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A1.04 - Hydrogenated Amorphous Silicon Germanium by Hot Wire CVD as an Alternative for Microcrystalline Silicon in Tandem and Triple Junction Solar Cells 
April 22, 2014   9:30am - 9:45am

The manufacturing cost of thin film Si based tandem and triple junction cells and modules is at present too high to meet current module market prices. Conventionally, microcrystalline silicon is used as the low-bandgap absorber in micromorph solar cells (a-Si/µc-Si tandem cells). However, due to the considerable thickness needed for the µc-Si:H absorber, it takes three to four times as many deposition reactors compared to single junction cells to produce tandem cells, leading to high cost of ownership. One of the approaches to reduce processing time of the low-bandgap layer(s) in multijunction silicon-based solar cells is the use of hydrogenated amorphous silicon germanium (a-SiGe:H). In general however, a-SiGe:H has not been considered a viable option because of (i) the high defect density for PECVD a-SiGe:H, at band gaps < 1.4 eV, and (ii) the cost of GeH4. On the other hand, due to its direct gap nature, the thickness of an a-SiGe:H absorber layer can be kept 10 times smaller than that of µc Si:H. We are investigating whether a-SiGe:H can be reconsidered for inexpensive production of multijunction thin film Si based solar cells if HWCVD is used as the deposition method. HWCVD is a simple and low cost deposition technique allowing high deposition rates while maintaining good defect passivation. Early results reported by NREL include the achievement of material with a band gap close to that of µc Si:H (1.2 eV) with an equivalent photoresponse (in excess of two orders of magnitude). Their work has led to 8.64% single junction cells without any band-gap profiling in the absorber layer. We now continued this development to provide a novel thin film alloy for the struggling micromorph technology. We have produced a-SiGe:H materials with Tauc band gaps ranging from 1.6 eV down to 1.2 eV. Due to the efficient dissociation of silane and germane gases at the hot filament, a high deposition rate is achieved. Moreover, the dissociation rate of germane is three times faster than that of silane. The deposition rate for the lowest-gap material is 0.7 nm/s and is always higher than 0.5 nm/s. With this deposition rate, an active absorber layer (i-layer) of 150 nm is readily deposited within 5 minutes. This should be compared to the roughly one-hour long deposition time needed to deposit a 2-µm thick µc Si:H film at the commonly used deposition rate of 0.5 nm/s. Using a GeH4/SiH4 ratio of 1, we deduce from Raman spectroscopy that the films already contain 60-70% Ge, showing that Ge is preferentially incorporated in the film. We will report an extensive microstructure analysis and will present our first cells.

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Keynote Address
Panel Discussion - Different Approaches to Commercializing Materials Research
Business Challenges to Starting a Materials-Based Company
Fred Kavli Distinguished Lectureship in Nanoscience
Application of In-situ X-ray Absorption, Emission and Powder Diffraction Studies in Nanomaterials Research - From the Design of an In-situ Experiment to Data Analysis