Influence of Lithium Additives in Small Molecule Light-Emitting Electrochemical Cells.
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.
Department of Physics, The University of Texas at Dallas , 800 West Campbell Road, PHY 36, Richardson, Texas 75080-3020, United States.
Kuo-Yao
KY
Lin
Department of Physics, The University of Texas at Dallas , 800 West Campbell Road, PHY 36, Richardson, Texas 75080-3020, United States.
Lyndon D
LD
Bastatas
Department of Chemistry, The University of Texas at Austin , 105 East 24th Street, Stop A5300, Austin, Texas 78712-0165, United States.
Kristin J
KJ
Suhr
Department of Chemistry, The University of Texas at Austin , 105 East 24th Street, Stop A5300, Austin, Texas 78712-0165, United States.
Matthew D
MD
Moore
Department of Chemistry, The University of Texas at Austin , 105 East 24th Street, Stop A5300, Austin, Texas 78712-0165, United States.
Bradley J
BJ
Holliday
Department of Mechanical Engineering, The University of Texas at Dallas , 800 West Campbell Road, EC 38, Richardson, Texas 75080-3020, United States.|Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080-3020, United States.
Majid
M
Minary-Jolandan
Department of Physics, The University of Texas at Dallas , 800 West Campbell Road, PHY 36, Richardson, Texas 75080-3020, United States.
Jason D
JD
Slinker
double layer formation; ionic transition metal complexes; iridium; mixed conductors; scanning Kelvin probe microscopy