Ballif, Christophe

Fesquet, Luc

Olibet, Sara

Shah, Arvind

Vallat-Sauvain, Evelyne

Infoscience - École polytechnique fédérale de Lausanne

Ns QsHIGH QUALITY SURFACE PASSIVATION AND HETEROJUNCTION FABRICATION BY VHF-PECVD DEPOSITION OF a-Si:H ON c-Si THEORY AND EXPERIMENTSLuc Fesquet, Sara Olibet, Evelyne Vallat-Sauvain, Arvind Shah, Christophe BallifUniversity of Neuchâtel, Institute of Microtechnology, Rue A.-L. Breguet 2, CH-2000 Neuchâtel, Switzerland Tel: +41 32 718 3321, Fax: +41 32 718 3201 e-mail: luc.fesquet@unine.ch2DV.2.14?Reach high Voc-high efficiency with a-Si:H/c-Si heterostructures: need of high quality c-Si surface passivation.?Investigation of a-Si:H layers passivation properties: analysis of lifetime measurements using an amphoteric model.?Double side passivating layer structures speed up the development of high efficiency solar cell.MotivationModeling interface recombination through amphoteric defects at the a-Si:H/c-Si interface1.From double side passivation…Recombination through amphoteric states: 2 paths coexist.…to final solar cellSample Ns(109.cm-2)Qs(1010.cm-2)i 50 nm 5.2 -7n-μdoped 50 nm 53 -85i 5 nm + p 20nm 3.2 150n-μdoped 5 nm + n 25nm 40 -50? Symmetrical passivation structures ? determine quality of the passivating layer.? Fitting of Seff(ECD) curves using the physical model.?Intrinsic a-Si:H layer ? Low Ns.?Doped a-Si:H layers ?Qs tuning.?Combination of both ? Low Seff.? Seff : down to 6 cm/s with a-Si:H/ a-Si:H layers.? The optimization of the passivation layers is facilitated.Example of charge and band diagram at the a-Si:H/c-Si interfaceFit to measured Seff for various a-Si:H layerFit parametersLifetime measured by QSSPC and Suns-Voc of a solar cell based on previous layers. qnkTJJV phoc ×⎟⎟⎠⎞⎜⎜⎝⎛≈0lnτb bulk lifetimeW wafer width.Deposition of VHF PECVD a-Si:H on c-Si Surface passivationThis work is supported by the Swiss National Science Foundation (FN-200021-107469 ) and by the Axpo Naturstrom Fonds a-Si:Hc-Si a-Si:Hc-Si a-Si:H<doped> a-Si:H<doped> a-Si:H a-Si:Hc-Si a-Si:H<n> a-Si:H a-Si:H a-Si:H a-Si:H layer /<doped> a-Si:H layersphJnkTqVJJ −⎥⎦⎤⎢⎣⎡ −⎟⎠⎞⎜⎝⎛= 1exp0J0 : saturation current densityJph : photogenerated current densityn: diode ideality factor?Dark IV characteristics of the cells are fitted by the 1 diode equation.?1-Sun Voc depends on J0 and n.?Set of a-Si:H/c-Si solar cells on various c-Si substrates:?n constant ≈ 1.2.?ln(J0) is linearly dependent to 1-Sun Voc .?Dark IV characteristics (J0): supplementary tool for passivation quality control?Example of symmetrical structures on FZ 2.5Ωcm p-type c-Si.2Wτ1τ1Sbeffeff ⋅⎟⎟⎠⎞⎜⎜⎝⎛ −=Passivation theoryBest Efficiency Solar cellsWafer Type Voc (mV) FF η (%)n 1Ωcm 682 82 19.1n 1Ωcm 713 74 16.9p 0.5Ωcm 690 74 16.3Relation between 1-Sun Voc and J0Conclusion?Development of deposited a-Si:H layers is facilitated by analysis of the passivation property using an amphoteric model.?Solar cells benefit from the development of these passivating layers, resulting in Voc and efficiency improvement.? Effective lifetime (τeff) measured by QSSPC ? injection level (ECD) dependent effective surface recombination velocity Seff.? High quality stacked layers allow the fabrication of high Voc solar cells.?In this example: Voc= 675mV.? Limited by the a-Si:H/<n> a-Si:H stack.?Suns-Voc measurements are made after front and back contact depositions.? To check if the contact deposition damages the a-Si:H passivating layers.? High efficiencies are achieved on both p and n-typed c-Si, partially due to high Voc’s? A trade-off exists between Voc and FF. However, c-Si surface passivation is necessary to achieve high efficiency.(1) S. Olibet, E. Vallat-Sauvain, C. Ballif, Phys. Rev. B 76 (2007) 35326. ? This passivation scheme is based on the recombination through amphoteric defects on a-Si:H, at the a-Si:H/c-Si interface.• Dangling bonds at the interface (Ns )? Recombination.• Average charge density at the interface (Qs ) ? Field effect passivation.

High quality surface passivation and heterojunction fabrication by VHF-PECVD deposition of amorphous silicon on crystalline Si: theory and experiments

http://infoscience.epfl.ch/record/133986/files/paper_459.pdf

High quality surface passivation and heterojunction fabrication by VHF-PECVD deposition of amorphous silicon on crystalline Si: theory and experiments

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