This Small Business Innovation Research (SBIR) Phase 1 project describes a Raman imaging spectrometer based on the proprietary Volume Bragg Grating (VBG) technology. The instrument will also employ different wavelength Raman sources in order to perform the analysis of multilayer thin films solar cells. This approach is based on the dependency of the penetration depth on the laser source wavelength. The commercial thin film solar cell panels such as a-Si and CIGS [CuInGaSe], reach only ~ 60% of the conversion efficiencies demonstrated in the laboratory, mostly due to various material quality and uniformity problems. The proposed Raman instrument will be eventually used as an online process control sensor to improve the yields and quality of the solar cell thin film deposition processes. The main tasks of the Phase 1 are the design of the VBG based Raman imaging instrument and the assembly of a bench top prototype instrument. Several samples of thin film solar cell structures (a-Si and CIGS) will be analyzed with the prototype instrument. We expect to demonstrate the ability to analyze multilayer structures and thus create foundation for the design and construction of an online process control unit in Phase 2.
The broader impact/commercial potential of this project is the significant advance in the conversion efficiency of commercial solar panels, based on a-Si and CIGS thin film technology. Market projections for thin film based solar panels predict sales of $6 billion in 2012. Thus, any improvement in the conversion efficiency brought by the proposed Raman process control sensor will have a huge commercial impact, amounting to perhaps hundreds of millions of dollars. While Raman spectroscopy has been used to analyze solar materials, it has not been used directly on the thin film manufacturing line as we proposed here. The on-line monitoring will yield new insights into causes of material quality and uniformity problems encountered in thin film solar panels and lead to their reduction and thus higher manufacturing yields. Higher yield will benefit the manufacturers as well as the users of solar panels produced by this technology.
PHASE I FINAL REPORT 1.Summary The main objective of the Phase I of this program was to demonstrate feasibility of novel instrumentation based on Raman spectroscopy, as applied to the manufacturing and quality control of thin film solar cells. Our objectives were ambitious and the task proved to be quite difficult, prompting the extension of the program by 2 months. By the end of the program most of the objectives of Phase I were achieved, and moreover, the unanticipated problems, which were considerable, were solved by the development of a novel analytical method, the Shifted Excitation Raman Differential Spectroscopy, or SERDS. We were forced to develop SERDS in order to mitigate the effects of strong fluorescence of glass substrate materials. The fluorescence almost completely obscured the Raman signal of very thin layers of amorphous silicon (< 1 micron thick), typical of commercial solar cells. The problem was particularly acute with the 785 nm Raman source, which penetrates well beyond the thickness of amorphous silicon. SERDS has been practiced before, but almost always by using the expensive laboratory size Raman spectrometers, equipped with tunable lasers. We demonstrated SERDS based on PD-LD’s patented Volume Bragg Gratings® (VBGs®), wavelength-stabilized laser diodes. This development not only allowed us to reach the objectives of Phase 1, but also resulted in prototypes of compact, low cost portable Raman instrumentation, applicable to a very wide range of analytical problems. These instruments will find its uses in solar cell industry, and also in biotechnology, pharmaceuticals, petrochemicals and general materials analysis, i.e. wherever the fluorescence presents a problem. The development of the low cost, portable SERDS instrumentation significantly increases chances for the commercial success of this project, because it opens much wider markets for our products, than originally anticipated. We believe that the results of Phase I established an excellent starting point for the Phase II work. ???2. Specific results achieved Most of the tasks of Phase I were accomplished. Raman sources at 532, 647 and 785 nm, respectively, were developed and applied to solar cell studies. Prototype Raman spectrometers and Raman imaging microscopes were assembled and applied to studies of commercial solar cells based on amorphous silicon thin films. Capability to assess material characteristics of thin film solar cells was demonstrated. Feasibility of Raman imaging utilizing compact, low cost band-pass filters was demonstrated. The instrumentation for SERDS, based on VBG wavelength stabilized laser diodes was developed and successfully applied to study solar cell samples, otherwise obscured by the strong fluorescence background. Software for interpretation of SERDS spectra, based on multivariate statistical methods, such as principal components analysis (PCA) and partial least squares regression (PLSR) was developed. The main overall objective of Phase II will be the refinement and redesign of current prototype instruments and components, demonstrated in Phase I, to the level of commercial products. The market research conducted with leading manufacturers of solar cell thin film deposition equipment clearly indicated interest in Raman instrumentation we are developing. The development of the low cost, portable SERDS instrumentation significantly increases chances for the commercial success of this project, because it opens much wider markets for our products, than originally anticipated. 3.Commercialization prospects We performed a significant amount of preliminary marketing research, regarding the need for Raman instrumentation in solar industry. We spoke to some leading manufacturers of deposition systems for the thin film solar cells, as well as to smaller equipment manufacturers. From these conversations we learned that there is a considerable interest in portable low cost Raman spectrometers as a quality control tool, but less interest in its uses as process control unit during the film deposition. This does not have a negative effect on the commercial prospects of this project, since there are many more QC points than deposition chambers in solar cell industry. We already mention the applicability of SERDS instrumentation in areas other than solar cell industry and thus the increased sales opportunities for products developed in this project. We also had frequent discussions wit Ms. Xiao-Wei Zhu from LARTA, which helped us to develop the commercialization focus for the project.
