As of today multicrystalline silicon wafer based solar cells form the major part of commercial large scale photovoltaic cell production. Fast solution etching is conventionally used for saw damage removal but induces randomly distributed surface indentations of a few microns. An enhancement of the overall light absorption of the solar cell surface is obtained by excimer laser ablating a regular etch barrier pattern and subsequent etching leading to a textures surface that traps incoming sunlight and minimizes reflection losses. LPXpro Series LEAP Series
Excimer Lasers
By large-area, mask-projection machining of a silicon nitride etch barrier with excimer laser wavelengths of 308 nm or 248 nm, a regular pattern of holes is created. Consecutive solution etching through the excimer laser machined silicon nitride etch barrier ultimately translates into a very regular hexagonal honeycomb-like surface structure with features of the order of some ten microns in size. Openings of diameters of 10 µm in the SiN layer at a pitch of 25 µm are obtained on the SiN etch barrier using a laser fluence of 1.8 J/cm² on the substrate surface. Figure 1 demonstrates the homogeneity of the laser texture on multicrystalline Si wafers after the etching process. The pitch of the holes influences the depth and the diameter of the texture and thus the aspect ratio. As expected, deeper structures yield a lower reflectance than flat structures as shown in figure 2. A significant overall solar cell efficiency gain of 0.4% is achieved with excimer laser texturing.

Fig. 1 - Honeycomb-like surface induced by etch barrier patterning with UV excimer laser.

Click on image for larger view. Fig. 2 - Surface reflection of laser textured multicrystalline silicon cell versus untextured cell.

With state-of-the-art high power excimer laser models providing hundreds of Watts average output power and multihundred Hertz of pulse repetition rate, process speeds for large-area, silicon nitride UV patterning are as fast as a few seconds per solar cell area (156mm x 156mm).

Material	Semiconductor
Process	Ablation-Material Removal