Displays and lighting systems made from organic light-emitting diodes (OLEDs) offer superior characteristics over their inorganic counterparts and are also compatible with flexible substrates. Although OLEDs are already ubiquitous in cell phones, their use in a wider variety of applications is currently limited by their short lifetimes. Now, Jwo-Huei Jou, Jing-Jong Shyue and colleagues at National Tsing Hua University and the Academia Sinica, both in Taiwan, have reported a simple method of increasing the efficiencies and lifetimes of OLEDs.1

OLEDs emit light when positive and negative charges, injected from electrodes at either end of the device, recombine with each other to emit a photon. Because organic semiconductors are generally better at transporting positive charges (or holes), there are usually many unpaired electrons inside the device, which reduces the emission efficiency. The researchers had previously demonstrated that the efficiency of a white OLED could be increased by placing polymeric nanoscale dots in the pathway of holes travelling from the positive electrode. This reduces the hole density, which in turn increases the chance of electrons pairing with a hole, and thereby improves the device efficiency. However, this technique was demonstrated only for a relatively dim device.

Fig. 1: The incorporation of polymeric nanodots leads to marked enhancements in efficiency for red, green, blue and white organic light-emitting devices.

In the current work, the researchers functionalized the surface of their nanodots by covering them with either a positive or negative charge. The result was an increase in the efficiency of high-brightness OLEDs across the entire color spectrum; the largest increase was observed for a blue OLED, which nearly doubled in efficiency. Green, red and white devices also improved significantly (Fig. 1). A higher efficiency means that a lower current density can be used to achieve the same brightness, thus increasing the device lifetime.

The team found that both positively and negatively charged nanodots caused an increase in efficiency. Positively charged nanodots repelled holes and thus preventing them from flowing smoothly, whereas negatively charged nanodots attracted and trapped holes; hole densities were therefore reduced in both cases. In addition, greater densities of charge (either positive or negative) led to higher efficiencies.

The technique is compatible with a variety of substrates, as well as with solution-processed OLEDs, which are cheap to make in large volumes. It can also be applied to other kinds of devices, says Jou: “The same nanodots can be incorporated into organic solar cells to modify the carrier transportation and thus enhance efficiency.”