Organic solar cell

An organic solar cell is a solar cell, which consists of materials of organic chemistry, i.e., hydrocarbon compounds ( plastics). The efficiency is converted into electrical energy by solar energy, is special, produced in laboratories by means of individual production cells having areas of about 1 cm ², with 12.0 % (as of January 2013) still below that achieved in solar cells made of inorganic semiconductor material. Organic solar cells and plastic solar cells, as they are also called, are a current research topic ( see Advantages and disadvantages ) because of the opportunities to potentially cheaper and more versatile manufacturing process.

Material

The material for this type of solar cell based on organic hydrocarbon compounds with a special electronic structure, the conjugated π -electron system, which gives the essential properties of amorphous semiconductor materials in question. Typical representatives of organic semiconductors are conjugated polymers and small molecules, which also specifically synthesized hybrid structures such as copper phthalocyanine are used. The first organic solar cell was prepared in 1985 by Tang consisting of copper phthalocyanine, and a derivative of PTCDA. The first plastic solar cells of conjugated polymers ( electron donors ) and fullerene ( electron acceptors ) were prepared, were also two-layer solar cells. These cells consisted of a thin layer of the conjugated polymer to which a further thin layer of fullerenes has been applied. The photoactive substance in these solar cells are the conjugated hydrocarbons that can go under light irradiation in excited states. These states can give their excitation energy in the form of an electron to a fullerene. Since the charges are completely separated metastable, these charges can be collected and removed via metallic electrodes. From a technological point of view, conjugated polymers and functionalized molecules due to the producibility of layers of liquid phase attractive base materials for low-cost mass production of flexible PV elements with relatively simple structure dar. Molecular semiconductors, however, are usually used in vacuum vapor deposition processes ( cf. thermal evaporation or generally physical vapor deposition ) processes into well-defined multilayer systems and allow the production of sequentially deposited semiconductor layers and thus more complex cell types (eg, tandem cells).

Principle of operation

The representative efficient organic solar cells based on the use of a so-called donor-acceptor system, ie on the skilful combination of different semiconductors, which after absorption of light an extremely fast transfer ( much less than 1 ps ) of the resulting charge carriers to donor and acceptor show (eg thin films of conjugated polymers and fullerenes ). Such DA pairs differ in their relative positions to each other shifted the electrochemical potentials: HOMO ( highest occupied molecular orbital) and LUMO ( lowest unoccupied molecular orbital ). These orbitals are similar to the inorganic semiconductor band diagram in a certain way. After the absorption of photons whose energy exceeds the distance between the HOMO and LUMO, called excitons formed ( electrostatically bound pairs of positive and negative charges ), which, inter alia, isolated by the local electric field at a donor-acceptor interface for some time be. After separation of the charge transport in the two semiconductors is effected selectively. The charge carriers move by " hopping " through the semiconductor; to enforce this is through their movement in the present disordered ( amorphous or microcrystalline ) environment with a lot of energy barriers. Meet The charges on many molecular and phase boundaries and thus to have substantive and structural defects, which means the recombination and thus the loss of the two charges.

In an organic solar cell is the ( from the liquid phase and / or by vacuum method applied ) absorber layer is usually made ​​of a mixture by volume of donator and acceptor-type organic semiconductors. This layer is applied onto a transparent conductive electrode (coated with a transparent conductor float glass). The transparent electrode makes it possible to couple as much light as to maximize the yield of photons absorbed in the active layer itself. At the same time they should have a low electrical sheet resistance. However, the most important feature is its work function that determines which of the two they preferred semiconductor charge carrier exchange (negative or positive, corresponding to electrons or electrical defects). On the other side of the absorber layer, a metal electrode is deposited. Collecting the charge carriers of opposite sign from those flowing through the transparent electrode.

The back reflection of unabsorbed light from the metal electrode increases the yield, because the reflected light will have another chance to absorption when re passing through the absorber layer. The thickness of the absorber layer in the resonator between the glass electrode and metal electrode can be optimized for maximum absorption of a certain wavelength; However, the effect in comparison to electrical reflection is low, as shown below.

The terminal voltage of such a solar cell is essentially 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 as high mobilities for charge carriers of both signs, so that they can be spatially separated by absorption as soon as possible and, depending on the sign, to drain their electrode. Since the organic semiconductor currently used have low charge carrier mobility of about 0.01 to 0.001 cm2/Vs, the optimal thickness of the absorber layer is in the range of a few 100 nm

Pros and Cons

The potential benefits of a solar cell based on plastic compared to conventional silicon solar cells are:

• Lower production costs due to cheaper production technologies ( roll-to -roll process, partially vacuum- free) and lower material costs (eg company Heliatek: 1 g wool for 1 m² cell area )
• Flexibility, transparency and ease of use ( the mechanical properties of plastics)
• Energy efficient production possible, no high-temperature processes necessary
• Satisfy the requirements of the EU Directive 2002/95/EC ( RoHS), as will dispense with the use of hazardous substances

Cons:

• So far, only a relatively low efficiency obtained ( 12.0 % of minicells were prepared and selected as a single unit in a laboratory )
• Low efficiencies require a higher area requirement, which is associated with a correspondingly high installation cost.
• The long-term stability of the organic compounds in sunlight is still insufficient (decomposition).

Outlook

The current efficiency of organic solar cells is in the laboratory below the other thin film technologies. For commercial breakthrough, both the efficiency and the long-term stability, especially on flexible substrates and large areas are still significantly increased. The technological potential of organic photovoltaics to keep as a good energy source entry into the mobile power, is supported by the target mass production based on established printing processes. In such a scenario of organic photovoltaics would particularly important in untapped application areas with low investment.

The company Konarka Technologies GmbH, Nuremberg, had brought in 2009 first organic collectors for mobile devices on the market. The efficiency is less than 3%. A module of 0.45 m has a power output of 7.8 watts at full sunlight. However, the company announced on June 1, 2012 bankruptcy.

The company Heliatek has put into operation in March 2012 a production plant for organic solar cells made of small molecules ( small molecules ).