CA2655294A1 - Method for producing photoactive layers and components comprising said layers - Google Patents

Abstract

The invention relates to a method for producing photoactive layers, and to components such as solar cells or photodetectors comprising said layer. The photoactive layer is produced according to the invention by synthesizing inorganic semiconductor particles with microwave irradiation, photoactive layers being formed from said particles in combination with organic semiconductor compounds.

Description

METHOD FOR PRODUCING PHOTOACTIVE LAYERS AND COMPONENTS
COMPRISING SAID LAYERS

The invention relates to a process for the production of photoactive layers that comprise inorganic semiconductor particles as well as organic semiconductor compounds as well as components that comprise this (these) layer(s).
Photoactive layers are functionally significant components of photoactive elements, such as solar cells or photodetectors. The nanoparticles that are integrated in the photoactive layers in this case quite significantly influence the degree of efficiency of the photoactive elements.
The invention is primarily important in the field of inorganic-organic hybrid solar cells. The design of such hybrid solar cells can be described as follows based on Figures 1 to 4:
The photovoltaic cell consists of a transparent carrier 1, which preferably consists of glass or a polymer such as polyethylene terephthalate (PET). A transparent electrode layer 2 that consists of a conductive oxide, for example indium tin oxide (ITO), a transparent conductive polymer or another transparent material with high conductivity is applied on the carrier. This electrode layer in general has a comparatively rough surface structure, so that it optionally is covered with a smoothing layer 3 that consists of a polymer that is made electrically conductive by a doping, usually PEDOT:PSS
(polyethylene dioxythiophene:polystyrene sulfonate). A photoactive layer 4 that consists of semiconductor particles and an organic semiconductor matrix with a layer thickness, depending on the application process, of, for example, 100 nm to several micrometers can be applied directly on the smoothing layer 3.
Relative to the photoactive layer in such solar cells, as described in the literature -see, for example, C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct.
Mater. 11 (2001) 15-26) - two concepts are available, namely the bulk heterojunction concept, see Figure 1: diagrammatic representation, as well as Figure 3: diagram without PEDOT:PSS
(polyethylene dioxythiophene:polystyrene sulfonate) and the bilayer heterojunction concept, see Figure 2: diagram, as well as Figure 4: diagram without PEDOT:PSS. In the case of the bulk heterojunction concept, the photoactive layer consists of a mixture of an electroactive polymer and semiconductor particles or low-molecular electroactive molecules and semiconductor particles 4. In the bilayer heterojunction concept, the photoactive layer consists of an electroactive organic layer 6 and a subjacent inorganic semiconductor layer 7. In principle, the two systems can also be combined with one another.
The production of the hybrid solar cells is concluded with the application of metal electrodes. Frequently used electrode materials are silver, aluminum, gold or a combination of calcium and aluminum, calcium and gold, magnesium and gold.
The function of semiconductor particles in hybrid solar cells is described in the literature, see E. Arici, N. S. Sariciftci, D. Meissner, Adv. Funct. Mater. 13 (2003) 165-171) in the example of CuInS2 nanoparticles. Here, the nanoparticles increase the efficiency of polymer solar cells by preventing a recombination of the generated charge carriers and by carrying out the transport of the negative charge carriers to the electrodes.
The use of additional semiconductor particles is known from the literature. A
few are mentioned by way of example below: W. U. Huynh, J. J. Dittmer, and A. P.
Alivisatos thus describe in Science 2002, 295, 2425 the use of CdSe nanoparticles; N. C.
Greenham, X. Peng, and A. P. Alivisatos thus describe in Physical Review B 1996, 54, 17628 the use of CdS nanoparticles; W. J. E. Beek, M. M. Wienk, and R. A. J. Janssen thus describe in Advanced Materials 2004, 16, 1009 the use of ZnO nanoparticles; and K. M.
Coakley, Y.
X. Liu, C. Goh, and M. D. McGehee thus describe in Mrs Bulletin 2005, 30, 37 the use of Ti02 nanoparticles.
The inorganic semiconductor nanoparticles that are necessary for the production can be produced with the most varied methods. Such methods are, for example, colloidal syntheses, solvothermal syntheses (high-pressure syntheses in the autoclave), gas phase reactions (chemical vapor synthesis), as well as electrochemical production methods.
Many known processes for the production of semiconductive nanoparticles are relatively expensive, however, from the processing standpoint. On the one hand, for many colloidal syntheses and gas phase reactions, often complex precursor materials are used, and, on the other hand, solvothermal syntheses are very time-consuming and, based on the high pressure that is necessary, require reactors that are especially designed for this purpose. Here, the invention is intended to correct this.
According to the invention, a process of the above-mentioned type is proposed, which is characterized in that inorganic semiconductor particles are synthesized under microwave radiation, from which photoactive layers are formed in combination with organic semiconductor compounds.

If these photoactive layers are used in solar cells according to the bulk heterojunction principle, the possibility exists to produce all the material necessary for the photoactive layer (nanocomposite) material under microwave irradiation by the semiconductor particles being produced under microwave irradiation directly in a solution of an electroactive polymer or from electroactive low-molecular molecules.
This polymer-semiconductor particle solution can then be used directly without an additional process step for the production of the active layer of a hybrid solar cell. In addition, the advantage exists that the concentration ratios between a polymer and a nanoparticle portion in the nanocomposite material can be varied as desired.
Advantageous configurations are disclosed according to subclaims.
The invention also relates to components such as solar cells or photodetectors that comprise the photoactive layers that are produced according to the invention.
The advantages of the process according to the invention lie in a significant simplification in the production of semiconductor particles, in particular of semiconductor particles that have a diameter of only a few nanometers. In this connection, this is a very simple and economical process, since by the use of microwave radiation, the reaction time can be greatly shortened and economical starting compounds can also be used. In many cases, the nanoparticle syntheses can be performed starting from the respective elements and simple metal salts. In contrast to this, metal complexes with organic ligands that are expensive and difficult to handle have to be used for many colloidal nanoparticle syntheses. The syntheses of Cd-containing nanoparticles, such as CdS, CdSe and CdTe, are an example of nanoparticle syntheses starting from organometallic precursor compounds, see: C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc.

(1993) 8706.
The semiconductive nanoparticles that are produced via microwave-supported synthesis and that are present in the photoactive layers according to the invention have been exposed to a microwave radiation at least once before or during the production of the layer. In this case, the microwave radiation is characterized by a wavelength of I m to 1 mm and a frequency range of between 0.3 GHz and 300 GHz.
The semiconductive nanoparticles produced by microwave-supported synthesis can be present in the photoactive layer as single particles, as agglomerates of single particles or as percolating networks of single particles or particle agglomerates.

The semiconductor particles can be produced via microwave-supported synthesis methods with stabilizing organic cappers, but also without cappers.
Surfactants that act as stabilizers for the nanoparticles are referred to as cappers. The great advantage of the syntheses without cappers is that the particle surfaces are not surrounded, in most cases, with insulating organic layers, by which satisfactory results can be achieved when using the photoactive layer in a solar cell.
Chalcogenidic particles are important and advantageous semiconductive particles for the production of hybrid solar cells. The process for the production of hybrid solar cells that is described here therefore primarily relates to the use of chalcogenidic semiconductor particles of the type: ABX2, AB5X9, AB5X8, CX, D2X3, and the like, whereby: A = Cu, Ag, Zn, Cd; B= In, Ga, Al; C= Cu, Ag, Zn, Cd, Pb, Hg, Eu, Tm, Yb;
D = Al, In, Ga, Ti, Y, La, B; and X = S, Se, Te.
As additional components in the photoactive layer, organic semiconductor polymers according to the invention are used. This refers to polymers that have a conjugated re-electron system, such as trans-polyacetylene, polypyrrole, polythiophene, polyaniline, poly-p-phenylenevinylene, poly-p-phenylene, polyfluorene, polyaromatic amines, poly(thienylenevinylene) and derivatives thereof.
The invention is explained in more detail below based on embodiments and based on Figures 1 to 10.
Here, Figure 1 shows the diagram of the design of a hybrid solar cell according to the bulk heterojunction principle, Figure 2 shows the diagram of the design of a hybrid solar cell according to the bilayer heterojunction principle, Figure 3 shows the diagram of the design of a hybrid solar cell according to the bulk heterojunction without PEDOT:PSS, Figure 4 shows the diagram of the design of a hybrid solar cell according to the bilayer heterojunction principle without PEDOT:PSS, Figure 5 shows the current/voltage characteristic of a bulk heterojunction solar cell with MEH-PPV (poly[2-methoxy-5-(2-ethyloxy)-p-phenylvinylene]) and CuInSz nanoparticles from a microwave-supported synthesis in dichloromethane, Figure 6 shows the current/voltage characteristic of a bulk heterojunction solar cell with P3HT (poly-3-hexylthiphene) and ZnS nanoparticles from a microwave-supported synthesis in toluene/pyri dine, Figure 7 shows the current/voltage characteristic of a bulk heterojunction solar cell with MEH-PPV and CuInS2 nanoparticles from a microwave-supported synthesis in ethylenediamine, Figure 8 shows the current/voltage characteristic of a bilayer heterojunction solar cell with P3HT and CuInS2 nanoparticles from a microwave-supported synthesis in triethylene glycol/pyridine, Figure 9 shows the current/voltage characteristic of a bulk heterojunction solar cell with MEH-PPV and CuInS2 nanoparticles from a microwave-supported synthesis in triethylene glycol/pyridine, and Figure 10 shows the XRD analysis (x-ray structural analysis) of the ZnS
nanoparticles produced according to Example 2.

Example 1:
Production of hybrid solar cells with CuInS2 nanoparticles produced under microwave irradiation in dichloromethane:
The synthesis is performed with the reactants Cul (I equivalent), InC13 (1 equivalent) and thioacetamide (2.2 equivalents) in dichloromethane as a solvent. The reaction parameters are: 180 C, 38 bar as well as microwave radiation within 15 minutes.
The reaction time can be kept very short by adjusting pressure, temperature and duration as well as intensity of the microwave radiation.
The black, fine-powder particles that are obtained are centrifuged off from the reaction solution, washed and mixed with an MEH-PPV solution. This suspension can now be used as a photoactive layer in a hybrid solar cell.
For example, hybrid solar cells according to the diagram of Figures 1 to 4 can be produced from the synthesized photoactive layers. As a result, hybrid solar cells according to the invention can be implemented both according to the bulk heterojunction concept and also according to the bilayer heterojunction concept.
For the production of solar cells, a portion of the ITO (indium tin oxide) layer is etched off with Zn/HCI, and ITO flakes, i.e., ITO-coated glass carriers: 15 mm x 15 mm x 1.1 mm, are poured into a beaker with isopropanol (p.a.) and purified for 15 minutes in an ultrasound bath at 60 C.

Then, a PEDOT:PSS layer and the photoactive layer that is produced according to the invention are applied.

To this end, the photoactive layer can be applied from a solution or suspension by spin-coating. In the suspension that is used, a polymer concentration (MEH-PPV) of 3 mg/ml and a ratio between a polymer and CuInS2 nanoparticles of 1 to 7 (parts by weight) are present.
Then, the photoactive layer is dried at 150 C in an inert gas atmosphere.
Finally, the metal electrodes, for example aluminum, are applied.
In Figure 5, the current/voltage characteristic of the solar cell produced according to Example 1 is depicted. This indicates a clear photoeffect, whereby the measured photoelectric current is 5.7 A/cm2, and the measured photoelectric voltage is 200 mV.
Example 2:
Production of hybrid solar cells, whose photoactive layer consists of a nanocomposite material made of P3HT (poly-3-hexylthiophene) and ZnS
nanoparticles, which was produced directly under microwave radiation:
According to this example, ZnS nanoparticles are produced under microwave irradiation directly in a solution that consists of an organic semiconductor polymer. For this purpose, anhydrous zinc acetate (1 equivalent) and thioacetamide (1.2 equivalents) were dissolved in a mixture of toluene and pyridine, and P3HT is suspended in this solution as a semiconductor polymer. The reaction is performed in a synthesis microwave oven. Thus, the polymer is completely dissolved without ZnS nanoparticles being formed before the end of the reaction; the reaction mixture is first kept under microwave radiation for 20 minutes at 80 C, then for 10 minutes at 120 C and brought under microwave radiation for 30 minutes at 180 C. By this microwave-supported synthesis method, the suspension for the application of the nanocomposite layer is produced in one step as a photoactive layer in the solar cell. The XRD analysis of the semiconductive particles produced in the polymer solution (see Figure 10) clearly shows - based on the two wide peaks of 27 to 34 and 48 to 55 - that this is nanocrystalline ZnS. Primer crystallite sizes of approximately 3 to 4 nm were found by the Debye-Scherrer analysis.
Using this photoactive layer produced according to the invention, the solar cell was produced analogously to Example 1.
The current/voltage characteristics of this solar cell in dark and illuminated conditions are depicted in Figure 6. The solar cell shows an excellent diode characteristic, a low dark current, a photoelectric current of 54 A/cm2, and a photoelectric voltage of 660 mV.

Example 3:
Production of hybrid solar cells with CuInS2 nanoparticles from a microwave synthesis in ethylenediamine as a solvent and as a stabilizer for the semiconductive nanoparticles that are produced:
The CulnS2 particles were produced directly from the elements Cu (1 equivalent), In (1 equivalent) and S (2 equivalents). As a solvent and simultaneously as a capper, anhydrous ethylene diamine was used. The reaction was performed in a sealed Teflon liner at 160 C for 60 minutes under microwave radiation.
Fine-powder black particles were obtained, which were centrifuged off from the reaction solution, washed and mixed with an MEH-PPV solution. This suspension with a polymer concentration of 3 mg/ml, a ratio of polymer to nanoparticle of 1 to 5 (parts by weight), can now be used as a photoactive layer for hybrid solar cells.
The primary design of this hybrid solar cell is carried out analogously to Example 1.
In Figure 7, the current/voltage characteristic of the solar cell is depicted.
The latter shows a clear photoeffect, whereby the measured photoelectric current is 3.6 A/cm2, and the measured photoelectric voltage is 290 mV.
Example 4:
Production of hybrid solar cells with CuInS2 nanoparticles:
For the production of CuInS2 nanoparticles, CuCI (98 mg, I mmol) is weighed in a Teflon liner (microwave reaction vessel), and 30 ml of triethylene glycol as well as 20 ml of pyridine are added. Then, the mixture is heated for 15 minutes in the synthesis microwave to 180 C. In this case, the CuCl is completely dissolved, and a green solution is produced.
After cooling, metallic indium powder (114 mg, I mmol) and sublimated sulfur (64 mg, 2 mmol) are weighed in a Teflon liner.

The reaction is performed for 40 minutes or 60 minutes at 180 C under microwave irradiation. During the reaction, a slight overpressure builds up in the vessels.

Black, fine-powder particles, which temporarily form a stable suspension in a red-colored reaction mixture, are produced.
These particles are centrifuged off, washed three times with ethanol and dried overnight at 60 C in a drying oven. The particles are then taken up in toluene and treated with ultrasound for 20 minutes for better distribution.
The synthesis can be modified by adding various solvents, for example, pyridine or tetraethylene glycol, to the standard solvent triethylene glycol.
In addition, the synthesis of the semiconductive nanoparticles can be altered by adding cappers, such as, for example, TPP (triphenylphosphite), TOP
(trioctylphosphine) or hexadecylamine.
For the production of solar cells, poly-3-hexylthiophene (P3HT), poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylvinylene] (MEH-PPV) and poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylvinylene] (MDMO-PPV) are used in combination with the semiconductive nanoparticles as semiconductive polymers.
The bilayer heterojunction solar cell is produced with the following parameters:
Polymer solution for the active layer: P3HT: 3 mg/ml in toluene Nanoparticle suspension for the active layer: 15 mg/ml in ethanol The bulk heterojunction solar cell is produced with the following parameters:
Solution for the active layer:
MEH-PPV: Nanoparticles = about 40 : 60 m% (polymer concentration: about 3 mg/ml in toluene) As examples, current/voltage characteristics of a bilayer heterojunction solar cell are depicted in Figure 8, and current/voltage characteristics of a bulk heterojunction solar cell are depicted in Figure 9. The parameters that describe the solar cells can also be found in the figures. The bilayer heterojunction solar cell produces a photoelectric current of 4.5 A/cm2 and a photoelectric voltage of 270 mV. The hybrid solar cell according to the bulk heterojunction principle shows a very slight dark current, a good diode characteristic, and produces a photoelectric current of 21 A/cm2 and a photoelectric voltage of 755 mV.
Going beyond the cited examples, still other tests were performed that led to the following findings:
Semiconductive organic compounds, for example phthalocyanines or perylenes or oligomers of the semiconductor polymers instead of semiconductor polymers, were used in the hybrid solar cells. The solar cells that were produced provided similar photoelectric currents and photoelectric voltages to the solar cells described in Examples 1 to 4.
In addition, it was found in other experiments that in addition to the elements Cu, In andlor Zn, the elements Ag, Cd, Ga, Al, Pb, Hg, Se and/or Te can also be used.
In summary, it can be stated that the process according to the invention is uncomplicated and energy-efficient as far as processing is concerned, since by using microwave radiation, the reaction time can be greatly shortened. In addition, with this process, the polymer/semiconductor particle suspension that is necessary for applying a photoactive nanocomposite layer can be produced in one step by the production of the semiconductor particles in the polymer solution under microwave irradiation.
This has the additional advantage that the semiconductor particles are distributed especially homogeneously in the electroactive polymer.
With this process, photoactive layers, whose applications in the case of photoactive elements, such as solar cells, result in satisfactory degrees of efficiency, can be produced.

Claims (19)

1. Process for the production of photoactive layers that comprise inorganic semiconductor particles and organic semiconductor compounds, characterized in that inorganic semiconductor particles are synthesized under microwave irradiation, from which photoactive layers are formed in combination with organic semiconductor compounds.

2. Process according to claim 1, wherein the synthesis of the inorganic semiconductor particles and the formation of the photoactive layer is carried out in two steps.

3. Process according to claim 1, wherein the synthesis of the inorganic semiconductor particles and the formation of the photoactive layer are carried out in one step.

4. Process according to claim 3, wherein the production of the inorganic semiconductor particles is carried out directly in a solution or suspension of the organic semiconductor compound under microwave irradiation and the thus obtained suspension is further processed directly to a photoactive layer.

5. Process according to any one of claims 1 to 4, wherein the inorganic semiconductor particles have a diameter of 1 nm to 1 µm, preferably 2 nm to 100 nm.

6. Process according to any one of claims 1 to 5, wherein the inorganic semiconductor particles are produced from metal chalcogenides.

7 Process according to claim 6, wherein as metal chalcogenides, those of type ABX2, AB5X9, AB5X8, CX, D2X3, and the like are selected, whereby: A= Cu, Ag, Zn, Cd; B = In, Ga, Al; C = Cu, Ag, Zn, Cd, Pb, Hg, Eu, Tm, Yb; D = Al, In, Ga, Tl, Y, La, B, and X = S, Se, Te.

8. Process according to any one of claims 1 to 5, wherein the inorganic semiconductor particles are produced from the respective elements of the periodic table.

9. Process according to any one of claims 1 to 5, wherein the inorganic semiconductor particles are produced from metal salts and/or metal compounds and a chalcogenide compound.

10. Process according to any one of claims 1 to 9, wherein as organic semiconductor compounds, semiconductor polymers and/or semiconductive oligomers are used.

11. Process according to claim 10, wherein the semiconductor polymers and/or the semiconductive oligomers are selected from the group trans-polyacetylene, polypyrrole, polythiophene, polyaniline, poly-p-phenylenevinylene, poly-p-phenylene, polyfluorene, polyaromatic amines, poly(thienylene-vinylene) and/or the derivatives thereof.

12. Process according to any one of claims 1 to 9, wherein as organic semiconductor compounds, phthalocyanines and/or perylenes are used.

13. Process according to any one of claims 1 to 12, wherein the production of the inorganic semiconductor particles is performed with the aid of surfactants (cappers).

14. Process according to any one of claims 1 to 13, wherein the inorganic semiconductor particles are present in the form of agglomerates.

15. Process according to any one of claims 1 to 13, wherein the inorganic semiconductor particles are present in the form of percolating particle networks.

16. Process according to any one of claims 1 to 15, wherein the microwave radiation that is used is in a wavelength range of 1 m to 1 mm and a frequency range of between 0.3 GHz and 300 GHz.

17. Component that comprises a photoactive layer, which is produced according to any one of claims 1 to 16.

18. Component according to claim 17, wherein the photoactive element is a hybrid solar cell.

19. Component according to claim 17, wherein the photoactive element is a photodetector.