The magic of molecular glue –
A layer of molecules can tell the difference

The very rapid development of conducting polymers since the report of the first electrically conducting doped polyacetylene has already led to the emergence of polymer electronics with products in the market. There is nearly no exception that all these polymer electronics involve polymer/inorganic (e.g. polymer/indium–tin oxide (polymer/ITO), polymer/metal, polymer/inorganic-semiconductor) interfaces. The intimacy, durability and reliability of these contacts critically affect the ultimate device performance because there are substantial charge transfers and varying induced-strain distributions across these interfaces and during device operation. The most elementary criterion of device quality is that the polymer layer must adhere tightly and firmly onto its substrate. Taking polymeric light emitting devices (PLED, Figure 1) as an example, the device performance of PLED (e.g. brightness, lifetime, stability) is always dependent on the intimate contact between the polymer layer and the substrate (e.g. ITO). On the other hand, there is no standardized quantitative method to measure the adhesive strength of polymer layer on substrate. This report addresses this critical problem in new generation polymer electronics (e.g. PLED, polymeric transistors) by providing a tailor-designed measuring method and apparatus for quantitative measurement of adhesive strength and energy, and also through dedicate surface engineering using self assembly monolayers in order to improve the adhesion of the polymer layer on substrate.

Figure 1. Typical structures and examples of PLEDs

Background technical information relevant to this case:
A. Self assembly monolayer (SAM)

Self assembly monolayer (SAM) molecules is a special type of molecules that have an inorganic head group and an organic tail group. Silanes (Figure 1) are examples of SAMs.

Figure 2. Silane type SAMs

B. Scotch tape test

Scotch tape test is a traditional and well-known test method being used in the industry for “qualitative measurement on adhesion”. The adhesion is evaluated by the degree of the film removal from the substrate. Standardized tape test method and its well-established rating scale are described in the ASTM D3359-97 standard. Due to the qualitative nature of this test, it is commonly used to establish whether the adhesion of a film to a substrate is at a generally adequate level.

Our approach to solve the problem:
A.Modified scotch tape method (MSTM) for “quantitative” measurement of adhesive strength

The modified Scotch tape method (MSTM) is a quantitative version of the originally qualitative Scotch tape test. A special adhesion measurement instrument, as shown in Figure 3, has been developed to implement our MSTM to quantify the common Scotch-tape test of adhesion. The instrument consists of a QTEST QT/1L universal loading machine and a homemade sample-mounting device. In a typical test, the substrate is mechanically fixed to the lower metal fixture (100 mm × 100 mm) and a segment of pre-cut Scotch tape of 6 mm × 6 mm is applied onto the polymer face of the substrate. The Scotch tape (3M Scotch Magic tape, ref. no. 810EG) is then fixed to the upper metal fixture by a double-sided adhesive tape (Eagle tape, 665 double-coated film tape). Both the lower and upper metal fixtures are made of stainless steel. The contact surfaces of both fixtures were so machined and polished that the two fixture surface planes are flat and parallel, with an inclination angle < 0.5º between them before they are in contact. In each test, the upper metal fixture is pressed against the lower metal fixture with a constant force of 30 N for 5min; a pull-up load is then applied with a constant crosshead speed of 80 mm/min to the sample. Figure 4 shows a typical force curve measured. Region I corresponds to the release of the compressive force, which acted on the sample before the pull-up motion. Region II corresponds to the first stage of the PFO delamination during which more than half of the film is still in contact with the substrate. Region III corresponds to the residual film delamination. Region IV corresponds to the total detachment of the film, with the recorded constant load representing the weight of the detached upper fixture. From this set of measurements, adhesion can be expressed either by the ‘maximum’ force per unit area or by the total energy required in the film detachment process.

Figure 3. Tailor-designed instrument for MSTM, and a schematic showing the operation principle.

Figure 4. Typical force curve measured by MSTM

B.Insertion of SAM interlayer for increased adhesive strength

We insert silane-based SAMs into the polymer/substrate interface for molecular optimization of the interfacial properties. Two classes of silane-based SAMs are studied. The first group consists of alkyl-silanes with 1 to 8 carbon atoms in the alkyl chains, for revealing the chain-length effects. SAMs with alkyl chain lengths longer than those in this group are not selected because it is expected that the contact resistance across the PFO/ITO will be compromised by the insertion of an insulating layer of more than 1 nm in thickness at the interface.The second group of selected SAMs consists of several phenyl-silanes in which a phenyl ring is attached to the silane group, for revealing the π–π interactions between the SAM and the polymer of interest in this work, polyfluorene (PFO). The chemical structures of these silanes are summarized in Figure 2.

The relative changes in adhesion energy for the SAMs listed in Figure 2 are summarized in Table 1, together with the raw adhesion energy data measured using MSTM we developed. The relative changes in adhesion energy are also plotted in Figure 5, which clearly shows some marked differences in the relative changes of adhesive energy for the SAMs selected in this work. First, insertion of a SAM in the PFO/ITO interface can indeed improve interfacial adhesion. Second, the relative increases in adhesion energy associated with aromatic SAMs are significantly higher than those induced by saturated alkyl SAMs.

Table 1. Summary of raw adhesion energy data and relative adhesion changes for the SAMs in Figure 1 as adhesion promoter between PFO and ITO

As clearly showed by Figure 5, the adhesive energy between PFO and ITO with an interlayer of SAM (SAM-Z) can increase by near 4 times when compared to the case without the use of any SAM as the molecular glue in between.

Figure 5. Relative changes in adhesion energy for PFO/SAM/ITO as a function of SAMs

In conclusion, insertion of a SAM between PFO and ITO can significantly improve the adhesive strength of this important overlayer structure in polymer electronics. Only a layer of molecules (less than 1 nm) can act as a magical glue to hold the polymer firmly onto the substrate. In addition, we have established a tailor-designed instrument and methodology for “quantitative” measurement on adhesive strength, which should find profound applications in other areas.