Materials Science

Freezing in the Glow

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Science  23 Feb 2007:
Vol. 315, Issue 5815, pp. 1054
DOI: 10.1126/science.315.5815.1054c

Polymer light-emitting electrochemical cells (PLECs) have mobile ions within the polymer layer, a feature that fosters low turn-on voltages and skirts the need for low-work-function cathodes or interfacial layers between the cathodes and polymer. However, in comparison with light-emitting diodes, PLECs tend to have slow response times and short operating lifetimes. Ion mobility limits the device speed, and performance can degrade as phase separation occurs between the emitting polymer and the second polymer used to store the mobile ions.

Shao et al. have fabricated PLECs with a simple sandwich structure, in which an organic ionic liquid, methyltrioctylammonium trifluoromethanesulfonate (MATS), acts as the reservoir for the mobile ions. Because MATS has a melting temperature of 56°C, the authors could freeze p-type-intrinsic-n-type (p-i-n) junctions into the devices at room temperature through heating/cooling cycles under an applied voltage bias. The consequent improved contact between the mobile ions and the luminescent polymer led to fast response times. Moreover, the compatibility of MATS with the luminescent polymer—in this case a substituted poly(para-phenylene vinylene) compound—precluded phase separation. The devices functioned with stable high brightness over days of continuous operation. — MSL

Adv. Mater. 19, 365 (2007).

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