The Rheology and Processing of Light Emitting Polymer (LEP) Solutions
In 1989, light emitting polymers (LEPs) were first discovered by R. Friend and his colleagues at Cavendish Laboratory, Cambridge University (Holton, 1997). The team found yellow-green light emitted from poly(p-phenylene vinylene), PPV, when subjected to an electric field (Burroughnes et al., 1990) Since then, the studies of LEPs have rapidly grown in both from academic and industrial research.
LEPs belong to a special class of polymers called "conjugated" polymers (Gostick, 1996). These polymers posses delocalised pi-electrons along the backbone. In the sense of mobility of delocalised pi-electrons, LEPs show the potential of semiconductors.
The quality of colours and the efficiency of devices are key issues of the display manufacturing. Since there is a variety of organic conjugated polymers, the possibility of getting a wide range of the visible spectrum from LEPs is very obvious. This is a good sign and makes LEPs become more attractive to display technology. Besides, there are several benefits of LEPs. For instance, unlike LCDs, LEPs have no restrictions on viewing angles and do not require backlight and colour filters. LEPs also can achieve an efficiency of 5% which is comparable to conventional LEDs at low voltages, but provide higher contrast and faster switching speed. In addition, the patterning process of LEPs is simple due to only one transparent substrate coated with polymer layers is required. Therefore, thin, conformable display screens become possible, using flexible plastic substrates.
Fig. I: The basic structure of LEP
In order to pattern LEP devices, polymer films are deposited by spin coating polymer solutions on the glass substrate as shown in the figures below. The spin coating process has been widely used to produce thin films for several applications, such as, photoresists, lithography resists, and protective coatings (Jenekhe and Schuldt, 1984). This technique is simply operated by spinning a disk which is the glass substrate coated with ITO at an angular velocity, w. A small amount of polymer solution is dropped on the top of the disk as shown in Figure II. The solution flows radically outward due to centrifugal force, reducing fluid thickness. The continuous stages of the fluid during the process are pictured in Figure III.
Fig. II: The spin coating process
Fig. III: Film formation
According to a number of researches studied on mechanism of the spin coating process, it is believed that the rheological properties of polymer solutions can effect the quality and thickness of thin films. As a result, the aim of this project is to study the effects of rheological properties of LEPs on the spin coating process.
- Burroughnes, J.H., Bradley, D. D. C., Brown, A. R., Marks, R. N., Friend, R. H., and Gymer, R. W., Chemical tuning of electroluminescent copolymers to improve emission efficiencies and allow patterning. Nature 356, 47-49 (1982)
- Gostick, M., Applications for Light Emitting Polymer Displays. Electronic Engineering, November, 67-69 (1996)
- Holton, W. C., Light-emitting polymers: Increasing promise, Solid State Technology, May, 163-169 (1997)
- Jenekhe, S. A., and Schuldt, S. B., Coating Flow of Non-Newtonian Fluids on a Flat Rotating Disk. Industrial and Engineering Chemistry Fundamentals 23, 432-436 (1984)