Intermediate Bonding Layer

In addition to testing alternative PEDOT:PSS brands, the use of a bonding layer will also be investigated. It is important that the material used as the bonding layer does not disrupt the electroluminescent mechanism; it would be useless to produce a well adhered part if it does not light up.

The material trialled as the intermediary layer between the PEDOT:PSS and the phosphor materials was a mixture of the two materials. Three different ratios were tested; 1:4 (A), 1:1 (B) and 4:1(C) (the ratios given are volume of diluted PEDOT:PSS

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to the volume of diluted phosphor). The miscibility, drying time, transparency and adhesion to adjacent layers are all important factors when considering if this mixture and in what ratio is suitable for use in the EL devices; but more importantly the inclusion of a bonding layer must still produce a working device.

Figure 8-3 shows the bonding layers against light and dark backgrounds respectively; it can be seen that from A to C the mixtures are darker in colour but more transparent. The layers sprayed evenly and were dried at 130 C in 3 minutes.

Figure 8-3: The three ratios of bonding layer (made using the Clevios PEDOT:PSS) shown against light and dark backgrounds.

Electroluminescent devices were produced using all three different mix ratios (A, B and C) along with a device with no bonding layer (D) for comparison; these can be seen in Figure 8-4. These tests highlighted one possible problem when using an additional bonding layer; some of the mixtures did not mix completely homogeneously and this had implications in the appearance of the device and caused blockages in the airbrush. This is discussed further in section 8.1.4.

Figure 8-4: EL devices made using an additional bonding layer in between the PEDOT:PSS and phosphor. The ratios of PEDOT:PSS to phosphor are A (1:4), B (1:1), and C (4:1); D has no bonding layer.

The devices made all illuminated successfully (an example can be seen in Figure 8-5); this shows that including a PEDOT:PSS-Phosphor mix bonding layer in the structure is a viable route to follow so this option will also be used in the adhesion tests.

Figure 8-5: A working EL device made with an additional bonding layer in between the PEDOT:PSS and phosphor.

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Adhesion tests were then carried out investigating both the use of different PEDOT:PSS brands and the addition of a bonding layer (using each transparent electrode material); the aim of these tests was to quantify the adhesion to determine the best combination to use in the injection moulding process.

8.1.3 Adhesion Tests

Since the problems experienced during in-mould layer application and insert moulding are due to poor adhesion between the transparent electrode and the emissive layer, a set of experiments were designed to determine the layer combination with the best adhesion by varying the electrode material and use of a bonding layer.

An experimental plan was designed to compare the adhesion of different layer combinations; three transparent electrode materials (ITO-R, Orgacon™ and Clevios™) and four bonding layer options (1:4, 1:1, 4:1 electrode-phosphor mixes and no bonding layer) were considered. Each bonding layer option was replicated with each electrode material (giving 12 different permutations) and repeated 3 times (totalling 36 test samples).

Since the layers to be tested were thin (in the region of 10’s of microns) and the forces required to separate them were small, a peel test was deemed unsuitable. It was decided that a cross cut adhesion test would be the most suitable in this situation. Layers would be applied using an airbrush, a 10 x 10 grid is scored into the surface and then an adhesive tape is applied and removed; this test is defined in ISO 2409 [153].

Counting adhered squares would provide a percentage adhesion result and a comparison between samples could be made. The only layers to be applied in the tests were the transparent electrode, the phosphor and the bonding layer (where appropriate); this was because during experimentation there were no adhesion problems at the silver-dielectric and dielectric-phosphor boundaries. Since only the PEDOT:PSS-phopshor interface is of interest, test can be carried out on any suitable substrate material.

8.1.3.1 Cross Cut Test 1

The layers were applied to a polycarbonate substrate as this is compatible with the EL materials and the same material that was used to make the traditional, thick layer and airbrush devices; this differs from the injected substrate material but the important measurement in this test was solely the adhesion between the PEDOT:PSS and the phosphor so it was irrelevant what substrate material the layers are mounted on. In order to match the adhesion test as closely as possible to the method used during experimentation, the EL materials were applied in the same order as both the in-mould method and insert production; first the PEDOT:PSS transparent electrode, then the bonding layer and phosphor respectively (see Figure 8-6). This was done because any curing/drying that occurs during the process may affect adhesion so the drying sequence was replicated exactly.

Figure 8-6: A diagram showing the sequence of layer application in cross cut test 1; PEDOT:PSS, bonding layer (where applicable), then phosphor.

One set of samples were made (using the Clevios™ brand of PEDOT:PSS) and testing began; the sample was scored and the tape applied and removed. Despite the fact that this was a layer combination (Clevios™ and no bonding layer) that separated during experimentation there was 100 % adhesion in this test (the sample is shown in Figure 8-7). Two other samples of the same combination were tested but they both had 100% adhesion, it was clear the test was not valid.

Figure 8-7: A Clevios™/no bonding layer/phosphor sample showing almost 100% adhesion in cross cut test 1.

In order to improve the adhesion test some modifications to the method were made. Since the ejection and insert removal normally occurs when the sample is still relatively hot, following production, the samples were all scored then heated in an oven to 130C (temp. of part approx. 30s after injection). The adhesive tape was

added and removed while still hot but the result was the same, 100 % adhesion was achieved between materials that do not adhere in-mould. The test was redesigned.

8.1.3.2 Cross Cut Test 2

After further consideration it was decided to perform the same cross-cut test (The ISO 2409 [153]) but with the layers applied in the reverse order; first the phosphor, then the bonding layer and the transparent electrode respectively (see Figure 8-8). This was justified because during ejection and insert removal, the transparent electrode is the outermost layer, so this was replicated in this adhesion test.

As in cross-cut test 1, a layer combination that separated during experimentation (Clevios™ transparent electrode and no bonding layer) remained 100 % adhered after the tape was removed (as seen in Figure 8-9); again this meant that the test was not valid.

Figure 8-8: A diagram showing the sequence of layer application in cross cut test 2: phosphor, bonding layer (where applicable), then PEDOT:PSS.

Even incorporating similar heat conditions the devices undergo during part ejection and insert removal, the test samples are not adequately imitating the process conditions of the entire process occurring during experimentation. Other factors such as temperatures, pressures and shear forces that occur during injection moulding must be affecting the adhesion between the transparent electrode and the phosphor, and these need to be considered when designing an adhesion test.

The ISO 2409 [153] cross cut test was clearly unsuitable and a new (non-standard) test was designed. A test was needed that incorporates injection moulding conditions but retains the variable combinations and the easily measured percentage adhesion result.

8.1.3.3 Insert Moulded Cross Cut Test

Initial thoughts to improve the adhesion test were to transfer the cross cut test in- mould, but there would be problems when scoring the layers with the 10x10 grid; either the mould tool could be damaged if the test was performed directly in-mould or the substrate film could be perforated if an insert moulding method was used.

The problem associated with scoring the grid was overcome by using a masking template producing a 10 x 10 matrix of squares when applying the transparent electrode layer (see Figure 8-10); this provides the one hundred square grid that will allow for an easy percentage measurement/quantification of adhesion. The use of a template limits the test to using an insert as the template does not follow the contour of the mould tool so a PTFE film insert method was used.

Figure 8-10: The 10x10 masking template grid used when making the inserts for the in-mould cross cut test.

The layer order was the same as if a working device was being produced (first the grid patterned PEDOT:PSS electrode, then the bonding layer (where applicable) and the phosphor respectively).

It was soon clear that this method of testing is also not valid. Figure 8-11 shows a test that again measures 100 % adhesion between Clevios™ PEDOT:PSS and the phosphor (no bonding layer); once again, this differs greatly to the adhesion achieved in-mould during experimentation.

Figure 8-11: Clevios™/no bonding layer/phosphor showing almost 100% adhesion in the insert mould cross cut test.

Since none of the methods used so far have adequately recreated the conditions 40mm

terminated, and an alternative method of measuring the best layer adhesion will be used.

8.1.3.4 In-mould Adhesion Observations

The adhesion problem still remains and since none of the cross cut tests adequately recreate the conditions that occur in-mould, the solution must be to manufacture full structure devices in-situ and compare the resulting layer adhesion. Full structure devices (PEDOT:PSS, phosphor, dielectric and silver layers) were applied in-mould onto a 65 °C tool using an airbrush and moulded over using PP at 220 °C; the ACMOS release spray was used in each case as it had already been established that this significantly improves the release of the PEDOT:PSS from the tool. Each transparent electrode material was used both with and without a 1:1 (PEDOT:PSS phosphor mix) bonding layer; further bonding layer ratios could be tested if the presence of a bonding layer shows a significant improvement to adhesion. To quantify the adhesion, a 20 x 10 grid was drawn onto a transparent film, this was placed over the moulded part and size of the successfully adhered area was visually measured; this can be seen in Figure 8-12.

Figure 8-12: A moulded part and the grid used to measure adhesion; this part was moulded with ITO-R

Measurements were made for all PEDOT:PSS and bonding layer options and combined with other observations to analyse the effectiveness of the adhesion improvement ideas.