FR2919428A1 - OPTOELECTRONIC COMPONENT ELECTRODE COMPRISING AT LEAST ONE LAYER OF A TRANSPARENT OXIDE COATED WITH A METAL LAYER AND CORRESPONDING OPTOELECTRONIC COMPONENT. - Google Patents

Abstract

The subject of the invention is an optoelectronic component electrode, comprising at least one layer of at least one conductive transparent oxide carried by a substrate, characterized in that said conductive transparent oxide is coated with a layer of at least one material metal, said layer having a thickness between 0.5 nm and 1.5 nm.

Description

Optoelectronic component electrode, comprising at least one layer

  a transparent oxide coated with a metal layer, and corresponding optoelectronic component. The field of the invention is that of the design and production of optoelectronic components. More specifically, the invention relates to an electrode of optoelectronic components of the type comprising at least one layer of at least one transparent conductive oxide carried by a substrate. Conductive transparent oxides (OTC) have the distinction of being simultaneously electrical conductors and transparent to light in the visible range. In the field of the invention, OTC layers are increasingly used as electrodes in organic and inorganic optoelectronic components. The most used OTCs are indium oxide, tin oxide, zinc oxide ... Among these OTCs, the most efficient (and therefore the most commonly used) is the oxide of indium doped with tin, called ITO (indium tin oxide). The applications of these electrodes are various and varied: flat screen, light emitting diodes, photo voltaic cell ..., this with regard to the organic components. ITO is also used as anode in organic photovoltaic cells and organic electroluminescent diodes. In this case, the ITO achieves much better performance than that obtained with another OTC. With reference to FIGS. 1 and 2, reference is made to the conventional structure of optoelectronic components. These components are based on the use of at least two organic components: one is an electron donor and the other is an electron acceptor.

  As regards the photovoltaic cells, the components are either mixed (this is referred to as an interpenetrated medium as shown in FIG. 1) or superimposed in the form of thin layers (this is called multilayer cells as illustrated by FIG. Figure 2).

  The superimposed layer structure is used for the light-emitting diodes, the thickness ratios being variable. In both cases, the number of superimposed layers is greater than two. In the case of interpenetrating media, the electron donor is a conjugated polymer such as polyparavinylene or polythiophene, and the electron acceptor is, for example, a fullerene derivative or a perylene derivative. A typical structure is then such that glass / ITO / PEDOT: PSS / conjugated polymer mixture: fluorene derivative / cathode, the cathode being generally a thin layer of aluminum. In the case of multilayer structures, the electron donor is an organic dye, often a phthalocyamine (Pc) such as CuPc, and the electron acceptor is fullerene or a perylene derivative. Often an exciton blocking layer is introduced between the electron acceptor and the cathode. There is then a layer superposition such as glass / ITO / PEDOT: PSS / electron donor / electron accepting molecule / exciton / cathode blocking (AI). In this context, the ITO, despite its performance, turns out not to be an ideal material, since it consists mainly of indium whose terrestrial reserves are very limited. In fact, today's demand for indium is growing extremely rapidly. Thus, with the very large growth of the flat panel market, the price of ITT thin films has risen sharply. As indium nature reserves are severely constrained, as is global production capacity, economists and industrialists expect impending supply difficulties. This effect is reinforced by the rapid growth of the photovoltaic energy market. One consequence of this increase in demand is the development of new thin film channels. These new channels, constituting a new generation, concern photovoltaic cells consisting of a stack of inorganic thin layers such as amorphous silicon, cadmium tellurium and sue: out semiconductors with chalcopyrite structure (l, Ill, IV, ), such as CulnSe ,, CuGaSe2, CuluS, and their alloys such as Cu (In, Ga) Se2 (otherwise referred to as CIGS). However, these also contain indium. Forecasts indicating difficulties in the supply of indium are confirmed by a recent report from the New Energy and Industrial Technology Development Organization (NEDO), which states that: global resources of indium will be exhausted between 2011 and 2019, when the 6000 tons of land on this rare item will have been used; Indium has been used extensively in the industry since the late 1990s to manufacture transparent indium tin oxide (ITO) electrodes for flat screens; - while aware of how limited global resources are, indium production has steadily increased to support demand, which has recently reached 300 to 400 tonnes per year, which has flying lessons.

  NEDO suggests using the available resource better and achieving a recycling rate greater than or equal to 70% in order to push back the shortage to 2025. NEDO adds that it will be necessary to accelerate the search for alternative materials so that these come into service in 2015. According to this report, it will be necessary to improve the manufacturing process of flat screens in order to limit the amount of material lost (representing 15% of indium), the number of defective panels when of their manufacture (representing 2% of indium) and the proportion of appliances that do not pass the quality test (representing 3% of indium). Finally, the report recommends that by 2010, efficient end-of-life indium recovery technologies be developed and that the amount of indium used for each screen be reduced by one third from 0.9 grams. at 0.06 grams per 15 inch screen. At the same time, concerning the design and manufacture of optoelectronic and organic components, it is found in practice that it is difficult to optimize the charge transfer between the electrode (in particular the anode) and the organic material. To do this, one technique is to insert a thin layer between the two components to adjust the electronic behavior of adjacent materials. Thus, as mentioned hereinabove, a thin layer of poly (ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT: PSS), which is a conductive polymer, is deposited by rotary coating on the cladding layer. ITO before the deposit of the organic material. This layer is very effective, allowing adjustment of extraction work, passivation of surface defects and smoothing of the ITO surface. However, the PEDOT: PSS poses the following problems in particular: it is unstable under ultraviolet irradiation (present in the solar spectrum); he introduces traces of water into the active organic layer; it is slightly acidic; the performances obtained through it lack reproducibility from one component to another. Furthermore, it is known that the problems posed by the OTC / organic material contact are related to the difference in energy between the OTC extraction work and the highest occupied molecular orbital energy of the material. organic (HOMO level). A poor agreement between these two parameters results in the formation of a resistive contact which increases the series resistance accordingly. In addition, it is difficult to obtain an OTC having a homogeneous surface state (morphology, chemical properties), hence the existence of zones causing the presence of leakage currents and therefore a low short circuit resistance.

  All of this justifies the need for chemical treatments of the TBT surface. However, this does not allow to emancipate completely the problems mentioned hence the use of an interface layer such as PEDOT: PSS, it posing the problems mentioned above. In the case of ZnO and SnO, it should be noted that the value of the work of extraction is of the order of 4-4.3 eV against 4.5-5 eV for the ITO, which justifies at least partly the poor results when used as an electrode. The invention particularly aims to overcome the disadvantages of the prior art.

  More specifically, the invention aims to provide an electrode of optoelectronic components incorporating at least one transparent conductive oxide, which is more efficient than those proposed by the prior art. The invention also aims to provide such an electrode whose performance can be improved, both by using ITO with other transparent conductive oxides. In this sense, an object of the invention is to provide such an electrode, which allows to consider a significant reduction in the use of ITO for the manufacture of electronic components. Another object of the invention is to provide such an electrode which makes it possible to obtain reproducible performances from one optoelectronic component to another. Another object of the invention is to provide such an electrode which is stable under UV. Another object of the invention is to provide such an electrode which is simple in design and easy to implement. These objectives, as well as others which will appear later, are achieved by virtue of the invention, which relates to an optoelectronic component electrode, comprising at least one layer of at least one conductive transparent oxide carried by a substrate. characterized in that said conductive transparent oxide is coated with a layer of at least one metallic material, said layer having a thickness between 0.5 nm and 1.5 nnl. Thus, in view of the invention, the presence of the thin metal layer on the surface of the CT allows a significant improvement in the performance of the optoelectronic components using such OTC. The thickness of the metal layer between 0.5 nm and 1.5 nm makes it possible to preserve the transparency of the layer while taking advantage of the specific properties of the metal (extraction work, conductivity, reproducibility, etc.). To discuss the influence of the very thin metal layer on the properties of the OTC / electron donor contact, we recall the electrical diagram of the photovoltaic cells (Figure 3) and the corresponding 1-V characteristics according to the values of the series resistance. Rs and short-circuit resistance Rsh. These characteristics are illustrated by Figures 4 to 6 which show: an ideal cell (Figure 4), in which Rsh tends to infinity and Rs to zero; a cell having a low resistance Rsh short circuit (Figure 5); a cell with a high Rs series resistance (Figure 6). It should be noted that the value of the series resistance. Rs, can be estimated from the inverse of the slope of the characteristics I-V around J = O, while the short-circuit resistance Rsh, is estimated in the same way but for V = 0. Recall also the following definitions: (Dm (extraction work): the work of extraction is the energy necessary to tear an electron from the upper layer of the material, that is to say situated at Fermi EE, to bring it to the vacuum level (to release it): HOMO, LUMO: in an organic semiconductor the HOMO (highest energy occupied molecular orbital) and LUMO (lowest energy unoccupied molecular orbital) levels correspond respectively at the upper part of the valence band and the lower part of the conduction band of a conventional inorganic semiconductor.The energy difference between these two levels is the band gap. the anode and the organic material requires a good agreement (a quasi-alignment) between the Fermi E level, the anode (here the COT or the COT covered with metal) and the HOMO level of the organic material. side of the cathode, the align With these reminders, the advantage obtained thanks to the thin layer of metal can be expressed as follows: the very thin layer of metal makes it possible to increase the work of extraction, which improves all the more, the contact with the organic (the Fermi level of the metal tending to bring the anode closer to the HOMO level of the OTC) and decreases the resistance of 2.5 contacts by the same amount. However, the fact that a very significant improvement in the efficiency of certain structures using a relatively low extraction work metal, such as silver, shows that the work of extraction does not explain everything. distributed metal layer (the homogeneous way on the surface of the OTC implies a better distribution of exchanges at the interface avoiding the effects of short circuit and thus increasing the resistance Rsh.

  Thus, not only is there homogenization of the properties of the surface of the COT, but also this is perfectly reproducible, since the properties of the metal are not subject to variation from one point to another, nor to one sample to another.

  According to a first embodiment, said conductive transparent oxide is tin-doped indium, as will be apparent later on, the performance of an electrode according to the invention with indium, can be improved by about 10 "/ c.

  In general, in this case, the essential contribution of the metal layer is an increase in the short-circuit current. According to a second embodiment, said transparent conductive oxide comprises a zinc oxide, which is preferably doped with aluminum.

  Thus, as will be explained in more detail below, the invention makes it possible to produce electrodes for efficient optoelectronic components without ITO. Given the limits of the nature reserves in indium mentioned above, the invention therefore provides a particularly advantageous solution in that it allows the use of transparent conductive oxides other than I 'ITO. According to a third embodiment, said transparent conductive oxide comprises tin dioxide, the latter being preferentially doped with fluorine.

  It is noted that, whatever the embodiment envisaged among those indicated above, all the metals can be used with regard to the metal layer deposited on the CT: all those that have been tested allow, to varying degrees , a significant improvement in the performance of optoelectronic components.

  According to an advantageous solution, said metallic material is gold, the corresponding layer preferably having a thickness of 0.5 nm. According to another advantageous solution, said metallic material is copper, the corresponding layer preferably having a thickness of Inm. According to yet another advantageous solution, said metallic material is nickel, the corresponding metal layer preferably having a thickness of 0.5 nm. The invention also relates to an optoelectronic component comprising an electrode having at least one layer of at least one conductive transparent oxide carried by a substrate, characterized in that said conductive transparent oxide is coated with a layer of at least one metallic material said layer having a thickness of between 0.5 nm and 1.5 μm.

  Other features and advantages of the invention will appear on reading the following description of several embodiments of the invention, given by way of illustrative and nonlimiting examples, and the appended drawings among which: FIGS. 2 are schematic representations of the conventional structure of the electronic components, respectively in an interpenetrated configuration and in a multilayer configuration; Figure 3 is an electrical equivalent diagram of a photovoltaic cell; FIGS. 4 to 6 are curves of the I-V characteristics of different photovoltaic cells; FIG. 7 is a schematic representation of a basic structure of an optoelectronic component used to show the performances obtained with an electrode according to the invention; FIG. 8 illustrates a conventional figure of performance calibration of optoelectronic components; FIG. 9 is a measurement curve of the electrical characteristics under illumination of a cell employing zinc oxide as transparent conductive oxide; FIG. 10 is a table of values from the curves of FIG. 9; FIG. II is a graph of measurement of the electrical characteristics under illumination of a cell using tin dioxide as conductive transparent oxide; FIG. 12 is a graph of measurement of the electrical characteristics under illumination of a cell using ITO as transparent conductive oxide; FIG. 13 is a table of values derived from the curves of FIG. 12: FIGS. 14 to 17 are tables of values characterizing various anodes produced according to the invention; FIGS. 18 to 20 are the electroluminescent response measurement graphs of various diodes, some of which are produced according to the invention, In the following description of the invention, the basic structure used is that illustrated in FIG. shows a component whose anode comprises an oxy (the conductive transparency (hereinafter referred to as "TCO"), a metal layer M, copper phthalocyamine (CuPc) as electron donor, fullerene (C6)) as an acceptor of electron and tris (8-hydroxyquinoline) (Alg3) as an exciton blocking layer, the cathode being constituted by an aluminum layer.Of course, the structure illustrated in FIG. 7 is a basic structure, and more complex assemblies can be achieved by the introduction of a layer at the interface with the cathode, or by superposition of several junctions in the framework of multilayer structures.The principle of the invention lies in the fact of coating the transparent oxide conductive by a metal layer with a thickness of between 0.5 nm and 1.5 μm, in order to optimize the performance of the optoelectronic components, which these integrate with ITO or another transparent conductive oxide; . Following the description, the properties of these new structures are validated by comparison in their performance with those obtained by conventional structures. As will be shown more clearly later, the invention makes it possible to improve the performance of these components, this being illustrated by examples of structures using the ITO as anode, and in parallel by examples using cathodes without ITO. .

  Referring to Figure 8, we recall what is the measurement that calibrates the performance of electronic components. Figure 8 shows the current-voltage characteristics of a photovoltaic cell in the dark and under illumination. The presence of a signal in the fourth dial IV shows that there is energy production when the cell is illuminated. The cell is illuminated by means of a solar simulator calibrated on an illumination known as AMI I.5, ie Pi = 100 mW / cm2. The power supplied corresponds to the hatched surface, Jcc being the short-circuit current density, Vco being the open circuit voltage, Vmax, Jmax corresponding to the power points Ma xiniuiu. So. we have the efficiency> 1, which is a fundamental value for the cell, according to: ri = (Vco x Jcc x FF) / pi, with FF = (Jmax x Vmax) / (Jcc x Vco) For a light-emitting diode, in in addition to the current-voltage characteristic in the darkness of the diode, we are interested in the light emitted as a function of the value of the bias voltage. In the present case, this value is materialized by the potential measured at the terminals of a photodiode used to measure the intensity of the emitted light. As indicated above, the invention therefore allows the use of a conventional OTC whose properties nevertheless meet the criteria required for their use as an electrode in an optoelectronic component, ie a square resistance of the order of 10 to 25 S2. and a visible light transmission of the order of 85 Ic at least. Thus, for the tests described later, ITO, ZnO and SnO2 were used as OTC.

  According to the invention, the presence of the thin metal layer on the surface of the OTC allows a significant improvement in the performance of the optoelectronic components using such improved OTCs. This phenomenon is illustrated later with the help of several examples. The examples presented show that efficient components can be obtained by substituting the pair ZnO / M (where M = Au, Cu, or Ag) with ITO (with or without PEDOT: PSS). The performance of the components is then measured with SnO2 as OTC. It is then shown that the properties of ITO are improved by the presence of a thin metal layer. FIG. 9 shows the typical electrical characteristics of a glass / ZnO type photovoltaic cell (500 nm / Au (0, Snm) / CuPc (40 nm) / C60 (40 nm) / Alg3 (10 nm) / Al (70 nm). nm), and those of a cell made without gold, as well as that using ITO The thicknesses of the layers are estimated in situ, during the deposition of the layers, using a quartz monitor.

  It should be noted that only the thickness of gold has been really optimized. The other thicknesses make it possible to obtain energy yields which place the cells in the average standard for this type of structure. In general, and this is valid for the other examples described later, more than the performances themselves, it is sought to show that it is the performance ratio that is significant.

  As appears in FIG. 9, as well as in the table of values (FIG. 10) derived from the curves of FIG. 9, or states that: the presence of a very thin layer of gold considerably improves the performance of the photovoltaic cells whose anode is in ZnO; these performances are similar to those obtained with ITO itself covered with gold. Considering an average obtained over several series of samples, it is found that the energy efficiency of a cell with a ZnO / Au electrode is multiplied by 10 compared to that obtained without addition of gold. For a 1 nm layer of Cu, the measured improvement is of the same order of magnitude. However, some cells have a VCO substantially lower than the usual value which is of the order of 0.45 to 0.5 V. Note that the CuPc can be replaced by other electron donors, for example pentacene , with identical results, that is to say a great improvement in ZnO yield when it is covered with a thin layer of gold. Referring to Figure II, a study of the same type is conducted taking SnO7 as OTC. This figure shows the results obtained for glass / SnO7 / Au (x nm) / CuPc / C60 / Alg3 / Al structures with x = 0 nm and x = 0.5 nm. As in the case of ZnO, the improvement provided by the thin layer of gold is spectacular. The average cell yield is increased here again by a significant order of magnitude. All of the above results show that the deposition of a very thin metal layer on ZnO and on SnO2 makes it possible to use them as an electrode in an optoelectronic component equal to the ITO. The following example illustrates the improvements in the performance of photovoltaic cells made according to the invention by studying the influence of thin gold layers deposited on the ITO in terms of cell performance.

  Figure 12 shows a sampling, in the form of curves, of the representative results for the ITO. The typical values resulting from these curves are reported in the table of FIG. 13. In general, the increase in performance obtained thanks to the presence of a very thin layer of gold is of the order of 25 C 70 , which is significant. In addition, the comparison of the tables of FIGS. 6 and 9 shows that the invention makes it possible to substitute ZnO and SnO for ITO, the improvement in the performance provided being much greater in the case of ZnO and SnO 2. It is also noted that the curves of FIGS. 9, 11 and 12 make it possible to deduce that the thin metal layer increases the short-circuit resistance Rsh and decreases the series resistance Rs. Similar studies have been carried out as For example, with copper (ITO / Cu), nickel (ITO / Ni): the results obtained also show a significant improvement in the performance of the cells. In the case of nickel, as in the case of copper, a limitation of the The thickness of the metal appears very quickly (table in Figure 10). This materializes on the electrical characteristics of the cells by a dramatic decrease in the open circuit voltage Voc. Thus, for a thickness of 1.5 nm of Ni, there is no more photovoltaic effect, for a thickness of 1 nm, the effect is present but the results are lower than those obtained without Ni, the voltage open circuit being lower than that generally obtained. Finally, for a thickness of 0.5 nm, the improvement obtained with respect to the ITO alone tends to 10%. Copper behaves similarly with a rapid decrease in open circuit voltage as the thickness of the metal layer increases. Thus the Voc is 0.15 to 0.20 volts for 1.5 nm and 0.25 to 0.35 volts for 1 nm. In general, in the context of the ITO, the essential contribution of the metal layer is an increase in the short-circuit current.

  A summary of these results is presented in the tables of FIGS. 10 to 12 which show the results obtained with: an ITO / Ni anode (FIG. 14) with increasing thicknesses of Ni; an ITO / Au anode (Figure 15) with increasing thicknesses of gold (compared to a PEDOT layer: PSS); a ZnO / Au anode with increasing thicknesses of gold (Figure 16); Different anodes with ZnO, Sno, and ITO (FIG. 17). Furthermore, it is shown hereinafter that the invention described above makes it possible to improve not only the performances of the photovoltaic cells but also those other optoelectronic components such as light-emitting diodes. To do this, a well-known light-emitting organic molecule is used, tris (8-hydroxyquinoline (Alq 3), which is an electron conductor and is associated with a hole conductor. : Glass 'ITC' M (x nm) / electron donors / Alq; / LiF / AL with M = Au, Cu or Ni and 0 <x <1.5 nm The thick LiF layer of I nm is known to improve Diode performances Two donors were tested, a thiophene derivative (a representation of which is inserted in Figure 18) and poly (N-vinylcarbazole) (PV K) Figure 18 shows the results obtained for diodes of the type: glass / ITO / N1 (x nm) / thiophene derivative / Alq; / LiF / Al, where M is either Ni or Cu, the thickness is 1.5 nm. for a lower voltage when the ITO is covered with a very thin layer of metal and this especially in the case of Cu.On the other hand, we can see that the inten The signality is increased by the presence of this thin layer and, again, especially in the case of Cu.

  In the same way, tests were carried out on Glass / ITO / M (x nm) structures PVK / Alq / LiF / AL with M = Au, Cu or Ni and 0 <x <1 nm. The results obtained are presented in FIG. 19. Here again, on the one hand, the metal layer allows the light signal to appear at a lower voltage and its intensity is much greater than a given voltage. Note the low efficiency of Ni in this case. Nevertheless, it has already been shown, in the case of the Ni in particular, that the fact of inserting micro-aggregates (the metal in the surface of the ITO makes it possible to improve the emission. overall is lower than that of a conventional diode because Ni aggregates, if they allow a better punctual yield (at the level of aggregates of Ni), do not improve the emission between the aggregates while they screen a part because of their size (100 nm), this is not the case with ultra thin metal layers, which avoids any untimely absorption.

  As with photovoltaic cells, replacement of ITO with another OTC was evaluated. The results obtained are shown in FIG. 20. Here again, there is a decrease in the thresholding voltage of the light signal and an increase in its intensity for a given bias voltage. In the case of ZnO, a similar result is obtained with passage of 8 V to 5 V of the threshold voltage according to which the COT is not or is covered by 5 nm of gold. In addition, Figures 19 and 20 clearly show that after adding a thin metal layer to the SnO2, the results are much higher than the ITO alone.

  It should be noted that the invention also allows satisfactory recycling of the ITO. After using thin layers of ITO for the production of organic photovoltaic cells, they were cleaned chemically to remove all the deposits excluding the ITO. This, after having been covered with a thin layer of gold according to the invention, was again used as an anode in an organic cell and the results obtained were at the same level as those resulting from a new layer. ITO. In addition, it is again noted that the invention allows an optimization of the properties of OTC (ITO and therefore In, O3, SnO ,, ZnO) in the context of their use as an electrode in an electronic component, and that this improvement is 'more operational than the OTC used has properties far removed from those required for this application. This means that the invention opens up the possibility of substituting Zn () and SnO, with ITO, as an electrode in the optoelectronic components, which is all the more remarkable as, for example, zinc is inexpensive. and widespread on earth unlike indium which is a high price because of its scarcity. 15

Claims (10)

  An optoelectronic component electrode, comprising at least one layer of at least one conductive transparent oxide carried by a substrate, characterized in that said conductive transparent oxide is coated with a layer of at least one metallic material, said layer having a thickness of between 0.5 nm and 1.5 nm.   Optoelectronic component electrode according to claim 1, characterized in that said conductive transparent oxide is indium doped with tin.   An optoelectronic component electrode according to claim 1, characterized in that said conductive transparent oxide comprises zinc oxide. 15   Optoelectronic component electrode according to claim 3, characterized in that said zinc oxide is doped with aluminum.   Optoelectronic component electrode according to claim 1, characterized in that said transparent conductive oxide comprises tin dioxide. 20   Optoelectronic component electrode according to claim 5, characterized in that said tin dioxide is doped with fluorine.   An optoelectronic component electrode according to any one of claims 1 to 6, characterized in that said metallic material is gold.   Optoelectronic component electrode according to claim 7, characterized in that said gold layer has a thickness of 0.5 nm.   9. Optoelectronic component electrode according to any one of claims 1 to 6, characterized in that said metallic material is copper.   10. Optoelectronic component electrode according to claim 9, characterized in that said copper layer has a thickness of 1 nm. Optoelectronic component electrode according to any one of claims 1 to 6, characterized in that said metallic material is nickel. Optoelectronic component electrode according to claim 1, characterized in that said nickel layer has a thickness of 0.5 nm. An optoelectronic component comprising an electrode having at least one layer of at least one conductive transparent oxide carried by a substrate, characterized in that said conductive transparent oxide is coated with a layer 10 of at least one metallic material, said layer having a thickness between 0.5 nm and 1.5 nm.