Understanding the mechanisms of light emission from DCJTB charge-transfer states in OLEDs using organic magnetic field effects

Abstract

Although energy transfer (ET) and direct charge trapping (DCT) are two competing mechanisms that are known to occur in dye-doped organic light-emitting diodes (OLEDs), which of these processes governs the electroluminescence is not clear. Additionally, each process influences the formation of excitons, and the interactions between excitons, differently. If ET is the dominate process, the mechanism that governs an OLED involves both the host and guest materials. In contrast, if DCT is the dominant process, the mechanism that governs an OLED will only involve the guest material. Magneto-electroluminescence (MEL) is an effective technique for exploring and understanding the mechanisms that occur within OLEDs. It exhibits sensitive fingerprint responses to intersystem crossing (ISC), reverse intersystem crossing (RISC), triplet-triplet annihilation (TTA) and triplet-charge annihilation (TQA). In this work, we fabricated three different OLEDs and measured their MEL curves. Tris-8-hydroxyquinoline aluminum (Alq3), 4,4′- N , N ′-dicarbazolebiphenyl (CBP) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) were used as the host materials, each of which had a different triplet exciton energy. 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4- yl -vinyl)- 4 H -pyran (DCJTB) was used as a charge-transfer (CT) dopant that had a small energy difference between the singlet and triplet excited states. By analyzing the energy level structure of the different devices, the absorption spectrum of the dopant, and the emission spectra of the host materials, we determined that ET was the dominant process when DCJTB was used as a dopant with Alq3. However, DCT dominated when DCJTB was used as a dopant with both CBP and PO-T2T. The current- and temperature-dependent MEL curves from all devices were composed of low field components (| B | ≤| B | B because both TTA and TQA were competing processes, but their low-field changes were completely different. The MEL curves from the DCJTB/Alq3 device exhibited a positive MEL because polaron-pair (PP) states can occur on the host material via ISC (PPS→PPT). Conversely, the DCJTB/CBP and DCJTB/PO-T2T devices exhibited negative MEL responses because RISC (1CT←3CT) occurred in the CT states of the dopant. However, the triplet energy levels of the host material simultaneously impacted RISC. Hence, these devices exhibited a diverse range of MEL curves. Low temperatures enhanced RISC and TTA of the triple CT state within DCJTB, which was attributed to the increased concentration and lifetime of triplet excitons at low temperature. This work serves as a reference for improving the internal quantum efficiency of fluorescent OLEDs by enhancing the use of triplet states, and importantly gives a deeper understanding of the mechanisms that govern light-emission from devices that incorporate DCJTB CT materials.