Influence of the anion on the kinetics and stability of a light-emitting electrochemical cell

• The selection of the anion is shown to be crucial for the response time and the lifetime of light-emitting electrochemical cells (LECs) fabricated with [co-block phenyl-substituted poly(para phenylene vinylene)]-PEO-LiX (X = Tf, TFSI, Tf-TFSI). With a mixed-anion (X = Tf-TFSI) salt. the LEC turned on in less than 0.4 s at ΔU = 4.0 V, with X = TFS1 it turned on in a few seconds, while X = Tf produced devices with significantly larger response times with a strong dependence on the film thickness. We attribute this significant variation in the response to the morphology of the PEO-LiX phase at room temperature: PEO-LiTf is largely crystalline with a correspondingly limited ionic mobility: PEO-LiTf-TFSI is amorphous with a relatively high mobility; and heat-treated PEO-LiTFSI is typically partially amorphous. For lifetimes. we focused on 3.0 ≤ ΔU ≤ 4.0 V since this range coincided with a crossover in performance for the X = Tf devices. At ΔU = 3.0 V, these lasted the entire measurement cycle (=55 h) with constant efficiency and only a small decrease in light output. At ΔU ≥ 3.25 V. the same devices exhibited a limited lifetime with a linear dependence on the salt concentration and a decrease in the efficiency with time. For X = TFSI. Tf-TFSI short lifetimes and a decrease in efficiency with time were obtained independent of the applied voltage. Considering recently published data on electrochemical stability, we propose that an irreversible overreduction of the anion is a significant side reaction that limits the lifetime of LEC systems.

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

Nyckelord

lifetime

response time

LEC

Tf anion

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