Organic light-emitting diodes (OLEDs) are being increasingly used in commercial applications such as displays and mobile phones, where their light weight and flexibility are considerable advantages. However, while OLEDs are very cost-efficient, their overall emission efficiency and device performance remain lower than those of their inorganic LED counterparts.

In a collaboration between the South China University of Technology in Guangzhou, China, the Japan Optoelectronic Industry and Technology Development Association, and Yamagata University in Japan, Shi-Jian Su, Junji Kido and co-workers1 have now developed a novel organic material that optimizes energy levels for enhanced electron transport and ultralow operating voltages. “The extremely low voltages we have achieved translate into a simplified OLED structure without compromising efficiency,” says Su.

Fig. 1: Schematic illustration of the structure of an OLED. Optimizing the combination of organic molecular blocks used for the electron transport layer lowers the operating voltage to near the theoretical limit.

An OLED consists of a number of layers (Fig. 1) that transport either electrons or electron vacancies (‘holes’) from electrical contacts to the active region (emissive layer), where they recombine to emit light. Efficient charge transport is therefore crucial in order to obtain good device performance.

Each layer is made from specifically selected organic compounds with energy levels designed to channel electrons and holes to the active layer. Tailoring the behavior of these layers can be complex. For example, holes must be blocked from entering the electron-transporting layer by inserting a layer with energy steps that the holes cannot overcome. However, these energy steps also cause voltage drops across the structure, increasing the operating voltage and the overall power consumption of the OLED. It is therefore important to optimize the energetic barriers between each layer.

In their study, the researchers focused on the electron-transporting layers. They studied a range of organic materials based on building blocks made from triazine, benzene and pyridine components. The combination of these building blocks made it possible to fine-tune the energy states of the organic materials so that an ideal candidate material for electron transport could be designed. The operating voltage of OLEDs made using this material is extremely low, approaching the theoretical limit and becoming comparable even to conventional inorganic LEDs.

Although lowering the operating voltage is an important step toward efficient OLEDs, the transport of holes remains a problem, and improved materials for hole transport are the next target, says Su. “With a better hole-transport material, we expect to achieve improved efficiency while maintaining low operating voltage.”