Light‐emitting electrochemical cells (LECs) are solid‐state lighting devices that convert electric current to light within electroluminescent organic semiconductors, and these devices have recently attracted significant attention. Introduced in 1995, LECs are considered a great breakthrough in the field of light‐emitting devices for their applications in scalable and adaptable fabrication processes aimed at producing cost‐efficient devices. Since then, LECs have evolved through the discovery of new suitable emitters, understanding the working mechanism of devices, and the development of various fabrication methods. LECs are best known for their simple architecture and easy, low‐cost fabrication techniques. The key feature of their fabrication is the use of air stable electrodes and a single active layer consisting of mobile ions that enable efficient charge injection and transport processes within LEC devices. More importantly, LEC devices can be operated at low voltages with high efficiencies, contributing to their widespread interest. This review provides a general overview of the development of LECs and discusses how small molecules can be utilized in LEC applications by overcoming the use of traditional lighting materials like polymers and ionic transition metal complexes. The achievements of each study concerning small molecule LECs are discussed.
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.
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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