Photovoltaic cell

p0001The photovoltaic cell

p0002[0001] The invention relates to a photovoltaic cell with a photoactive layer of two molecular components, namely, an electron donor and an electron acceptor, in particular a conjugated polymer component and a fullerene component, and with two on both sides of the photoactive layer provided metallic electrodes.

p0003[0002] Plastics with extended π-electron systems, where changing succession of single and double bonds are called conjugated polymers. These conjugated plastics have in terms of the electron energy with semiconductors to comparable energy bands, so that they can also be converted by doping from the non-conductive, in the metallic conductive state. Examples of such conjugated polymers are poly phenylene, polyvinylphenylenes (PPV), polythiophenes or polyanilines. The energy conversion efficiency of photovoltaic polymer cells of a conjugated polymer is, however, typically between 10^"3 and 10^"2 %. Although heterogeneous layers were to improve this efficiency already proposed two conjugated polymer components (US 5,670,791 A), are used by which a polymer component as an electron donor and the other polymer component as an electron acceptor. Through the use of fullerenes, especially buckminsterfullerenes Cεo, as electron acceptors (US 5,454,880 A) was the usual charge carrier recombination in the photoactive layer can be largely avoided, resulting in an increase in efficiency of 0.6% to 1% under AM (Air Mass) led 1.5 conditions. Nevertheless, there remains the achievable efficiency for economic, technical use of such photoactive layers for the construction of photovoltaic cells is generally too low.

p0004[0003] The invention is thus based on the object, a photovoltaic cell of the kind mentioned above so that a further increase in the efficiency of energy conversion is possible.

p0005[0004] The invention solves this problem in that an electrically insulating transition layer is provided with a thickness of at most 5 nm, at least between one electrode and the photoactive layer.

p0006[0005] The invention is based on the fact that is established in the transition region between the photoactive layer and the electrode, a considerable resistance to a charge carrier transfer, which is probably due to reactions between the metal electrode and the organic photoactive layer. Can therefore these immediate effects are inhibited, so to be expected under otherwise identical conditions with an improvement of the charge facing downgrading, which leads to an increase in efficiency. By providing an electrically insulating transition layer these immediate reactions between photoactive layer and the electrode can now be largely eliminated, but the thickness of the electrically insulating transition layer must be limited to not more than 5 nm, lest the high electrical resistance of this transition layer Facilitated transfer of charge carriers between photoactive layer and electrode prevented. Due to the very small layer thickness, which can not expect continuous, closed intermediate layer between the photoactive layer and the electrode, surprisingly, the otherwise occurring between the electrode and the photoactive layer barrier can be largely reduced without complicating the carrier transfer in addition. Using this electrically insulating junction layer, the efficiency of photovoltaic cells could be increased as compared to otherwise identically constructed cells without these transition layer up to 20 to 25%. For this purpose, however, an optimization of the electrically insulating junction layer is required. [0006] Such an optimization can be achieved 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 have halide of a salt, in particular an alkali metal, proven, and particularly good properties could be determined with a transition layer of lithium fluoride also in terms of processing, the vacuum-deposited on the photoactive layer or the electrode in the desired layer thickness can be.

p0007[0007] In the drawing, the subject invention is illustrated, for example. Show it

p0008Fig. 1 is a photovoltaic cell according to the invention in schematic section, and

p0009Fig. 2 shows the current-voltage characteristic of a conventional and a photovoltaic cell according to the invention.

p0010[0008] The photovoltaic cell is shown in FIG. 1 from a light-transmissive glass substrate 1, is applied to the one electrode layer 2 made of an indium / tin oxide (ITO). This electrode layer 2 generally has a comparatively rough surface structure, so as to layer 3 of an electrically conductive by a doping polymer, typically PEDOT, is covered with a smoothing. In this smoothing layer 3, the photoactive layer 4 is composed of two components applied in a layer thickness depending on application method, for example, 100 nm to several microns. The photoactive layer 4 is composed of a conjugated polymer, preferably a PPV derivative, as an electron donor and a fullerene, particularly functionalized fullerene PCBM, as an electron acceptor. The term polymer is to be understood both high polymers and oligomers. The two components are mixed with a solvent and applied as a solution to the smoothing layer 3 z. B. by a spin-coating or dropping. For coating large surfaces with such a photoactive layer 4 and squeegee or printing method can be used. preferably a Feinungsmittel such as chlorobenzene is used as solvent instead of the conventional toluene used to ensure a fine structure of the heterogeneous layer 4 then having a mean particle size less than 500 nm. Thus, the number of contact points between the electron donor and the electron acceptor be significantly increased, which is reflected in an improved charge separation and an increase in efficiency of approximately 2.6% 1.5 affects conditions under simulated AM.

p0011[0009] However, before the counter electrode 5 is applied, is a thin transition layer 6 coated on the photoactive layer 4 having a layer thickness of for example 0.6 nm, the work must be electrically insulating. This transition layer in the exemplary embodiment of an alkali metal halide, namely a lithium fluoride, in a vacuum of 2 x 10 the^"6 Torr is deposited at a rate of 0.2 nm / min, but can be expected not continuously closed cover of the photoactive layer 4 due to the low layer thickness.

p0012[0010] With the use of ITO as lochsammelnde electrode is used as electrode elektronensammelnde aluminum, which is deposited on the electrically insulating layer transition the sixth As a 5-disruptive effect forming reactions in the immediate border region between electrode 5 and active Photo layer 4 can be largely avoided by the interposition of an electrically insulating transition layer 6 between the photoactive layer 4 and the electrode 5 on the charge transfer between the photoactive layer 4 and the electrode, precisely because the electrode 5 is not adjacent in large areas directly on the photoactive layer 4, the charge carriers transfer from the photoactive layer 4 is improved in the electrode 5 under the assumption that the transition layer 6 is not itself an additional barrier between the photoactive layer 4 and the electrode 5 builds, which can be ensured by restricting the thickness of the transition layer. 6 The electrical insulation properties of the transition layer 6 thereby prevent obvious that particular transition layer 6 the transfer of charge carriers inhibitory influences to take effect in the transition region of the photoactive layer. 4 [0011] In FIG. 2, the current density I to the voltage U at an excitation energy of 80 mW / cm² under simulated AM 1, 5 conditions of two photo- cells tovoltaischer applied, which differ only by the presence of a 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 of an 8 with the exception of the transition layer 6 coincident constructed reference cell shows that at approximately the same short circuit current of approximately 5.2 mA / cm² an increase in the open circuit voltage of 770mV to 810 mV could be measured. In addition, since the filling factor of 0.52 improved to 0.62, the efficiency of the photovoltaic cell of 2.6% compared to the cell of the invention could be increased to 3.2%, representing an improvement of the energy conversion of 20 to 25%.

p0013[0012] The invention is of course not limited to the illustrated embodiment, showing the electrically insulating transition layer 6 between the electron collecting electrode 5 and the photoactive layer. 4 So could the electrically insulating transition layer 6 between the hole collecting electrode 2 and the subsequent organic layer in the embodiment, the smoothing layer 3, are provided. In addition, the electrically insulating transition layer 6 may be provided only in the area of ​​the electrode second Since the effect of the electrically insulating layer 6 does not transition to conjugated polymers as an electron donor and fullerene as an electron acceptor is limited, the effect of the invention can also be observed in all the photovoltaic cells with a molecular layer of a two-component electron donor and an electron acceptor.