The silane depletion fraction as an indicator for the amorphous/crystalline silicon interface passivation quality

Christophe Ballif, Loris Barraud, Stefaan De Wolf, Antoine Descoeudres
American Institute of Physics, 2010
Article

Résumé

In silicon heterojunction solar cells, thin amorphous silicon layers passivate the crystalline silicon wafer surfaces. By using in situ diagnostics during plasma-enhanced chemical vapor deposition (PECVD), the authors report how the passivation quality of such layers directly relate to the plasma conditions. Good interface passivation is obtained from highly depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells, yielding aperture efficiencies up to 20.3% on 4 cm2

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https://infoscience.epfl.ch/record/155068?ln=fr

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Silane plasma diagnostics for high-efficiency silicon heterojunction solar cells

Christophe Ballif, Loris Barraud, Priscille Marguerite Bôle Rothen, Stefaan De Wolf, Antoine Descoeudres

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In silicon heterojunction solar cells, the passivation of the crystalline silicon wafer surfaces and fabrication of emitter and back surface field are all performed by intrinsic and doped amorphous silicon thin layers, usually deposited by plasma-enhanced chemical vapor deposition (PECVD). Since the properties of materials deposited by PEVCD are directly linked to the plasma properties, plasma diagnostics are very useful tools to optimize such devices. A novel diagnostic has been developed to measure in-situ the molecular silane depletion fraction in the plasma during deposition. It is found that the silane depletion strongly depends on the process parameters, and appears to be a relevant parameter for the quality of the passivating layers. Good passivation is indeed obtained from highly depleted silane plasmas. Based on this, layers deposited in a large-area PECVD reactor working at very high frequency (40.68 MHz) were optimized for heterojunction solar cells. All other fabrication steps were also fully industry compatible, using sputtering for transparent conductive oxide layers and screenprinting for the front grid. The best 2 x 2 cm2 cell shows a high open-circuit voltage of 717 mV, yielding a conversion efficiency of 20.3% (aperture area). Keywords: Heterojunction, PECVD, High-Efficiency

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21% efficiency silicon heterojunction solar cells produced with very high frequency PECVD

Christophe Ballif, Loris Barraud, Priscille Marguerite Bôle Rothen, Stefaan De Wolf, Bénédicte Demaurex, Antoine Descoeudres, Jonas Geissbühler, Johannes Peter Seif

Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition (PECVD). By using in-situ plasma diagnostics and ex-situ film characterization, we show that the best a-Si:H films for passivation are produced from deposition regimes close to the amorphous-to-crystalline transition. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells. 4 cm2 solar cells were produced with fully industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%.

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