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Influence of random pyramid surface texture on silver screen-printed contact formation for monocrystalline silicon wafer solar cells
Most industrial monocrystalline silicon wafer (mono-Si) solar cells are metallised by screen printing. Silver (Ag) pastes are commonly used to form electrodes to phosphorus-doped n+ Si. However, there is still ambiguity about the influence of the random-pyramid surface texture on Ag screen-printed contacts for mono-Si solar cells. We present an experimental study to investigate this influence using screen-printed p-type mono-Si cell groups fabricated with a controlled variation in the alkaline surface texturisation process. It is observed that cell groups fabricated on Czochralski (Cz) wafers with smaller pyramids achieve higher average fill factor (FF) and lower average specific contact resistance than cell groups fabricated on Cz wafers with larger pyramids. To explain these observations, the pyramid texture height distributions are characterised statistically and the distribution statistics are correlated to electrical solar cell measurements and microstructure investigations of the Ag/n+ Si contact interface. Microstructure investigations reveal that most Ag crystallite growth is concentrated around the upper part of the pyramids and hence pyramid density is identified as an important parameter influencing contact formation. The influence of average pyramid height and pyramid height uniformity within a pyramid texture height distribution is also clarified with regards to contact formation. It is further observed that direct Ag crystallite contacts to the bulk Ag metallisation are not a prerequisite for achieving high FF (>80%). Based on the study, guidelines are developed for tailoring random-pyramid surface textures to optimise Ag screen-printed contact formation to n+ Si for mono-Si solar cells.