The explosion in research into organic light-emitting diodes (OLEDs) has led to the design of efficient light emitting molecules of almost every color in the visible spectrum. However, the search for high-efficiency infrared emitters continues, not least because devices that could address the wavelength window for communications well into the infrared—1310nm or 1550nm—would be ideal for applications.

Fig. 1: The electroluminescence spectra and the multilayered structure (inset) of the OLED devices.

Now Zhi Yuan Wang from Carleton University in Canada and colleagues at the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences1 have synthesized a set of near-infrared-emitting chromophores that have strong emission at wavelengths greater than 1000nm. When used in OLEDs the devices have high efficiency, showing promise for future applications including eye-invisible displays.

The researchers created molecules with a donor–acceptor structures, because their bandgap levels, and therefore their emission colours, can be tuned by changing the strengths of the donors and acceptors. The molecules used here had donor groups on either end and acceptor groups in the middle, all connected together with pi-conjugated spacers. These structures have the added advantage that they are not totally planar molecules, so they do not have a tendency to stack on top of each other in aggregates. Avoiding aggregation is important, because it would lead to quenching of emission, which would in turn lower the emission efficiency of the chromophores.

After synthesis, layered devices were made using charge-conducting layers on either side of the layer of emitting molecules. The researchers demonstrated devices with 0.28% efficiency with an emission peak at 1080nm, and no emission below 1000nm, which causes problems with some chromophores.

Wang says, “through molecular engineering we develop a class of chromophores whose bandgap can be readily tuned using different electron donor and acceptor groups. We have demonstrated a nearly tenfold increase in the efficiency of non-doped OLEDs in comparison with devices based on ionic dyes.”

For applications, the emission wavelength is vital. “The longest emission at 1220 nm, very close to the telecommunication wavelength of 1310 nm, is achieved for the first time for non-doped OLEDs,” says Wang. “The chromophores should also have potential for applications such as infrared fluorescent tags for bio-imaging and sensing,” he continues, because of their “strong emission in the solid state and highly efficient photoluminescence.”