The two main categories of technology are defined by the choice of the semiconductor: crystalline silicon in a wafer form or thin-films of other materials.

Silicon wafer

Crystalline silicon (c-Si) has been used as the light-absorbing semiconductor in most solar cells, even though it is relatively poor absorber of light, requiring considerable thickness (several hundred microns) of material. Nevertheless, it has proved convenient because it yields stable solar cells with good efficiencies (11-16%, half to two-thirds of the theoretical maximum) and uses the same process technology developed from the huge knowledge base of the microelectronics industry.

Two types of crystalline silicon are used by the industry. The first is mono-crystalline, produced by slicing wafers (up to 150mm diameter and 350 microns thick) from a high-purity single crystal. The second is multi-crystalline silicon, made by sawing a cast block of silicon first into bars and then wafers. The main trend in crystalline silicon cell manufacture is toward multi-crystalline technology.

For both mono- and multi-crystalline Si, a semiconductor homo-junction is formed by diffusing phosphorus into the top surface of the boron doped (p-type) Si wafer. Screen-printed contacts are applied to the front and rear of the cell, with the front contact pattern specially designed to allow maximum light exposure of the Si material with minimum electrical (resistive) losses in the cell.

The most efficient production cells use mono-crystalline c-Si with laser grooved, buried grid contacts for maximum light absorption and current collection.

Some companies are working on technologies to overcome some of the inefficiencies of the crystal growth/casting and wafer sawing route. One route is to grow a ribbon of silicon, either as a plain two-dimensional strip or as an octagonal column, by pulling it from a silicon melt.

Another is to melt silicon powder on a cheap conducting substrate. These processes may bring with them other issues of lower growth/pulling rates and poorer uniformity and surface roughness.

Each c-Si cell generates about 0.5V, so 36 cells are usually soldered together in series to produce a module with an output to charge a 12V battery. The cells are hermetically sealed under toughened, high transmission glass to produce highly reliable, weather resistant modules.

| 01.01.2009 | Read more | Print |

Thin-film

c-si

The high cost of crystalline silicon wafers (about 40-50% of the finished module cost) has led the industry to look for alternatives. The thin film technologies are complex but a promising research to produce a better and yet affordable material.

The most common materials are amorphous silicon (a-Si, still silicon, but in a different form), or the polycrystalline materials: cadmium telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS). These materials are all strong light absorbers and only need to be about 1micron thick, so materials costs are significantly reduced. Amorphous silicon is the most well developed of the thin film technologies.

In order to build up a practically useful voltage from thin film cells, manufactures include a laser scribing sequence that enables the front and back of adjacent cells to be directly interconnected in series, with no need for further solder connection between cells.

Thin film cells are laminated to produce a weather resistant and environmentally robust module. Although less efficient than silicon-wafer, thin films are potentially cheaper than c-Si because of their lower materials costs and larger substrate size.

Amorphous silicon is the most well-developed thin film technology to-date and has an interesting avenue for further development through the use of "micro-crystalline" silicon which seeks to combine the stable high efficiencies of crystalline Si technology with the simpler and cheaper large area deposition technology of amorphous silicon.

However, conventional c-Si manufacturing technology has continued its steady improvement year by year and its production costs are still falling as well.

The emerging thin film technologies are starting to make significant in-roads in to grid connect markets, particularly in Germany, but crystalline technologies still dominate the market. Thin films have long held a niche position in low power, less than 50W, and consumer electronics applications, and may offer particular design options for building integrated applications.

Currently, thin film panels are available in a multitude of form and shapes depending on the application.

They have been used to build solar power skylights, solar power building façade, solar power windows and atrium.

Thin film solar modules are available in transparent multi-functional panels. They can be used in all normal windows in the same way as conventional insulating glass however, they also comprise a solar power unit delivering up to 60 kWh per m² per year. Most products in that category are available in a variety of versions from semi-transparent to opaque, including a graduated version.

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