Efficient light-emitting diodes from organic radicals with doublet emission

Abstract

Organic light-emitting diodes (OLEDs) with doublet-spin radical emitters have emerged as a new route to efficient display technologies. In contrast to standard organic semiconductors, radical materials have unpaired electrons. This feature results in the most well-known examples of organic radicals being where they are reactive species in chemical reactions 1 . Stabilised radicals can be used in optoelectronic applications which exploit their optical and spin properties, allowing up to 100% internal quantum efficiency (IQE) for electroluminescence 2 . Highly efficient OLEDs have been demonstrated which operate in the doublet-spin electronic state manifold with doublet emission 2,3 . The radical-based devices present a departure from the singlet-and triplet-level considerations which impose efficiency limits in OLEDs for typical organic semiconductors (25% IQE). This Perspective focuses on radical doublet emitters for optoelectronics, outlining how the photo-and spin-physics of unpaired electron systems present new avenues for research in light-emitting applications. 3 I. DOUBLET EMISSION FOR OLED DEVICESElectron and hole recombination from conduction and valence bands results in light emission for semiconductor systems. These electron-hole excited states are known as excitons. Organic semiconductors intrinsically screen the coulombic interaction of electric charges less than their inorganic counterparts, a result of the lower dielectric constant in organic molecular solids 4 . The stronger interaction of charges within organic semiconductors can give rise to Frenkel excitons, where electron-hole pairs are more tightly bound than those in the Wannier-Mott excitons found in more classical inorganic semiconductors. Frenkel excitons generally have stronger transition dipole moments for more efficient light emission in optoelectronic devices. The 'organic' approach allows more flexible manufacture of light-emitting layers than devices based on III-nitride semiconductors 5,6 , as well as easily tuneable properties from chemical synthesis.However, strong coulomb interactions in organic semiconductors also impose efficiency limits for light emission from charge recombinationa consequence of the quantum-mechanical spin properties of singlet (S1) and triplet (T1) excitons. Singlet and triplet electronic states have total spin quantum numbers, S = 0 and S = 1, respectively. Due to a singlet ground state in typical organic semiconductors, triplet excitons should be dark and non-emissive due to the rule of spin conservation in transitions for light emission. As triplet excitons are formed in 75% of charge recombination events for such organic semiconductors 4 , spin statistics would limit the electroluminescence efficiency of OLEDs to 25%. The generally larger coulomb interaction in organic semiconductors compared to inorganic systems results in a larger exchange interaction and singlet-triplet exciton energy gap; triplets act as excitonic, non-luminescent traps if their emissive properties are not enhanced because T1 exciton...

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