Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Bottom-to-top process: buried grid metallisation

Using this hybrid screen-print/buried contact process, which has been specially optimised for multi-crystalline silicon material, an efficiency of 18.1% on a large area cell has been achieved. The front side is made by the conventional buried contact approach, whereas the rear side has a back surface field made by screen printing and firing aluminium paste.

The front side was mechanically texturied by a single blade resuluting in parallel v-grooves. The cell was made on Polix material from Photowatt, which had a Voc of 636 mV, a Jsc of 36.91 mA/cm2 and a FF of 77%. Fraunhofer ISE certified the efficiency. The formerly reached efficiency of 17.6% was clearly topped.

The improvement was mainly contributed by a better spectral response in the long wavelength region due to a more effective back surface field and more homogeneous material quality. Cell thickness was higher, differences could also be seen at the front surface reflectance, the texture was deeper and pointier for the 18.1% cell.

Applying a similar process to TOPSICLE material resulted in best efficiency of 15.8%. This lower efficiency was partly due to the fact that the TOPSICLE wafers were only NaOH etched rather than being textured. However, there is also evidence that the efficiency of the LGBC cells on TOPSICLE wafers was limited by relatively poor minority carrier diffusion length.

Analysis of LGBC cells made on TOPSICLE I wafers showed that the inclusion of MIRHP in the process increased the mean bulk diffusion length. However, screen-printed cells fabricated on neighbouring wafers had an even higher mean diffusion length. It was apparent that MIRHP as applied in the buried contact process is not effective in realising the optimum bulk lifetime of the TOPSICLE material.

A batch of 50 high quality wafers was used to process high efficiency buried contact cells. Textureing, BSF formation and MIRHP passivation were done by UKON, all remaining process steps by NaREC. Due to technical contraints the cells could not be finished in time.

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