This international collaborative research project between investigators at the University of Utah and the University of Luxembourg addresses the synthesis of high-quality chalcopyrite semiconductors for thin film photovoltaics using electrodeposition and pulsed laser annealing. The investigation examines two hypotheses: 1) relatively-high total fluence pulsed laser annealing is effective at annealing electrodeposited precursor films and can result in chalcopyrite semiconductor phases having good photovoltaic and crystalline properties and, 2) relatively-low total fluence pulsed laser annealing is effective at reducing specific electronic and structural defects in previously-annealed chalcopyrite thin films. The effects of pulsed laser annealing on the composition, phase, crystalline, optoelectronic, and electronic defect properties of a model chalcopyrite, CuInSe2, prepared by electrodeposition, as well as the incorporation of Ga as substitute for In in the electrodeposited films, is investigated.
Anticipated results from this work are relevant to the rapidly-developing field of earth-abundant materials for thin film photovoltaic cells. Undergraduate students at Utah and graduate students at both Utah and Luxembourg take part in interdisciplinary, materials science and chemistry research activities at the participating laboratories.
This award is co-funded by the Division of Materials Research and the Office of International Science and Engineering.
This Materials World Network project collaborated with Prof. Phillip Dale's group at the University of Luxembourg to investigate the use of laser annealing to synthesize thin films of Cu(In,Ga)Se2 (CIGSe) from electrodeposited precursor films. The project was comprehensive in terms of investigating different types of electrodeposited precursor films – stacked elemental layers, stacked binary phases, and codeposited – as well as different laser wavelengths from ultraviolet through near-infrared, instantaneous powers, and pulse or dwell times from nanoseconds to tens of seconds. In terms of Intellectual Merit, we found that only coelectrodeposited precursor films combined with near-infrared laser heating to temperatures above the 400-600 C typically used in furnace approaches for tens of minutes. These higher temperatures applied for short dwell times of 1 second or less can yield structural and optoelectronic improvements in quality sufficient for use in photovoltaic devices. A key finding was that it is not possible to effectively laser anneal without maintaining a sufficient overpressure of Se even though the process is rapid and excess Se is included in the precursor films. We proved that the seemingly-attractive route of melting the precursor to drive liquid phase growth is impractical because the liquid rapidly dewets on substrates of interest (e.g. Mo or MoSe2). Practical constraints on the processing window related to cracking of glass substrates or warping of metal substrates were identified also. We studied the changes in photoluminescence yield as well as trap state populations accompanying laser annealing. The first solar cell having power conversion efficiency >1% produced by electrodeposition and laser annealing was produced by the end of the project. It was shown that this device suffered from spatially inhomogeneous material properties such as photoluminescence yield and spectral shape arising from the inhomogeneous degree of annealing caused by the rastered laser beam. Thus the use of more uniform laser annealing achieved by scanning high-aspect-ratio laser spots should result in improved results in terms of optoelectronic properties and thus device results. This project yielded Broader Impacts in terms of a hand-in-glove level of international collaboration that would not have been possible otherwise by supporting the collaboration and research exchange with the group in Luxembourg. The project also gave meaningful research opportunities for over a dozen undergraduate students and especially to two students who became the most active undergraduate researchers. A deliverable of the broader impacts section was the creation of materials properties websites for earth abundant photovoltaic materials which were added to the well-known PVEducation.org website. These data will facilitate the development of alternative photovoltaic materials scalable to terawatt installed levels.
