Harnessing Metallic Nanoparticle-Based Anodes for Red-Shifting and Reshaping Electroluminescence toward the Near-Infrared Region

Leveraging conventional red-emitting materials, a color-tuning strategy is used to develop high-performance near-infrared (NIR) organic light-emitting diodes (OLEDs). This study presents a practical approach for achieving color-tunable optically impaired OLEDs through localized surface plasmon resonance (LSPR), which extends the emission of red-emitting materials into the NIR region, thereby eliminating the need for dedicated NIR emitters. The method utilizes silver (Ag) and gold (Au) nanoparticle-based anodes modified with titanium dioxide (TiO₂) coatings under various rapid thermal annealing (RTA) treatments (200 °C, 400 °C, and 600 °C). Three TiO₂ coating techniques─overcoating, stacking, and sandwiching─were compared in terms of their influence on the LSPR behavior and electroluminescence (EL) spectra. UV–vis spectroscopy, including transmittance and absorbance analyses, confirmed that the different coating methods facilitated LSPR formation and thus modulated EL characteristics and induced distinct spectral features. Ag nanoparticles exhibited stronger LSPR responses and greater color-tunability than Au nanoparticles. Scanning electron microscopy revealed that RTA promoted island-like nanoparticle growth and minimized agglomeration, while TiO₂ coatings enhanced island formation, resulting in well-defined and narrower absorbance peaks. The overcoating approach successfully red-shifted the primary emission peak of red phosphorescent OLEDs from 663 to 723 nm, achieving a maximum external quantum efficiency of 12.91% and a low turn-on voltage (V_on) of 2.41 V. Furthermore, a broad EL emission spectrum (full width at half maximum = 199 nm) spanning 643–842 nm was achieved using the TiO₂ stacking configuration. These findings highlight the potential of Ag nanoparticle-based anodes for color-tunable NIR OLEDs and introduce a stacking coating technique with potential for broad-wavelength applications.