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.
Advanced semitransparent CdTe PV a Phase I NSF SBIR grant to Xunlight 26 Solar, LLC with subaward to The University of Toledo Alvin D. Compaan, P.I. Thin-film solar modules based on cadmium telluride (CdTe) have shown some of the fastest market growth of any photovoltaic (PV) technology and have become the low-cost leader in PV. However, like most PV, traditional CdTe modules are opaque and are not suitable for many applications which require some light to be transmitted. Xunlight 26 Solar, LLC, with its partner The University of Toledo, has developed the science and technology for making CdTe cells and modules thin enough to transmit light and to generate power with light from either side of the module. These "ultrathin" cells have active CdTe layers that are only about 300 nm thick. This is only 1/5 to 1/10 the thickness of standard cadmium telluride cells and modules. This new technology is ideally suited for applications in building-integrated PV (BIPV) windows and skylights and in PV sunroofs for automobiles. The goal of this Phase I SBIR project was to develop methods to make further improvements in ultrathin CdTe solar-cell and solar-module performance. Ultrathin cells provide major advantages by using fewer natural resources, but also if cells are thin enough they can transmit light and be used in windows. It is simple enough to deposit thinner layers, but the challenge is to keep the cell efficiency high when the back contact is very close to the front junction. In this project, Xunlight 26 Solar and its subawardee, The University of Toledo, succeeded in developing an improved back contact together with a special interfacial layer a few nanometers thick between the CdTe and the back contact that substantially improves the ultrathin cell performance, especially when the CdTe is less than 500 nm thick. Xunlight 26, in this project, also developed technology to improve the color balance of the light transmitted through the ultrathin cell. The transmitted light naturally has a yellowish or bronze hue, similar to many ski goggles. In auto sunroof applications and in building design situations, control of the color balance of light transmitted through windows is important and this project identified ways to control that color without diminishing the solar-cell performance. Thus, this project has led to a window prototype that has generated excitement from designers, architects, and window suppliers in both the automotive sunroof and BIPV markets.