In recent years thin film Cadmium Telluride (CdTe) solar cell recombination rates have been reduced by increasing bulk crystal quality so that a greater number of charge carriers can participate in conduction. As devices have improved and more minority carriers reach the back surface contact layers, surface recombination has become a greater limiter of device efficiency. This inefficiency is likely a result of an acidic surface treatment for bulk CdTe material that is intended to prepare the material for contacting but leaves a residual Te-rich layer on the surface that does not provide sufficient passivation for current charge carrier concentrations. New methods of preparing bulk CdTe material for contacting that reduce the portion of Te present on the surface of the device are necessary for further efficiency gains.
Researchers at NREL have invented a method of treating CdTe materials with a basic solution so that surface recombination is minimized. The treatment consistently leaves non-stoichiometric Cd-rich surfaces that can be reconstructed into a crystalline lattice that exhibits a significant increase in carrier lifetimes. The invention is valid over a range of temperatures and times that are suitable for industrial processing (5s – 5min, 20C – 65C) and has been demonstrated successfully in both air and inert environments. Devices created using this method increased near-surface charge carrier lifetimes enough that the true bulk lifetime of the material was observed.
For more information, please contact Bill Hadley at Bill.Hadley@nrel.gov
U.S. Patent # 9,722,111 B2
Applications and Industries
- CdTe photovoltaics
- Increased device efficiency through decreased surface recombination
- Easily implemented in industrial environments
- Method works consistently at low cost