The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

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We demonstrate that three Ir͑III͒ complexes used as principal dopants in organic electrophosphorescent diodes have very high photoluminescence quantum efficiency ͑ PL ͒ in a solid-state film. The green emitting complex, fac-tris͑2-phenylpyridinato͒iridium͑III͒ ͓Ir͑ppy͒ 3 ͔, the red-emitting bis͓2-͑2Ј-benzothienyl͒pyridinato-N , C 3 Ј͔ ͑acetylacetonato͒iridium͑III͒ ͓Btp 2 Ir͑acac͔͒, and the blue complex bis͓͑4 , 6-difluorophenyl͒pyridinato-N , C 2 ͔͑picolinato͒iridium͑III͒ ͑FIrpic͒ were prepared as codeposited films of varying concentration with 4,4 Ј-bis͑N-carbazolyl͒-2 , 2 Ј-biphenyl, a commonly used host material. The maximum PL values for Ir͑ppy͒ 3 , Btp 2 Ir͑acac͒, and FIrpic were, respectively, 97% ± 2% ͑at 1.5 mol%͒, 51% ±1% ͑at 1.4 mol%͒, and 78% ± 1% ͑at 15 mol%͒. Furthermore, we also observed that the maximum PL of FIrpic reached 99% ± 1% when doped into the high triplet energy host, m-bis͑N-carbazolyl͒benzene, at an optimal concentration of 1.2 mol%.

In this study, we investigate the lasing properties of 4,4′-bis[(N-carbazole)styryl]biphenyl thin films under electrical pumping. The electroluminescent devices incorporate a mixed-order distributed feedback SiO2 grating into an organic light-emitting diode structure and emit blue lasing. The results provide an indication of lasing by direct injection of current into an organic thin film through selection of a high-gain organic semiconductor showing clear separation of the lasing wavelength from significant triplet and polaron absorption and design of a proper feedback structure with low losses at high current densities. This study represents an important advance toward a future organic laser diode technology.