Everything you wanted to know about solar cells – Part 2

Dr. Robert N. Castellano, The Information Network, USA.

NEW TRIPOLI, USA: Manufacturing a solar cell requires a substantial investment in equipment and materials. Since there are a variety of types of solar cells, each requires its own set of materials and manufacturing equipment. For example, there is nearly no synergy in equipment to make a crystalline silicon solar cell versus a thin film amorphous silicon solar cell.

The top 13 vendors of solar equipment registered sales of $6.3 billion in 2010, representing 61 percent of the $10.3 billion market, as shown in Fig. 1.Source: The Information Network, USA.

Monocrystalline/polycrystalline solar cell manufacturing steps
Fig. 2 is an illustration of the steps to manufacture a crystalline/polycrystalline solar cell.Source: The Information Network, USA.

Etching and texturing
Wafers are cleaned with industrial soaps and then etched using hot sodium hydroxide to remove saw damage. The texturization helps reduce the reflection of sunlight. Left untreated, the surface of the PV cell can act like a mirror, reflecting more than 30%of the light that strikes it. The same process doesn’t work as well in the case of polycrystalline wafer.

Light with an 80 percent probability of being absorbed by a flat surface will have a 96 percent chance of being absorbed by a textured surface. Texturing has become routine on high-quality solar cells. Chemical etching creates texturing on the cell's surface. It makes a pattern of cones and pyramids, which capture light rays that might other-wise be deflected away from the PV cell, and redirects them down into the cell.Source: The Information Network, USA.

Diffusion and edge isolation
Since the wafers are pre-doped with boron (p-type), an n-type material is diffused into the wafer, to achieve n-p junction, Phosphorous is the usual diffusant and is achieved by subjecting the wafers at high temperature to a phosphorous source like phosphoric acid, phosphorus oxychloride, etc…

Phosphorous diffuses not only into the desired wafer surface but also into the side and the opposite surface, to some extent. This gives a shunting path between the cell front and rear.

The POCl3 diffusion furnace can be equipped with four automatic cantilevers to load and unload each process tube separately. One tube can handle four quartz boats per run, each loaded with 50 wafers back to back. After the drive-in and the heat-up step, the process starts with temperature stabilization. After that, nitrogen is used as carrier gas for the POCl3 liquid. An additional oxidation steps follows to finish the process. After the adjusted diffusion time the tube opens to get unloaded and reloaded again.

In this process the one-sided edge isolation is combined with the removal of the phosphor-silicate glass layer. The emitter layer created on the under side of the wafer during the diffusion process is isolated on one side in order to prevent malfunctions of the solar cell. Removal of the path around the wafer edge, “edge junction isolation”, can be performed by “coin stacking” the cells and exposing them to a plasma etching chamber to remove exposed edges. An alternative method is an optical system that steers a laser beam precisely close to the wafer's edge, thus removing the exposed edge as well as providing a large active cell area.

Anti-reflection coating
To further reduce the surface reflection, an anti-reflection coating (ARC) is done on the surface like silicon nitride, titanium oxide, etc…

Antireflection coatings, to increase the amount of light coupled into the solar cell, are typically next applied. Silicon nitride has gradually replaced titanium dioxide as the antireflection coating because of its excellent surface passivation qualities. It prevents carrier recombination at the surface of the solar cell.

One of the best ARC is silicon nitride deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The process not only deposits a layer of ARC but also improves the electronic properties of the silicon by injecting hydrogen and provides the passivation, useful to improve silicon quality. The highest-efficiency cells typically use a well designed, double-layer antireflection coating and a textured surface. This combination can lower reflection to less than 2 percent of the incoming light.Source: The Information Network, USA.

Metallization
Silver is the most widely used metal for contact formation due to its solderability. Silver in the form of a paste is screen printed onto the front and the rear. In Addition, aluminum paste is also used onto the rear to achieve Back Surface Field (BSF), which improves the performance of the solar cell. The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact with the silicon.

In the firing furnace the applied metal conducting tracks are first dried and then fired, to produce an electric contact on the front and rear sides of the solar cell. A new heating lamp technology, together with belt guidance and cooling technology, ensures an optimum distribution of the temperature which provides excellent contact characteristics and thus an optimal filling factor.

Instead of screen-printing technology, with laser transfer printing the metallisation is carried out contact-free. The metalliferous pastes are printed in various patterns in-line and entirely in digital format.

Using light-induced plating technology very fine-structured and low-shadowing contact grids can be produced on the solar cell. In the first step the front contact fingers, so-called seed layers, are applied, for example using inkjet printing technology. In the second step, the contact lines are silver-plated to improve their electrical conductivity. The illuminated solar cell supplies the required polarity of the cell front side.

As part of the metallization process, materials in the silver paste etch the SiN ARC coating so that the metal penetrates through the nitride to form a low-resistance ohmic contact. In addition, hydrogen diffuses into the bulk of the cell to passivate impurities and defects. The multipurpose role of the nitride demands that it be a low-absorption AR coating, serve as a barrier layer for control in metallization, and promote favorable electronic processes that can passivate the surface, as well as the bulk, of the device.