Influence of Lithium Additives in Small Molecule Light-Emitting Electrochemical Cells

Abstract

Light-emitting electrochemical cells (LEECs) utilizing small molecule emitters such as iridium complexes have great potential as low-cost emissive devices. In these devices, ions rearrange during operation to facilitate carrier injection, bringing about efficient operation from simple, single layer devices. Recent work has shown that the luminance, efficiency, and responsiveness of iridium-based LEECs are greatly enhanced by the inclusion of small amounts of lithium salts (≤0.5%/wt) into the active layer. However, the origin of this enhancement has yet to be demonstrated experimentally. Furthermore, although iridium-based devices have been the longstanding leader among small molecule LEECs, fundamental understanding of the ionic distribution in these devices under operation is lacking. Herein, we use scanning Kelvin probe microscopy to measure the in situ potential profiles and electric field distributions of planar iridium-based LEECs and clarify the role of ionic lithium additives. In pristine devices, it is found that ions do not pack densely at the cathode, and ionic redistribution is slow. Inclusion of small amounts of Li[PF6] greatly increases ionic space charge near the cathode that doubles the peak electric fields and enhances electronic injection relative to pristine devices. This study confirms and clarifies a number of longstanding hypotheses regarding iridium LEECs and recent postulates concerning optimization of their operation.

The Effect of the Dielectric Constant and Ion Mobility in Light‐Emitting Electrochemical Cells

“…Earlier work on iridium‐based LEEC devices showed that the dielectric constant and the thickness of the EDL changed with Li[PF 6 ] additive concentration . As the percentage of Li[PF 6 ] was increased, the dielectric constant also increased.…”

Color‐Stable, Efficient, and Bright Blue Light‐Emitting Electrochemical Cell Using Ionic Exciplex Host

“…It has been revealed that electron injection/transport (n‐type doping) are usually much less efficient than hole injection/transport (p‐type doping) in blue LECs incorporating ionic iridium complexes. [ 23,61,65,66 ] It is shown here that the host‐guest LEC with an exciplex host largely promotes electron injection and transport (thus n‐type doping) through the acceptor molecules. It can be concluded that enhancement of PQLY of the active layer and facilitation of n‐type doping (thus establishment of p‐i‐n junction) both contribute to the high performances of host‐guest blue LECs with an ionic exciplex host.…”

“…It has been reported that addition of ionic liquid or salt into the active layer of LEC could accelerate the formation of ohmic double layers at the electrode/active layer interfaces and the establishment of p‐i‐n junction, leading to enhanced device performances. [ 23,61 ] An ionic liquid, tetrahexylammonium tetrafluoroborate ([THA][BF 4 ]), was then added into the active layers of device B. The resultant device structure is ITO/PEDOT: PSS (40 nm)/tBuCAZ‐ImMePF 6 : TRZ‐ImEtPF 6 : [THA][BF 4 ]: 10 wt% [Ir(buoppy) 2 (dmapzpy)]PF 6 (100 nm)/Al (100 nm).…”

Stress Testing Polymer Light‐Emitting Electrochemical Cells: Suppression of Voltage Drift and Black Spot Formation

“…The test also showed a driving voltage that approached 4 V at the end of the 1000 h test as a result of voltage drift. In small‐molecule or ionic‐transition metal (ITMC) LECs, added lithium salt has been shown to improve the response and luminance of the cells . These studies all point to the importance of salt concentration in LEC operation.…”