Intellectual Merit: The research objective of this BRIGE award is the fundamental developmental of Cu2ZnSnS4 (CZTS) thin films and solar cells by non-vacuum, liquid-based techniques. Expensive materials and processes limit the potential for future long term cost reductions in photovoltaics. Thin film polycrystalline low cost solar cells have emerged as a significant step in reducing the materials cost in photovoltaics. CZTS has recently emerged as one of the most promising absorber layer materials for low-cost thin-film solar cells, providing both an economical and green solution to the current thin-film technologies, consisting of abundant, low-cost, non-toxic elements. CZTS has a suitable optical band gap of ~1.5 eV and a large optical absorption coefficient of over 104 cm-1. This research will develop a solution-particle (hybrid slurry) approach using the CTZS constituents to fabricate the CZTS thin films. Characterization and analysis of the thin films will focus on the composition ratios with the objective of growing Cu-poor and Zn-rich CZTS thin films, the formation of a stoichiometric compound, growth of large grains, and a stable crystal structure. High residual stress in the thin films can trigger significant undesirable consequences including deformation, fracture, delamination and device failure. This research will perform CZTS thin film adhesion and stress analysis and employ techniques and mechanisms that alleviate stress, promote adhesion and improve the overall quality of the films. This research will establish a relation between thin film deposition conditions, and electrical, optical and structural properties of CZTS thin films that will enable the fabrication of high efficiency solar cell devices.

Broader Impact: The proposed research of CZTS thin films enabling high efficiency solar cell devices will broaden the participation of underrepresented groups in an emergent area of national need ? alternative renewable energy sources. This project will implement educational and outreach activities including research experiences for undergraduate and graduate students. The outcomes of this research will be integrated into a new course, ?Fundamentals of Solar Power and Renewable Energy?. This course will be developed during the proposed project and offered to students in all science, technology, engineering and mathematics (STEM) disciplines. This research will be integrated into the outreach and educational activities beyond training of undergraduate and graduate students. Early awareness outreach programs that stimulate interest in science, engineering and solar energy will be developed. Outreach activities will include solar energy summer camps and programs targeted to young girls in middle school. In addition to supporting and mentoring undergraduate researchers, a postdoctoral researcher will be supported and mentored.

Project Report

Copper zinc tin sulfide - Cu2ZnSnS4 (CZTS) is a semiconductor material consisting of abundant, low-cost, low-toxic elements. CZTS is one of the most promising absorber layer materials for low-cost thin-film solar cells providing both an economical and green solution to the current thin-film technologies. CZTS has a suitable optical band gap of ~1.5 eV and a large optical absorption coefficient of over 104 cm-1. This research focuses on the fundamental development of Cu2ZnSnS4 (CZTS) thin films by non-vacuum, liquid-based techniques. CZTS is a new semiconductor material with a very high potential for use in solar cells. Little is known and reported about the properties of CZTS, including the defect physics, the effect of doping and dopants, bandgap tuning, and optimum fabrication processes. This research is developing well controlled deposition methods for CZTS that will result in a high quality absorber material for CZTS solar cells. The research is establishing the relation between film deposition conditions, and electrical, optical and structural properties of CZTS films that will result in low cost, high efficiency solar cells. The primary focus of this research plan is the fundamental investigation and development of CZTS thin films and solar cells by a non-vacuum, liquid-based thin film coating deposition method. Several of the CZTS crystal growth and thin film techniques are carried out at high temperatures. It is important for the determination of the stoichiometry of the compounds to control the vapor pressure of the components at these temperatures. The microstructure of the films is mainly determined by the substrate temperature, the lattice match of the compound, the substrate properties, the growth process direction, and the growth rate and pressure during deposition of the films. The electronic behavior of the films may also vary considerably with deposition conditions. CZTS polycrystalline thin films were prepared by a non-vacuum liquid-based coating method. Soda lime glass (SLG) was used as substrates. SLG substrates were cleaned with acetone and deionized water rinses, and dried with nitrogen (N2), to remove hydrocarbons and other contaminants, prior to the film deposition. In this study, film preparation began with the preparation of CZTS precursors by mixing of the metal sources copper (II) acetate monohydrate, zinc (II) acetate dehydrate, and tin (II) chloride dehydrate of 4.375 x 10-2, 2.1875 x 10-2 and 2.1875 x 10-2 mol, respectively, in a 50 ml solvent of 2-methoxyethanol. Sulfur precipitated powder was added as the sulfur source at 8.75 x 10-2 mol. Monethanolamine was used as a stabilizer and 5 ml was added to the solution. The solution was stirred and heated at 45°C for 1 hour to dissolve metal sources. The solution was deposited on SLG substrates by spin coating. The film was dried in air at 300°C for 15 minutes on a hot plate. This process was repeated 4 additional times resulting in a total of 5 layers of coated and dried films. After the fifth layer, samples were annealed varying the temperature from 400°C to 600°C. Characterization and analysis of the thin films were performed using the JEOL 7600F scanning electron microscope (SEM), the Panalytical X-ray diffractometer (XRD) to determine the crystal structure, orientation and crystallite size, Raman spectroscopy and transmission and absorption spectroscopy. Results show the formation of kesterite CZTS. We have demonstrated the use of a low cost approach for the development of CZTS thin films using a liquid-based, non-vacuum technique. CZTS thin films were spin coated and air dried in five successive layers, followed by annealing at temperatures ranging from 400°C to 600°C. The XRD diffraction peaks from the CZTS (112), (220) and (312) planes were observed in all the samples indicating the formation of the kesterite CZTS phase. SEM surface micrographs show the formation of uniform dense 200 nm grains for the sample annealed at 500 °C. The energy band gap of the sample annealed at 500°C is ~1.45 eV, which is very close to the optimum value of 1.5 eV required for the CZTS absorber layer in a thin film solar cell. Mo, ZnO, and ZnO:Al were sputtered deposited on SLG substrates for characterization and analysis of the resulting electrical material properties. Results show films exhibit the desired electrical properties required to achieve efficient thin film solar cells. XRD data show the polycrystalline formation of the Mo thin film and the ZnO:Al thin film with the desired crystal structure and orientation. Hall Effect measurements were performed at room temperature and at 77K to obtain the mobility, carrier density, and resistivity of the thin films.

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Southern Polytechnic State University
United States
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