Applications of Laser Micro Machining in Photovoltaics – Mono- and Polycrystalline Solar Cells

Laser micromachining processes being used with single crystalline or polycrystalline solar cells include laser edge isolation, laser micro via drilling, laser fired contacts, and laser surface structuring. All of the aforementioned machining technologies do guarantee high efficiency of the completed solar cell at a minimum of materials damage and least material’s loss.

Edge Isolation

For the achievement of high opto-electronic efficiency, differently doped front and rear sides of crystalline solar cells need to become isolated at the edges. The latter layers are electrically isolated by scribing with Q-switched solid-state lasers (Nd:YAG or Nd:YVO4). Lasers implemented in workstations made by 3D-Micromac AG provide power densities high enough to remove molten material from the groove and to avoid redeposition of debris in the groove. Cutting widths of 40 to 60 µm are reached at depth of 30 to 40 µm. Routinely achieved cutting speed amounts to 300-500 mm/s, in the high-speed mode even 1,000 mm/s.

Another advantage consists in the deployment of high-definition motion systems enabling to guide the isolation lines at highest precision along the cell edges, hence maximising the effective cell area and the efficiency of the cell itself

Grooving (Laser grooved buried contact)

In the Laser Grooved Buried Contact (LGBC) concept, the front-side contact of the solar cell is positioned in prescribed laser grooves. Shadowing effects are reduced and the efficiency of the solar cell is enhanced. The processing of such grooves is accomplished using diode-pumped solid-state lasers. Using high-capacity lasers, groove depths between 5 and 130 µm and a scribing speed of 300 mm/s are achieved.

Laser Drilling for Back-Contact Cell Design

The efficiency of solar cells can be significantly improved by a redesign of the front-side contacts that are covering considerable portions of the active area. The basic idea is to shift as much contact area as possible towards the back side of the cell. This is achieved by drilling holes of different size using a disc laser. Employing the EWT (emitter wrap-through) and the MWT (metal wrap-through) concept, electrical contacts are being transferred from the front side towards the rear side. In EWT technology, hole diameters amount to 30 to 80 µm and using percussion drilling, 5,000 to 15,000 holes can be generated per second. Using the trepanning approach, about 25 holes of some 100 µm diameter can be machined per second for MWT solar cells.

On the basis of long standing experience in the range of laser micromachining, the aforementioned highly effective processes could be established. Drilled holes are machined free of cracks, at minimum heat affects and without debris.

Laser Cutting and Laser Scribing

Monocrystalline and polycrystalline silicon wafers can be cut at highest precision and minimum heat load using q-switched disc lasers. This procedure is applied for separating squarish cells from circular wafers. Cutting speeds of 150 mm/s are reached for a wafer thickness of up to 200 µm. For wafers possessing a thickness in excess of 400 µm or in fully automated production environments, silicon wafers are not entirely cut but scribed and subsequently separated manually or in an automated fashion. Typical scribing speeds are on the order of 50 to 280 mm/s. Cutting and scribing edges processed this way possess superior surface quality.

Laser Structuring (Contact Structuring, Surface Structuring)

The efficiency of solar cells is considerably influenced by processes applied for structuring nitride layers deposited onto the wafer, like laser patterning, superficial laser structuring, and laser contact structuring. The technology of laser fired contacts (LFC) aims at the enhancement of the solar-cell efficiency by the realisation of a highly efficient, dielectrically passivated back contact. Using the latter approach, shadowing losses of a standard cell can be reduced from between 10 and 15 % to between 2 and 3 %.

Processing consists of the following steps:

(1) Deposition of the dielectric passivation layer,

(2) Deposition of an aluminium layer,

(3) Localised generation if contact points through the dielectric layer by laser.

The laser intensity is optimised such that a good contact is achieved and in parallel the aluminium layer is not evaporated.

Laser-Marking

For product tracking and tracing, solar cells are endued with marks generated by solid-state lasers. Besides alphanumeric symbols, data matrix codes and bar codes as well as manufacturer’s logos can be generated. The size of the markings ranges from semi visible (with a font size of about 75 µm) to an extension of several centi¬metres. The minimum font width amounts to 10 µm.