Chlorination Improves Organic Electronics

Chlorinating a common electrode material for organic light-emitting diodes (OLEDs) could make devices easier and less expensive to manufacture, researchers report (Science, DOI: 10.1126/science.1202992).

In a typical OLED, electrons move from an organic, light-emitting material to indium tin oxide. But the energy of the electrons removed from the light-emitting material is higher than the oxide can accept. Consequently, layers of other materials—for example, copper phthalocyanine—are used to bridge the gap and facilitate electron flow. The additional layers, however, add cost and complexity to manufacturing and reduce the electrical efficiency of electronic devices.

In an effort to do without those extra layers, a research group led by materials science and engineering graduate students Michael G. Helander and Zhibin Wang and professor Zhenghong Lu at the University of Toronto chlorinated the electrodes by exposing the material to o-dichlorobenzene and ultraviolet light. The treatment causes chlorine radicals from the solvent to displace oxygen and bind to indium on the electrode surface.

The resulting layer of polar In–Cl bonds increases the electrostatic potential just above the electrode’s surface. That change in potential increases the electron energy that the electrode can accept and closes the energy gap between the electrode and light-emitting materials, such as a phosphorescent iridium complex doped into 4,4´-N,N´-dicarbazole biphenyl. Electrons can then directly transfer between the light-emitting layer and the chlorinated electrode, making electronic devices easier to manufacture and more efficient to operate.

The prototype devices made by the Toronto group show a substantial improvement in operating voltages and efficiency, says Franky So, a materials science and engineering professor at the University of Florida. “This approach might lead to a paradigm shift in OLED technology.”