To improve the brightness, contrast and resolution of displays while reducing production costs and energy use, scientists have developed several types of leds, including OLED, quantum dot based LED(QLED), perovsk-based LED and Micro LED. A new study published in The journal Nature shows new advances in OLED using free radicals.

On November 22, The journal Nature published a cooperative paper titled "Efficient Bipartite State Free radical Light-emitting Diode" by the research team of Professor Li Feng and Professor Richard H. Friend of The State Key Laboratory of Supramolecular Structure and Materials, School of Chemistry, Jilin University and Professor Richard H. Friend of The Cavendish Laboratory, Cambridge University.

Professor Li Feng's research team used TTM free radical as the core and two carbazole derivatives PCz and NCz as donors to obtain two efficient charge transfer state (CT) red radical TTM-3NCZ and TTM-3PCZ (as shown in Figure 1). The molecular design of such Donor-Radical structure greatly improves the stability and luminescence efficiency of luminescent Radical molecules. The photoluminescence efficiency of the two molecules in toluene solution reaches 49% and 46% respectively, and the photoluminescence efficiency of the doped film reaches 90% and 61% respectively.

In jilin university, under the support of the plan with the university of Cambridge, Richard h. Friend professor team cooperation, with NCZ TTM, TTM - 3 PCZ doped thin films for the preparation of the OLED light-emitting layer EQE biggest reached 27% and 17% respectively (see figure 2), with 27% of the EQE limit value is close to 100% of ige theory, is now reported from the deep red/near-infrared light emitting diode (LED) of the peak. At the same time, transient spectra and theoretical calculation results show that the luminescence of the device comes from the transition of free radical bilinear exciton SOMO→HOMO. This research result is a major breakthrough in the field of OLED research, showing the application prospect of luminous free radicals in the field of organic photovoltaics, and opening up a new direction for OLED research.

Tetsuro Kusamoto and Hiroshi Nishihara explain that organic radicals have only an odd number of electrons and are therefore highly reactive and chemically unstable. But by changing the molecular structure, some radicals become stable enough in the air. Since 2006, studies have demonstrated the stability of free radical luminescence and the possibility of their use in lighting materials and devices.