Highly Efficient Blue Thermally Activated Delayed Fluorescence Top-Emitting OLEDs Achieved Via Combination of Multilayered Emissive Layer and Ag/WO3/Ag Transparent Cathode

Abstract

Thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) have attracted immense research interest because of the merits including 100% internal quantum efficiency and absence of heavy metals as dopants. However, great challenges, accomplishing the viewing angle characteristic, color shift, high efficiency, and color purity, simultaneously, remain [1, 2]. Herein, we propose a concept of combining the multiple quantum well-based emissive layer (MQW-EML) and multilayered transparent cathode to overcome these challenges. The Ag/WO3/Ag (AWA) multilayers (12 nm/65 nm/12 nm) was optimized based on optical simulation for use as a transparent cathode that exhibits high transmittance (~85% in the visible and blue wavelength region) and low sheet resistance of 5.3 Ω/sq on glass substrates. Compared to thin Ag-based TADF top-emitting OLEDs (TEOLEDS), the proposed AWA-based TEOLED having MQW-EMLs displayed a 40% improvement in external quantum efficiency (Figure. 1). Importantly, the device exhibited a lower angular dependence as well as a low full width at half maximum (FWHM) value (~60 nm), indicating high color purity. These improvements are attributed to the synergistic effect of the MQW-EML and AWA transparent cathode. The EML with quantum well structure effectively confines the charges and excitons, thereby significantly improving color purity and efficiency. On the other hand, the optimized AWA transparent cathode improved the charge injection and light coupling as well. Therefore, this work is expected to open an avenue for high efficient TADF-TEOLEDs, via the combination of the multilayered EML and transparent cathode. Figure 1