposing these newly formed surfaces to water showed that CdTe solar cells made using CdCl₂ and CdS:O emitters contain water-soluble components at their front surfaces, raising questions pertinent to cell reliability. We show that the SnO₂-emitter interface is also characterized by an extremely strong gradient in oxidation of the chalcogens present as well as a high fraction of oxidized tellurium. Selenium addition to the front of the device, done to improve carrier lifetimes, also affects the extent of chalcogen oxidation. Our work provides new and unanticipated details about the structure and chemistry of front surface interfaces and should prove vital to improving materials, processes, and reliability of next generation CdTe-based solar cells.