NL2006756C2

NL2006756C2 - Method for forming an electrode layer with a low work function, and electrode layer.

Info

Publication number: NL2006756C2
Application number: NL2006756A
Authority: NL
Inventors: Siegfried Christiaan Veenstra; Wilhelmus Johannes Hermanus Verhees; Yulia Galagan; Nadia Grossiord
Original assignee: Stichting Energie
Priority date: 2011-05-10
Filing date: 2011-05-10
Publication date: 2012-11-13
Also published as: WO2012154045A1; TW201251035A

Description

5

Method for forming an electrode layer with a low work function, and electrode layer Field of the invention

The present invention relates to a method for providing an electrode layer.

Prior art

The article ‘Metal grid/conducting polymer hybrid transparent electrode for inverted polymer solar cells’ by Jingyu Zou et al., Applied Physics Letters 96, 203301 (2010) discloses a method for forming an electrode for application in polymer photo-voltaic cells. This article 10 mentions the disadvantage of present day PV cells employing a transparent electrode with

Indium Tin Oxide (ITO): despite having excellent properties with respect to transparency in the visible range of the solar spectrum and a good electrical conductivity, the mechanical characteristics of the ITO layer in combination with plastic substrates is poor, and availability of especially Indium is becoming more and more relevant.

15 In the article, it is acknowledged that using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been demonstrated as replacement of ITO in certain PV cells, and that the relatively high sheet resistance of this material may be overcome by using a conductive metal grid in combination with the PEDOT:PSS layer. In the electrode manufacturing process described in this article, 20 part of the process involves deposition of Al and Ag on a glass substrate which still relies on a vacuum and a wet etching process to obtain a metal grid.

Summary of the invention

The present invention seeks to provide an improved manufacturing method for 25 providing an electrode with a low work function, which can be used in combination with organic active materials, e.g. in a polymer solar cell.

According to the present invention, a method according to the preamble defined above is provided, the method comprising applying a metal in a solution to a substrate to form a conducting layer, applying a conductive polymer in a solution on top of the conducting layer as 30 a second layer, and applying a contacting layer using a solution on top of the second layer.

The present invention allows to obtain an electrode layer or devices with such an electrode layer using layer formation techniques that can be applied in normal processing environments, without requiring any vacuum or high temperature steps.

It is noted that screen printing techniques as such are known for obtaining grids of 35 conductive material on a substrate. However, the combination of steps of the present invention resulting in the advantages as mentioned has not been contemplated yet.

In a further aspect, the present invention relates to an electrode layer comprising a substrate, a conducting layer, a second layer of a conducting polymer, and a contacting layer 2 wherein the conducting layer comprises a metal selected from the group comprising Ag, Cu, Al, wherein the conductive polymer is PEDOT:PSS, and wherein the contacting layer comprises zinc oxide, aluminum-doped zinc oxide, titanium oxide or cesium carbonate.

In an even further aspect, the present invention relates to a photo-active device 5 comprising an electrode layer according to the present invention embodiment, a layer of photoactive material, and a further electrode.

Short description of drawings

The present invention will be discussed in more detail below, using a number of 10 exemplary embodiments, with reference to the attached drawings, in which

Fig. 1 shows a cross sectional view of an electrode manufactured according to an embodiment of the present invention;

Fig. 2 shows a cross sectional view of a PV cell manufactured according to an embodiment of the present invention.

15

Detailed description of exemplary embodiments

The present invention provides a method for manufacturing an electrode layer which is very well suited for application in organic electronics, such as organic solar cells and organic light emitting devices, and which manufacturing process allows for high volume low cost 20 production of such electrode layers.

The basic idea is to provide a (semi) transparent electrode layer using a combination of three materials, more specifically a highly conducting metal grid (e.g. silver, aluminum or copper), a highly conducting polymer layer such as poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and a contacting layer using 25 e.g. a metal oxide like Zinc oxide (ZnO).

This results in an electrode layer having a work function of about 4eV, which makes it particularly suitable for the function of an electron collecting contact layer in a polymer solar cell. More generally, the electrode layer manufactured according to the present invention embodiments allows a good electrical contact with the conduction band of a semiconductor 30 material (especially of an organic semiconductor material). The (ZnO) contacting layer e.g.

forms a selective contact to a bulk hetero-junction layer (e.g. P3HT:[C60]PCBM, see below) to collect the electrons from the conduction band of the electron acceptor material (i.e. the [C60JPCBM in the example described below).

The highly conducting polymer (PEDOT:PSS) forms a good contact with the highly 35 conducting metal grid (Ag). PEDOT:PSS is a highly doped p-type semiconductor. Holes are injected from the Ag contact into the PEDOT:PSS layer and recombine with electrons at the PEDOT:PSS/ZnO interface.

As will be described in more detail below with reference to various embodiments, the electrode can be formed from various solutions on different types of substrates. No processing 40 steps are needed which require a vacuum, or a high temperature (>150°C). The resulting 3 electrode is mechanically and chemically stable, which allows to add further layers using wet chemical processes, as will be described below in more detail. Also a protective environment, such as an oxygen free protective environment, is not needed for this electrode. The manufacturing steps of the present invention also allow to obtain a high speed of production, 5 using various printing and/or coating techniques to apply the various layers.

In the art, it is already possible to manufacture flexible organic electronic devices, wherein a light sensitive layer can be printed, resulting in a cost-effective manufacturing process. However, the additional layers in the electronic devices, e.g. electrode layers for PV cells, are added or made using various techniques involving high temperatures, vacuum 10 environment or other costly processing steps.

According to the present invention embodiments, all the layers can be manufactured using solution processing in ambient conditions, such as a printing or coating type of processing step, which is far more cost-effective. Alternative processing steps using solution processing in ambient conditions are e.g. spray deposition, ambient plasma assisted deposition 15 (atmospheric pressure), electrolytic or electrochemical deposition. At most, the layer formation steps according to the present invention embodiments are executed in a controlled environment with a controlled relative humidity and temperature.

For PV cells, the present invention proposes an electron collecting layer, which can be manufactured entirely from solutions. In an embodiment, this electron collecting layer 20 comprises Ag, PEDOT:PSS and ZnO. The Ag layer can be made semi-transparent by printing the Ag in a pattern (finger pattern, grid pattern, honeycomb pattern, etc.). Furthermore, a hole collecting layer can be formed using similar techniques, comprising Ag and PEDOT material. Again, the Ag can be e.g. printed in a pattern to allow forming a semi-transparent layer.

In Fig. 1 a cross sectional view is shown of an electrode layer 10 manufactured 25 according to one of the present invention embodiments.

On a substrate 1, Ag in a liquid solution is provided as a conducting layer 2, in this embodiment as a patterned layer. E.g. a commercially available ink (SunTronic U5603 ink of SunChemical) is used to inkjet print the Ag grid as the conducting layer 2. This ink comprises 20 wt% Ag nanoparticles in organic solvents such as ethanediol.

30 The conducting layer2 is e.g. applied using an inkjet printing technique, which is known as such. The conducting layer 2 can thus be applied with a well defined thickness of the conducting layer 2, e.g. with a thickness of about 500 nm. In further embodiments, the thickness of the conducting layer 2 is between 200 and 1000nm. When thicker (higher) lines are used, the line resistance will decrease, further enhancing the efficiency of the electrode 35 layer 10.

On top of the conducting layer 2, a solution is applied for forming a second layer 3 of a highly conducting polymer, such as the PEDOT:PSS material mentioned above. In order to have a good lateral conductivity, a highly conductive variant of the PEDOT:PSS material (HC PEDOT:PSS) is used in a further embodiment. In an exemplary embodiment, the thickness of 4 this second layer 3 is about 70 nm. In further embodiments, the thickness of the second layer 3 is between 30-300nm, e.g. between 70-150nm.

The solution e.g. comprises a solvent and other constituents, which allows the second layer 3 to be applied using inkjet printing or other in-solution techniques. After applying, the 5 solvent evaporates and the polymer layer (second layer 3) is formed.

After this step, the final layer (contacting layer 4) of the electrode layer 10 is formed by applying a solution which results in formation of a e.g. a ZnO layer, e.g. using wet chemical coating techniques such as spin coating techniques. The ZnO layer (contacting layer 4) has a thickness of about 30 nm in an exemplary embodiment. The ZnO layer (contacting layer 4) 4 in 10 further embodiments has a thickness in the range from 5-200nm.

The contacting layer 4 is of an n-type semiconductor material such as ZnO, and can be solution processed. E.g. a ZnO layer 4 may be processed using ZnO nanoparticles in a solution, as e.g. described in W. J. E. Beek, M. M. Wienk, M. Kemerink, X. Yang, R. A. J. Janssen, J. Phys. Chem. B 109, 9505 (2005).

15 In an alternative embodiment, aluminum-doped zinc oxide (AZO) is used alternatively or additionally for forming the contacting layer 4. In even further alternative embodiments, a contacting layer 4 is provided further comprising titanium oxide (TiOx) or cesium carbonate (CS2CO3).

The second layer 2 is allowed to dry before applying the contacting layer 4. As the 20 contacting layer 4 is applied to the second layer 3 after the latter has dried, no problems can arise with respect to possible chemical reactions between the material of the second layer 3 (and its solvents) and the ZnO. In known electrode structure manufacturing techniques, the PEDOT:PSS material is applied to an existing ZnO layer, and due to the acid nature of the PEDOT:PSS in solution, irregularities or faults may result in the ZnO layer in this sequence of 25 process steps. To be able to provide a stable and reliable contacting layer 4, the contacting layer 4 is applied in a controlled environment, wherein the relative humidity is kept low, and the temperature is also controlled.

As an alternative, the contacting layer 4 may be applied to the second layer 3 using a precursor technique, e.g. as described in ‘A facile route to inverted polymer solar cells using a 30 precursor based zinc oxide electron transport layer’, P. de Bruyn et al.Organic Electronics 11, 1419, 2010. The contacting layer 4 is e.g. made by spin casting of a zinc acetylacetonate hydrate solution as precursor material, followed by low temperature annealing under ambient conditions (pyrolysis) resulting formation of ZnO. This has the advantage that only a single processing step is needed to make the contacting layer 4.

35 The resulting electrode layer 10 has a working function of about 4eV, which makes it particularly useful in combination with further semi-conductor materials, e.g. as mentioned above as an electron collecting layer in polymer solar cells.

In an embodiment of the present invention, the substrate 1 is a flexible substrate, e.g. a plastic substrate. Alternatively the substrate 1 is a glass substrate or a metal foil substrate 40 having an insulating layer. Examples of material for the substrate 1 include, but or not limited to 5 glass, Si02, foil materials like PET, PEN, kapton, metal (Ti, Al, Fe, stainless steel etc.), foils with a planarization layer, a plastic substrate with a barrier layer such as SiNx, SiOx, AIOx with or without a planarization layer.

As an alternative, the conducting layer 2 is screen printed, e.g. using Inktec TEC-PA-5 010 hybrid nano silver paste with a silver content of 55+/-10 wt%, a blend of silver nano particles and a soluble silver complex. Using this technique it is e.g. also possible to embed the conducting (patterned) layer 2 in the substrate 1, which allows to obtain a substantially flat surface for applying the next layer. Furthermore, the height of the lines in the conducting layer 2 can be increased (e.g. even up to 0.3 to 20jj,m or even more) allowing e.g. to use Ag lines 10 with large cross sections and thus lower line resistance.

A further exemplary device which may be manufactured using the present invention embodiments is shown in the cross sectional view of Fig. 2. Again, an electrode 10 is formed as described with reference to Fig. 1. On top of the electrode 10, more specifically on top of the ZnO layer 4, an active layer 5 is formed, e.g. a photo active layer of a (polymer) semiconductor 15 material, which active layer 5 is formed by using spin coating technique.

Alternative deposition techniques (also for the other layers described) include printing methods (such as flexo, gravure, reverse gravure, offset, reverse offset, inkjet, pad printing, flat bed or rotory screen printing), coating methods (such as (ultrasonic) spray coating, dip, kiss, slit, wire bar, flow coating, slot die coating, doctor blade, curtain coating etc), spray 20 techniques, plasma (assisted) deposition techniques, electrolytic or electrochemical deposition techniques.

The active layer 5 e.g. comprises P3HT:[C60]PCBM material (a combination of poly(3-hexylthiophene), a p-type semiconductor, and Methanofullurene phenyl-C61-butyric acid methyl ester, an effective solution processable n-type organic semiconductor), which is known 25 as such. A device with such an active layer 5 could be widely applied in photovoltaics (and other (organic) electronic devices. In an exemplary embodiment, the thickness of the active layer 5 is about 260 nm. In further embodiments, the thickness of the active layer 5 is between 20 and 1000nm.

A further electrode 11 is formed comprising a further layer 6 of PEDOT:PSS on the 30 layer of semiconductor material 5 and a further metal layer 7 of Ag on the further layer 6 of PEDOT:PSS. The further layer 6 of PEDOT:PSS material is applied using e.g. spin coating, and in an exemplary embodiment has a thickness of about 1000nm. In further embodiments, the further layer 6 has a thickness of between 900 and 1000nm.

The further metal layer 7 is applied using e.g. a screen printing technique, and in an 35 exemplary embodiment has a thickness of between 100 and 17000nm. In case the further metal layer 7 is screen printed, the thickness may be in the high end of this range. E.g. the commercially available UV curable Rexalpha RA FS FD 018 ink of Toyo Inc may be used, which is a solvent free, UV curable ink. In case the further metal layer 7 is applied as a grid layer as well, e.g. using inkjet printing technique as discussed above with reference to the 40 conducting layer 2, the layer thickness may be in the lower end of the range (100-1000nm).

6

The further metal layer 7 (as solid layer) provides for a reflective layer in the inverted PV cell structure as shown in Fig. 2. The result is a complete PV cell which functions effectively and is produced entirely without any vacuum or high temperature processing steps. In further alternative embodiments, the further metal layer 7 may be made of Al or Cu materials.

5 In the PV cell embodiment shown in Fig. 2, the combination of conducting layer 2 and second layer 3 functions as an charge collection layer, and the contacting layer 4 as an electron transport layer, in contact with the bulk hetero-junction formed by the active layer 5. The further layer 6 of PEDOT:PSS acts as hole transport layer, and the further metal layer 7 as hole collecting layer.

10 In further embodiments, the electrode layer 10 or the further electrode 11 is embodied as a semi-transparent layer using a grid printing technique such as inkjet printing. This allows to manufacture the inverted PV cell structure as shown in the embodiment of Fig. 2, but might also allow to manufacture a ‘normal’ PV cell structure. In an even further embodiment, the electrode layer 10, which in the embodiments described above is a (semi-)transparent 15 electrode, is adapted to be a reflective electrode layer 10. This allows to influence light characteristics in layers above (or below) the electrode layer 10.

The electrode layer 10 may in further embodiments comprise the same type of layers, but in a different order. For single junction organic PV cell devices, triple layer electrode layers 10 may be envisaged. When designating the conducting layer 2 as M (metal), second layer 3 20 as PEDOT, and the contacting layer 4 as MO (metal oxide), the following variations are possible for an inverted PV cell design: M-PEDOT-MO (see description above, M can also be a full layer); PEDOT-M-MO; PEDOT-MO-M. For a regular PV design the conducting layer 2 (M) can be patterned or full, and the electrode layer can be MO-PEDOT-M, MO-M-PEDOT or M-MO-PEDOT.

25 The electrode layer 10 as described above may also be combined with (nano-) structured layers (not shown), which may also be effective in influencing light propagation characteristics in adjacent layers (when present). These structured layers may be added using techniques which are known as such, e.g. from nano-imprint lithography (NIL) techniques.

By careful tuning the layer thicknesses of the high conductive PEDOT layers 3, 6 and 30 the printed Ag layer 2, 7, one can optimize the optical and electrical properties of both the top electrode 11 and bottom electrode 10. This enhances the freedom of design for cell and module structures. For example, modeling shows that effective sheet resistances of 1 Ohm square can be reached with PEDOT:PSS/Ag grid electrodes 10 while maintaining a high transparency (>80% over the visible spectrum). This effective sheet resistance is significantly 35 lower compared to typical transparent conductive oxide (TOO) electrodes.

The freedom of design could be beneficial for aesthetic reasons, to ease integration of a PV function in a product and/or to increase the performance of a PV module.

Increased module performance is possible when the cell size in a PV module increases as this increases the ratio between active area over the total area of a module (less area is 40 used for interconnections).

The triple layer electrode layer 10, 11 in the embodiments as described above may be used for organic PV cells as mentioned, but also for organic LED’s, tandem cells, triple junction cells.

7

In general the steps relating to the various invention embodiment using an all-solution 5 based process can be used in a variety of applications, including but not limited to (Organic) Photo-Voltaic cells, (Organic) LEDs, Organic lighting devices, organic material based electronic circuitry, etc.

The present invention embodiments have been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative 10 implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

A method for providing an electrode layer (10) comprising applying a metal in a solution to a substrate (1) to form a conductive layer (2), applying a conductive polymer in a solution on the top of the conductive layer (2) as a second layer (3), and applying a contacting layer (4) using a solution on the top of the second layer (3). 10

The method according to claim 1, wherein the conductive layer (2), second layer (3) and contacting layer (4) are applied using solution processing in normal environmental conditions.

The method of claim 1 or 2, wherein the metal is selected from the group consisting of Ag, Cu, Al.

The method of claim 1,2 or 3, wherein the conductive polymer is PEDOT: PSS. 20

The method according to any of claims 1-4, wherein the second layer (3) can dry before applying the contacting layer (4).

6. A method according to any one of claims 1-5, wherein the contacting layer (4) comprises zinc oxide, aluminum-doped zinc oxide, titanium oxide or cesium carbonate.

The method of any one of claims 1-6, wherein the substrate (1) is selected from the group of: a plastic substrate; a flexible substrate; a glass substrate, a metal foil substrate with an insulating layer. 30

The method of any one of claims 1-7, further comprising adding a layer (5) of semiconductor material, such as an organic material.

9. Method according to claim 8, wherein the semiconductive material forms a photo-active layer (5).

The method of any one of claims 1-9, further comprising applying a further layer (6) of PEDOTPSS to the layer (5) of semiconductor material and applying a further metal layer (7) to the further layer (6) of PEDOTPSS. 40

The method of claim 10, wherein the further metal layer (7) is a reflective layer.

12. Method as claimed in any of the claims 1-14, further comprising applying a structured layer.

13. An electrode layer comprising a substrate, a conductive layer (2), a second layer (3) of a conductive polymer, and a contacting layer (4) wherein the conductive layer (2) comprises a metal selected from the group comprising Ag, Cu A1, wherein the conductive polymer is PEDOT: PSS, and wherein the contacting layer (4) comprises zinc oxide, aluminum-doped zinc oxide, titanium oxide or cesium carbonate.

14. A photoactive device comprising an electrode layer (10) according to claim 13, a layer (5) of photoactive material, and a further electrode (11).