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Sacrificial high-temperature phosphorus diffusion gettering process for lifetime improvement of multicrystalline silicon wafers
Iron is among the most deleterious lifetime-limiting impurities in crystalline silicon solar cells. In as-grown material, iron is present in precipitates and as point defects. To achieve solar cell conversion efficiencies above 20%, bulk minority-carrier lifetimes in excess of 300 µs (p-type) and 900 µs (n-type) are required [1]. For cost-effective multi-crystalline silicon wafers, achieving this lifetime requires gettering. Gettering at higher temperatures for longer times is often necessary to fully dissolve and remove precipitated impurities. However, such time-temperature profiles can result in unacceptably deep emitters, affecting the blue response of the finished device. Here, we explore a “sacrificial” gettering step in which gettering and emitter-formation are decoupled and optimized independently. The optimization is guided by the Impurity-to-Efficiency simulation tool [2] and explores high-temperature regimes. While models predict that increasing the gettering temperature decreases total iron concentration resulting in an increased lifetime, experimental results show that for the highest temperatures tested, the minority carrier lifetime is reduced.
Nanocrystalline silicon based solar cell technology for large volume manufacturing
In this paper, we report the progress of nanocrystalline silicon (nc-Si) based a-Si/nc-Si double-junction solar cell technology development at Hanergy Solar. We conduct the experiments mainly for optimization and development on 1) high quality intrinsic amorphous silicon (a-Si) films used for top cells, 2) p-layers and buffer layers, and 3) intrinsic nc-Si film used for bottom cells. We have achieved initial total area efficiency of 12.9%, and stable total area efficiency of 11.1% for a-Si/nc-Si double-junction modules (0.79m2). Experimental results including study of individual component cell optimization, crystal volume fraction optimization, and development of superior doped layers are presented.
A Novel Transient Control Strategy for VSC-HVDC Connecting Offshore Wind Power Plant
This paper proposes a novel steady-state and transient management scheme for voltage source converter based high voltage direct current (VSC-HVDC) connecting permanent magnet synchronous generator (PMSG)-based offshore wind power plant (WPP). The proposed control arrangement aims to fully utilize the HVDC converters' normal loading capabilities during steady-state operation. Furthermore, it targets the employment of the available converters' overloading capabilities to enhance the fault ride through (FRT) performance during faults. The positive and negative sequence components are controlled to 1) neutralize dc-link voltage ripples due to asymmetrical grid faults, 2) inject reactive current support for the grid voltage, and 3) deliver the maximum possible active power during different faults. Novel mathematical and time-variant representations of the positive and negative sequence components are introduced to adapt the converter current limits for full utilization of the converter limit. Comprehensive simulation studies using PSCAD/EMTDC are presented to verify the functioning of the proposed control strategy.
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