© Fraunhofer FEP

© Fraunhofer FEP

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Thin-film solar cells

Solar modules produced using thin-film technology are based on semiconductors that are extensively and cost-effectively applied to substrates such as glass, metal or plastic film. Their main advantages are their low material consumption and the comparatively simple production technology for covering large areas. In addition to amorphous and microcrystalline silicon, different compound semiconductors are also utilised that have specific advantages and disadvantages. Their efficiencies are still considerably less than those for crystalline Si-cells.

Materials and components

With amorphous silicon, the silicon atoms are not arranged in a regular pattern but form instead a continuous random network that contains approximately 10% hydrogen atoms. The material’s band gap is larger than with crystalline silicon, which means that the solar cells produce high cell voltages. The efficiency of the solar cells initially reduces slightly when exposed to light but then remains stable. Microcrystalline silicon is a mixed-phase material consisting of very small silicon crystals and amorphous silicon. The material also contains hydrogen, which as with amorphous silicon inherently pacifies electronic defects. Microcrystalline silicon only deteriorates insignificantly when exposed to light and, in contrast to amorphous silicon, it also absorbs sunlight from the nearinfrared range.

Technologies for producing the components

Using plasma enhanced chemical vapour deposition (PECVD), both amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon are produced by decomposing silane (SiH4) in combination with hydrogen. Since the process is conducted at low temperatures of approximately 200 °C, this enables cost-effective substrate materials to be used such as glass or metal and plastic films. However, the relatively low deposition speed, which ranges between 0.5 and 2 nm/s, limits the throughput of the current industrial production.

In addition to the silicon layer, the transparent and conductive front contact – which is mostly a doped metal oxide such as tin or zinc oxide – is essential for achieving maximum efficiencies.

At the Research Centre Jülich (FZJ), a front contact made of nanotextured zinc oxide layers has been developed for a-Si/μc-Si tandem cells. Together with the reflecting rear contact, they almost completely scatter the light in the cell, which enables high efficiencies. The LIMA project (light management for industrially produced silicon thin-film modules) is researching how light management can enable incident light in the solar cell to be directed along longer paths and thus produce more solar power.

Thin polysilicon cells on glass

In order to achieve higher efficiencies, micrometre-thin polycrystalline silicon layers on glass are also being investigated as an alternative to the a-Si/μc-Si tandem concept. Because of its much larger crystallite size (μm instead of nm), this polycrystalline material shows considerable similarities to crystalline silicon. For example, it has a higher electronic quality and lower absorption. The Helmholtz Zentrum Berlin is working on a cell concept for polysilicon cells on glass where the silicon layer is deposited using electron beam evaporation. Efficiencies of up to 7% have been attained so far.

The industrialisation of thin-film silicon technology

A large production capacity has been developed in recent years. Modules with efficiencies between 9 and 10% are available on the market. The largest producer is currently the Japanese company Sharp, which has built up its capacity in Japan and Italy to 1 GWp.

Researchers are currently working on raising the module efficiency to 12%, increasing the deposition rate and improving the front contacts.

CIS / CIGSe cells

The highest efficiencies in the thin-film PV area are currently being provided by solar cells with a highly absorbent compound semiconductor made of Cu(In,Ga,Al)(S,Se)2. Because of their crystal structure, these belong to the chalcopyrite series of compounds. Several universities and research institutes are working on manufacturing and characterising chalcopyrite-based solar cells. In 2010, the ZSW in Stuttgart claimed the world efficiency record for CIGSe laboratory solar cells for Germany with an efficiency of 20.1%, thus surpassing the longstanding record previously held by the National Renewable Energy Labs in the USA.eltrekord zur Effizienz von CIGSe-Labor-Solarzellen nach Deutschland.

Materials and components

A p-type Cu(In,Ga,Al)(S,Se)2 absorber layer with a thickness between 1.8 and 2 μm is deposited on a substrate, which is generally glass. A CdS buffer layer above it ensures the high quality of the interface between the p- and the n-components of the semiconductor heterojunction in CIGSe thin-film solar cells. Sputtered on it is a transparent n-type ZnO front contact, which is approximately 300 nm thick. This is used for collecting the current. The finished cells are structured and connected in series to form modules. In order to ensure a 25-year service life on roofs or in fields, the solar modules must be protected against environmental influences. They are therefore encapsulated with a polymer film and a cover plate.

The industrialisation of CIGSe technology

Many new companies have successfully developed a pilot production line and some have already begun ramping up to an industrial scale. In Germany these are Würth Solar, Solibro, Avancis, Sulfurcell (now Soltecture) and Global Solar Energy Deutschland. Considerable efforts are also being made worldwide. For example, the Japanese company Solar Frontier announced in 2010 that it intends developing the currently largest production of around one gigawatt.

In recent years, several companies in Germany and the USA have begun specialising in flexible CIGSe thin-film technologies. These include Global Solar Energy, Solarion, Odersun, CIS Solartechnik, Solopower, Miasole and Nanosolar. All these companies rely on roll-to-roll manufacturing processes.

Indium-free cells

Indium is among several raw materials that could become scarce and expensive in future as a result of their diverse range of possible technical applications. More recent research has shown that the semiconductor compounds Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) can also be deployed. These so-called kesterites provide an absorber material that consists of abundantly available non-toxic components. Initial attempts to construct photovoltaic componentswith CZTS absorbers have resulted in efficiencies of between 7 and 10%.

Flexible CIGSe cells

The manufacture of CIGS cells on flexible films has recently made considerable progress. Various companies use steel film or temperature resistant plastics (polyimides) for mass production purposes. The current efficiency record for CIGSe thin-film solar cells on polyimide film is 17.6% (2010, EMPA, CH).

CIGS cells have to date reached efficiencies of around 20%, but in principle efficiencies of more than 30% are possible. By means of computer simulations of the socalled indium-gallium puzzle, scientists at the Johannes Gutenberg University Mainz have made an important breakthrough in the search for more efficient thin-film solar cells. The findings from the comCIGS project have highlighted a new way of increasing the efficiency.


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