Photovoltaic cell

A photovoltaic cell with a photoactive layer (4) made of two molecular organic components, namely a conjugated polymer component as electron donor and a fullerene component as electron acceptor, with two metallic electrodes (2, 5) provided on both sides of the photoactive layer (4) and with a Intermediate layer described at least between an electrode (5) and the photoactive layer (4). In order to achieve an increased energy yield, it is proposed that the intermediate layer form an electrically insulating transition layer (6) with a thickness of at most 5 nm, which does not result in a continuously closed cover of the photoactive layer (4).

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DVR 0078018

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The invention relates to a photovoltaic cell with a photoactive layer of two molecular organic components, namely a conjugated polymer component as electron donor and a fullerene component as electron acceptor, with two metallic electrodes provided on both sides of the photoactive layer and with an intermediate layer between the photoactive layer and at least one Electrode.

Plastics with extensive ττ-electron systems, in which single and double bonds alternate, are called conjugated plastics. These conjugated plastics have energy bands that are comparable to semiconductors in terms of electron energy, so that they can also be converted from the non-conductive to the metallic-conductive state by doping. Examples of such conjugated plastics are polyphenylenes, polyvinylphenylenes (PPV), polythiophenes or polyanilines. However, the efficiency of the energy conversion of photovoltaic polymer cells made of a conjugated polymer is typically between 10 '³ and 10 '² %. To improve this efficiency, heterogeneous layers of two conjugated polymer components have already been proposed (US Pat. No. 5,670,791 A), one of which is used as an electron donor and the other polymer component as an electron acceptor. Through the use of fullerenes, i.e. carbon clusters with a steric framework structure of carbon atoms, in particular Buckminster fullerenes C.₆o, as electron acceptors (US Pat. No. 5,454,880 A), the otherwise customary charge carrier recombination in the photoactive layer could largely be avoided, resulting in an increase in efficiency of 0.6% to 1% under AM 1.5 conditions (irradiation of the 1.5 times larger air mass (Air Mass) due to exposure to sunlight at an angle of 41.5 °). In spite of this, the degree of efficiency that can be achieved remains generally too low for an economical, technical use of such photoactive layers for the construction of photovoltaic cells.

In connection with light-emitting cells, it is known (US Pat. No. 5,247,190A) to provide a contact layer made of aluminum oxide between a photoactive layer containing at least one conjugated polymer and an aluminum electrode in order to improve the injection of electrons into the photoactive layer when an external voltage is applied. Since in a photovoltaic cell, however, electrons are not injected into the photoactive layer, but rather withdrawn from the photoactive layer, an electroluminescent cell cannot be compared with a photovoltaic cell in this regard.

In the case of a photovoltaic cell with a multilayer photoactive layer made of organic components, it is also known (US Pat. No. 5,350,459 A) to increase the open circuit voltage and the short-circuit current, between a layer of the photoactive layer forming an electron acceptor and the adjoining electrode, an intermediate layer made of an inorganic semiconductor provide, which has an excess of electrons. Such an intermediate layer, however, requires the use of inorganic semiconductors.

The invention is therefore based on the object of designing a photovoltaic cell of the type described at the outset in such a way that a further increase in the efficiency of the energy conversion is possible.

The invention achieves the stated problem in that the intermediate layer forms an electrically insulating transition layer with a thickness of at most 5 nm, which does not result in a continuously closed cover of the photoactive layer.

The invention is based on the fact that in the transition region between the photoactive layer and the electrode there is a considerable resistance to the transfer of charge carriers, which is presumably due to reactions between the metallic electrode and the organic photoactive layer. If these direct influences can therefore be prevented, an improvement in the charge transfer must be expected if the conditions remain the same, which leads to an increase in the degree of efficiency. By providing an electrically insulating transition layer, these direct reactions between photoactive layer and electrode can now be largely switched off, but the thickness of the electrically insulating transition layer must be limited to a maximum of 5 nm so that the high electrical resistance of this transition layer does not facilitate the transfer of charge carriers between photoactive layer and electrode prevented. Due to the very small layer thickness, there is no continuous, closed intermediate layer between the photoactive

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Layer and the electrode results, surprisingly, the barrier that otherwise occurs between the electrode and the photoactive layer can be largely broken down without making the charge carrier transfer more difficult. With the help of this electrically insulating transition layer, it was possible to increase the efficiency of photovoltaic cells by up to 20 to 25% compared to cells with the same structure without this transition layer. For this purpose, however, an optimization of the electrically insulating transition layer is necessary.

Such an optimization can take place by reducing the thickness of the transition layer to a maximum of 2 nm. Of course, the desired effect can also be influenced via the chemical layer structure. Thus, transition layers made of an alkali halide have proven to be useful, with particularly good properties also being found with regard to processing with a transition layer made of lithium fluoride, which can be vapor-deposited in a vacuum onto the photoactive layer or the electrode in the desired layer thickness.

The subject matter of the invention is shown in the drawing, for example. Show it

1 shows a photovoltaic cell according to the invention in a schematic section and

2 shows the current-voltage characteristic of a conventional photovoltaic cell and a photovoltaic cell according to the invention.

According to FIG. 1, the photovoltaic cell consists of a transparent glass substrate 1 on which an electrode layer 2 made of an indium / tin oxide (ITO) is applied. This electrode layer 2 generally has a comparatively rough surface structure, so that it is covered with a smoothing layer 3 made of a polymer which is electrically conductive through doping, usually PEDOT (polyethylene dioxythiophene). The photoactive layer 4 made of two components is applied to this smoothing layer 3 with a layer thickness of, for example, 100 nm to a few μm, depending on the application method. The photoactive layer 4 consists of a conjugated polymer, preferably a PPV derivative, as an electron donor and a fullerene, in particular functionalized fullerene PCBM ([6.6] -phenyl-C₆₁-butylic acid methyl ester), as electron acceptor. The term polymer is understood to mean both high polymers and oligomers. The two components are mixed with a solvent and applied as a solution to the smoothing layer 3, e.g. B. by spinning or dripping applied. Squeegee or printing processes can also be used to coat larger areas with such a photoactive layer 4. Instead of conventional toluene (methylbenzene), the solvent used is preferably a fining agent such as chlorobenzene, for example 1,2-dichlorobenzene, in order to ensure a fine structure of the heterogeneous layer 4, which then has an average grain size of less than 500 nm. This allows the number of contact points between the electron donor and the electron acceptor to be increased considerably, which results in improved charge separation and an increase in efficiency of approx. 2.6% under simulated AM 1.5 conditions.

However, before the counter electrode 5 is applied, a thin transition layer 6 with a layer thickness of, for example, 0.6 nm, which has an electrically insulating effect, is applied to the photoactive layer 4. In the exemplary embodiment, this transition layer consists of an alkali halide, namely lithium fluoride, which in a vacuum of 2 10⁶ Torr is evaporated at a rate of 0.2 nm / min, but due to the small layer thickness, there is no continuously closed cover of the photoactive layer 4.

When ITO is used as the hole-collecting electrode, aluminum is used as the electron-collecting electrode, which is vapor-deposited onto the electrically insulating transition layer 6. Since the interposition of an electrically insulating transition layer 6 between the photoactive layer 4 and the electrode 5 can largely avoid the reactions affecting the transfer of charge between the photoactive layer 4 and the electrode 5 in the immediate boundary area between the electrode 5 and the active photo layer 4, because the electrode 5 does not directly adjoin the photoactive layer 4 in large areas, the transfer of charge carriers from the photoactive layer 4 to the electrode 5 is improved provided that the transition layer 6 does not itself build up an additional barrier between the photoactive layer 4 and the electrode 5, which can be ensured by limiting the layer thickness of the transition layer 6. The electrical insulating properties of the transition layer 6 obviously prevent the transition from the photoactive layer 4 to the transition layer 6, particularly in the transition region

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Influences that inhibit charge carriers become effective.

In FIG. 2, the current density I is plotted against the voltage U at an excitation energy of 80 mW / cm² applied under simulated AM 1.5 conditions of two photovoltaic cells which differ only in the presence of a thin transition layer 6 according to the invention. From the comparison of the characteristic curve 7 for a photovoltaic cell according to the invention with the characteristic curve 8 of a comparison cell constructed in the same way with the exception of the transition layer 6, it follows that with approximately the same short-circuit current of approx. 5.2 mA / cm²an increase in the open circuit voltage from 770 mV to 810 mV could be measured. Since the fill factor, i.e. the ratio between the maximum output cell power and the product of short circuit current and open circuit voltage, improved from 0.52 to 0.62, the efficiency of the photovoltaic cell according to the invention was able to go from 2.6% of the comparison cell to 3.2 %, which corresponds to an improvement in energy conversion of 20 to 25%.

The invention is of course not restricted to the exemplary embodiment shown, which shows the electrically insulating transition layer 6 between the electron-collecting electrode 5 and the photoactive layer 4. Thus, the electrically insulating transition layer 6 could also be provided between the hole-collecting electrode 2 and the adjoining organic layer, in the exemplary embodiment the smoothing layer 3. In addition, the electrically insulating transition layer 6 could only be provided in the area of ​​the hole-collecting electrode 2.