Process Introduction

Our silicon solar cells are manufactured from thin p-doped multicrystalline silicon wafers supplied by, among others, ScanWafer. They are produced by casting large ingots of high purity silicon. The ingots are wire-cut into wafers that typically measure 156 mm x 156 mm x 0.2 mm.

Damage Etch

The wire-cutting process generates damages on the surface of the wafers. This damage is removed by etching in a strong acidic or alkaline solution. Typically 40-50 microns are etched away creating a flat surface.

Texturing

To minimise reflection from the surface, the wafers are textured. This means creating a roughened surface so that incident light will have a larger probability of being adsorbed into the solar cell. This is performed by etching in a weak acidic or alkaline solution.

P-doping

To create the pn-junction, a thin n-type layer is made by in-diffusion of phosphorus (P) from the surface. A liquid glass containing P is uniformly distributed on one side of the wafer by spraying or spinning. This side now defines the front of the solar cell. The wafers are then heated in an oven to harden and evaporate the solvents in the liquid glass.

Emitter Diffusion

To make the P-atoms diffuse into the silicon from the glass, the wafers are placed in a belt furnace and heated to about 900°C. The glass which contained the P-atoms act as a thin insulating layer and is therefore removed by etching.

Anti Reflection Coating

At this stage in the process, the solar cell has reflectivity of about 30%. To reduce losses due to surface reflection, a thin layer of silicon nitride (H:SiN_x) is deposited on the front of the cell in a PECVD chamber. The silicon nitride works as an anti-reflective coating due to a combined effect of its favourable refractive index and the layer thickness. The solar cell front now has a dark blue colour.

Contact Formation

To extract current from the solar cell, metal contacts have to be made. These are applied by screen printing, in which a paste is pressed though a pre-defined pattern mask onto the wafer. While the back is totally covered by metal, the front of the wafer is covered in a finger-pattern allowing light to reach the surface of the silicon.

Sintering and Firing

As of now, the metal contacts in the front lie on top of the anti-reflection coating, whereas on the back the metal is in touch with the thin n-type layer caused by gas diffusion. In the sintering and firing process, the metallic contacts are rapidly heated up to about 900°C, allowing the front contact to etch through the anti-reflection coating. At the back-side, the metallic region depletes the thin n-type layer, and forms a heavily doped p-type layer referred to as a back surface field.

Ready for Use!

Now the solar cells are finished and ready to be mounted into solar panels.