Photovoltaic cell

It is 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, 6) provided on both sides of the photoactive layer (4) and with a Intermediate layer (5) between the photoactive layer (4) and at least one electrode (2 or 6) described. In order to improve the charge transport between the photoactive layer (4) and the electrodes (2, 6), it is proposed that the intermediate layer (5) between the photoactive layer (4) and at least one electrode (2, 6) consists of a conjugated polymer with a doping corresponding to the electrode potential and that the polymer of the intermediate layer (5) has a band gap between the valence band and the conduction band of at least 1.8 eV with regard to the electron energy bands.

6

<img file="AT410729B_D0001.tif" />

DVR 0078018

AT 410 729 B

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 p-electron systems in which single and double bonds alternate are referred to as 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 10² %. 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. By using fullerenes, i.e. carbon clusters with a steric framework structure of carbon atoms, in particular Buckminster fullerenes Qo, as electron acceptors (US Pat. No. 5,454,880 A), the otherwise common charge carrier recombination could largely be avoided, which led to a considerable increase in efficiency. Good charge separation is a necessary, but not sufficient, condition for achieving a good degree of efficiency, because it must also be ensured that the separated charges also reach the corresponding electrodes of the photovoltaic cell. In conventional photovoltaic cells of this type, a hole-collecting electrode made of indium / tin oxide (ITO) and an electron-collecting electrode made of aluminum have proven successful.

In a photovoltaic cell with a multilayer photoactive layer made of organic components, it is known (US Pat. No. 5,350,459 A) to provide an intermediate layer of an inorganic semiconductor between a layer of the photoactive layer which forms an electron acceptor and the subsequent electrode to increase the open circuit voltage and the short-circuit current that 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 above so that the charge transport between the photoactive layer and the electrodes can be increased in the sense of increasing the short-circuit current without having to use inorganic semiconductor layers.

The invention solves the problem in that the intermediate layer between the photoactive layer and at least one electrode consists of a conjugated polymer with a doping corresponding to the electrode potential and that the polymer of the intermediate layer has a band gap between the valence band and the conduction band of at least 1 with regard to the electron energy bands , 8 eV.

Since the conjugated polymer of the intermediate layer is doped in terms of the electrode potential, which means oxidative doping in the area of ​​the hole-collecting electrode and reductive doping in the area of ​​the electron-collecting electrode, the conjugated polymer provides an excess of holes in the area of ​​the hole-collecting electrode, but in the area of ​​the electron-collecting electrode a surplus of electrons sure, so that the hole conduction is supported in the area of ​​the oxidatively doped polymer and the electron conduction is supported in the area of ​​a reductively doped polymer. However, since the conjugated polymer of the respective intermediate layer is constructed in such a way that it has a comparatively large band gap between the valence band and the conduction band of at least 1.8 eV with regard to the electron energy bands, there is a correspondingly high activation energy for the intrinsic conduction, which leads to that in the case of an oxidatively doped polymer layer the electron conduction from the photoactive layer to the hole-collecting electrode and in the case of the reductively doped intermediate layer the hole conduction from the photoactive layer to the electron-collecting electrode are hindered. With the help of these special intermediate layers, a valve effect can thus be achieved, which allows the conduction of the charge carriers to be collected at the respective adjacent electrode from the photoactive layer

AT 410 729 B supports the electrode, but prevents the diffusion of opposing polarity charges in the same direction. As a result of these special layers, the charge conduction to the electrodes can be improved accordingly, which has the direct effect of increasing the short-circuit current. Depending on the application, photovoltaic cells with such an intermediate layer made of a conjugated polymer can be used between the hole-collecting electrode and the photoactive layer, between the electron-collecting electrode and the photoactive layer or in the area of ​​both electrodes.

Although different conjugated polymers can be appropriately doped oxidatively or reductively to form the intermediate layers, particularly advantageous conditions result when the intermediate layer consists of a doped polythiophene derivative, for example poly-3hexylthiophene.

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

1 shows a partial schematic section of a photovoltaic cell according to the invention, and FIG

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

The photovoltaic cell according to FIG. 1 has 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 forms a comparatively rough surface structure, so that it is covered with a smoothing layer 3 made of a polymer that is electrically conductive through appropriate doping, usually PEDOT (polyethylene dioxythiophene). In contrast to conventional photovoltaic cells of this type, according to the invention, the photoactive layer 4 is not applied directly to the smoothing layer 3, but rather to an intermediate layer 5, which consists of a conjugated polymer, preferably a poly-3-alkylthiophene, which, according to the Applying to the smoothing layer 3 was oxidatively doped with nitrosonium tetrafluoroborate in order to obtain a corresponding excess of holes.

The photoactive layer 4, which is applied to the intermediate layer 5 in the form of a solution, consists of a conjugated polymer, preferably a polythiophene 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 electron-collecting electrode 6 consists of aluminum which, in the case of the exemplary embodiment shown, is vapor-deposited onto the photoactive layer 4 without the interposition of a further intermediate layer, which, however, would be entirely possible. In this case, the conjugated polymer of the intermediate layer would have to be reductively doped in accordance with the negative potential of the electron-collecting electrode 6 in order to ensure a corresponding excess of electrons.

Due to the selected intermediate layer 5, which has a band gap of at least 1.8 eV between the valence band and the conduction band, which, in contrast to inorganic semiconductors, can be adjusted by known chemical measures, the entry of electrons from the photoactive is due to this comparatively wide band gap Layer 4 in the intermediate layer 5 made difficult, without impairing the hole conduction between the photoactive layer 4 and the hole-collecting electrode 2. The conduction band of the conjugated polymer of the intermediate layer 5 is e.g. B. in contrast to the PEDOT layer 3 at a significantly higher energy level than the energy band of the electron acceptor of the photoactive layer 4. This means a unipolar charge transfer from the photoactive layer 4 to the hole-collecting electrode 2, which is noticeable in a corresponding increase in the short-circuit current, as can be seen from FIG. In FIG. 2, the current density I is plotted against the voltage U at an excitation energy of 80 mW / cm² plotted under simulated AM 1.5 conditions (irradiation of the 1.5 times larger air mass due to solar radiation at an angle of 41.5 °) for a photovoltaic cell according to the invention in comparison to a cell with the same structure with the exception of the intermediate layer 5. It can be seen that the characteristic curve 7 associated with the photovoltaic cell according to the invention results in a short-circuit current measured at the voltage U = 0V which is approximately twice as large as the short-circuit current of the comparison cell according to the characteristic curve 8.

Needless to say, in an arrangement of the intermediate 3

AT 410 729 B

Layer 5 between the photoactive layer 4 and the electron-collecting electrode 6, the distance between the valence bands and not the conduction bands is decisive.