CAREER: Using Photonic Crystals to Control the Emission of Rare Earth-Doped Semiconducting Polymers

Stanford University, Stanford CA

Investigators

Abstract

Light-emitting diodes (LEDs) made with semiconducting (conjugated) polymers now have quantum efficiencies as high as 4 % and operating lifetimes of 50,000 hours. Several companies are developing flat panel displays with these LEDs. One of the limitations of polymer LEDs is that triplet excitons are non-emissive. Initial steps towards solving this problem by transferring energy to phosphorescent molecules and rare earth complexes have recently been taken. One of the goals of this project is to optimize energy transfer and charge transport in "doped" polymer films so that high quantum efficiencies, low operating voltages and high device stability can be obtained. Another limitation of current polymer LEDs is that most of the emitted photons are trapped in the device by total internal reflection and that the photons which do escape cannot conveniently be collimated into a beam or coupled into a waveguide. The second major goal of the project is use photonic crystals, e.g. one-dimensional dielectric stacks and two-dimensional gratings, to control the directionality of emission. Previous attempts to do this have not been fully successful because it was not possible to make photonic crystals with a photonic band gap wide enough to completely control the emission of conjugated polymers, which have an emission spectrum with a width of more than 100 nm. By using rare earth-doped polymers, which have emission bandwidths of less than 4 nm, it will be possible to use photonic crystals to control the directionality of emission. This project will not only increase the efficiency and functionality of polymer-based LEDs, but will also provide a convenient light source for developing the science and technology of photonic crystals. The project provides many excellent research opportunities for students. They will work with a team of chemists to design new rare earth complexes and learn the quantum mechanics that regulate energy transfer, charge transfer and light emission. They will interact with members of industry to learn how to optimize polymer LEDs. They will use computer modeling to design one-, two-, and three-dimensional photonic crystals to control the emission of light. They will also get to interact with a team of researchers at 3M and have the opportunity to push polymer-photonic crystal science and technology into new directions. Most of the research will be done by two graduate students, but there will be many opportunities for undergraduates and M.S. students to take on short-term projects. New courses on nanotechnology and organic optoelectronics will be developed to prepare students for research. The course on nanotechnology is designed to make students in several departments aware of the opportunities in this exciting area and to foster multidisciplinary research. The course on organic optoelectronics will have several lab sessions in the PI's labs so that students can reinforce what they learn in class. High school teachers will visit the labs during the summer and be trained to use a kit of optical equipment so that they can demonstrate photonics experiments to their students.