High-Performance Organic Light-Emitting Diodes Based on Intramolecular Charge-Transfer Emission from Donor-Acceptor Molecules: Significance of Electron- Donor Strength and Molecular Geometry

• Published in: Advanced functional materials 2006-05, Vol.16 (8), p.1057-1066
• Main Authors: Kulkarni, A. P., Kong, X., Jenekhe, S. A.
• Format: Article
• Language: English
• Subjects: Electron transport; Hole transport; Light-emitting diodes; Light‐emitting diodes, organic; Luminescence; organic
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200500722

Organic light‐emitting diodes based on intramolecular‐charge‐transfer emission from two related donor–acceptor (D–A) molecules, 3,7‐[bis(4‐phenyl‐2‐quinolyl)]‐10‐methylphenothiazine (BPQ‐MPT) and 3,6‐[bis(4‐phenyl‐2‐quinolyl)]‐9‐methylcarbazole (BPQ‐MCZ), were found to have electroluminescence (EL) efficiencies and device brightnesses that differ by orders of magnitude. High brightness (> 40 000 cd m–2) and high efficiency (21.9 cd A–1, 10.8 lm W–1, 5.78 % external quantum efficiency (EQE) at 1140 cd m–2) green EL was achieved from the BPQ‐MPT emitter, which has its highest occupied molecular orbital (HOMO) level at 5.09 eV and a nonplanar geometry. In contrast, diodes with much lower brightness (2290 cd m–2) and efficiency (1.4 cd A–1, 0.66 lm W–1, 1.7 % EQE at 405 cd m–2) were obtained from the BPQ‐MCZ emitter, which has its HOMO level at 5.75 eV and exhibits a planar geometry. Compared to BPQ‐MCZ, the higher‐lying HOMO level of BPQ‐MPT facilitates more efficient hole injection/transport and a higher charge‐recombination rate, while its nonplanar geometry ensures diode color purity. White EL was observed from BPQ‐MCZ diodes owing to a blue intramolecular charge‐transfer emission and a yellow–orange intermolecular excimer emission, enabled by the planar molecular geometry. These results demonstrate that high‐performance light‐emitting devices can be achieved from intramolecular charge‐transfer emission, while highlighting the critical roles of the electron‐donor strength and the molecular geometry of D–A molecules. Emissive phenothiazine‐quinoline and carbazole‐quinoline donor–acceptor (D–A) molecules yield high‐performance organic light‐emitting diodes (see figure). The electron‐donor strength and molecular geometry of the D–A molecules are critical factors governing the intramolecular charge transfer and electroluminescence efficiency.