Modification of charge balance in light emitting electrochemical cells for efficient solution-based devices
Light-emitting electrochemical cells (LECs) are attractive candidates for the next generation of large-area, low-cost and flexible solid-state light-emitting devices that can be processed from solution by techniques like spin-coating, printing or slot-coating. The unique feature of LECs is the incorporation of mobile ions into the active (emissive) layer that facilitate the injection of electrons and holes from the electrodes. The main focus of this work was to improve the charge carrier balance of the LECs and therefore enhance their overall performance by the use of minority charge carrier injection and transport support layers.
This peculiar property enables the use of air-stable electrodes and low driving voltages close to the band gap potential of the light-emitting species, in both forward and reverse bias direction or even from an AC power supply. Despite these promising characteristics, the typical efficiency values of LECs remain still comparably low for luminance levels above 500 cd m-2.
One of the reasons therefor is considered to be the charge imbalance within the device, which can be tuned with a careful design of the emissive layer materials and/or the device architecture to obtain long-living and efficient LECs.
For this propose a new all solution-based LEC architecture using a Zinc Oxide (ZnO) nanoparticle layer at the cathode was developed. The brightness and efficiency of the LEC increased in average by more than 70 % by the implementation of the additional inorganic layer, which is attributed to an improved electron / hole balance in the device due to enhanced electron injection into the active emissive layer.
In a second step, the ZnO nanoparticle layer was also used with a different emitter to demonstrate the generality and applicability of the new architecture.
Despite the above-mentioned benefits, the nanoparticle layer has demonstrated a critical impact on the LEC lifetime, correlated with the ZnO load and the layer morphology. To overcome this issue, a new solution-based architecture comprising a thin film of 1,3,5-Tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) on top of the emissive layer of a LEC was investigated.
The TmPyPB-LEC was showing in constant voltage mode an efficiency improvement of almost a factor of 2 compared to the reference device, suggesting a pronounced hole blocking effect of the additional organic supporting layer as the origin of improvement.
In addition, as expected for pure hole-blocking properties of the support layer, the implementation of the TmPyPB layer did not significantly affect the device lifetime.
The main focus of this work was to improve the charge carrier balance of the LECs and therefore enhance their overall performance by the use of minority charge carrier injection and transport support layers.