Light-emitting
electrochemical cells (LECs) are composed of blends
of semiconducting polymers and electrolytes, in which a unique cooperative
action of ions and electrons induces a dynamic p–n junction
for efficient emission. One of the crucial issues remaining in LECs
is uniformity in blends of polymer and electrolyte; phase separation
in between the two components results in poor performance or failure
of operation. Here, we overcome this issue by developing an ionic
liquid-based electrolyte of alkylphosphonium-phosphate that shows
notable compatibility high enough to dissolve even light-emitting
polymers. This exceptional compatibility enabled us to prepare uniform
film blends with various blue to red emitting polymers, and offered
bright and efficient LECs. Especially, a blue-emitting LEC showed
excellent performance: the luminance reached ∼20 000
cd m–2 with a high luminance efficiency of ∼5
cd A–1, of which performances significantly exceed
a light-emitting diode using the same polymer. The ionic liquid was
further applied to the LECs with state-of-the-art light-emitting dendrimers
showing thermally activated delayed fluorescence under electrical
excitation, giving a high efficiency of 11 cd A–1. These demonstrations remind us of the great importance of the polymer–electrolyte
compatibility and the usefulness of ILs for electrolyte of LECs.
The electrolytic properties of a lithium battery electrolyte based on triethyl(2-methoxyethyl)phosphonium bis(trifluoromethylsulfonyl)imide are described. Testing charge–discharge performance of the cell containing the lithium-binary phosphonium ionic liquid electrolyte showed excellent capacity and rechargeability, compared to the cell containing a comparative ammonium ionic liquid electrolyte.
An extended displacement current measurement (DCM) is performed to analyze the operation mechanisms of a poly(p‐phenylenevinylene) derivative (Super Yellow)‐based light‐emitting electrochemical cell (LEC). The characteristics of the actual current, displacement current, and electroluminescence (EL) intensity during the relaxation processes of electrochemical doping are simultaneously measured. The transient characteristics indicate correlations between the conductance, capacitance, and EL efficiency. The conductance and capacitance decrease with proceeding doping relaxation, whereas the EL efficiency increases. The EL efficiency is dominated by the electrochemical doping state, rather than the actual current density. Moreover, the EL efficiency deteriorates with increasing direct current (DC) voltage applied for electrochemical doping. Since the DCM reveals that the electrical properties of the device obey the trap charge limited current regime in the intrinsic region and are almost independent of the applied DC voltage, efficiency deterioration is not responsible for the intrinsic region but for the doped region. The results suggest that the self‐absorption in the doped region causes efficiency loss, otherwise the emitting zone is formed close to the doped region. The extended DCM enables us to comprehensively analyze the operation mechanisms of LECs on the basis of the simultaneous observations of electrical and luminous characteristics, including their transient changes.
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