Enhancing extracted electroluminescence from light-emitting electrochemical cells by employing high-refractive-index substrates

Solid-state light-emitting electrochemical cells (LECs) show several advantages over conventional organic light-emitting devices (OLEDs) such as simple device structure compatible with solution processes, low operation voltage and capability of utilizing inert cathode metals. However, device performance of LECs must be improved, e.g. enhancing light extraction, to meet the requirements for practical applications. Among the optical modes trapped in LECs, light trapped in substrate mode is easier to be extracted, e.g., by simply roughing the output surface. Therefore, increasing the percentage of substrate mode is beneficial in improving light extraction. In this work, the contributions of optical modes in LECs employing substrates with various refractive indices are analyzed. Higher-refractive-index substrates are shown to trap more light in the substrates. Smaller refractive index difference between higher-refractive-index substrate and indium tin oxide (ITO) layer also increases the cutoff spectral range of light waveguided in ITO layer. Furthermore, light intensity in surface plasmon mode significantly reduces as the refractive index of the substrate increases. Reducing the percentage of surface plasmon mode facilitates light extraction since it requires more complicated methods for outcoupling light in this mode. With commercially available unpolished sapphire substrates, light output of LECs is enhanced by 56%. When a scattering layer was inserted between ITO and sapphire substrate, more light in substrate mode can be extracted and 71% enhancement in light output is realized. High external quantum efficiency up to 5.5% is consequently obtained in LECs based on a ruthenium complex. Such device efficiency is among the highest reported values for red-emitting LECs and thus confirms that utilizing higher-refractive-index substrates would offer a simple and feasible approach to improve light output of LECs. In comparison to OLEDs, increased EL trapped in substrates of LECs mainly comes from surface plasmon mode rather than waveguide mode.