Polymer light-emitting electrochemical cells for high-efficiency low-voltage electroluminescent devices

Organic light-emitting devices exhibiting high power conversion efficiency and long operating lifetime may potentially be achieved with the polymer light-emitting electrochemical cell (LEC) configuration. An LEC device typically uses a thin layer of conjugated polymer sandwiched between two contact electrodes. The polymer layer contains an ionically conductive species that are essential in the formation of a light-emitting p-i-n junction. LEC devices are characterized with balanced electron and hole injections, high current density at relatively low bias voltages (2-4 V), and high electroluminescent power efficiency. We will describe the working mechanism of the LECs and review the recent developments in LEC materials, device fabrication and performance. Among the important developments are planar (surface-typed) LECs, bilayer LECs that emit different colors at forward and reverse biases, frozen p-i-n junction LECs that functions like diodes, and phosphorescent LECs. Extensive efforts have been made to improve the LEC performance by controlling the blend morphology, including the use of bipolar surfactant additives and new electrolytes, the synthesis of conjugated polymers with ion-transporting main chain segments or side groups and polyelectrolyte. Degradation mechanisms that limit the lifetime of the LECs will also be discussed.