Conclusions for the production of multicrystalline silicon solar cells

The experiments described in section 3.3 clearly show that the oxidation at 1050 °C deteriorates the bulk lifetime of multicrystalline silicon. Nevertheless, oxidation of the silicon surface plays a key role in high-efficiency processing of solar cells. It serves as a masking layer which simultaneously passivates the surface (and can act as an anti-reflection coating if no encapsulation is required).

Therefore the sequence of phosphorus diffusions and oxidations has to be chosen correctly if an oxidation is necessary.

The performed experiments result in clear guidelines how to maintain a high bulk-lifetime in multicrystalline silicon when applying high-temperature processes.

Gettering

The phosphorus diffusion is capable of effectively gettering mobile impurities such as iron into the phosphorus layer. This gettering layer should be etched away in order to avoid the release of impurities from the gettering site during subsequent high-temperature processes.

Oxidation

The high-temperature processes, namely oxidations, should be applied after gettering since they increase the dislocation density and therefore the number of sites where impurities can accommodate. Oxidation at 1050 °C dissolves precipitated impurities and should, if possible, be followed by a phosphorus diffusion (e.g. emitter diffusion) to recover carrier lifetime (at least partly). It is not beneficial to have a high-temperature oxidation at the end of the process sequence (e.g. for emitter drive-in) since carrier lifetime is decreased again. Furthermore it is advisable to reduce the oxidation temperature and decrease up and ramp-down gradients to minimise degradation effects.

3.4 Chapter summary

The change of the minority carrier bulk lifetime in multicrystalline silicon during gettering processes and oxidations was investigated. For the examined material phosphorus gettering at temperatures between 860 °C and 900 °C achieved higher minority carrier lifetimes than phosphorus-aluminium co-gettering. One of the reasons for the improved lifetime after the phosphorus diffusion was the removal of iron. This was concluded from injection dependent lifetime spectroscopy on large measurement spots. With spatially resolved measurements of the minority carrier lifetime the local effect of the phosphorus diffusion was investigated. The gettering efficiency was not homogenous all over the wafer: In regions with many dislocations no increase in lifetime was detected whereas in regions of low dislocation densities very high bulk lifetimes were measured. A microscopic model explaining the results was discussed.

The oxidation of multicrystalline silicon at 1050 °C clearly deteriorated the material quality. Besides the negative effect of a rise in dislocation density

precipitated impurities were suspected to have dissolved and spread out in the bulk leading to homogeneously low carrier lifetimes. These could at least partly be recovered by a subsequent phosphorus diffusion.

Nevertheless, the degradation could be prevented by adjusting the oxidation process. Lowering the oxidation temperature to 800 °C significantly reduced the degradation. The ramp-up and ramp-down profiles of the process were shown to have a strong impact, too. Using slow ramps the carrier lifetime of multicrystalline silicon was nearly perfectly maintained.

From the conducted experiments conclusions were drawn with respect to the processing sequence for multicrystalline silicon solar cells: The first process should be the permanent removal of impurities from the material by etching away the gettering layer of a phosphorus diffusion. If an oxidation of the surfaces is necessary this has to be done at low temperatures and should be followed by another phosphorus diffusion.

4 Texture and front surface structure of multi-crystalline silicon solar cells

4.1 Introduction

As early as 1960 solar cells were textured on the front surface to decrease the reflectance [53] and to boost the efficiency. In those days the front surface was shaped by ultrasonic cutting into inverted pyramids. Later on, wet-chemical etching was introduced [54] and the first commercial applications appeared in 1975 for space solar cells [55]. Nowadays all high-efficiency single crystalline silicon solar cells are textured on the front surface to reduce reflection losses and to improve the light-trapping properties. The most prominent way to obtain good results is the etching of pyramids into <100> orientated monocrystalline wafers.

This method uses the anisotropic etch rates of potassium (KOH) or sodium hydroxide (NaOH), where <111> planes are etched two orders of magnitude slower than <100> and <110> planes. The result is a surface covered with intersecting <111> planes that form randomly distributed pyramids of varying size in the case of “random pyramids” or, when a cross-hatched masking layer is used, form the “inverted pyramids” structure. This method is an elegant and effective way to texture the surface of monocrystalline wafers with <100> orientation, but it is not applicable for multicrystalline wafers since the multiple grains have different and generally unknown orientations. Therefore it is necessary either to use isotropic etches or to structure the surface by other means which are independent of crystal orientation.

Since an increased absorption of light in the solar cell directly increases the short-circuit current density and consequently the efficiency, the texturing of multicrystalline silicon has become a topic of high importance and is studied by various research groups. Several approaches are described in section 4.2. A special case of high-efficiency texturing for multicrystalline silicon is a hexagonal pattern of etched bowls, the so called “honeycomb” structure. The optimisation of low-temperature plasma processes to achieve a damage-free surface and very good light trapping properties is described in section 4.3.

To enable an easy comparison of different textures, the highly wavelength-dependent reflectance is weighted with the solar spectrum and integrated from λ0 = 300 to λ1 = 1200 nm via

with q being the elementary charge, h Planck's constant, c the velocity of light and S(λ) the spectrum AM1.5g. Thus the distribution of the standard spectrum is taken into account and a single value represents the optical quality of the texture for the incident light. The weighted reflectance values Rw found in literature often vary in the integration boundaries λ^{0 and} λ^1.