Figure a. Molecular structures of TTM, TTM-3NCz and TTM-3PCz; b. EQE curves and energy levels of radical-based OLEDs.

With the support of the National Natural Science Foundation of China (Project No. 51673080，91233113), an international research team led by Prof. Feng Li from Jilin University and Prof. Richard H. Friend from Cambridge University has discovered that semiconducting molecules with unpaired electrons, termed ‘radicals’ can be used to fabricate very efficient organic-light-emitting diodes (OLEDs) with external quantum efficiency (EQE) up to 27%, exploiting their quantum mechanical ‘spin’ property to overcome efficiency limitations for traditional, non-radical materials. The research results were published in Nature on November 22, 2018, entitled "Efficient radical-based light-emitting diodes with doublet emission ". The website link is: https://www.nature.com/articles/s41586-018-0695-9.

Light-emitting diodes (LEDs) are the key components in the field of lighting and display. Compared to traditional LEDs, OLEDs have important application values and commercial prospects because of the advantages such as ultra-thinness, high contrast and flexibility. For traditional compounds (i.e. non-radicals without an unpaired electron) used in OLEDs, quantum-mechanical-‘spin’ considerations dictate that charge injection forms 25% bright-‘singlet’ and 75% dark-‘triplet’ states in OLED operation. Thus, the upper limit of internal quantum efficiency (IQE) of traditional OLEDs is 25%. In the past three decades, how to utilize triplet excitons to achieve 100% IQE is always the challenge. So far, scientists mainly focus on two ways, phosphorescence and thermally activated delayed fluorescence (TADF) to achieve 100% IQE. However, Li et al. didn’t confine themselves to the paths mentioned above, they demonstrated that electrical excitation of a luminescent radical leads to the formation of bright ‘doublet’ excited states which emit deep-red light with near-100% IQE.

Based on their previous research, Li et al. designed and synthesized two highly efficient charge-transfer (CT) luminescent radicals with deep-red emission, TTM-3NCz and TTM-3PCz (Figure a). The donor-acceptor-type and asymmetric molecular structure significantly increases the photo luminescent quantum efficiency (PLQE) and stability of radicals. The PLQE values of their toluene solutions and doped films in 4,4-bis(carbazol-9-yl)biphenyl (CBP) matrix (3.0 wt%) are 49%, 46% and 90%, 61%, respectively. The maximum EQEs of TTM-3NCz- and TTM-3PCz-based OLEDs can be up to 27% and 17%, respectively (Figure b), such an EQE of 27% is the highest value reported for deep-red/infrared LEDs so far. Transient absorption and fluorescence spectra and theoretical calculations verified that the emission of radical-based OLEDs comes from the transition between singly occupied molecular orbital (SOMO) and highest occupied molecular orbital (HOMO). This significant breakthrough in the field of OLED exhibits the great application potential of luminescent radicals and also paves a new way for the research of OLED.