Optimisation of wet-chemical conditioning of Si substrates for heterojunction solar cells

A crucial pre-condition to avoid recombination losses of charge carriers on hetero-interfaces is to optimise the wet-chemical substrate pre-treatment. In heterojunction solar cells the a-Si:H layer is used to form the p/n junction. Therefore, the substrate surface directly becomes part of the electronic interface. In textured mono- and polycrystalline substrates which are commonly used to optimise the light trapping properties in Si solar cells, the interface area is increased and consequently interface defects become more and more critical to the quality of subsequent processing like thin oxide film preparation or deposition various passivation layers.

Wet-chemical processes are used in solar cell manufacturing mainly for three purposes:

The recombination losses of charge carriers on Si interfaces are mainly controlled by surface charge, and the density and character of rechargeable interface states Dit . These electronic interface states result from stretched and dangling bond defects localised in a very small interlayer extended over only a few Å. Therefore, the density of these states is strongly related to the surface morphology and micro-roughness.

To reduce interface recombination losses, different approaches are utilised to minimise the density of electronically active defects at the Si interface: (i) removal of damaged regions from the Si surface by wet-chemical smoothing, (ii) saturation of dangling bonds at the surface and near surface region by wet-chemical hydrogen (H‑termination), ultra-thin oxide layers or other substituents, (iii) the engineering of the surface band bending to separate the electronic junction from the crystallographic interface.

In order to evaluate in detail the effect of these processes on the solar cell performance, we investigated standard wet-chemical processes, typically used in solar cell or microelectronic device manufacturing as well as newly developed wet-chemical methods.


Combined application of surface sensitive tech­niques, the field-modulated surface photovoltage (SPV), ex-situ and in-situ photoluminescence (PL) measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM), spectroscopic ellipsometry in the ultra-violet and visible region (UV-VIS-SE) and Fourier-Transform infrared ellipsometry (FTIR-SE), total hemispherical UV-NIR-reflectance measurements, microwave detected photo-conductance decay (µW-PCD) and quasi-steady-state photo conductance (QSSPC) provides detailed information about the influence of wet-chemical treatments on preparation induced micro-roughness, surface charge, energetic distribution of interface states Dit(E) and the resulting interface recombination behaviour of wet-chemically passivated Si substrates with special surface morphology.

references

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