Emitter Wrap Through - EWT

Emitter wrap-through” (EWT) and “Metallisation wrap through” (MWT) are high-efficiency back-contact solar cell technologies. Vias allow the emitter on the front of the cell to be “wrapped-through” to the back surface. This takes the metallization for the bus bars (MWT) or the metallization for both bus bars and fingers (EWT) to the back of the cell,

• Metallisation is moved to the back of the cell reducing shadowing
• EWT also increases emitter area (n doped regions of the back side) 
• A high power IR laser with low M2 is ideal for silicon drilling 
• EWT: 300 wafer/module/hour, MWT: 1200 wafer/module/hour
• 15mm 3s via position available with the MSV-Wx system to enable correct contacting to iterdigitated rear electrodes.

Laser micro machined emitter wrap through technology is the creation of vias to wrap the emitter on the front surface to contacts on the back of the cell. Before the n-dopant diffusion process lasers are used to  drill via holes using pulsed laser ablation.

The process of laser via drilling creates minor surface damage (micro-defects) which may subsequently be removed by flash etching material away from the via hole. Sodium Hydroxide (NaOH) is one of several etchants that may be used in the process of clearing debris from exposed surfaces.

The n-doped emitter is allowed to wrap through the vias during the implantation process, the area of p-type material on the back of the cell is minimised and therefore maximises the collector area after n-doping,  The technique can be used with lower quality and thinner silicon without significant losses in efficiency. Having the provision of a large cost advantage over conventional cell configurations this system has the potential to be implemented in a cost effective process flow.