The NSF Sustainable Energy pathways (SEP) Program, under the umbrella of the NSF Science, Engineering and Education for Sustainability (SEES) initiative, will support the research program of Prof. Hugh Hillhouse and co-workers at the University of Washington to develop new solar cell technology from earth abundant elements. A sustainable energy pathway must yield high-value energy products at or below the price from non-renewables, with a net environmental and societal impact that is known and benign, and at a scale sufficient to become a major contributor to worldwide energy production (terawatts). Based on crustal abundance of the elements, mineral production rates, cost of the mineral feed stocks, the material defect chemistry, stability of the material and likely p-n junctions, and the photovoltaic electricity generation potential, Cu2ZnSnS4 (CZTS) appears to be a most promising candidate for terawatt-scale low-cost solar energy harvesting. Here, we will use a rigorous life cycle assessment (LCA) to analyze the sustainability of CZTS solar cells and avoid indirect market effects that have plagued other large-scale energy production routes. The LCA will affect the synthetic routes to fabricating the devices and help steer a body of fundamental research focusing on: (1) Developing a fundamental understanding of defects at interfaces and their passivation by applying combinatorial surface treatments, state-of-the-art electrical scanning probe methods, photoluminescence, and ab initio calculations; (2) Exploring the possibility of using novel voltage-boosting electron-dopant Auger de-excitation processes in the buffer layer; (3) Pioneering a method to utilize the unique properties of graphene to create a field-effect back contact. Using this 3-terminal back-contact architecture, the back-surface field, for the first time, will be tuned. This extra degree of freedom will allow the effects of surface chemistry to be separated from Fermi level equilibration. The increase in efficiency that could be achieved would bolster this technology to a level that would be economically competitive with current non-sustainable approaches.
In 2009 the National Research Council estimated that fossil fuel use accounts for $120B/yr in non-climate damages and 20,000 premature deaths in the U.S. As a result, the development of a sustainable energy pathway to low-cost solar cells from Earth abundant elements will have a substantial impact on the environment and human health. In addition, the solar cell market is predicted to reach $44B/yr by 2017, and currently, U.S. companies lead the world in manufacturing and technology development for thin film devices. Advances such as those being pursued by the SEP team could shift thin film technology into the lead of the solar cell market, and help U.S. companies secure a long-term leadership position. The project will also train a skilled set of scientists and engineers for this growing manufacturing sector who understand the necessity of sustainable processes and appreciate the collaborative interdisciplinary approaches needed to achieve them. The impact of the project will be broadened by efforts including: (1) setting up a new children's exhibit at the Pacific Science Center called Light Racing, (2) deploying experimental solar cell technologies at the new UW Solar Energy Testbed Facility; (3) staffing the SunDawg Solar Kiosk which will be viewed by thousands at University of Washington and Seattle sporting events, (4) increasing the already substantial effort the investigators have in undergraduate research experiences (70 long-term research projects with undergraduates in the last decade); (5) expanding a high-school outreach program that within the past two years has already resulted in greater than 20 classroom visits to Seattle high schools, reaching more than 800 students; and (6) supporting efforts to broaden participation working with the MESA outreach program to provide mentorship to underrepresented minorities.
This project has the transformative potential to solve key fundamental and technological challenges that will enable a new sustainable energy pathway for thin film solar cells that have economic advantage over current technology and are competitive with utilizing fossil fuel resources. If successful, the project could transform the solar industry from one currently dominated by silicon solar cells that are manufactured overseas to a new technology based on copper, zinc, tin, and sulfur that are manufactured in the U.S.
