In a step that could lead to longer battery life in smartphones and lower power consumption for large-screen televisions, Stephen Forrest of the University of Michigan and his research team have extended the lifetime of blue organic light-emitting diodes (OLEDs) by a factor of 10.
Blue OLEDs are one of a trio of colors used in OLED displays such as smartphone screens and high-end TVs. The improvement means that the efficiencies of blue OLEDs in these devices could jump from about 5% to 20% or better in the near future, according to the researchers.
Phosphorescent OLEDs, also known as PHOLEDs, produce light through a mechanism that is four times more efficient than fluorescent OLEDs. Green and red PHOLEDs are already used in these new TVs—as well as in Samsung and LG smartphones—but the blues are fluorescent.
“Having a blue phosphorescent pixel is an important challenge, but they haven’t lived long enough,” said Forrest, the Paul G. Goebel Professor of Engineering.
Forrest’s group, in collaboration with researchers at Universal Display Corp., have previously shown that, for blue PHOLEDs, a concentration of energy on one molecule can combine with that on a neighbor, such that the total energy is enough to break up one of the molecules. It is less of a problem in green- and red-emitting PHOLEDs because it takes lower energies to make these colors.
“That early work showed why the blue PHOLED lifetime is short, but it didn’t provide a viable strategy for increasing the lifetime,” said Yifan Zhang, a recent graduate from Forrest’s group who is first author on a new study published in the September 24 issue of Nature Communications (DOI: 10.1038/ncomms6008). “We tried to use this understanding to design a new type of blue PHOLED,” says Zhang.
The solution, which was demonstrated by Zhang and Jae Sang Lee, a current doctoral student in Forrest’s group, spreads out the light-producing energy so that molecules are not as likely to experience the bad synergy that destroys them.
The blue PHOLED consisted of a thin film of light-emitting material sandwiched between two conductive layers—one for electrons and one for holes. Light is produced when electrons and holes meet on the light-emitting molecules.
If the light-emitting molecules are evenly distributed, the energetic electron–hole pairs tend to accumulate near the layer that conducts electrons, causing damaging energy transfers. Instead, the team arranged the molecules so that they were concentrated near the hole-conducting layer and sparser toward the electron conductor. This drew electrons further into the material, spreading out the energy.
The new distribution alone extended the lifetime of the blue PHOLED by three times. Then, the team split their design into two layers, halving the concentration of light-emitting molecules in each layer. This configuration increased the lifetime tenfold.
