White Light Emission of Polystyrene (PS) Thin Films Deposited by Spray Pyrolysis

The optical, structural and luminescent characteristics of polystyrene PS films doped with luminescent organic pigments (LOP) and deposited by spray pyrolysis technique are reported. The films are deposited on glass, quartz and GaN at low temperatures, using chemical solutions of recycled PS (from stayfoam) and LOPs as precursors, and N.N-dimethylformamide, as solvent.

The thickness of the films was around 2.5 μm films showed at deposition rates between 100 and 200 Å/s. Doped films have roughness between 100 and 2500 Å depending on the kind of pigment used and the surface morphology shows that the organic pigments incorporate into the polystyrene host forming globular particles of about 2 µm in diameter. The organic pigments used have luminescent emissions peaked at 445, 510 and 590 nm.

Non-doped films are very transparent and smooth; on the other hand, doped films are opaque and rough. The LOPs luminescent emission wavelengths were preserved when they were incorporated in the PS films. Using an appropriate mix of OPs it was possible to obtain films that have white light emission. These multi doped films were deposited on violet and blue light emitting LED chips, achieving a complete white light source device. It was determined that the exposure to the violet light from the LED degrades the luminescent characteristics, however, the blue light has is not energetic enough to cause a degradation of the luminescent layer over similar period of time.

Emissions intensity behavior of photoluminescence depends of doping concentration and deposition temperature. The luminescence intensity had a maximum when the films were deposited at 240 ⁰C in every case, save for the use of different concentrations of  LOPs (~0.025 to 4%) in solution.

The surface morphology and the root mean square surface roughness for PS thin films were determined by Atomic Force Microscopy, By UV-Vis technique, %T was measured. In addition, infrared spectra, Scanning Electron Microscopy images and EDS chemical composition results are also shown.

Acknowledgments:

The authors would like to thank SECITI (before ICyTDF) project PICSO12-119, and CONACyT México because of financial support.

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