Manipulation of Hole and Exciton Distributions in Organic Light-Emitting Diodes with Dual Emission Layers

The efficiency improvement of organic light-emitting diodes (OLEDs) is important but challenging. Here, we introduce a unique OLED with a hole modulation layer (HML) in the middle of its emission layer (EML). The external quantum efficiency and power efficiency can be improved by approximately 58% when an HML with optimized thickness is inserted. HML insertion can efficiently retard hole flow, thus improving (i) exciton distribution uniformity and (ii) local electron–hole charge balance. A systematic study of the individual contributions of two EMLs separated by the HML shows that the former factor dominantly works at low current densities (< 10 mA/cm²), whereas the latter factor functions over the entire current density range of the OLED. Therefore, the efficiency improvement is greatest at low current densities, which aligns with the typical operating range in display applications. The results provide a deeper understanding of the OLED emission mechanism, and the proposed OLED structure can significantly benefit high-performance OLED displays. Graphical Abstract: (Figure presented.)