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a-Si:H application to Solar Cells Jonathon Mitchell Semiconductors and Solar Cells
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Overview Fundamentals Process Where we’re at…
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Fundamentals Photovoltaic effect results from incident light on some materials PV effect promotes electrons into higher energy conduction bands, leaving holes behind Separation of carriers, electrons (-ve) and holes (+ve) important to solar cells +ve and –ve carriers transported through material in all directions Surface recombination Bulk recombination
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Fundamentals Recombination of these carriers occurs in a variety of ways End Result: Nothing useful Need to separate these charges Recombination occurs at the surface as well due to free opposing charges (defects) Passivation of this surface is necessary to solar cells
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Surface Passivation High density of defects at surface of c-Si Defect passivating layers: SiO x, SiN x, a-Si:H SiO x Best results Good insulator Temperatures >900 o C High risk of impurity contamination SiN x Very good results Good insulator Temperatures 400 o C Industrial BSF used Risk of impurities a-Si:H Equal or better than others Good conductor Temperatures <250 o C Doped layers Heterojunctions possible Thin layers <10nm Expensive Cheaper Cheapest/Easiest
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Process c-Si wafers etched, RCA cleaned a-Si:H deposited by plasma enhanced chemical vapour deposition (PECVD) Non-homogenous Deposition difficult to control Lower quality layer homogenous layer quality and deposition conditions improved
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PECVD Deposition initiates reactions at surface Desorption/abstraction Absorption HF
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QSSPC ? ? ? ? ? Carrier lifetime measured within materials Quasi-Steady State Photoconductance (QSSPC) Transient Photoconductance (PCD)
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Where we’re at… Post-deposition thermal anneal greatly improves passivation quality of a-Si:H layer Carrier lifetimes equivalent or better than those reported by other groups Non-diffusion process defined for surface passivation
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Where we’re at… Post-deposition thermal anneal Thermal annealing near deposition temperature significantly improved results Thermally stable once saturation reached Optimal a-Si:H layer thickness ~10-20nm Other thicknesses work well
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Where we’re at… Non-diffusion surface passivation process measured and defined Surface passivation believed to occur from hydrogen diffusing from within these thin layers towards the surface Less energy needed for surface passivation than for diffusion A re-configuration of the surface fits these results Energy ~1.5eV Surface Passivation Activation Energy ~ 0.69 ± 0.1eV
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Conclusion a-Si:H thin film layers provide excellent surface passivation for c-Si solar cells Ultra-clean, state of the art, high power systems aren’t necessary for these results Bulk diffusion of hydrogen is insufficient to explain surface passivation 1.5eV Non-diffusion surface passivation reactions suggest surface reconfiguration is the underlying process 0.69 ± 0.1eV Thermal annealing improves the surface passivation provided by the deposited a-Si:H Solar cell are possible with the work that has been done so far
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Acknowledgements ARC for providing funding Murdoch University for use of PECVD Questions?
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