One of the challenges in the development of advanced full-color displays and solid-state lighting systems is in finding luminescent materials that emit intense, high-quality blue light with high efficiency. To this end, Chien-Hong Cheng and colleagues at the National Tsing Hua University, Hsinchu1 describe the properties of a group of blue host emitters that satisfy all three of these requirements.

Light emitting organic materials are promising as light sources—such as organic light emitting diodes (OLEDs)—that are more versatile and efficient than conventional incandescent or fluorescent light sources, as wells as being cheaper and easier to produce than devices based on inorganic semiconductors.

Fig. 1: Plot of external quantum efficiency versus current density for the DMPPP-based device (ITO/CuPc/NPB/DMPPP/TPBI/Mg:Ag). Inset: The corresponding CIE color coordinates of this device.

The purity of a color produced by a light source is defined by the Commission Internationale d'Énclairage (CIE) in terms of three color coordinates, x, y and z (Fig. 1). For OLEDs intended for full-color displays, the difficulty has been to find fluorescent materials that emit blue light with a CIE y-coordinate of less than 0.15.

Cheng's group studied pyrene-based diarylbenzenes and the most promising was found to be 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP). Using this dipyrenylbenzene, they fabricated OLEDs that emitted intense blue light with x- and y-coordinates of 0.15 and 0.11, respectively, and a high external quantum efficiency of up to 5.2%.

“The main benefits of our dipyrenylbenzene emitters are their simple preparation, one-step synthesis from commercially available starting materials, and that they satisfy the key criteria for blue OLEDs, the high efficiency as well as good colour purity,” says Cheng.

The high performance of the group's dipyrenylbenzene emitters was mainly due to the twisted conformation of the molecules. “This reduces intermolecular π-π stacking that would cause non-radiative decay and also shortens the effective conjugation length of molecules,” explains Cheng. “Both of the two effects help to move the emission peak towards higher energy in the solid state enabling the efficient emission of light at the far blue end of the colour spectrum.”

Another important issue to consider in the design of OLED materials is their long-term stability. In this respect also, dipyrenylbenzene emitters are promising, exhibiting high glass transition and melting temperatures in the ranges of 97–137 ºC and 303–388 ºC, respectively.