Solar panel types - Thin film - Monocrystalline - Polycrystalline solar modules

The energy needs of our planet are increasing every year by a growing world population and a greater economic activity. To reduce CO2 emissions, we have to use more alternate sources of energy, such as solar, wind or hydropower. The total incident amount of photovoltaic energy exceeds the world’s needs energy by more than a factor of a thousand.

Solar panel types – Thin film – Monocrystalline – Polycrystalline solar modules

By the government support in countries such as Germany and Japan is the production of photovoltaic solar systems have increased by more than 30% per year during the last ten years year. This resulted in production capacity of 5.5 GW PV panels in 2018. More than 90% of the solar modules produced use mono- or multi-crystalline silicon with a thickness between 200 and 300 μm. Multi-crystalline silicon possesses the excellent semiconductor properties for use in PV cells and is also non-toxic, widely available and stable. Commercial photovoltaic systems based on crystalline silicon have efficiencies of 14 – 18%.

The strong demand for photovoltaic energy systems resulted in a declining availability of purified silicon and so in a substantial price increase of the silicon slices. The shortage of silicon and strong demand offer great potential for thin film silicon cells. Photovoltaic cells based on thin film polycrystalline silicon are cheap alternative to the current “wafer-based” silicon PV cells. The low absorbance of light in the thin polysilicon layer is to obtain a good cell efficiency, a big challenge.

Solar modules are divided into monocrystalline, polycrystalline and thin film modules. They differ in terms of materials, production and efficiency.

Thin film modules

For thin film modules, a carrier material is coated with the semiconductor material. The semiconductor material used so far is primarily amorphous silicon. Other semiconductor materials are also used, such as cadmium telluride, CdTe, gallium arsenide, GaAs, or copper indium selenide, CuInSe2. There are also thin film modules with microcrystalline silicon. Organic solar panels cells in which the absorber layer consists of a polymer also belong to the thin film cells. The production of thin film modules is relatively simple. They are less expensive than monocrystalline or polycrystalline modules, because the wafer production is eliminated. The carrier is evaporated with a very thin layer of semiconductor material only a few microns thick. As a substrate next to glass or metal and flexible materials such as plastic in question, which widens the scope significantly.

The efficiency of thin film modules is significantly lower than that of monocrystalline or polycrystalline modules. Average efficiencies of modules available on the market are between 5 and 10%. However, significantly higher efficiencies of up to 18% could be achieved in the laboratory. Degradation of thin-film modules is around 20 to 25% in the first 1,000 operating hours, but this is already taken into account in the manufacturer’s data on efficiency.

Inexpensive production
Low acquisition costs
Less raw material consumption
Low weight
High yield even for diffused light
No efficiency losses due to heat

Thin film modules have a number of advantages despite the lower efficiency. The low efficiency is compensated by a correspondingly larger area. Especially the high photosensitivity even with a high proportion of diffuse radiation ensures high yield even in the winter months. At the same time, thanks to the high-temperature resistance, the current yields of thin-film modules in summer remain high even at temperatures above 25 ° C. More about the difference between mono and poly.

Solar panel types – Thin film – Monocrystalline – Polycrystalline solar modules

Monocrystalline photovoltaic cells

Crystalline solar panel cells are made from the basic material silicon, in contrast to thin-film or dye cells.

For the production of monocrystalline cells, silicon is melted and cleaned. From this melt, a rod is then drawn, which forms a uniform crystal lattice, a so-called single crystal or monocrystal. These ingots are then sawn into slices: the wafers. Wafers for monocrystalline PV cells are only a few micrometers thick. The next production step involves cleaning the wafer surface by chemical treatments. Since the silicon was already doped with boron during wafer production, doping with phosphorus still has to take place for the other half of the wafer.

After wafer production, a solar cell is built with the p- and n-doped wafers, which has a high efficiency because of the low impurities. Due to the manufacturing process, the maximum size of a monocrystalline cell is fixed. The ingots are manufactured in diameters of 150, 200 and 300 mm. Monocrystalline panels cells are easy to recognize for two reasons: Due to the uniform crystal structure, they appear uniformly dark, moreover, they are not quite square, because they are sawn from round ingots and thus have rounded corners.

Solar modules equipped with monocrystalline cells achieve significantly higher efficiencies than other cells. Efficiencies more than 20% are already achieved under laboratory conditions. Monocrystalline solar cells are therefore always particularly useful if the highest possible yield is to be achieved on a small area. Even the uniform look is appreciated by many as an advantage.

PV modules with monocrystalline cells are expensive to produce and this is reflected in the price: They are significantly more expensive than other photovoltaic modules. In addition, the energy-intensive production and the energy amortization of the modules with monocrystalline PV cells is worse than polycrystalline cells. Compared to thin-film cells, the efficiency in diffuse light is significantly lower.

Solar panel types – Thin film – Monocrystalline – Polycrystalline solar modules

Polycrystalline PV modules

Silicon is the basic material for polycrystalline modules. They are the most widely used photovoltaic modules.

The first step in the production of polycrystalline solar panel cells is the purification of the silicon. Silicon is an almost infinite raw material, but for the production of polycrystalline photovoltaic modules you need pure silicon, which is now also produced in sufficient capacity. This silicon will then be melted and processed. There are then various production methods for polycrystalline photovoltaic cells.

The casting methods all have in common that the molten silicon is poured into crucibles. In order to produce the ingots, the silicon has to cool down and solidify in a certain way, namely from bottom to top. The silicon crystals grow upwards.

In the Bridgeman process, the silicon is heated by induction heating, just as in the block casting process. However, the targeted cooling takes place in the same crucible. This allows larger edge lengths of the blocks for the future polycrystalline modules. The silicon blocks produced in this way are first sawn into columns. Then horizontal slices (wafers) are sawn from them. Even with polycrystalline solar cells, the doping of silicon with boron takes place even before the casting. The wafers are cleaned after sawing, and further processed into polycrystalline modules.

Solar panel types – Thin film – Monocrystalline – Polycrystalline solar modules

In the case of polycrystalline modules, several crystals are formed during the casting or Bridgeman process, hence the name polycrystalline. The production methods described are one of the great advantages of polycrystalline photovoltaic modules, since they are significantly cheaper than, for example, the production of monocrystalline cells. Therefore, the price-performance ratio of polycrystalline modules is very good. It is not without reason that the market share of polycrystalline modules is well over 80% of the modules.

Polycrystalline photovoltaic modules have a lower efficiency than monocrystalline modules. They are also heavier than thin film modules. Polycrystalline PV modules are easily recognizable by their bright, bluish glittering surface. In contrast to some thin-film modules, there is no possibility to adapt them to the own design in the color.