Improved Light Extraction Efficiency and High Color Purity of Indium Phosphide Top Emission Quantum Dot Light‐Emitting Diodes Using a Metal–Insulator–Metal Color Enhancement Filter

Abstract Top‐emitting quantum‐dot (QD) light–emitting diodes (TE‐QLEDs) are promising candidates for next‐generation display technologies because of their high aperture ratio and compatibility with opaque substrates. However, their optical performance is limited by the intrinsically low transmittance of conventional semi‐transparent metal electrodes. To overcome this challenge, a metal–insulator–metal color enhancement filter (MIM‐CEF) is proposed as a top electrode to enhance the light outcoupling efficiency and narrow the spectral linewidth, thereby improving the color purity without causing carrier injection imbalance owing to changes in the functional layer thickness. The MIM‐CEF operates via tunable microcavity resonance governed by the thickness of a poly(methyl methacrylate) spacer, which selectively amplifies the optical transmittance at target wavelengths. Photoluminescence measurements confirm that QD films integrated with the MIM‐CEF exhibit significantly higher emission compared to their Ag electrode counterparts for both red and green indium phosphide (InP)‐based QDs. In mixed red–green QD films, the MIM‐CEF simultaneously suppresses non‐resonant emission and enhances the emission intensity and color purity at the resonant wavelength. Utilizing these properties, TE‐QLEDs integrated with the MIM‐CEFs demonstrates higher luminance and external quantum efficiency than conventional devices. Thus, MIM‐CEFs offer a promising electrode platform for next‐generation high‐performance QD display technologies.