Beam splitter plate for rapid oxide etch mask texturing applications

Custom diffractive beam splitter plate enables rapid oxide etch mask texturing for innovative Solar Panels

Holo/Or is a member of the BURST consortium where we are responsible for the optics used to texture PV cells for improved efficiency. In this article we share some of the challenges in the optical design task for this interesting application.

Introduction: Producing solar cells with improved efficiency by using a Beam splitter plate

Solar cells efficiency has improved dramatically in recent years, reaching 24.8% in n-type IBC commercially available panels. Further improvement in efficiency is highly desirable for benefits such as reduction of footprint per watt, maintenance and cleaning cost etc. One avenue being explored for solar cell improvement is by creating a photonic crystal pyramidical holes array structure on the front side, with a potential to improve efficiency dramatically. To create such pyramid structures, the BURST consortium intends to employ an innovative approach where the front side of the Si PV cell is first covered by an oxide layer (typically SiO2) and then holes are ablated into the oxide to generate an etch mask, followed by etching that generates the pyramid structures. In this article we review the optical system design required to achieve industrially relevant structuring rates (>2000cm2/min), including the challenges, and different beam splitter plate design options.

The optical challenge of large area laser texturing

The ideal optical structure for photonic crystal aimed at maximizing light trapping in the PV cell is in the range 1.5-3.1µm. Thus, holes in the oxide mask layer must be opened with a similar pitch and the 515nm laser spot on the oxide must be of similar magnitude (1-2µm diameter). As the fluence needed to remove the oxide per hole is rather low, thousands of such holes can be simultaneously structured with a single laser pulse. The main challenge is that the energy cannot be spread by scanning over the field, as scanning optics do not typically reach a spot size of 1-2µm. The way to spread the energy is by using a diffractive beam splitter plate to generate an array of spot that will simultaneously structure multiple holes, and scanning this line using a fast stage.

Diffractive beam splitters- principle of operation

A diffractive beam splitter plate is a flat optical component that splits an incoming laser beam into a fan  of pre-defined output orders, each having a pre-defined fraction of the input energy and its own deflection angle. When focused by an external lens, this generates an array of spots with uniform separations, each having a profile identical to the spot created by the un-split input laser. Diffractive beam splitters are periodic phase gratings, typically transmissive, etched into a transparent material (fused silica in our case). Using advanced iterative algorithms, the phase is designed to maximize the efficiency of splitting to a certain diffraction order, while maintain uniform energy in the desired orders. Sometimes, a lens function is added to the diffractive grating, resulting in a beam splitter lens, but for large area laser oxide texturing, a special large field focuser must be used.

Large field focuser for oxide on Si laser texturing

As previously mentioned, the main challenge in oxide laser texturing is the spreading of laser power over the surface in a structured way, while scanning rapidly perpendicularly to the line. For this purpose, the Focus optics must support a large field. For a texturing line of 2mm wide, if scanning is done at 2m/s (fast axis stage), the texturing rate would be 2400cm2/min, i.e. an industrially relevant rate of 10 typical cells per minute. Therefore, a laser focuser that can achieve 2µm spots of a 2mm field is needed. As such objectives are not commercially available, innovative optical design of custom optics must be used. Furthermore, best performance is achieved by placing the beam splitter plate diffractive optical element (DOE) at a pre-designed position within the focusing system, requiring simultaneous refractive and diffractive design of the system as a whole.

Possible texturing schemes

A straightforward scheme for texturing the oxide is a line of uniform spots scanner perpendicular to the line. However, due to inherent mutual interference constraints, designing a line of 2um spots with 3.1um will result in mutual interference between the spots, reducing intensity uniformity. To mitigate this, different spot patterns can be used, such as a staggered spot pattern where there is much less overlap between neighboring spots. Due to the inherent design flexibility of diffractive beam splitters, the same focusing system can be used with different beam splitter plates, resulting in different texturing patterns.

Conclusion

Diffractive beam splitter plates can enable rapid laser texturing of oxide masks on PV material, creating hundreds of 2µm holes in the oxide with controlled separations over fields as large as 2mm. As this is a challenging optical task due to the large NA and field required, the refractive focusing objective must be designed around diffractive element requirements, so the system achieved its intended specs.