Pyrite is a potentially attractive and sustainable semiconductor for photovoltaic (PV) applications because of its low cost, abundance in the earth's crust, environmentally benign source material, and desirable electronic properties, such as 0.95 eV bandgap, high optical absorption coefficient, ease of n/p doping, high carrier mobility, and relatively long carrier lifetime; however, there has been only limited progress in producing high-quality pyrite thin films needed to explore the potential of this material for solar PV applications. The overall goal of the proposed research is to develop a fundamental understanding of the electronic and defect properties of doped and undoped pyrite thin films for solar photovoltaics.
The PIs will develop a fundamental understanding of the thermodynamic and kinetic factors involved in producing high-quality epitaxial pyrite thin films, and determine how growth conditions influence the structural, chemical, electrical, and optical properties of the films. Furthermore, methods will be developed to produce high-quality pyrite thin-films with controlled n- and p-type doping, high carrier mobility, and long diffusion length that are useful for PV applications. Towards this end, a Molecular Beam Epitaxy (MBE) process will be developed to enable film growth at high sulfur pressures, insuring that high quality pyrite films can be produced. A wide range of structural, chemical and electrical measurements, defect studies, and band-structure calculations will be used to characterize and analyze film properties. Growth studies will measure the sticking coefficient and resonance time of the impinging S and Fe as a function of temperature and reactant impingement rate, and the decomposition rate of pyrite will be measured in vacuum and under growth conditions. These experiments will be used to determine the rate-limiting steps for the forward and reverse reactions and, their influence on achieving high quality epitaxial pyrite films.
The proposed activities are designed to educate and inspire K-12 and graduate students in solar PV topics. First, a hands-on education module on solar PV technology will be developed and then presented to over 10,000 Arizona K-12 students through the Arizona State University (ASU) Science is Fun program. The module involves having students configure solar cells with various light sources to power an electronic device. The module will be delivered to K-12 classrooms by at least 15 undergraduate interns that are trained by the ASU LeRoy Eyring Center for Solid State Science. At least one third of the students that participate in the Science is Fun program will come from underrepresented groups. Second, for each year of the project, two high-school students will participate in the ASU Southwest Center for Education and the Natural Environment (SCENE) high?school program, where they will spend ten hours each week for a term in the PIs laboratory to develop a science-fair project involving the synthesis and characterization of pyrite materials for energy generation applications. Finally, graduate students will be trained in the fundamental aspects of PV technology and in the synthesis and characterization of thin films needed to be successful in the solar PV workforce.
Pyrite (FeS2) is an attractive semiconductor for use in terawatt-scale photovoltaic (PV) systems. It is composed of inexpensive, non-toxic elements, has a bandgap of ~0.95 eV and an absorption coefficient higher than conventional direct bandgap semiconductors such as GaAs. In this project, we invented a new method to produce high structural quality and low contamination pyrite thin-films directly from the inexpensive elemental constituents, Fe and S. Then we will produced and characteried materials that can ultimately be incorporated into large area inexpensive solar cells. Specifically, project (a) developed a fundamental understanding of the thermodynamic and kinetic factors involved in the synthesis of high-quality epitaxial pyrite thin-films, (b) developed a scalable, unique method and apparatus to reliably grow pyrite thin films in a high-vacuum chamber using only Fe and S, (c) investigated how growth conditions influence the structural, chemical, electrical, and optical properties of pyrite films, and (d) produced pyrite with dopants that resulted in electron conduction (i.e. n-type material) and hole conduction (i.e. p-type material). Finally, our patent-pending sequential-growth method can be used to make thin films of mateirals that contain one or more volatile constituents, including oxides, sulfides and nitrides Two graduate students and a post-doctoral researcher were trained in the fabrication and use of advanced ultra-high-vacuum synthesis systems and the characterization of semiconductor materials, and two undergraduate students and two high-school students worked with us carrying out science projects focused on characterizing the properties of pyrite materials and contacts. Mahmoud Vahidi, graduated with a Ph.D. in August 2013 and is now employed by Lawrence Semiconductor Research Laboratory. His thesis focused on the thermochemical analysis of pyrite thin film growth and the development of our patent-pending pyrite sequential growth technique. Another student, Alex Wertheim, will graduate with a Masters degree in December 2014. His thesis focused on improving the morphology of pyrite using low-temperature buffer layers, capping layers, and other forms of thermal processing.