Dipole reorientation and local density of optical states influence the emission of light-emitting electrochemical cells

Controlling the Emission Zone by Additives for Improved Light-Emitting Electrochemical Cells

The position of the emission zone (EZ) in the active material of a light-emitting electrochemical cell (LEC) has a profound influence on its performance because of microcavity effects and doping- and electrode-induced quenching. Previous attempts of EZ control have focused on the two principal constituents in the active material-the organic semiconductor (OSC) and the mobile ions-but this study demonstrates that it is possible to effectively control the EZ position through the inclusion of an appropriate additive into the active material. More specifically, it is shown that a mere modification of the end group on an added neutral compound, which also functions as an ion transporter, results in a shifted EZ from close to the anode to the center of the active material, which translates into a 60% improvement of the power efficiency. This particular finding is rationalized by a lowering of the effective electron mobility of the OSC through specific additive: OSC interactions, but the more important generic conclusion is that it is possible to control the EZ position, and thereby the LEC performance, by the straightforward inclusion of an easily tuned additive in the active material.

Novel patterned sapphire substrates for enhancing the efficiency of GaN-based light-emitting diodes

In this study, a novel patterned sapphire substrate (PSS) was used to obtain mesa-type light-emitting diodes (LED), which can efficiently reduce the threading dislocation densities. Silicon nitride (Si₃N₄) was used as a barrier to form the PSS, replacing the commonly used silicon dioxide (SiO₂). The refractive index of Si₃N₄ is 2.02, which falls between those of sapphire (1.78) and GaN (2.4), so it can be used as a gradient refractive index (GRI) material, enhancing the light extraction efficiency (LEE) of light-emitting diodes. The simulation and experimental results obtained indicate that the LEE is enhanced compared with the conventional PSS-LED. After re-growing, we observed that an air void exists on the top of the textured Si₃N₄ layer due to GaN epitaxial lateral overgrowth (ELOG). Temperature-dependent PL was used to estimate the internal quantum efficiency (IQE) of the PSS-LED and that of the PSS-LED with the Si₃N₄ embedded air void (PSA-LED). The IQE of the PSA-LED is 4.56 times higher than that of the PSS-LED. Then, a TracePro optical simulation was used to prove that the air voids will affect the final luminous efficiency. The luminous efficiency of the four different structures considered is ranked as Si₃N₄ (PSN-LED) > PSA-LED > PSS-LED with SiO₂ (PSO-LED) > PSS-LED. Finally, we fabricated LED devices with different thickness of the Si₃N₄ barrier. The device shows the best luminance–current–voltage (LIV) performance when the Si₃N₄ thickness is 220 nm.

A novel patterned sapphire substrate composed of a silicon nitride barrier and air voids was developed for enhancing the efficiency of GaN-based light-emitting diodes.