report
5.2.3 State of the art

Different research axis are under development[i],[ii],[iii]

Thin-Layer Solar Cells

To optimize thin layer solar cells, nanotexturized transparent conductive oxide layers are analyzed as front electrodes. It shall help optimize light dispersion in the substrate and minimize reflection losses.

Further approaches lie in the improvement of the rear reflectors for which usually metal layers (e.g. silver) are used, for example through the application of photonic crystals or non-metal nanolayer systems to further increase the light yield in the substrate.

Zinc oxide nanowires in Dye Sensitized Solar Cells

Some new approaches are now developed to improve the efficiency of the dye sensitized cell. One is the use of ZnO nanowires instead of TiO₂ nanoparticles. Single crystalline nanowires arrays have indeed excellent electron transport property and provide therefore a faster carrier extraction. They provide also a larger surface area for dye loading. So there is an overall increase in carrier collection efficiency.[iv]

Nanostructured Antireflection Layers

A relatively low-cost method of increasing energy yields of solar cells and solar collectors is the application of antireflection layers. These layers can indeed enhance solar cell performance by increasing light coupling. Marketable developments are antireflection layers for flat glass based on a nanoporous coating of silicon dioxide. The layers are made on the basis of a sol-gel process with ensuing dip coating. The porosity allows the adjustment of the effective refraction index between glass and ambient air, which helps reduce reflection losses of glass panes of usually 8% to 2%. Thus it is possible to increase the annual heat yield of solar collectors by up to ten percent.

Some others research direction lies in the development of antireflection layers based on ZnO nanostructures for Si solar cells. The proposed solution achieves a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC.[v]

Fullerene Derivates as Electron Acceptors in Polymer Solar Cells

Polymer solar cells use organic semiconductors for energy conversion. Conjugated polymers like P3HT are used as light-absorbing electron-donors, while fullerene derivates like PCBM are used as electron acceptors. Both substances are integrated into the sandwich-like cell structure between charge-transport layers and electrodes (ITO, metals) as 100 to 300 nm thin composite layers. This contact improves efficiency by allowing charge transfer to take place at the sub 10-nanometer scale, on the order of the diffusion length of an exciton generated from organic semiconductors. Advantages of such cells are low-cost materials and manufacturing processes as well as their mechanical flexibility. Mass production of large scale modules in a continuous roll-to-roll- printing process is aspired. The optimization of materials and cell structure should bring about medium-term efficiencies of approx. 10% and a lifespan of several years. The most recent cells exhibit conversion efficiencies of ~ 5%. New research approaches concern the replacement of ITO-layers for transparent polymer composites. Apart from cost savings, this also provides the possibility to increase light trapping through imprinting nanostructures. The extensive periodical surface structures produced through holographic exposure processes can be transferred into the polymer layer of the solar cells in a low-cost imprinting process.

Nanolayers in Stack Cells

With approximately 40%, stack cells of III/V-semiconductor systems show the highest conversion efficiencies of all solar cell types. Such solar cells are currently mainly applied in aerospace industry, since due to costly manufacturing processes the cells are too expensive for terrestrial applications. Application will become more efficient, when sunlight is concentrated through relatively inexpensive optics, and the efficiency of the cells is thus increased. Such concentrator modules are already commercially available, and there are prospects for a reduction of costs through further system optimizations and economies of scales in production, so that this kind of power generation will quite be competitive against grid prices in the long run. Stack cells are also feasible on the basis of other material systems, such as silicon or polymers.

Nanocrystalline

These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption). Using nanocrystals allows one to design architectures on the length scale of nanometers, the typical exciton diffusion length. In particular, single-nanocrystal devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.

Transparent Electrodes

To improve the electrode efficiency, several developments are in progress. The first one consists in the use of carbon nanotubes. Compared to existing transparent conductor technologies such as Indium Tin Oxide (ITO), CNT films have a number of potential advantages. These include the use of an abundant material (carbon), high film flexibility, and solution processibility.[vi]

It is also possible to use ultra thin graphene films which possibly exhibit a high conductivity and a transparency of more than 70% over 1000-3000 nm.[vii]

The second approach is the use of nanostructured ZnO.[viii]

Electrolyte in Dye Sensitized Solar Cells

The technology of dye sensitised solar cells made an enormous progress the last decade. Though the stability increased significantly, it still does not meet the standards of other solar modules (longer than 20 years for temperatures down to -40°C and up to 85°C). Intrinsically the cell should be able to pass this criterion. However, with a liquid electrolyte, sealing and phase changes make it technologically very hard. A solid state version would be more robust.

[i] Application of nanotechnologies in the Energy Sector, Aktionslinie Hessen-Nanotech of the Hessian Ministry of Economy, Transport, Urban and Regional Development - http://www.hessen-nanotech.de/mm/NanoEnergy_web.pdf

[ii] Nanoroad SME: Roadmap Report concerning the use of nanomaterials in the energy sector; Steinbeis-Europa-Zentrum, Karlsruhe (2006) - http://www.nanoroad.net/

[iii] European Roadmap for PV R&D; H. de Moor, A. Jäger-Waldau et al., PVNET

[iv] Nanowire dye-sensitized solar cells; M. Law, L.E. Greene, J.C. Johnson, R. Saykally & P Yang: Nature Materials, 4, 455 (2005)

[v] ZnO Nanostructures as Efficient Antireflection Layers in Solar Cells; Y.-J. Lee, D.S. Ruby, D.W. Peters, B.B. McKenzie, and J.W.P. Hsu; Nano Letters 8, 1501 (2008)

[vi] Carbon nanotubes as integrative materials for organic photovoltaic devices; V. Sgobba & D.M. Guldi: Journal of Materials Chemistry, 18, 153 (2008)

[vii] Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells; X Wang, L. Zhi, & K. Müllen: Nano Letters 8, 323 (2008)

[viii] Nanostructured ZnO as a solution-processable transparent electrode material for low-cost photovoltaics; A. Salleo, L. Goris: The Global Climate and Energy Project (GCEP) - Stanford University (2007) - http://gcep.stanford.edu/index.html