A phenanthroimidazole derivative is used as an emitter in light emitting electrochemical cells (LECs).

We report a versatile approach to harvest electroluminescence from a nondoped light-emitting electrochemical cell (LEC) using an easily accessible phenanthroimidazole derivative. The authors investigated two different types, (i) ionic and (ii) neutral phenanthroimidazole derivatives by modifying our previously reported LEC emitter. Sky-blue electroluminescence was achieved by applying these modified emitter in LEC devices. In comparison to the parent molecule, a highly contrasting performance was exhibited by all the modified emitters except the neutral butyl derivative (nbpypn). By employing an ionic molecule (ihpypn) in a fully solution-processed typical LEC device structure, a peak brightness of 711 cd/m 2 was observed at a current efficiency of 0.18 cd/A. Our champion device (ihpypn-LEC) presented a 5-fold increase in maximum brightness at a ten times higher current density than its parent molecule. These peak brightness values are among the best comparing to those reported for LECs with the corresponding emission colors. Even though the neutral molecules did not show any high electroluminescence, their current efficiency at maximum brightness has improved 20 times when compared to its parent molecule utilized device. The study reveals that substituents on imidazole nitrogen has a critical impact on its performance in the LEC devices. This result is even more encouraging, considering that our molecular design can be applied to the majority of the imidazole derivatives and may open-up a plausible way of enriching the library of emitters for LECs with efficient and easily obtainable small organic molecules.

A series of cationic iridium complexes (1−6) were synthesized using alkylated imidazole-based ancillary ligands, and the photophysical and electrochemical properties of these complexes were subsequently evaluated. Lightemitting electrochemical cells (LECs) were fabricated from these complexes, and the effects of the alkyl chain length on the electroluminescent properties of the devices were investigated. The LECs based on these complexes resulted in yellow emission (complexes 1, 3, and 5) and green emission (complexes 2, 4, and 6) with Commission Internationale de L'Eclairage (CIE) coordinates of (0.49, 0.50) and (0.33, 0.59), respectively. Our results indicate that the luminance and efficiency of the LECs can consistently be enhanced by increasing the alkyl chain length of the iridium complexes as a result of suppressed intermolecular interaction and self-quenching. Subsequently, a high luminance of 7309 cd m −2 and current efficiency of 3.85 cd A −1 were achieved for the LECs based on complex 5.

Albeit their easy accessibility and low cost, small organic molecules are not known for their high electroluminescence in light-emitting electrochemical cells (LECs). To construct a bright low-cost LEC device, the functions of charge transport and charge recombination should be separated in the active layer of LEC devices. Herein, we demonstrate that the widely used host–dopant strategy in organic light-emitting diodes (OLEDs) can significantly improve the electroluminescence from small organic molecule fueled LEC devices, provided the host molecules are carefully selected. Furthermore, performance of host–dopant small-molecule LEC devices hugely relies on the properties of host materials rather than the emitting luminophores. Conversely to the high performance of intramolecular charge-transfer (ICT) molecular systems in OLEDs, doped ICT fluorophores having a low-lying charge-transfer state can behave like exciton loss channels in the high ionic environment of LEC-active layers. Similar to the behavior of previously reported ICT molecules in polar solvents, our synthesized D−π-A−π-D phenanthroimidazole derivative exhibited fluorescence quenching and a huge blue shift of emission in the doped thin film of the ionic host. However, even with a less efficient emitter, high electroluminescence was achieved from a host–dopant LEC system. Our best device exhibited a maximum brightness of 5016 cd/m2 at a current efficiency of 0.73 cd/A. This device outplays our previously reported nondoped LEC (ihpypn-LEC) with a 7-fold increase in the maximum brightness and over a 3-fold increase in the current efficiency at peak brightness. To the best of our knowledge, these peak brightness values recorded here (device 2) are the best among those reported by small organic molecule LEC devices so far. This report reveals the potential of small organic molecules, especially phenanthroimidazole derivatives, in casting bright and efficient low-cost host–dopant LECs with minimum effort and appreciable sustainability.

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