Let There Be Light

[2]: In a typical organic light-emitting device, luminescent molecular excited states are generated in the electron transport and luminescent layer near its border with the hole transport layer.

Both polymeric and small molecule OLEDs [Fig. 2] operate by accepting charge carriers of opposite polarities, electrons and holes, from the cathode and anode contacts, respectively. An externally applied voltage drives these carriers into the recombination region where they form a neutral bound state, or exciton. There are two types of excitons formed, called singlets and triplets. On average one singlet and three triplets are formed for each four electron-hole pairs injected into the exciton formation region of the OLED.

Quantum mechanics allows de-excitation (or recombination) of the singlet within a few nanoseconds of formation. This leads to a photon emission and is called fluorescence. Recombination of the triplet exciton is slow (taking about 1 ms to 1 second) and when it does occur, usually results in heat rather than light. But If a heavy- metal atom such as iridium or platinum is placed in an otherwise organic molecule, the characteristics of singlet and triplet excitons mix, speeding the emission of light to within 100 ns-100 µs. This kind of emission is called phosphorescence.

Recent advances in boosting the efficiency of OLED light emission have led to the possibility that OLEDs will find early uses in many battery-powered electronic appliances such as cell phones, gameboys, and personal digital assistants. Typical external quantum efficiencies of OLEDs (defined as the ratio of the number of emitted photons to the number of injected electrons) made using a single fluorescent material that both conducts electrons and radiates photons are greater than 1 percent.

But by using guest-host organic materials systems where the radiative guest fluorescent or phosphorescent dye molecule is doped at low concentration (typically between 0.5 and 5 percent) into a conducting molecular host thin film, the efficiency can be substantially increased to 10 percent or higher for phosphorescence or up to approximately 3 percent for fluorescence.

Currently, efficiencies of the best doped polymer and molecular OLEDs exceed that of incandescent light bulbs. Efficiencies of 20 lumens per watt have been reported for yellow-green-emitting polymer devices, and 40 lm/W attained for phosphorescent molecular OLEDs, compared to less than 20 lm/W for a typical incandescent light bulb. It is reasonable to predict that soon, efficiencies of 80 lm/W—a value comparable to that of fluorescent room lighting—will be achieved using phosphorescent OLEDs.

A fundamental difference between small -molecule and polymeric devices is the manner in which they are constructed. Small molecules are deposited in thin layers around 5-100 nm by high-vacuum thermal sublimation. That technique heats powder sources of the molecules in a vacuum to a temperature high enough to evaporate, but low enough to avoid their decomposition (about 250 ^o to 450 ^o C). The evaporated molecules then collect on a cool substrate to form a film whose thickness can be precisely controlled to near monolayer accuracy.