Conjugated Polymer Exhibits Phosphorescence
Conjugated polymers are a class of materials that hold promise as a basis for electro-optical applications, but that promise has been blunted by the fact that electrical injection creates a substantial proportion of long-lived dark states. Now metallic contamination in ladder-shaped poly(para-phenylene) polymer (LPPP) has been shown to display electrically induced phosphorescence.
When two monomers assemble into the poly(para-phenylene) structure shown here and the units combine into a long, ladderlike structure, triplet excitons travel along the "ladder" to a metal atom, where they recombine and phosphoresce. Courtesy of John M. Lupton.
John M. Lupton at Max Planck Institute for Polymer Research in Mainz, Germany, and his colleagues at Graz University of Technology in Austria and Universität Potsdam in Golm, Germany, created diphenyl-substituted LPPP (PhLPPP) in the search for new LED materials. During the formation of PhLPPP, palladium contamination is introduced as a byproduct of polymer synthesis -- a good thing, as it turns out. The new polymer has provided a tool with which to investigate the physics of triplet excitons, which are notoriously difficult to measure because of their lack of coupling to readily accessible states.
In the researchers' work with electrically stimulated luminescence, they observed in the material not only the expected singlet peaks at 460 and 500 nm, but also phosphorescence in the neighborhood of 600 nm. The 460-nm peak rapidly decreased after a voltage pulse ended, but the 600-nm phenomenon was longer-lived. The intensity of the 460-nm peak also varied as a function of bias voltage, while the longer-wavelength luminescence was unaffected.
This implied that the two peaks are created through significantly different mechanisms: specifically, that the 460-nm peak arises through singlet-state excitons, created by recombination of charge carriers, while the 600-nm peak is due to triplet-exciton coupling to a previously unseen radiative state. In this model, the triplet excitons travel along the ladder structure of the PhLPPP until they reach a site where local distortions of the electronic structure allow coupling to a radiative state.
By examining the elemental abundance of the polymer, Lupton's group identified palladium contamination at 79 ppm, or approximately one atom of palladium for every 1700 monomer units of the polymer structure.
A general investigation
The triplet electroluminescence depends upon a balance between the distance between the palladium sites and the exciton diffusion length. In LPPP with lower palladium concentrations, significant numbers of excitons do not reach the metal atom sites. In LPPP with higher concentrations, the bulk electronic structure of the polymer is distorted such that triplet exciton formation is enhanced in the first place.
Because LPPP is the only essentially pure hydrocarbon conjugated polymer that allows direct observation of triplet excitons, the group will continue to use it to investigate the dynamics of triplet excitons. "This will certainly have implications for the design of materials for polymer LEDs, but we are presently taking a more basic approach to this," Lupton said.
Such a general investigation should yield other interesting results. "This method of triplet-exciton deactivation provides a way for dealing with triplet excitons in solid-state organic dye lasers, where triplets generally accumulate and ruin the gain," he said.
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