This Small Business Innovation Research (SBIR) Phase I project aims to develop ultra-thin CdTe solar cells by improving the back-contact interface, which facilitates the transport of positive charge carriers (holes) into the back contact electrode and simultaneously reflect negative charge carriers (electrons) toward the main junction where they are collected by the transparent conducting electrode. Two complementary approaches will be taken to achieve this objective. Suitable dopant elements such as phosphorous will be added to create a heavily doped p-type layer before applying the back contact. The heavy doping will create an electron reflector and facilitate a hole-conducting semiconductor or metal contact. The second pathway will involve magnetron sputter deposition of suitable p-type semiconductors such as ZnTe doped with nitrogen or copper. Preliminary studies have shown promise with thick CdTe layers and CdTe crystals. This project will extend the studies to ultra-thin, semitransparent CdTe solar cells to validate the efficacy of either or both approaches as applied to PV window structures. The broader/commercial impact of this project will be the potential to provide an improved back-contact interface for semitransparent photovoltaic windows, which will help move these power-producing windows toward 10% efficiency while maintaining good transparency. The estimated annual market for curtain-wall glass applications in the U.S. is about 150 million square feet (14 million square meters). Value-added PV window coatings for high-rise office buildings can provide a substantial reduction of solar heat loads while delivering a major fraction of the electricity needed for these buildings
