GB2340658A

GB2340658A - Solar cell

Info

Publication number: GB2340658A
Application number: GB9919227A
Authority: GB
Inventors: Klaus Kohrs; Dieter Meissner
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1998-08-14
Filing date: 1999-08-13
Publication date: 2000-02-23
Also published as: FR2782416A1; FR2782416B1; GB9919227D0; DE19837019A1; DE19837019C2

Abstract

The solar cell comprises at least one active zone 2 which is constituted from molecules stacked on top of one another. The molecules have at least one n-conducting side and at least one p-conducting side. The molecules are arranged on top of one another in such a way that the p-conducting and the n-conducting sides each develop stacks of their own. The molecules may be synthesised from p-conducting phthalocyanines and n-conducting perylenes and evaporated onto a layer 3 such as p-conducting phthalocyanine. A material such as n-conducting perylene forms the upper layer 1.

Description

2340658 SOLAR CELL The invention relates to a solar cell with a p-n

contact, and to the manufacture and the method of use of the solar cell.

It is known from DE 196 40 065 Al to convert photons in a dye contained in a solar cell into an electric current.

Incident photons generate a charge separation in the active zone (active region) formed here by dyes - As a result of the charge separation a pair of charge carriers is produced. If a charge carrier, such as for example an electron, is moved out of the active zone, this will cause an electric current to flow.

The process of successfully separately moving out charge carriers of a pair of charge carriers in order to generate an electric current relies on the provision of suitable means, for example the provision of a suitable electric field and/or suitable positioning of the energy states in which the charge carriers find themselves. For example, a solar cell is said to have a suitable electric field as a result of a p-n contact having been prepared with a depletion zone between the n-conductor and the p conductor. The depletion zone then constitutes the active zone.

2 The photovoltaically active contact developing at the contact of organic n-semiconductors and p-semiconductors is normally a few nanometres thick. This thickness has proved to be too thin for practical applications in solar cells. In the active region necessary for charge separation, too little light is absorbed to be able to achieve good efficiencies.

To solve this problem it has been attempted, as disclosed in the publication by G. Yu, J. Gao, J.C. Hummelen, F.

Wudl, A.J. Heeger, 'Polymer Photovoltaic Cells: Enhanced Efficiencies via a network of Internal Donor-Acceptor Heterojunctions', Science 270 (1995), 1789, to manufacture so-called heterolayers, gradient layers or so-called interconnected networks out of n-conducting and p-conducting molecules. These layer systems have the drawback that the rectifying contacts which develop between the individual molecules, but also between the crystallites of homogeneous molecules, are not orientated and thus it is not possible to adjust a privileged direction in the layer. The hoped-for efficiencies therefore cannot be realised.

From the publication by J. Simon, J.-J. Andr6, 'Molecular Semiconductors', Springer Publishers, Heidelberg 1985 it is known to manufacture solar cells based on phthalocyanines. Phthalocyanines have a very high light- 3 absorbing capacity. The absorbed light is converted into pairs of charge carriers with a yield of up to 100%.

Nevertheless the efficiencies of the phthalocyanine-based photoelements, or solar cells, that have so far been manufactured are only around 1%.

Phthalocyanines with trivalent central atoms form stacked p-cyano complexes which are distinguished by very good conductivities of 10-2 -2s 10-2 S in the direction of the stack.

As a rule, phthalocyanines are organic p-semiconductors.

However, according to the publication by D. Woehrle, L.

Kreienhoop, D. Schlettwein, 1 Phthalocyanines - Properties is and Applications', eds. C.C. Le=off and A.B.P. Lever, VCH Publishers, Inc., New York, 1996, vol. 4, pp. 219 284, it is possible to produce n-semiconductors by substituting the ring with highly electron-attracting groups (electron-attracting groups are lateral groups that lower the electron density of the central part of the molecule), or by using the corresponding pyridine or pyrazine derivatives in place of the phthalic acid derivatives as the starting material for phthalocyanine synthesis.

From the publication by D. Woehrle, D. Meissner, 'Organic Solar Cells', in Adv. Materials 3 (1991), p. 129, it is 4 known that other organic compounds such as for instance perylene compounds (perylenes) also possess n-conducting characteristics.

N-conducting and p-conducting organic molecules can be chemically combined with one another by organic syntheses. The chemical bond of the molecules may be covalent or ionic. It is possible to synthesis p-n dimers, p-n trimers or larger units whose n-conducting 10 and p- conducting sides are joined together by bridges of different lengths. Such compounds are also known as donor-acceptor molecules. From the publication by J. Simon, P. Bassoul, 15 'Phthalocyanines - Properties and Applications', eds. C.C. Leznoff and A.B.P. Lever, VCH Publishers, Inc., New York 1989, vol. 2, pp. 223-299, it is known to provide liquid crystal phthalocyanine stacks as submicron wires for contacts in semiconductor chips. 20 It is an object of the invention to provide and manufacture a solar cell of the type mentioned in the introduction that exhibits enhanced characteristics. It is further an object of the invention to indicate a 25 practical method of using the solar cell provided.

These objects are achieved by a solar cell having the features contained in the main claim, and by a method and use having the features contained in the corresponding subsidiary claims.

At least one active zone of the solar cell as claimed in the claims is formed from molecules stacked on top of one another. The molecules have at least one n-conducting side and at least one p-conducting side (in other words n-conducting and p-conducting regions, respectively). A 10 plurality of the molecules are arranged one above the other in such a way that the stack is divided into at least one p-conducting and at least one n-conducting side. The p-conducting sides of the molecules therefore form p-conducting sides of the stack. The n-conducting sides of the molecules form n-conducting sides of the stack. In principle any n-conducting or pconducting side may in turn be viewed respectively as an nconducting or pconducting stack. N-conducting and pconducting stacks are for example joined together 20 ionically. The solar cell includes means which move out the charge carriers to generate an electric current. Fundamentally any means known from-the prior art are suitable - for 25 example those indicated in the introduction.

In particular the aforementioned requirements can be 6 satisfied using organic molecules.

If the solar cell as claimed in the claims is positioned in such a way that incident photons, that is to say incident light, travel through the stacked molecules, in other words that the light is incident parallel to the length of the stack, it then travels along a path in the active zone of the p-n junction which may lie substantially above 10 nm. A molecule stack is then higher than 5 nm, as may be taken from an estimate given in the exemplifying embodiment. Typically it will be 100 nm high, though heights of 1000 nm are also possible.

The invention avoids the drawbacks that stem from excessively thin p-n junctions. This results in enhanced efficiency of the solar cell as claimed in the claims, compared to the prior art indicated in the introduction.

Advantageously, the n-side(s) are covalently joined to the p-side(s) in the case of the stack-forming molecules.

The covalent bond reliably remains intact whilst the stack is being produced.

In a further form of embodiment of the invention, metal complexes are provided as covalently linked organic molecules. The metal atom interacts with the ligands of the molecule stacked above and/or below it.

7 If the p-sides and/or n-sides of two molecules interact more strongly with one another (attracting interaction) than is the case between a p-side and an n-side of the individual molecule, then the molecules virtually automatically become stacked on top of one another as claimed in the claims, if the molecules are for example evaporated on an object or applied thereto in dissolved form.

The stack is also automatically created in the manner outlined earlier if side-chains of the molecule interact sufficiently strongly with the side-chains of the nearest molecule (attracting interaction). The interactions concerned may be hydrophobic, hydrophilic or Coulomb interactions.

In a further form of embodiment of the invention, an active zone configured as a layer is constructed from covalently linked organic molecules. The covalently linked organic molecules form stacks with n-conducting sides (charge transfer in the lowest unoccupied molecular orbitals or LUMOs) and p-conducting sides (charge transfer in the highest occupied molecular orbitals or HOMOs).

In a further form of embodiment, the means which moves z charge carriers out of the active zone in order to 8 generate a current includes at least one selectively barrier-forming material which adjoins the stacks or layers as claimed in the claims, for example on the lower face and the upper face. What is meant by "selectively barrier- forming" is that the material constitutes a diode in combination with n-conducting or p-conducting stacks as claimed in the claims. To put it another way, the material must be chosen so that it constitutes a barrier either for the positive or for the negative charge carriers that are formed in the active layer by light irradiation. Also the barrier is pervious to either the negative or the positive aforesaid charge carriers.

A selectively barrier-forming material may be a metal which adjoins stacks, and which together with the stacks forms a Schottky contact. A Schottky contact is taken to mean a rectifying metal semiconductor contact.

Under another form of embodiment of the invention, in the active layer (active zone) the stacks are orientated substantially perpendicular to the layer's surface. The active layer is then constituted from a large number of parallel stacks. When light falls vertically onto the surface of the layer, the light then advantageously travels through a very large contact area between the p conducting and the n-conducting stack. Moreover, unlike in the prior art cited in the introduction, virtually all

9 the light is absorbed inside the active layer. This therefore increases the efficiency.

Stacks as claimed in the claims can, inter alia, be manufactured as follows. A p-conducting material, for example p- conducting phthalocyanine, is evaporated onto an electrically conductive substrate. The substrate may be transparent. 10 The light needed to operate the solar cell can then pass through the transparent material into the active zone. From intermediates, such as for example p- conducting phthalocyanines and n-conducting perylenes, molecules are 15 synthesised which have a p-conducting side and an nconducting side. The resulting molecules are evaporated onto the evaporated p-conducting material. 20 Alternatively they are dissolved in a solvent and applied in layers to the substrate coated with the p-conducting material. This for example involves immersing the substrate in the solution and taking it out. 25 Alternatively the substrate is rotating whilst the solution drips onto the surface coated with the p- conducting material.

The substrate may be heated during the application, in order to rapidly evaporate off the solvent.

What remains is the desired layer, in which the stacks as claimed in the claims have formed virtually automatically.

Next an n-conducting material is applied, say for example an n-conducting phthalocyanine or perylene. Onto this is applied an electrically conductive contact material, for example a metal or a transparent, electrically conductive oxide, to make the contact. is In the case of another example a layer of transparent, electrically conductive oxide is applied, say for example evaporated, onto the substrate, followed by the application of n-conducting perylene molecules. Stack20 forming molecules are applied, say for example evaporated, onto the perylene molecules, which are now present as layers. The stacks then grow substantially perpendicular to the layer consisting of perylene. The n-conducting stacks, or n-conducting sides, of the 25 stacked molecules then form an ohmic contact with the perylene layer. The p-conducting stacks, or pconducting sides of the stacked molecules, then form a p-n contact 11 with the perylene layer. A p-conducting top layer is applied to the stacked molecules. The n-conducting stacks, or n-conducting sides of the stacked molecules, then form a p-n contact with the p-conducting top layer.

The p-conducting stacks, or p-conducting sides of the stacked molecules, then form an ohmic contact with the p conducting top layer.

To make the contact, a metal such as for example gold may again be evaporated onto the p-conducting top layer, and forms an ohmic contact with the p-conducting layer. A second transparent contact may also again be applied here, consisting of a conductive oxide.

is Noble metals are the preferred metals for the contacts, for they are resistant to corrosion. The aim is to provide an electrical contact that will be stable over a long period of time.

Embodiments of the invention will now be explained in detail with the help of examples:

Figures la and 1b are plan views showing a pentamer molecule made up of covalently linked units of phthalocyanine molecules that have n-conducting and p conducting sides. In Figure 1b the p-conducting and n conducting sides of the molecule, which form rectifying diodes with respect to one another, are symbolised. In 12 the centre is an n-conduct-Ling side (region). Four pconducting sides are grouped around the n-conducting side.

Figure 2a shows a section through a layer system comprising an nconducting top layer 1, an intermediate layer 2 and a p-conducting substrate layer 3. The intermediate layer is built up from the n-p-npentamer molecules shown in Figure 1. The layer system is a 10 central component of a solar cell in accordance with the invention. Figure 2b shows clearly the arrangement of the molecules in the layer. The nconducting top layer is symbolically is constituted from three nconducting molecules. Below these are symbolised three of the molecules shown in Figure ib, arranged in a stack. The section thus shows an nconducting stack constituted from three n-sides stacked on top of one another. Depicted to the left and 20 right of the n-conducting stack are p-conducting stacks. These are constructed from two of the four pconducting sides of molecules seen in Figure 1 (1a, lb). The n-conducting stack is connected via an ohmic contact 25 to the n-conducting top layer 1. The two p-conducting stacks each form a p-n contact with the top layer 1. The contact with the p-conducting substrate layer takes place the other way around. In Figure 2b corresponding symbols for resistance illustrate how the contact is made.

Within the layer 2 the stacks are arranged perpendicular to the surface of the layer. The superposed phthalocyanine units are linked by symbols for resistance in the molecule stacks. 10 Figures 3a and 3b show a dimer molecule. This has covalently linked units of phthalocyanine molecules which form n-conducting sides and p-conducting sides of a stack (Figure 3a). The sides are determined via the terminal 15 groups and/or the central atoms. Figure 3b shows its schematised view as p-conducting and nconducting units which form rectifying diodes with respect to one another. By way of example the case is shown here in which the nmolecule carries long aliphatic chains as terminal 20 groups, which bring about the growth of the stack. Figure 4 shows another layered construction consisting of dimer units, here produced by orientating the dimers depicted in Figure 3b on top of one another via the 25 terminal groups in the case where the dimer stacks are statistically assembled in the layer. The layer construction for example corresponds to that in Figure 14 2a.

The size of the interface between the p-conducting and n conducting material will now be estimated. If we assume that within a molecule stack as seen in Figure lb, with an edge length of approx. 5 nm a respective four "active" contacts (zones or regions) develop with a lateral length of approx. 1.5 nm, this produces a contact surface of one square centimetre on a base surface of 1 cm.2 even in the case of a layer just 4 nm thick. This denotes a "roughness factor" of 1000 even in the case of an overall layer thickness of just 4 mm (or a factor of 250,000 in the case of a layer 1 cm thick. what is meant by roughness factor is the ratio of the area of active is contact of the molecule stacks to the area of a p-n junction orientated parallel to the substrate under the prior art.

In one example the n-sides and p-sides in a phthalocyanine stack are arranged alongside one another in such a way that situated alongside each n-side are four p-sides which are insulated from all the other stacks. Contact with the n-sides and p-sides, which ultimately in turn represent n-stacks and p-stacks, is in effect made separately, by using selectively barrier forming materials to make the contact.

The contact may for example be made via thin layers of n conducting perylenes on the one hand (for example on the substrate below the molecule stacks) and p-conducting phthalocyanines on the other hand (for example above the molecule stacks). Since these form a blocking contact to a respective side of the p-n molecules, the result is that the respective side of the molecule stack is insulated.

It is also possible to first apply a p-conducting phthalocyanine and then an n-conducting perylene.

The synthesis procedure can be divided into three parts:

9 synthesis of the intermediates construction of the pentameric phthalocyanine molecule a synthesis of the n-complex stacks It is also possible to force the orientation of the stacks by the use of an electric field, for the positive and negative charges produce a dipole moment which can be utilised for alignment if necessary.

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Claims

1. Solar cell with at least one active zone constituted f rom molecules stacked on top of one another, the molecules having at least one n-conducting side and at least one p-conducting side and the molecules being arranged on top of one another in such a way that the p-conducting and n-conducting sides each develop stacks of their own.

2. Solar cell according to claim 1, wherein the stack or stacks consist of phthalocyanine molecules.

3. Solar cell according to either of the preceding claims, wherein the stack or stacks of molecules are contacted by selectively barrier-forming materials.

4. Solar cell according to any of the preceding claims, wherein the molecule stacks form a layer in which the stacks are arranged perpendicular to the surface of the layer.

5. Method for operating the solar cell having the features contained in any of the preceding claims, wherein the cell is positioned in the light in such a way that incident light travels through the stacked molecules.

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6. Method for manufacturing the solar cell having the features contained in any of the apparatus claims, comprising applying a selectively barrier-forming material in layers onto a substrate, next applying molecules having a p-conducting side and an n conducting side in layers onto the applied selectively barrier-forming material, and then applying in layers a further selectively barrier forming material onto the molecules applied in layers, which molecules have a p-conducting side and an n-conducting side.