Ionic Additives in Light Emitting Electrochemical Cells from Ionic Transition Metal Complexes




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A light emitting electrochemical cell is an alternative technology for display and solid state lighting. It can either be based on conjugated polymers or ionic transition metal complexes. The ease in their fabrication process and simplicity of their architecture make them appealing in industrial processing. However, for this technology to be put into practical use, more research efforts are still needed to reduce their long turn on times, and to increase the duration of high luminance. Here we focused our attention in the effect of ionic additives and couterions on the performance of devices made from ionic transition metal complexes. With proper complex engineering and using appropriate additives, we were able to fabricate devices that meet the US DOE benchmark for luminance level with appreciable lifetimes. We applied spectroscopic techniques coupled with simulation studies to probe the origin of the enhancement. By employing these methods, we were able to prove our previous hypothesis that due to the immobility of the positively charged complex emitter in salt-free devices, the packing of accumulated positive ions in the negative electrode is less dense than the accumulation of negative ions in the positive electrode. These accumulations of uncompensated charges near the electrodes are called electric double layers (EDLs). The discrepancy in EDL formation leads to an imbalance of electron and hole injection because the EDLs aid in reducing the width of injection barriers. Addition of ionic additives of appropriate size and concentration remedies the offset and promote balanced densities of charge carriers thereby optimizing the device performance. Adding a higher fraction of ionic additive above the threshold concentration disrupts the balance and adversely impact the performance of the device, presumably due to side reactions such as incomplete dissociation or reassociation of ions. We also found out that the width of the EDLs for optimal device performance should not be too thick for facile injection and transport of charges through the bulk of the device. Large EDL widths induce a very strong Coulomb force that could hinder injected charges from reaching the middle of the device. Our finding also shows that the dielectric constant (ε) has an effect on the device performance and has a connection with the thickness of EDL formed. For materials considered, active layers with lower dielectric constants are favorable in the recombination of hole and electron as lower-ε materials exhibit strong exciton binding energy and greater bimolecular recombination strength; however, they also result in lower film conductivity thereby affecting the turn-on time of the devices. Consequently, as far as the materials considered in the study, a “sweet spot” of the dielectric constant is necessary for optimal device performance. Some simple methods in tuning the value of the dielectric constant as illustrated in our works are through addition of salts or choosing the right negative counterion to complement the complex emitter. As a recommendation, more studies by employing different complexes and additives should be conducted to verify the results mentioned above. In conclusion, careful implementation of the considerations mentioned above must be done to achieve optimal device performance.



Light-emitting electrochemical cells--Additives, Transition metal complexes, Electric double layer, Dielectrics, Impedance spectroscopy, Ions


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