The sun represents the most abundant potential source of sustainable energy on earth. Solar cells for producing electricity require materials that absorb the sun's energy and convert its photons to electrons, a process called photovoltaics. While solar cells made from crystalline silicon currently have about 90 percent of the worldwide solar photovoltaics (PV) market, alternative solar cells made from very thin layers of non-silicon materials abundant in the earth's crust offer a number of advantages. These include dramatically reduced materials consumption and low-cost fabrication, as well as new form factors that are flexible or foldable, ultralight in weight, and enable facile integration into building structures. Unfortunately, new materials based on elements abundant in the earth's crust cannot match the sunlight-to-electric power conversion efficiency of rigid crystalline silicon-based materials, and so are presently not competitive. This project seeks to gain fundamental understanding of the power conversion performance one promising class of thin-film photovoltaic materials based on zinc blends, which are made from a mixture of nontoxic elements abundant in the earth's crust and are relatively inexpensive. The research will focus on controlling the atomic level disorder within these materials to ultimately provide a viable pathway to low-cost, scalable, high-performance solar photovoltaics. The research will also engage and train a new generation of undergraduate, graduate and postdoctoral scientists in the important area of sustainable energy materials development. Results from the research will also be incorporated into undergraduate and graduate level materials science courses at Duke University.
Current thin film solar photovoltaic materials in commercial use, including CIGS and Cd-Te based materials, contain elements that are either costly or rare in the earth's crust (e.g., indium, tellurium) or present toxicity issues (e.g., cadmium). These sustainability issues potentially impose limits on future cost reduction and market share. Recently, kesterite-based thin-film solar cells made of (Cu)2-ZnSn-(S,Se)4-(CZTSSe), in which indium/gallium in CIGS-Se materials are replaced by more readily available and lower-cost elements zinc/tin, have achieved conversion efficiencies of up to 12.6%. However, this efficiency is still only about one-half that of the best crystalline silicon or thin-film commercial solar PV materials. The proposed research is based on the hypothesis that anti-site structural disorder in current zinc-blende material matrices (e.g., Cu on Zn, Zn on Cu) lead to electrical potential energy fluctuations and band tailing that effectively limits the open-circuit voltage. The overall goals of this research are to understand the nature of anti-site disorder in copper chalcogenide based zinc-blende materials for solar PV, and to design new blende materials in which the level of anti-site disorder can be controlled and reduced through a combination of rational computational and experimental materials design approaches. The research plan has three objectives. The first objective is to computationally evaluate prospective zinc-blende-related materials for PV absorbers, focusing on identifying the materials features that lead to more benign recombination-based defects and grain boundaries. The second objective is to design and synthesize stable CZTSSe analogs that have a lower propensity towards anti-site disorder and band tailing. Finally, the third objective is to study the impact of varying amounts of anti-site disorder on materials and their device properties. Achievement of the project goals will provide a framework for understanding defect control in complex energy harvesting materials and offer one pathway to new scalable and high-efficiency solar PV devices.
