Organic solar cell

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A commercially available flexible module of a polymeric organic solar cell

An organic solar cell is a solar cell made of materials used in organic chemistry ; H. from hydrocarbon - compounds ( plastics ). The efficiency with which solar energy is converted into electrical energy is in the case of special cells manufactured individually in laboratories with areas of around 1 cm², at 17.3% (as of August 2018), still below that of solar cells made of inorganic semiconductor material . Organic solar cells or plastic solar cells, as they are also called, are a current research topic due to the possibilities in terms of potentially cheaper and versatile manufacturing processes (see advantages and disadvantages ).

material

Structure of a single-layer solar cell

The material for this type of solar cell is based on organic hydrocarbon compounds with a special electronic structure, the conjugated π-electron system , which gives the materials in question the essential properties of amorphous semiconductors . Typical representatives of organic semiconductors are conjugated polymers and small molecules, whereby specially synthesized hybrid structures such as copper phthalocyanine are also used. The first organic solar cell was manufactured in 1985 by Ching W. Tang, consisting of copper phthalocyanine and a PTCDA derivative. The first plastic solar cells made from conjugated polymers (electron donors) and fullerenes (electron acceptors) were also two-layer solar cells. These cells consisted of a thin layer of the conjugated polymer on which another thin layer of fullerenes was applied. The photoactive substances in these solar cells are the conjugated hydrocarbons, which can change into excited states when exposed to light. These states can transfer their excitation energy to a fullerene in the form of an electron. Since the completely separated charges are metastable, these charges can be collected and removed via metallic electrodes. From a technological point of view provide conjugated polymers and functionalized molecules due to the producibility of layers of liquid phase attractive base materials for the low cost mass production of flexible PV elements with a comparatively simple structure. Molecular Semiconductors, however, are usually employed in vacuum vapor deposition processes (see FIG. Thermal evaporation or generally physical vapor deposition ) are processed into well-defined multilayer systems and allow the production of sequentially deposited semiconductor layers and thus more complex cell types (e.g. tandem cells ).

Working principle

Section through a multilayer solar cell

The efficient representatives of organic solar cells are based on the use of a so-called donor-acceptor system, i.e. H. on the skillful combination of different semiconductors, which after absorption of light show an extremely fast transfer (much less than 1 ps) of the charge carriers formed to the donor and acceptor (e.g. thin layers of conjugated polymers and fullerenes). Such DA pairs differ in their positions of the electrochemical potentials that are shifted relative to one another: HOMO ( highest occupied molecular orbital ) and LUMO ( lowest unoccupied molecular orbital ). These orbitals are somewhat comparable to the band scheme of inorganic semiconductors. After the absorption of photons whose energy exceeds the distance between the HOMO and LUMO, so-called excitons (electrostatically bound pairs of positive and negative charges) arise . a. be separated for some time by the local electric field at a donor-acceptor interface. After the separation, the charge transport in the two semiconductors takes place selectively. The charge carriers move through the semiconductor by “hopping”; this is enforced by their movement in the disordered (amorphous or microcrystalline) environment with a large number of energy barriers. The charges hit many molecular and phase boundaries and thus substantial and structural defects, which means the recombination and thus the loss of the two charges.

In an organic solar cell, the absorber layer (applied from the liquid phase and / or by a vacuum process) generally consists of a volume mixture of donor-like and acceptor-like organic semiconductors. This layer is applied to a transparent, conductive electrode ( float glass coated with a transparent conductor ). The transparent electrode allows as much light as possible to be coupled in in order to maximize the yield of absorbed photons in the actual active layer. At the same time, it should have a low electrical sheet resistance. The most important property, however, is their work function, which determines with which of the two semiconductors they prefer to exchange charge carriers (negative or positive, corresponding to electrons or electron defects). A metal electrode is vapor-deposited on the other side of the absorber layer. It collects the charge carriers of opposite signs from those flowing through the transparent electrode.

The back reflection of the unabsorbed light from the metal electrode increases the yield because the reflected light has a further chance of absorption when it passes through the absorber layer again. The thickness of the absorber layer in the resonator between the glass electrode and the metal electrode can also be optimized for maximum absorption of a certain wavelength; however, the effect is small compared to electrical considerations, see below.

The terminal voltage of such a solar cell is largely determined by the different work functions of the two electrodes. In order to achieve a high photocurrent, the organic semiconductors used in the absorber layer should have the highest possible mobility for charge carriers of both signs so that they can be spatially separated as quickly as possible after absorption and, depending on the sign, flow off to their electrode. Since the currently used organic semiconductors have low charge carrier mobilities of approx. 0.01 to 0.001 cm² / Vs, the optimal absorber layer thickness is only a few 100 nm.

Advantages and disadvantages

The potential advantages of a plastic-based solar cell over conventional silicon solar cells are:

Disadvantage:

  • So far, only a relatively low level of efficiency has been achieved (17.3%)
  • The low efficiency results in a higher space requirement.
  • The long-term stability of the organic compounds in sunlight is still insufficient (decomposition).

outlook

The current efficiency of organic solar cells in the laboratory is below that of other thin-film technologies. For a commercial breakthrough, both efficiency and long-term stability, especially on flexible substrates and large surfaces, must be significantly increased. The technological potential of organic photovoltaics to find its way into the mobile power supply as a cost-effective energy source is supported by the targeted mass production based on established printing processes. In such a scenario, organic photovoltaics would be of particular importance in previously untapped areas of application with low investments at the same time.

The company Konarka Technologies GmbH, Nuremberg, had taken 2,009 first organic panels for mobile devices on the market. The efficiency is less than 3%. A module with 0.45 m² has an output of 7.8 watts in full sunshine. However, the company filed for bankruptcy on June 1, 2012.

The company Heliatek GmbH in March 2012 a production facility for organic solar cells of small molecules ( small molecules taken) into operation.

The Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, together with partners, has succeeded in producing a cheap, organic solar cell on flexible film. Production takes place in a roll-to-roll process . The solar cells used do not require indium tin oxide (ITO).

The Danish start-up infinityPV ApS (founded in 2014) also sells organic solar cell modules, which completely dispense with ITO and vacuum processes. They are manufactured entirely in a roll-to-roll process with printing and coating processes. High-voltage modules can be assembled simply by cutting at any point.

literature

  • NS Sariciftci, L. Smilowitz, AJ Heeger, F. Wudl: Photoinduced Electron Transfer from Conducting Polymers onto Buckminsterfullerene. In: Science . 258, No. 5087, 1993, pp. 1474-1476, doi: 10.1126 / science.258.5087.1474 .
  • NS Sariciftci, AJ Heeger: Photophysics, charge separation and device applications of conjugated polymer / fullerene composites. In: HS Nalwa (ed.): Handbook of Organic Conductive Molecules and Polymers. Volume 1, Charge-Transfer Salts, Fullerenes and Photoconductors, Wiley, Chichester / New York 1997, ISBN 0-471-96593-6 , pp. 413-455.
  • Christoph J. Brabec, N. Serdar Sariciftci, Jan Kees Hummelen: Plastic Solar Cells. In: Advanced Functional Materials . 11, No. 1, 2001, pp. 15-26.
  • Christoph Brabec, Vladimir Dyakonov, Jürgen Parisi and Niyazi Serdar Sariciftci (eds.): Organic Photovoltaics. Springer-Verlag, Berlin 2003, ISBN 3-540-00405-X .
  • H. Hoppe, NS Sariciftci: Organic solar cells: an overview. In: J. Mater. Res. 19, No. 7, 2004, pp. 1924-1945.
  • Sam-Shajing Sun, Niyazi Serdar Sariciftci (eds.): Organic Photovoltaics: Mechanisms, Materials, and Devices (Optical Engineering). CRC Press, Boca Raton 2005, ISBN 0-8247-5963-X .
  • H. Hoppe, NS Sariciftci: Polymer Solar Cells. In: SR Marder, K.-S. Lee (eds.): Photoresponsive Polymers II. Springer, Berlin / Heidelberg 2008, ISBN 978-3-540-69452-6 , pp. 1-86.

Web links

Individual evidence

  1. ↑ New world record in organic photovoltaics: Heliatek achieves 12% solar cell efficiency. (No longer available online.) SolarServer, January 16, 2013, archived from the original on January 18, 2013 ; Retrieved January 18, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.solarserver.de
  2. ↑ CW Tang: Twolayer organic photovoltaic cell . In: Appl. Phys. Lett . tape 48 , no. 183 , 1985, pp. 183-185 , doi : 10.1063 / 1.96937 .
  3. ↑ Production by the Heliatek company
  4. ↑ Konarka announces availability of solar cells for portable chargers at the European Photovoltaic Solar Energy Conference ( Memento of October 21, 2009 in the Internet Archive ). Konarka (press release).
  5. ↑ Konarka Power Plastic® 620 Solar Charger - Product Specifications. (PDF; 129 kB) data sheet from Konarka, accessed on May 27, 2010
  6. ^ Konarka Technologies Files for Chapter 7 Bankruptcy Protection . Konarka press release (accessed June 2, 2012).
  7. ↑ Heliatek inaugurates worldwide unique production facility for the production of organic solar films ( Memento from December 24th 2012 in the Internet Archive ) (PDF; 1.1 MB). Heliatek (press release)
  8. ↑ www.ise.fraunhofer.de Organic photovoltaics by the meter, accessed on June 7, 2014
  9. ↑ Company homepage , accessed on July 7, 2015
  10. ↑ Organic solar cells - fast roll-to-roll (R2R) printing & coating
  11. ↑ infinityPV foil - printed organic solar cells - cutting & electrical contacting DIY , accessed July 7, 2015