Light Emitting Diodes (LEDs)
- Typical LED structure
- Evidence for spin-dependent recombination: More singlets are generated than expected which allows polymer LED to be more than 25% efficient
- CDT's Plastic LED Test Cells
The Group has pioneered the physics of semiconducting polymers as light emitting diodes. The Group is deeply involved in understanding the basic physics of the light-emitting processes, which involves techniques such as excited state spectroscopy.
Structure of LEDs
The diagram shows the structure of a typical polymer light emitting diode. Although "PPV"-related polymers are still of interest elsewhere, much of the effort in Cambridge is now directed at "polyfluorenes".
In a polymer LED, light is emitted when positive hole and negative electron charge carriers are injected into the organic semiconductor metallic contacts made to opposite sides of the semiconducting polymer films. When electrons and holes come under the influence of their mutual Coulomb attraction inside the device, they recombine with each other, emitting a photon. The wavelength of the emitted light depends on the band gap of the semiconducting polymer. Hence, the output colour can be altered by chemically tailoring the polymer.
Role of spin
Electron and holes are spin-1/2 particles, and the exciton that is formed can either be a S=0 singlet exciton or a S=1 triplet exciton. From simple spin statistics one would expect 25% of singlet excitons and 75% triplet excitons to be formed if the spin of the incoming charge carriers is random. In a polymer material with weak spin-orbit coupling only the singlet excitons can recombine radiatively, since the ground state is a singlet state. From this one would expect that the efficiency of polymer LEDs cannot exceed 25%. However, in practice many polymer LEDs exceed this limit, and we have recently been able to show that the process of electron-hole capture leading to exciton formation is spin-dependent. Approaches to improve the performance of polymer LEDs include the incorporation of heavy atoms to increase spin-orbit coupling and enable harvesting of triplet excitons.
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