By inserting platinum (Pt) atoms into an organic semiconductor, a group led by physicist Z. Valy Vardeny at the University of Utah (Salt Lake City, UT) has tuned the polymer to emit light of different colors—a step toward truly white organic LEDs (OLEDs) for lighting.1 Existing white-emitting OLEDs use other techniques for white output, such as combining a blue OLED with a yellow phosphor (Novaled; Dresden, Germany), or phosphorescent approaches such as those by Universal Display Corporation (Ewing, NJ) and Konica Minolta (Osaka, Japan).
Six types of spectroscopy used
The team of researchers from the University of Utah Los Alamos National Laboratory (Los Alamos, NM), and Nanjing University of Science and Technology (Nanjing, China) synthesized polymers containing intrachain Pt atoms separated by one (“Pt-1”) or three (“Pt-3”) organic spacer units. They then used a number of nonlinear types of spectroscopy—including broadband ultrafast, continuous-wave pump-probe photomodulation (PM), electroabsorption (EA), and two-photon absorption (TPA)—as well as absorption and photoluminescence spectroscopy (which are both linear) to characterize the polymer (see figure). The reason for so many types: some probe excited electronic states having either odd or even symmetry, while others probe those having both symmetries.
The Pt-1 arrangement emits violet and yellow light; the other version, Pt-3, emits blue and orange light. By varying the amount of platinum in the polymer, the physicists could create and adjust emissions of fluorescent and phosphorescent light, as well as adjust the relative intensity of one color over another. “This polymer emits light in the blue and red spectral range and can be tuned to cover the whole visible spectrum; as such, it can serve as the active [or working] layer in white OLEDs that are predicted to replace regular light bulbs,” says Vardeny.
Triplet state accessible
The new platinum-doped polymers can convert more energy to light than other OLEDs now under development, Vardeny says. This is because the addition of platinum to the polymer makes accessible more energy stored within the polymer molecules: in addition to the singlet state already accessible by conventional OLEDs, the normally inaccessible triplet state is usable in the new technique.
The researchers found from their spectroscopic studies that the lowest singlet state in the Pt-1 arrangement was a “metal-to-ligand charge transfer” (MLCT) state, which lies below the lowest excited exciton; however, the order is reversed in Pt-3. The Pt-1 arrangement has a larger “intersystem crossing” (ISC) rate (intersystem crossing is a radiationless transition in the state of an electron, here between singlet and triplet states).
What is needed for practical devices is actually a smaller ISC rate, which leads to a larger fluorescence emission; thus the Pt-3 arrangement might be better for use in white-light OLEDs. But the crux of the researchers’ results is that tailoring molecular details is a good approach for finding the optimum properties for a practical photonic device based on Pt and an organic semiconductor.
However, the polymers in the study are not yet OLEDs, as they are pumped optically rather than electrically. Vardeny predicts a span of about one year until the design of a “platinum-rich pi-conjugated polymer” that is tuned to emit white light when stimulated by light, and about two years until development of true white OLEDs.
1. C.-X. Sheng et al., Sci. Rep., 3, 2653 (2013); doi:10.1038/srep02653.