Photovoltaic cell

The invention relates to a method for producing a photovoltaic cell with a photoactive layer of two organic components, namely a conjugated polymer component as electron donor and a fullerene component as electron acceptor, a mixture of the two components and a solvent on a carrier layer provided with an electrode layer applied as a film and then this film is covered with a counter electrode.

Plastics with extensive Π-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 made up of carbon atoms, in particular Buckminster fullerenes C.₆₀As electron acceptors (US Pat. No. 5,454,880 A), the otherwise common 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 due to solar radiation at a wink! 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.

The invention is therefore based on the object of designing a method for producing 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 becomes possible.

The invention solves the problem in that, in order to reduce the average largest grain size of the two components and their mixed phases below 500 nm, chlorobenzene is added to the mixture of the two components as a solvent and refining agent.

The invention is based on the knowledge that effective charge separation can only be ensured in the contact area between the electron donor and the electron acceptor, so that after photoexcitation of the conjugated polymer component, the excitation energy is passed on to the fullerene component in the form of electrons only in the areas of contact with the fullerene component. If the average largest grain size of the components and mixed phases of the photoactive layer is kept smaller than 500 nm, the proportion of contact between the two components can be increased accordingly due to the associated increase in surface area, which leads to a significant improvement in charge separation. The efficiency, which is dependent on this charge separation, rose to a characteristic one

2.5% under simulated AM 1.5 conditions. However, ensuring a correspondingly reduced grain size is not easily possible. If the mixture of the two components and a solvent is applied in the usual way as a film to a carrier layer provided with an electrode layer, the requirement for a restriction of the grain sizes can surprisingly be met by the use of chlorobenzene as solvent and fining agent.

The effect of the fine-grain structuring of the photoactive layer of a photovoltaic cell according to the invention is explained in more detail with the aid of the drawing.

Show it

1 shows the basic structure of a photovoltaic cell according to the invention in a section,

2 shows the surface structure of a conventional photoactive layer,

3 shows the surface structure of a photoactive layer according to the invention,

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

AT 410 859 B

5 shows the charge yield related to the wavelength of the photo-excitation per incident light power on the one hand for a conventional and on the other hand for 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 made electrically conductive by doping, usually PEDOT (polyethylene dioxythiophene). The photoactive layer 4 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, before the counterelectrode 5 is applied. When ITO is used as the hole-collecting electrode, aluminum is used as the electron-collecting electrode 5 and is vapor-deposited onto the photoactive layer 4.

The photoactive layer consists of a conjugated polymer, preferably a PPV derivative (for example alkoxy-PPV), as an electron donor and a fullerene, in particular functionalized fullerene PCBM ([6.6] -phenyl-C₆i-butylic acid methyl ester), as electron acceptor. The term polymer is understood to mean both high polymers and oligomers. These two components are mixed with a solvent and applied as a solution to the smoothing layer 3 ζ. B. applied by spinning or dripping. Toluene (methylbenzene) is used as the usual solvent, but this cannot ensure the desired fine structure of the photoactive layer 4, as shown in FIG. 2 shows, in which the typical surface structure of such a photoactive layer with toluene as solvent is shown. It can be seen in an atomic force recording (tapping mode AFM images), as shown schematically in FIGS. 2 and 3, in particular the grain structure of the fullerene component 6 or a mixed phase, while the polymer component or a further mixed phase substantially Fills spaces between the pronounced grains. On the basis of the length unit shown, a maximum grain size results that is considerably larger than 500 nm.

However, if a chlorobenzene, for example 1.2-dichlorobenzene, is used as the solvent according to the invention, a considerably finer structure is obtained with an otherwise identical composition of the active layer 4, which, according to FIG. 3, results in a correspondingly smoother surface structure. The chlorobenzene thus acts as a refining agent, the achievable average grain size of less than 500 nm of the photoactive layer 4 significantly increasing the number of points of contact between the electron donor and the electron acceptor and thus a considerably improved charge separation and reduced charge recombination, which results from the voltage -Current characteristics can be read off immediately. In Fig. 4th the current density I of the photovoltaic cells to be compared is plotted against the voltage U, with an excitation energy of 80 mW / cm² under simulated AM 1.5 conditions. If one compares the characteristic curve 7 of the photovoltaic cell with the coarser-grained structure of the photoactive layer 4 with the characteristic curve 8, which was recorded for a photovoltaic cell with a fine-grained structure of the photoactive layer 4, one immediately recognizes the improved conditions in a photovoltaic cell according to the invention the characteristic 8. The short-circuit current measured at a voltage of 0 V was 2.79 mA / cm in the known cell², in the cell according to the invention 5.24 mA / cm². Since the open circuit voltage also increased from 710 mV to 770 mV, an increase in efficiency from approx. 1% to 2.6% could be achieved, whereby it must be taken into account that the fill factor, i.e. the ratio between the maximum output cell power and the product of short-circuit current and open-circuit voltage, increased from 0.40 to 0.52 due to the finer structure of the photoactive layer according to the invention.

The effects according to the invention can be seen particularly clearly on the basis of FIG. 5, in which the charge yield per incident light power IPCE [%] = 1240. I_k [μΑ / cm²] / A [nm] .l, [W / m²] is plotted against the wavelength λ for the photovoltaic cells to be compared. With l_k the short-circuit current and the light output with 1 must be entered in the above formula. It turns out that according to the characteristic curve 9 for a cell according to the invention compared to the characteristic curve 10 of the conventional cell there is approximately twice the charge yield per incident light output if the fine structure of the heterogeneous photoactive layer 4 has an average grain size smaller than