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New cell developments II

High efficiency OSCs with mixed donor and acceptor absorbers

Both vacuum and wet-chemical processes are suitable for producing mixed absorber layers. The aim is to blend the two materials as much as possible to create a large interface between the donor and acceptor material. This is achieved by co-evaporation in a vacuum and through wet-chemical deposition from a mixed solution. The resulting mixed D/A absorbers convert the light into charge carriers very efficiently.

At the research stage, the wet-chemical deposition of polymer mixtures is much cheaper than vacuum techniques. The large material diversity of polymer chemistry can be researched in standard chemistry laboratories. The large volumes of solvents are problematic, however, for industrial production. Nevertheless, the lower development costs (per laboratory) mean that most research is focussing on polymer solar cells, so that a diverse range of competing polymer solar cells can be expected on the market in the near future.

Relatively little research is being conducted on OSCs consisting of mixtures of small molecules. This is because complex plants are required for the thermal vacuum deposition and the materialsmust be thermally stable. These disadvantages are irrelevant, however, for later mass production. Vacuum processes enable organic mixed phases to be produced using co-evaporation and sequential layers to be produced very variably and precisely, enabling them to be tailored for special applications. Despite the different intensities with which the respective research is being conducted, both the polymer and small molecule research areas provide good results.

Donor-acceptor nanocomposites (DAN)

An ideal cell architecture is achieved by combining the above-mentioned complementary absorber architectures. Using sequentially deposited but highly interpenetrating donor-acceptor layers enables the advantages of both to be combined, namely the high purity and crystallinity of the individual layers and a maximised DA interface morphology. The limited transport properties of organic semiconductorsmean that the interpenetration typically has a domain size of 100 nm. For this reason, nanotechnology will play a key role in the development and industrial production of functional materials based on DAN. The manufacturing costs can be further reduced by utilising processes that exploit the self-organisation of the materials.

Hybrid systems use liquid or solid electrolytes. With dye-sensitised solar cells (Brian O’Regan and Michael Grätzel 1991), the absorber consists of a donor layer that is just a few nanometres thick and is based on ruthenium complexes. This covers the inner surface of a nanoporous and transparent film of titanium dioxide (TiO2) particles that acts as the acceptor. The resulting D/A interface extends across the entire volume of the several μm-thick TiO2 film. The charge carrier generation is comparable to that in organically mixed solar cells. In both cases, the complicated nanomorphology exacerbates the transport of the charge carrier ‘holes’ to the rear contact. The best results are provided by a liquid electrolyte as a hole transporting material. However, this affects the stability and toxicity. Alternative materials made from gelatinous or solid electrolytes are therefore becoming more widespread.

With extremely thin absorber (ETA) cells, the absorber is coated on a fairly compact, crystalline TiO2 layer that has a coral-like, highly structured surface morphology. A multiply folded but closed D/A interface layer is formed, enabling a standard metal rear contact to be used. Although ETA cells originally just used inorganic dyes, increasingly more organic absorbers are now being deployed.

Concepts for the market introduction

The above-mentioned cell concepts have demonstrated the potential of the new technologies. Research work and preliminary feasibility studies have been conducted in national networks since the year 2000. New approaches increased the initially low efficiencies to more than 3%. The rapid development finally aroused the interest of industry and the research was consequently intensified through the “OPV Initiative 2007–2012”. The cell efficiency in the research laboratories has meanwhile more than doubled (as of 2010). Although the reported records are not always comparable with one another, they show impressive values with 8.1% (polymers) and 8.3% (small molecules).

The new record cells integrate various individual innovations to form more complex and therefore realistic cell architectures with innovative layer materials:

  • Targeted nanostructuring and doping of absorber layers improve the transport properties, whereby absorbers with precise doping gradients (e.g. p-i-n structure) produce very good results.

  • The integration of optical spacers in the layer architecture helps to optimally couple the sunlight. This is because standing waves caused by interference form in ultra-thin layer systems when they are illuminated. The ability to adjust the distance to the reflecting metal layer on the rear contact using transparent, conductive buffer layers of a suitable thickness therefore enables the absorber layer to be positioned in the maximum of a wave caused by interference.

  • The absorption spectrum can be expanded by combining two different OSCs to form a tandem cell. Here, two solar cells are layered one above the other to obtain, in the ideal case, a component with a doubled open terminal voltage for the same photocurrent.


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