Patent Application
20120152303
The invention relates to a photoactive component, especially an organic solar cell or a photodetector, with a layer arrangement comprising an electrode and a counter-electrode and a sequence of organic layers arranged between the electrode and the counter-electrode.
Since the demonstration of the first efficient organic solar cell with an efficiency in the percentage range by Tang et al. 1986 (C. W. Tang et al., Appl. Phys. Lett. 48, 183 (1986)), organic materials have been investigated intensively for a variety of electronic and optoelectronic components. Organic solar cells consist of a series of thin layers, which are typically between 1 nm and 1 μm thick, of organic materials, which are vapour-deposited in a vacuum or applied from a solution. The electric contacts are as a rule provided by transparent, semitransparent or non-transparent layers of metal and/or transparent conductive oxides (TCOs) and/or conductive polymers.
The advantage of such organic-based components over the conventional inorganic-based components, e.g. semiconductors such as silicon or gallium arsenide, is the optical absorption coefficients, which can sometimes be extremely high and reach as much as 3×105 cm−1, so that the possibility is created of producing very thin solar cells with little material and energy input. Other technological aspects are the low costs, the possibility of producing flexible large-area components on plastic films, and the virtually unlimited possible variations available in organic chemistry.
In the following, three key points will be explained, which constitute central technical problems in the development and successful economic exploitation.
A solar cell converts the energy of light into electrical energy. In contrast to inorganic solar cells, free charge carriers are not created directly by the light in the case of organic solar cells, but instead bound Frenkel excitons first form, which are electrically neutral excitation states in the form of bound electron-hole pairs. These excitons can only be separated by very powerful electric fields or at suitable interfaces. Sufficiently powerful fields are not available in organic solar cells, so that all the promising concepts for organic solar cells are based on the separation of excitons at photoactive interfaces (Organic Donor-Acceptor Interface—C. W. Tang, Applied Physics Letters, 48 (2), 183-185 (1986)) or Inorganic Semiconductor Interface (cf. B. O'Regan et al., Nature 353, 737 (1991)). For this, it is necessary for excitons generated in the bulk of the organic material to be able to diffuse to this photoactive interface.
The low-recombination diffusion of excitons to the active interface therefore plays a critical role in the case of organic solar cells. In order to make a contribution to photoelectric current, the exciton diffusion length in a good organic solar cell must therefore be at least in the same range as the typical penetration depth of light so that the greater part of the light can be exploited. Organic crystals or thin films which are perfect in terms of their structure and chemical purity certainly satisfy this criterion. For large-scale applications, however, it is not possible to use monocrystalline organic materials, and the production of multiple layers with sufficient structural perfection is still very difficult, even today.
Instead of increasing the exciton diffusion length, it is also possible to reduce the mean distance from the closest interface. Document WO 00/33396 proposes the creation of an interpenetrating network: a layer contains a colloidally dissolved substance, which is distributed in such a way that a network forms via which the charge carriers can flow (percolation mechanism). In a network of this kind, the task of light absorption is performed either by only one of the components or by both.
The advantage of a mixed layer of this kind is that the excitons produced only have to travel a very short distance before they reach a domain boundary, where they are separated. The electrons and holes are transported away separately in the dissolved substance or in the rest of the layer. Since the materials in the mixed layer are in contact with one another everywhere, it is decisive with this concept that the separated charges should have a long life on the material concerned and that closed percolation paths are available from every location for both charge carrier locations to the respective contact. With this approach, it was possible to achieve efficiencies of 2.5% for polymer-based solar cells produced by wet-chemical means (C. J. Brabec et al., Advanced Functional Material 11, 15 (2001)), while polymer-based tandem cells already have an efficiency of more than 6% (J. Y. Kim et al., Science 13, 222-225 (2007)). Other known approaches for achieving or improving the properties of organic solar cells are listed below:
As described above, an inherent difficulty with organic solar cells is the fact that the exciton diffusion lengths in the organic absorber materials are in ranges from approx. 10 nm to 40 nm. So that the excitons do not recombine in the absorber layer and the energy in the context of the solar cell is lost, the layer thicknesses of the absorber should be in the same range as the exciton diffusion length.
This strict limitation of the absorber layer thicknesses to size ranges which as a rule are well below 60 nm also always limits the absorption (and hence also the photoelectric current and efficiency) in an organic solar cell. Even the above-mentioned interpenetrating networks can only partially compensate for this problem. It is therefore decisive that absorber materials should be found which either alone or in a combination of more than one material should optimally exploit a broad spectral range of visible light and have powerful absorption characteristics despite the small maximum layer thicknesses.
At present, standard materials used in organic solar cells in research and development are “metal phthalocyanines” (such as copper phthalocyanine, CuPc, or zinc phthalocyanine, ZnPc) and fullerenes (such as C60). Although these materials are known, easy to handle and easy to obtain, they alone will not offer a lasting solution; the reason for this is that, on the one hand, they do not absorb sufficiently strongly and. on the other hand, are only able to exploit a narrow range of the sunlight available. According to the present state of the art, it is possible to absorb preferably in a wavelength range around 450 nm with C60; with ZnPc, it is possible to absorb in a wavelength range around 650-700 nm. A great part of the energy of sunlight with wavelengths between 450 and 650 nm thus remains unused (M. Riede et al., Nanotechnology 19, 424001 (2008)). Furthermore, not all the light can be absorbed in the absorbing ranges, because the thin layers do not absorb sufficiently strongly.
To sum up, it can thus be said that when today's standard materials are used,
All in all, it can therefore be said that it will not be possible to compensate for the problem of the limited separation of excitons because of the short exciton diffusion length with the present absorbers and that new materials will be necessary.
Another major field of problems in research and development regarding organic solar cells is the subject of suitable energy levels. If the charge carriers produced are to be transported away efficiently, there must be no energy barriers between the absorber materials and the electric contacts of a solar cell. With the p-i-n architecture, or with doped organic layers, it is possible to a certain extent to ensure that energy barriers are reduced and that the electrons and holes generated are transported away well (K. Walzer et al., Chemical Reviews 107(4), 1233-1271 (2007); C. Falkenberg et al., Journal of Applied Physics 104, 034506 (2008); S. Pfützner et al., Proceedings of SPIE 6999, 69991M (2008); C. Uhrich et al., Journal of Applied Physics 104, 043107 (2008); J. Drechsel et al., Applied Physics Letters 86, 244102 (2005)). This method is well-known by now and has been tried and tested for the materials ZnPc and C60. Since, however, as described above, ZnPc and C60 do not have sufficiently good properties, it can be said in this case too that there is an urgent need for other materials, which not only possess good absorber properties, but also need to have suitable energy levels so that, in combination with doped transport layers, high open-circuit voltages and filling factors are ensured.
A key aspect in this connection is the production of tandem, triple or multiple cells in general, consisting of a stack of a plurality of solar cells, so that the multiple cell as a whole can absorb in a broad spectral range thanks to different absorber materials, each of which only absorb a specific part of the spectrum. In this way, it is possible to a certain extent to circumvent the problem of the only limited exciton diffusion length, since a multiple solar cell can be regarded as a layered stack of plural solar cells (so-called subcells), in which a plurality of absorber layers can co-operate. In this way, considerably higher efficiencies than with single cells can be achieved. It is, however, important in this connection, if a plurality of materials are used which have similar absorber characteristics (i.e. they absorb at similar wavelengths), that the layers also take photons away from one another and limit one another as a result of the fact that photons absorbed by one subcell are no longer available to other subcells. This problem can only be avoided if different absorbers are used which are complementary to one another and absorb in different wavelength ranges. The tandem/multiple-cell technology thus also makes it clear once again that a broad spectral range is necessary for efficient solar cells. Tandem/multiple cells made from the materials C60 and ZnPc are therefore not a solution.
In order to be able to satisfy the above-mentioned requirements regarding exciton diffusion length, energy levels and multiple cells, it is therefore urgently necessary to find new absorber materials which can fill the absorption gap between C60 and ZnPc, which have a strong absorption and have favourable energy levels (Highest Occupied Molecular Orbital [HOMO], Lowest Unoccupied Molecular Orbital [LUMO]). With the present state of the art, it is not possible to achieve sufficient efficiencies, which would be necessary in order to satisfy the economic and technological requirements of organic photovoltaic systems.
The present state of the art in the case of absorber molecules in organic photovoltaic systems for filling the gap between C60 and ZnPc is the class of substances of dicyanovinyl oligothiophenes (DCVTs) (K. Schulze et al., Advanced Material 18, 2872 (2006)), shown in
Employees at Sanyo reported on tetraphenyl dibenzoperiflanthene as a donor material in organic solar cells (Fujishima et al., Solar Energy Material and Solar Cells 93, 1029 (2009); identical to Kanno et al., Proc. PVSEC-17). They succeeded in creating a simple solar cell consisting of the organic materials tetraphenyl dibenzoperiflanthene, C60 and 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline. Thanks to the strong absorption of tetraphenyl dibenzoperiflanthene, Fujishima and Kanno achieved an efficiency of 3.56% for 0.033 cm2 and 2.58% for 1.60 cm2, though it is not clearly stated whether the data were already checked for a spectral mismatch. Despite this result, it is nevertheless clear that with the system chosen by Fujishima and Kanno, no further efficiency increase can be expected: ultimately, the absorption of the C60-tetraphenyl dibenzoperiflanthene compound limits the photoelectric current and photovoltage; it would be necessary to use doped dedicated charge carrier transporters in order to arrive at higher filling factors. Single cells with tetraphenyl dibenzoperiflanthene therefore do not provide a solution for the above-mentioned requirements.
The problem of the invention is to use a suitable thermally stable material, which is easy to synthesise, in order to satisfy the requirements described above with regard to achieving greater efficiencies in such a way that
This problem is solved by an organic photoactive component in accordance with claim 1 and the use of such components in accordance with claims 31 and 32. Preferred embodiments can be found in the dependent claims.
This problem is solved in accordance with the invention by an organic photoactive component, especially a solar cell or photodetector composed of a plurality of layers, wherein at least one of the layers comprises at least one di-indeno[1,2,3-cd:1′,2′,3′-lm]perylene compound of the general formula
wherein each R1-R16 is independently selected from hydrogen, halogen, unsubstituted or substituted, saturated or unsaturated C1-C20-alkyl, C1-C20-heteroalkyl, C6-C20-aryl, C6-C20-heteroaryl, saturated or unsaturated carbocycle or heterocycle, which may be the same or different. In addition, two adjacent radicals R1-R16 may be part of a further saturated or unsaturated, carbocyclic or heterocyclic ring or chain, wherein the ring or chain may comprise C, N, O, S, Si and Se. In the following, a material corresponding to the above description will be abbreviated to “di-indenoperylene compound”.
Advantageous embodiments are the subject matter of dependent claims. One subject matter of a further invention in this context is that the di-indeno[1,2,3-cd:1′,2′,3′-lm]perylene compounds mentioned are particularly advantageously combined with doped transport layers for electrons and holes. These surprisingly result in extremely high filling factors, which are not otherwise reported in organic solar cells.
A further, dependent invention is tandem solar cells with the above-mentioned di-indeno[1,2,3-cd:1′,2′,3′-lm]perylene compounds. It has surprisingly been found in this context that with the appropriate substitution, spectral absorption can be achieved, so that together with the known class of substances of phthalocyanines, there is no major overlap, and the two subcells do not in each case reduce the current of the other cells.
In accordance with the invention, the di-indenoperylene compound is used as a light-absorbing material in photoactive components, especially organic solar cells. The optical density of dibenzoperiflanthene, for example, as shown in
Other derivatives can be synthesised in a targeted manner in this way, such that their absorption is adapted precisely to the respective requirements. Specific examples here are (in the order from higher to lower wavelengths) 1,4,9,12-tetraphenyl-di-indeno[cd:lm]perylene, 2,3,10,11-tertaethyl-1,4,9,12-tetraphenyl-di-indeno[cd:lm]perylene and 2,3,10,11-tertabutyl-1,4,9,12-tetraphenyl-di-indeno[cd:lm]perylene. These derivatives absorb at even lower wavelengths in each case (shifted even further into the blue), as is shown further down in
Preferred applications of the invention are tandem, triple or multiple solar cells in general, in which the molecule is used as an absorber material. It is advantageous in the invention to provide further organic layers in order to optimise the energy levels between the absorber layer and the electric contact of the solar cell in a targeted way, so that with an efficient charge transport, high photoelectric currents, photovoltages and filling factors can be achieved. Advantageous applications of the invention therefore comprise the combination of the absorber materials with doped, non-absorbing or doped, absorbing organic materials. Advantageous applications of the invention in the use in tandem cells comprise the use of heavily doped layers as conversion contacts.
Examples of materials for the electric base contact are metals (for example, but not limited to: aluminium or silver), conductive polymers (for example, but not limited to: polyethylene dioxythiophene):poly(styrene sulphonate) [PEDOT:PSS]) or transparent conductive oxides (for example, but not limited to: aluminium-doped zinc oxide, tin-doped indium oxide, fluorine-doped tin oxide) or combinations of metal, conductive polymer or transparent conductive oxide.
Preferred examples of the materials conducting positive charges are 4,4′,4″-tris(1-naphthylphenylamino)-triphenyl amine (TNATA), N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (alpha-NPD), 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF), 4,4′-bis-(N,N-diphenylamino)-quaterphenyl (4P-TPD), N,N′-diphenyl-N,N′-bis(4′-(N,N-bis(naphth-1-yl)-amino)-biphenyl-4-yl)-benzidine (Di-NPB), N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD). Materials conducting positive charges may also be referred to as hole transport materials, which can be used in a hole transport layer (HTL), see also
An advantageous embodiment of the invention contains materials in the HTL which serve as dopants (acceptors) for the materials which preferably conduct positive charges (holes). An example of this is: 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ).
Preferred examples of the materials conducting negative charges are 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) or Buckminster fullerenes (C60). An advantageous embodiment of the invention contains materials in the ETL which serve as dopants (donors) for the materials which preferably conduct negative charges (electrons). An example of this is: (N,N,N′,N′-tetramethylacridine-3,6-diamine) (AOB). Materials that conduct negative charges are also referred to as electron transport materials, which can be used in electron transport layers (ETL).
Examples of p-dopants are phthalocyanines, especially, but not limited to zinc phthalocyanines (ZnPc), copper phthalocyanines (CuPc); Buckminster fullerenes (e.g. C60 or C70); dicyanovinyl-oligothiophene derivative (DCVxT); chlorine-aluminium phthalocyanine (ClAlPc or also AlClPe); perylene derivatives. An advantageous embodiment of the invention contains materials in the active layer which serve as dopants for the light-absorbing materials.
Preferred examples of heavily doped materials are bathocuproine (BCP) or 4,7-diphenyl-1,10-phenanthroline (BPhen).
Preferred examples of materials which absorb photons are SiN, SiO2.
Preferred examples of materials which absorb photons and are applied in a mixed layer are N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) or tris(8-hydroxy-quinolinato)-aluminum (Alq3).
Preferred examples of materials for an exciton-blocker layer are TiO2 or SiO2.
The invention is based on the surprising finding, obtained by experiment, that di-indenoperylene compounds and derivatives are characterised not only by powerful absorption and thermal stability, but, in combination with heavily doped hole-transport materials, can keep energy harriers to a minimum, which leads to very high filling factors. In addition, experiments with tandem cells have shown that high photovoltages can be obtained when the spectral sensitivities of different materials are combined in a suitable way. A decisive factor here is that the materials should have suitable bandgaps in order to be able to optimise the absorption and energy levels.
It is thus clear that tetraphenyl dibenzoperiflanthene—incorporated in an appropriate material system—is a suitable absorber for constructing solar cells efficiently. This is due to the easy synthesis and
Organic electronic devices, such as organic semiconductors, can be used to fabricate simple electronic components, e.g. resistors, diodes, field effect transistors, and also optoelectronic components like organic light emitting devices (e.g. organic light emitting diodes (OLED)), and many others. The industrial and economical significance of the organic semiconductors and their devices is reflected in the increased number of devices using organic semiconducting active layers and the increasing industry focus on the subject.
OLEDs are based on the principle of electroluminescence in which electron-hole pairs, so-called excitons, recombine under the emission of light. To this end the OLED is constructed in the form of a sandwich structure wherein at least one organic film is arranged as active material between two electrodes, positive and negative charge carriers are injected into the organic material and a charge transport takes place from holes or electrons to a recombination zone (light emitting layer) in the organic layer where a recombination of the charge carrier to singlet and/or triplet excitons occurs under the emission of light. The subsequent radiant recombination of excitons causes the emission of the visible useful light emitted by the light-emitting diode. In order that this light can leave the component at least one of the electrodes must be transparent. Typically, a transparent electrode consists of conductive oxides designated as TCOs (transparent conductive oxides), or a very thin metal electrode; however other materials can be used. The starting point in the manufacture of an OLED is a substrate on which the individual layers of the OLED are applied. If the electrode nearest to the substrate is transparent the component is designated as a “bottom-emitting OLED” and if the other electrode is designed to be transparent the component is designated as a “top-emitting OLED”. The layers of the OLEDs can comprise small molecules, polymers, or be hybrid.
The most reliable and efficient OLEDs are OLEDs comprising doped layers. By electrically doping hole transport layers with a suitable acceptor material (p-doping) or electron transport layers with a donor material (n-doping), respectively, the density of charge carriers in organic solids (and therefore the conductivity) can be increased substantially. Additionally, analogous to the experience with inorganic semiconductors, some applications can be anticipated which are precisely based on the use of p- and n-doped layers in a component and otherwise would be not conceivable. The use of doped charge-carrier transport layers (p-doping of the hole transport layer by admixture of acceptor-like molecules, n-doping of the electron transport layer by admixture of donor-like molecules) in organic light-emitting diodes is, e.g., described in US 2008/203406 and U.S. Pat. No. 5,093,698.
8,9-dibutyl-7,10-diphenyl fluoranthene: 3.56 g acecyclone (10 mmol), the same amount of 5-decin and 20 mL xylene were heated for 16 h in a sealed ampoule to 250° C. After all the volatile components had been removed by distillation, the residue was extracted from a layer of silica gel K60 with pentane. 2.91 g (6.24 mmol, 62% of theory) of a slightly yellowish solid were obtained. C36H34 Mw=466.66 g/mol. Elemental analysis: C, 92.22%; (adj. 92.66%), H, 7.42%; (adj. 7.34%).
ESI-MS (0.5 mM NH4COOH, +10 V): 467.3 (100) [M+H+], 950.6 (80) [2M+NH4+]. 1H-NMR (500 MHz, CDCl3): 7.61 (d, 3J=7.8 Hz, 1H), 7.60-7.52 (m, 3H), 7.48-7.46 (m, 2H), 6.26 (d, 33-6.8 Hz, 1H), 2.55 (t, 3J=8.4 Hz, 2H), 1.47 (quin., 3J=7.3 Hz, 3J=8.4 Hz, 2H), 1.21 (sex., 33-7.4 Hz, 3J=7.3 Hz, 2H), 0.77 (t, 3J=7.4 Hz, 3H). 13C-NMR (125 MHz, CDCl3): 140.7, 138.8, 137.9, 137.0, 135.2, 132.8, 129.4, 128.8, 127.5, 127.3, 125.8, 122.4, 33.6. 29.8, 23.2. 13.6.
2,3,10,11-tertabutyl-1,4,9,12-tetraphenyl-di-indeno[cd:lm]perylene; 3.8 g iron(III) chloride in 6 mL nitromethane were added drop-wise to a deoxygenated solution of 0.933 g 8,9-dibutyl-7,10-diphenyl fluoranthene in 40 mL dichloromethane and then stirred for 5 min. Nitrogen was introduced constantly throughout. After the addition of 60 mL methanol, the mixture was filtered, and the solid was washed with methanol until the wash solution was colourless. The product was obtained in an amount of 0.867 g (1.87 mmol, 92% of theory) as a purple powder. C36H34 Mw=926.30 g/mol. Elemental analysis: C, 92.18%; (adj. 93.06%), H, 6.95%; (adj. 6.94%).
ESI-MS (0.5 mM NH4COOH, +10 V): 929.5 (100) [M+H+], 872.5 (23) [M+H+−C4H9]. 1H-NMR (500 MHz, CDCl3): 7.66 (d, 3J=7.7 Hz, 1H), 7.57-7.51 (m, 3H), 7.44-7.43 (m, 2H), 6.14 (d, 33-7.7 Hz, 1H), 2.49 (t, 3J=8.4 Hz, 2H), 1.45 (quin., 3J=8.4 Hz, 3J=7.6 Hz, 2H), 1.18 (sex., 3J=7.6 Hz, 3J=7.3 Hz, 2H), 0.74 (t, 3J=7.3 Hz, 3H). 13C-NMR (125 MHz, CDCl3): 140.5, 138.9, 138.0, 136.9, 135.3, 133.9, 129.9, 129.4, 128.7, 127.3, 124.9, 123.2, 121.4, 33.6, 29.7, 23.2, 13.6.
(The figures in the text refer to Illustration 11)
Documentation of an organic solar cell with a di-indenoperylene derivative (more precisely: dibenzoperiflanthene as a preferred example) as the absorber material using p-doped charge carrier transport layers. The objective here was to obtain a combination of a high photoelectric current and high photovoltage.
A sample was produced on glass (0), with a transparent earthing electrode of tin-doped indium oxide (ITO, 1), with a 1-nm-thick layer of a p-dopant or acceptor material, such as NDP9 (Novafed AG) (2), followed by a 25-nm-thick layer of N,N′-diphenyl-N,N′-bis(4′-(N,N-bis(naphth-1-yl)-amino)-biphenyl-4-yl)-benzidine (Di-NPD), p-doped with 5% of a p-dopant, such as NDP9, (3). The light-absorbing layers were applied on top: 6 nm dibenzoperiflanthene (4), 30 nm mixture of dibenzoperiflanthene with C60 (mixing ratio 2:3) (5), 35 nm C60 (6), followed by an exciton-blocker layer of 6 nm 4,7-diphenyl-1,10-phenanthroline (BPhen) (7) and 100 nm aluminium as the back contact (8).
In characterising the samples (the characteristic curves are shown in Ill. 12), it is noticeable that even without the absorber ZnPc, which is otherwise standard, a high photoelectric current of 8.08 mA/cm2 can be achieved. The conclusion to be drawn from this is that C60 and dibenzoperiflanthene complement each other in their absorption characteristics and do not withdraw any photons from each other. A filling factor of 43.1% shows that the energy levels are not yet adapted optimally to one another, though the focus in this sample was on the photoelectric current. The high open-circuit voltage of 0.905V is almost twice as high as the voltage of conventional ZnPc:C60 systems, which indicates a favourable position of the energy levels between the absorber materials. All in all therefore, it is possible, by using di-indenoperylene in a simple cell structure, to achieve a notable efficiency of 3.15%, which is higher than the efficiencies of comparable C60:ZnPc solar cells (typical values here are 2-2.5%, see K. Walzer et al., Chemical Reviews 107(4), 1233-1271 (2007); C. Falkenberg et al., Journal of Applied Physics 104, 034506 (2008)). It was possible to achieve this figure here without absorption at wavelengths above approx. 650 nm, so that higher values can be expected if suitable red absorbers are added.
(The figures in the text refer to illustration 13)
Documentation of an organic solar cell with a di-indenoperylene derivative (more precisely: dibenzoperiflanthene) as the absorber material. The objective here was to obtain a combination of a high filling factor and high photovoltage.
A sample was produced on glass (0), with a transparent earthing electrode of tin-doped indium oxide (ITO, 1), with a 1-nm-thick layer of a p-dopant or acceptor material, such as NDP9 (Novaled AG) (2), followed by a 25-nm-thick layer of N,N′-diphenyl-N,N′-bis(4′-(N,N-bis(naphth-1-yl)-amino)-biphenyl-4-yl)-benzidine (Di-NPD), p-doped with 5% of a p-dopant, such as NDP9, (3). The light-absorbing layers were applied on top: 20 nm dibenzoperiflanthene (4), 35 nm C60 (5), followed by an exciton-blocker layer of 6 nm 4,7-diphenyl-1,10-phenanthroline (BPhen) (6) and 100 nm aluminium as the back contact (7).
The characteristic curve is shown in Illustration 14. The sample has an open-circuit voltage VOC=0.93V, a photoelectric current of ISC=4.54 mA/cm2 and a very high filling factor of 70.2%, which leads to a high efficiency of 2.96%. This provides the evidence that even extremely high filling factors can be obtained by a targeted choice of the layer structure, which is a great advantage for applications in tandem cells or multiple cells.
(The figures in the text refer to Illustration 15)
Documentation of an organic solar cell with a di-indenoperylene derivative (more precisely: dibenzoperiflanthene) as the absorber material in a multiple cell (here: a tandem cell, consisting of two subcells). The aim in this context was to demonstrate in principle that C60, ZnPc and di-indenoperylenes can be combined in a cell structure and that a combination of a high voltage and a high filling factor can be achieved.
A sample was produced on glass (0), with a transparent earthing electrode of tin-doped indium oxide (ITO, 1), with a 1-nm-thick layer (of a p-dopant or acceptor material, such as NDP9 (Novaled AG) (2), followed by a 25-nm-thick layer of N,N′-diphenyl-N,N′-bis(4′-(N,N-bis(naphth-1-yl)-amino)-biphenyl-4-yl)-benzidine (Di-NPD), p-doped with 5% of a p-dopant, such as NDP9, (3). That was followed by the absorber layer of the first subcell: 25 nm ZnPc:C60 (ratio 1:1) (4). After that, a “conversion contact” was used to obtain an efficient, low-loss recombination of 5 nm C60 (n-doped with an n-dopant, such as NDN1, Novaled AG, Dresden) (5) and 10 nm p-doped di-NPD (doped with 5% NDP9) (6). That was followed by a layer of 5 nm 4,4′-bis-(N,N-diphenylamino)-quaterphenyl (4P-TPD) (7) for a barrier-free charge carrier transport to the conversion contact. The second subcell consisted of 25 nm dibenzoperiflanthene (8), 30 nm C60 (9), followed by an exciton-blocker layer of 6 nm 4,7-diphenyl-1,10-phenanthroline (BPhen) (10) and 100 nm aluminium as the back contact (11).
The characteristic curve is shown in Illustration 16. This tandem solar cell has a high filling factor of FF=59.1%. The photoelectric current is still relatively low, at ISC=3.25 mA/cm2, which is due to the fact that no optimisation has been performed yet: in tandem cells the cell with the lower photoelectric current always limits the current of the overall cell, so that the two subcells have to be matched to one another optimally. All in all, embodiment 3 is a successful example of a tandem cell, since the photovoltage, at 1.38V, corresponds approximately to the sum of the voltages of single cells (typical C60:ZnPc—single cell: VOC=0.5V; solar cells from embodiments 1 and 2 approx. VOC=0.9V; the sum of the two subcells is thus approx. 1.4V, which is approximately reached by the tandem cell). As a result, with this non-optimised application, an efficiency of 3.25% was achieved.
The figures in the text refer to Illustration 8.
Documentation of an organic solar cell with a di-indenoperylene derivative (more precisely: 2,3,10,11-tertabutyl-1,4,9,12-tetraphenyl-di-indeno[cd:lm]perylene as the absorber material. The aim here was to achieve high filling factors and open-circuit voltages in a single cell without using a bulk heterojunction (mixed layer).
A sample was produced on glass (0), with a transparent earthing electrode of tin-doped indium oxide (ITO, 1), a 25-nm-thick absorber and electron transport layer of C60 (2), a 25-nm-thick layer of a di-indenoperylene derivative (more precisely: 2,3,10,11-tertabutyl-1,4,9,12-tetraphenyl-di-indeno[cd:lm]perylene, already described above in synthesis example 2) (3), followed by a 40-nm-thick layer of the hole-transport material BPAPF (9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene), which is heavily p-doped with 20% by weight of a p-dopant, such as NDP9 (Novaled AG, Dresden) (4), followed by 10 nm ZnPc, p-doped with 2.5% by weight (5), followed by a 40-nm-thick layer of gold as the back electrode (6).
The characteristic curve is shown in Illustration 17. In charactering the cell, it is noticeable that by using hole-transport materials which are suitable from the energy point of view, combined with heavy p-doping, an extremely high filling factor of more than 69% is obtained. Filling factors on this level are not otherwise reported for organic solar cells. The filling factor and voltage (0.99 V) show that the use of dedicated charge carrier transport layers and dopants is of decisive importance. This makes it possible to adjust the energy levels precisely and thus to reduce energy barriers considerably. Only in this way can both high photovoltages and high filling factors be achieved, while keeping the series resistance low, which is necessary for efficient solar cells.
The features of the invention disclosed in the claims, the description and the drawings can be essential to implementing the invention in its various embodiments both individually and in any combination.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 022 408.4 | May 2009 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE2010/000536 | 5/17/2010 | WO | 00 | 3/9/2012 |
