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Poster Award WCPEC-6

Our contribution „Influence of substrate morphology on the growth of thin-film microcrystalline silicon studied by in-situ Raman spectroscopy“ received a poster award at the „6th World Conference on Photovoltaic Energy Conversion“ (WCPEC-6, in Kyoto, Japan. Thematically, the presentation deals with the growth of microcrystalline silicon as absorber layer in thin-film silicon solar cells. Those absorber layers were grown on different textured substrates by plasma enhanced chemical vapor deposition (PECVD).

In state-of-the art thin-film silicon solar cells specific substrate materials with periodic or random surface structures are used to enhance the light path in the absorber layers and to improve the light coupling into the solar cell. In this way, the solar cell thickness can be reduced to save resources and reflection losses are diminished.

Beside the light management the substrate surface morphology determines the growth of deposited layers. In the case of microcrystalline silicon different substrate textures impact on the ratio of crystallites to amorphous contributions, which affects the current and voltage parameters of a solar cell. At IEK-5 photovoltaics, a unique setup was developed over the last few years that enables the monitoring of the absorber layer growth during deposition.

If light is scattered in a solid state body, photons may interact with solid-state vibrations (phonons). The wavelength of the scattered light depends on whether the scattering process took place in the crystallites or in amorphous domains. The intensity proportion of the Raman shifted light allows to draw conclusions on the phase mixture of the material.

From the technical point of view, the measurement setup was realized by integrating suitable optical components directly in the electrode of a PECVD reactor. It turned out that the accuracy of the measurement process is sufficient to detect even small impacts of the substrate morphology on the crystallinity of the growing layer in-situ. On the basis of the information that one obtains by this technique, the external measurement parameters can be adjusted to achieve beneficial crystallinity profiles in growth direction for different kinds of substrates.

As a result of this process control the solar cell performance can be optimized with high accuracy and reproducibility. Moreover, the comparability of light management of different textured substrates is significantly improved.