Novel industrial approaches in solar-cell production

The photovoltaic industry has experienced enormous growth in recent years. However, for solar-cell technology to become competitive in the long term, both an increase in energy-generating efficiency and a reduction in production costs is required. Several laser applications for solar-cell production are shown in Figure 1, of which three—hole drilling for back-contacted solar cells, silicon dioxide (SiO₂) removal for making grooves, and scribing of thin-film cells—are described here in more detail.

Laser drilling is a key technology used in many new back-contact solar-cell production concepts because alternative economically feasible drilling processes are not yet available.^1–4 Square 6in-wide (156×156mm²) and 250μm−thin wafers are standard in industry. Common industrial photovoltaic cells have a screen-printed contact layer on the front, which blocks 5–7% of the incoming light by shadowing. To overcome this performance degradation, many new cell concepts are being developed, usually with the emitter contact either completely or partially on the rear. This results in higher cell efficiencies for energy generation. For these emitter wrap-through (EWT) cells, laser drilling is the only suitable method to create the necessary holes from the front to the back. Typically, a drilling efficacy of 15,000 80μm-diameter holes is required.¹ Figure 2 shows a typical hole. Up to several thousand holes can be drilled per second.

Figure 1. Laser applications for solar-cell production: hole drilling of back-contact solar cells, SiO₂ removal for groove production, laser welding/soldering of contacts, edge isolation, wafer cutting, removal of dielectric layers for improved contact performance, texturing to increase the absorption of sunlight and therefore to enhance the efficiency of the cell, scribing of thin-film cells, and edge deletion.

Figure 2. A typical hole drilled with a pulsed-fiber laser, characterized by a burst of 20 pulses and a pulse energy of 1mJ (after etching). A human hair illustrates the small diameter of the holes.

For wafer-based solar cells, grooves are used in several novel designs.^1,5 Grooves typically have a depth of up to several tens of microns. The groove is obtained in two steps. First, a barrier layer, such as SiO₂ or silicon nitride (SiN_x), is removed using a laser beam. Subsequently, a chemical etching process is applied to remove the laser-damaged silicon and to obtain the desired depth. The residual SiO₂ or SiN_x layer functions as a barrier during the etching process. Figure 3 shows a typical groove result.

Figure 3. Groove processed with an excimer laser at a wavelength of 248nm. The rectangular laser spot is shown in the processed area. The groove depth is about 20μm. The laser-processed surface is smooth after chemical etching, and does not contain any debris.

Figure 4. Cross-section of a thin-film solar cell (not to scale). The layers are typically up to several microns thick. The different layers, P1–P3 (transparent conductive oxides or absorber material), are successively deposited and scribed during the production process. The current flow is shown by the arrows.

Figure 5. Deposition and scribing for a thin-film solar cell. (1) Substrate, (2) TCO deposition, (3) P1 TCO scribing, (4) absorber deposition, (5) P2 absorber scribing, (6) TCO deposition, and (7): P3 TCO scribing.

Scribing is required to obtain a monolithic series connection of modules.⁶ This essential technology has both allowed economically feasible solar-cell production and enabled the strong growth market for thin-film photovoltaic applications. Thin-film solar cells can be made using different material combinations. The substrate can either be glass, plastic foil, or metal foil. Generally, the cell itself consists of a back contact, a front-contact layer, and an absorber material in between. For the contacts, transparent conductive oxides (TCOs) or metals are used. Commonly used absorber materials include silicon, cadmium telluride, copper-indium di-selenide (CIS), and other combinations with copper indium, such as CIGS or CIGSSe. The layers are typically up to several microns thick. In Figure 4, a thin-film solar-cell cross-section is shown. Common referencing to the laser processes includes ‘patterning 1’ or P1 for the first contact, P2 for the absorber, and P3 for the second contact.

Thin-film solar-cell scribing is, in essence, based on the different transmissivities of the film materials at the laser wavelengths used. The deposition and scribing processes are shown schematically in Figure 5. The first TCO layer deposited can be zinc oxide, tin dioxide, or indium-tin oxide. Usually, when glass is used as the substrate, scribing is done from the glass side. An example is shown in Figure 6. Silicon as the absorber material is usually scribed with a laser wavelength of 532nm. For this setup, TCO is transparent, and silicon absorbs radiation in a thin layer. Scribing of the third layer can be done using the same laser wavelength as for the absorber. In this case, the second absorption layer is also removed. This is not necessary, but it does not affect the solar-cell function either.

Figure 6. Example of TCO scribing of P1 with a diode-pumped solid-state laser at a wavelength of 1047nm.

In summary, the role of laser technology in the solar photovoltaic industry is gaining importance. Laser applications enable economic and technical feasibility of new design concepts. To achieve the required performance quality in acceptably short processing times, further technological system development is necessary. Both system and process development of newly emerging laser sources and their applications are the subject of future research at our institute.