Oligomer Electrolytes for Light-Emitting Electrochemical Cells: Influence of the End Groups on Ion Coordination, Ion Binding, and Turn-on Kinetics

Abstract

The electrolyte is an essential constituent of the light-emitting electrochemical cell (LEC), since its operating mechanism is dependent on the redistribution of mobile ions in the active layer. Recent developments of new ion transporters have yielded high-performance devices, but knowledge about the interactions between the ionic species and the ion transporters and the influence of these interactions on the LEC performance is lacking. We therefore present a combined computational and experimental effort that demonstrates that the selection of the end group in a star-branched oligomeric ion transporter based on trimethylolpropane ethoxylate has a paramount influence on the ionic interactions in the electrolyte and thereby also on the performance of the corresponding LECs. With hydroxyl end groups, the cation from the salt is strongly coordinated to the ion transporter, which leads to suppression of ion pairing, but the penalty is a hindered ion release and a slow turn-on for the LEC devices. With methoxy end groups, an intermediate coordination strength is seen together with the formation of contact ion pairs, but the LEC performance is very good with fast turn-on. Using a series of ion transporters with alkyl carbonate end groups, the ion transporter:cation coordination strength is lowered further, but the turn-on kinetics are slower than what is seen for devices comprising the methoxy end-capped ion transporter.

Controlling the Emission Zone by Additives for Improved Light‐Emitting Electrochemical Cells

“…[ 15a,b ] Thus, a plausible explanation for the observed mobility behavior is that the anion mobility is essentially the same in both LECs, whereas the cation mobility is higher with the smaller TMPE‐OH ion transporter than the larger TMPE‐OC. However, we emphasize that other factors, such as the active‐material morphology, [ 15a,22 ] the initial spatial distribution of the electrolyte, [ 21a ] and the release kinetics of cations from the electrolyte complex, [ 23 ] also can influence the effective ion mobility. In the morphology context, we mention that an AFM study revealed that the active materials featured a very smooth surface morphology, regardless of the ion‐transporter selection (see Figure S4, Supporting Information).…”

Coordination Effects in Polymer Electrolytes: Fast Li+ Transport by Weak Ion Binding

“…As seen in Figure , addition of PEO gives a much stronger shift response than for any of the carbonyl-coordinating polymers, indicating its much stronger interactions with the Li + ions. PEO is known to strongly coordinate the lithium through chelating structures created from its ether oxygen atoms, whereas the coordination structures formed from the carbonyl oxygens in the polyester and polycarbonate electrolytes result in a much weaker coordination. ,,, …”

The role of coordination strength in solid polymer electrolytes: compositional dependence of transference numbers in the poly(ε-caprolactone)–poly(trimethylene carbonate) system

“…Yet another property that may affect the coordination structure and ion pairing in the polymer electrolyte system is the dielectric constant. Previously, a correlation between coordination strength and ion pairing has been observed, 44 but its interrelation with the dielectric constant remains unclear. The resulting ion paring should, however, not significantly affect the transference number, unless the ion clustering results in an extensive formation of ion triplets.…”