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New cell developments

With the third solar cell generation, organic dyes convert light into electricity, whereby the cells are produced with adapted established processes at low temperatures. These flexible solar cells can be easily deployed in a diverse range of areas and produce cheap electricity while mobile, on facades or even integrated in clothes. However, there is still scope for improving the efficiency and lifespan but interesting concepts such as tandem cells are being developed.

Organic solar cells are based on dye-like molecules such as phthalocyanine or polycyclic dyes that show very strong absorption in the wavelength range of visible sunlight. They can be manufactured very cheaply and enable large-scale use with lightweight and, if necessary, flexible modules. The most important research fields are concerned with the efficiency of the energy conversion and with extending the lifespan.

The lifespan depends on the material system (polymer, low-molecular or hybrid). Fraunhofer ISE has measured a lifespan of 1.5 years for polymer solar cells (with continuous illumination with 1,000 W/m² and at 50 °C).

Concepts, strategies, potential

In the area of photovoltaics based on organic materials (OPV) and hybrid systems, there is a diverse range of concepts that extent beyond traditional Si and thin-film technology. The developers are benefiting from the synergies with related organic electronic fields such as organic light-emitting diodes (OLED), organic field effect transistors (OFET) and products such as monitors, lighting and RFID (Radio Frequency Identification).

The aim is to evaluate and adapt base materials and production processes on a broad basis for use in OPV. Suitable substrates are already available on the market and can be used without being altered. These are mostly made of glass or flexible PET films coated with indium tin oxide as a transparent front contact (Transparent conductive oxide – TCO). Researchers are looking for cheaper TCO alternatives, e.g. based on ZnO.

It is envisaged that the third generation of absorbers will be made from ultra-thin layers and still be highly absorbent, whereby a wide range of organic dyes with semiconducting properties are available. The properties of molecules and polymers can be adapted to the needs of OPV with relatively little development effort. The same applies to the adaptation of established coating processes such as dipping, printing, spraying, vacuum deposition and roll-to-roll.

Corresponding OPV products are expected to be launched on the market in the near future. Prototypes are already available, for example in the form of handbags with organic 1 W modules.

New properties and potential applications

The new technologies differ from the traditional ones through the materials used (dyes) and the production methods. Consistently low-temperature processes are used, both in the vacuum process and with wet-chemical processes. The physical function principles show remarkable differences and are being intensively researched.

The absorbers in organic solar cells (OSCs) mostly consist of dye molecules with donor or acceptor properties that do not require an additional matrix (such as silicon). As a result, the active layers are ultra thin, being just 100 nm thick. Modules made of OSCs make efficient use of materials, are easy to produce, are lightweight and in mechanical terms very flexible.

OSCs have remarkable absorption properties in the spectral range of sunlight. They have relatively narrow absorption bands with extremely high absorption coefficients that are spectrally adjustable through the selection of molecules. The adjustable absorption range offers product designers new creative possibilities thanks to the varying transparency and colour of the modules.

Despite the still low efficiencies and short lifespans of just a few years, the properties of third generation solar cells open up an enormous range of new applications. Industry partners are looking into integrating the modules in clothing, handbags, mobile phones, car roofs, window panes, Venetian blinds and building facades (Fig. 24).

Basic cell architectures

The basic cell concepts presented here are all donoracceptor systems with either one or two organic absorber materials. Despite the shared principles, the cell concepts differ considerably – partly in terms of the cell architecture and partly in terms of the preparation methods.

Sequentially coated organic solar cells (OSCs)

In 1985, American researchers succeeded for the first time in producing a solar cell from a metal phthalocyanine as the donor (D) and a perylene derivative as the acceptor (A). The cell, which had a transparent contact layer made of indium tin oxide (ITO), had an efficiency of 1%. This was a double-layer solar cell, i.e. the two absorber layers (D, A) were arranged sequentially. Because doublelayer solar cells are solely produced using a vacuum process, allmaterials used must have the necessary thermal stability. The alternative means of production for such sequential cell architectures using wet-chemical deposition is less suitable since the solubility of the materials during the coating with the second layer has an adverse impact.

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