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Garralaga Rojas E.,Institute Fr Solarenergieforschung Hameln ISFH | Garralaga Rojas E.,European Space Agency | Hensen J.,Institute Fr Solarenergieforschung Hameln ISFH | Baur C.,European Space Agency | Brendel R.,Institute Fr Solarenergieforschung Hameln ISFH
Solar Energy Materials and Solar Cells | Year: 2011

We investigate sintering and reorganization of electrochemically etched mesoporous Ge layers. Sintering in nitrogen, forming gas, or argon atmospheres always yields a reorganized and oxidized porous layer. Water-insoluble Ge native oxides produced during etching do not hinder thermal reorganization, but evolve to water soluble oxides during annealing. Reorganized porous layers hence dissolve in subsequent water or HF dips. Sintering in hydrogen atmospheres removes native Ge oxides and prevents oxidation of porous layers. Porous layers with porosities less than 30% exhibit a compact closed surface after sintering, whereas porous layers with porosities exceeding 50% collapse. The porosity decreases linearly in a range of 57%45% for electrolyte concentrations and in a range of 3550 wt% for a specific resistivity of 0.013 Ω cm and an etching current density of 5 mA cm-2. We obtain porosities below 30% by using substrate resistivities in the 0.0150.030 Ω cm range, etching current densities below 2.5 mA cm-2 and an electrolyte concentration of HF 50 wt%. © 2010 Elsevier B.V. All rights reserved.

Hermann S.,Institute Fr Solarenergieforschung Hameln ISFH | Merkle A.,Institute Fr Solarenergieforschung Hameln ISFH | Ulzhfer C.,Institute Fr Solarenergieforschung Hameln ISFH | Dorn S.,Institute Fr Solarenergieforschung Hameln ISFH | And 11 more authors.
Solar Energy Materials and Solar Cells | Year: 2011

We report on RISE-EWT (Rear Interdigitated Single Evaporation-Emitter Wrap-Through) solar cells on full area (12.5×12.5 cm2) pseudo square boron doped Czochralski-grown silicon wafers. We investigate the main efficiency optimisation factors of these cells by investigating the dependence of RISE-EWT cell parameters on the base dopant concentration NA. We furthermore detail the effects of large feature sizes in base and emitter regions at the rear of the solar cell and investigate these effects with particular attention to the edge regions. EWT solar cells typically exhibit rather low fill factors. However, our results show that the improved fill factors can be achieved by increasing NA, which in return leads to optimised efficiency values. For our RISE-EWT solar cells made from boron doped Cz-Si wafers, this benefit is maintained even after light-induced degradation. Our investigation of edge area related effects shows the importance of proper cell design in these areas, leading to a further 2.8% absolute improvement in the fill factor. Combining increased base dopant concentration with optimised edge design, we achieve 19.0% efficiency on (12.5×12.5 cm2) boron doped Cz silicon wafers before light-induced degradation, resulting in 18.1% efficiency in the light-degraded state. © 2010 Elsevier B.V. All rights reserved.

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