Processing issues for thin film CdTe/CdS solar cells

Processing options addressing critical issues associated with fabrication of high efficiency thin film CdTe/CdS solar cells are presented. Particular focus is given to methods for: minimizing CdS loss during CdCl2 heat treatment; obtaining high open circuit voltages; obtaining spatially uniform properties; and forming ohmic contacts. Data is presented for cells deposited by physical vapor deposition (PVD) which demonstrates that CdS loss can be overcome by use of CdTe1-XSX absorber layers and by controlling the delivery of CdCl2 vapor species during heat treatment. The CdCl2 vapor treatments are shown to produce residue-free surfaces and uniform film properties which leads to uniform cell performance. State of the art open circuit voltages (>850 mV) are obtained with PVD cells by performing a short high temperature anneal prior to CdCl2 treatment. Low resistance contacts using diffused Cu doping followed by surface etching are demonstrated on CdTe/CdS thin films deposited by five methods.