Asian Dudes Talk White Power

Charges, excitons, and white organic LEDs

Happy April Fool’s Day! The linked article is a Chem. Soc. Rev. on controlling the properties of white OLEDs (for Jess and my mom: white lights made out of stuff containing carbon). Interesting stuff. Most of us chemists are probably familiar with the mobility problem for organic electronics—the fact that organic semiconductors tend to transport charge much worse than silicon. Improving OLED performance is more than just a matter of improving mobility, however, and managing charges and excitons within the active layers of OLEDs is a fundamental, deep way to improve OLED performance in a rational way. White OLEDs are particularly interesting, as they must span an extremely wide array of wavelengths and thus incorporate multiple active materials.

Emission in OLEDs occurs from excited states (excitons) of organic molecules. Excited states come in two flavors: singlets and triplets. Here’s the problem: excitation of organic molecules produces one singlet for every three triplets (on average), and only singlets can emit light efficiently! Emission happens via decay to a singlet ground state. The rule is “singlet-to-singlet OK, triplet-to-singlet NOT OK”. Triplets hang around until heat or some other non-emissive process kicks them back into a singlet state, or until they phosphoresce, which takes way too long for practical switching purposes (imagine turning on a light and waiting ten seconds for full brightness. Blah.). Some creative solutions have been devised to get around this problem, the most fundamental of which involves incorporating a heavy metal atom into the organic species, allowing violation of the rule and speeding up the triplet-to-singlet emissive process. In fact, considering the fact that triplets are longer lived and travel farther, you don’t even have to dope the entire device with metal—just place fluorescent materials close to the excitation site (to deal with singlets) and metal-doped phosphorescent materials farther away (to deal with triplets)! The singlets and triplets will separate spatially because of their differences in diffusion distances, and the triplets will travel farther. “Chromatography for excitonsTM!” In one cool application, different phosphors emitting different wavelengths of light are layered so that as triplets diffuse, multiple wavelengths of light are emitted, producing white light.

With regard to charges, an imbalance of negatively charged electrons and positively charged holes in an active layer can wreak havoc on device efficiency. Electrons and holes need to get together in order for LEDs to produce light. But most organic compounds treat electrons and holes differently, moving one along faster than the other…what to do?! Basically, existing solutions have relied on either helping the organic layer do what it can’t do on its own or slowing down what it does well. Either way, the way the material treats electrons and holes becomes more balanced.

Theories of WOLED operation are still in their infancy, say Wang and Ma, but this is clearly a fascinating area. My interest in organic electronics just got jump-started. Apologies to the blogosphere masters if I screwed anything up.

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3 Comments

  1. About damn time you picked up an interest in the coolest part of organic chemistry! 🙂 A few things, though…

    Most of us chemists are probably familiar with the mobility problem for organic electronics—the fact that organic semiconductors tend to transport charge much worse than silicon.

    Not so true. We’re easily on par with amorphous silicon…and silicon, with an indirect bandgap, isn’t emissive anyway 😉 And we have plenty of ambipolar organic semiconductors; you may want to check out the lit on light-emitting transistors. The trouble–and likely a contributing factor for years and years of sorry electron mobilities in organics–is that often one type charge carrier (generally electrons) is more sensitive to defects than the other.

    Triplets hang around until heat or some other non-emissive process kicks them back into a singlet state, or until they phosphoresce, which takes way too long for practical switching purposes (imagine turning on a light and waiting ten seconds for full brightness. Blah.).

    Phosporescence is generally faster than that, on the order of milliseconds to seconds I think. Wouldn’t be surprised if the quantum yields for that process weren’t fantastic, though–and it can be very difficult to tune the emission wavelengths for triplet emitters.

    Reply

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