Operation mechanisms of thin film organic electroluminescent diodes

Abstract

Optical and electrical characteristics of single-and double-layer (DL) organic electroluminescent diodes based on an aromatic diamine (TPD) and 8-hydroxyquinoline aluminium complex (Alq3) were investigated to compare their operation mechanisms. From the optical spectra of thin films and of single-layer (SL) and DL devices, the nature of emitting states is revealed and their energy level scheme is derived. The injection-controlled electroluminescence mode of the operation of SL and DL devices provides a powerful experimental approach for investigating the process of charge injection. It is demonstrated that the physical mechanism responsible for charge injection is the primary carrier penetration over the image force barrier at high electric fields, and field-assisted thermionic injection in the low-field regime of operation of these devices. The role of material and device structure for charge injection parameters is pointed out in this context.

A possible mechanism for enhanced electrofluorescence emission through triplet–triplet annihilation in organic electroluminescent devices

“…In Fig. 1, quenching such as deactivation of the excited species by energy transfer at a metal cathode 38 and voltage dependence of emission zone 39,40 are omitted. Figure 2 shows the luminance-current density (L -I) curves of the EL devices with various treated ITO.…”

Two peaks observed in the electroluminescence spectra of Alq3-based OLEDs

“…Later, this group found that the EL peaked at 528 nm in the Alq 3 /NPB-based OLEDs [3]. In Alq 3 /TPD-based OLEDs, however, the EL emission peak from Alq 3 was also reported at 520 nm by Kalinowski and co-workers [11], and at 510 nm by Curry and Gillin [12]. Our measurements demonstrate that the EL emission from Alq 3 is consistent with the reports in the literature: The EL emission envelope yields a maximum at 525 nm from Alq 3 in our fabricated OLED with a structure of ITO/Alq 3 (60 nm)/ Mg-Ag, as shown in Fig.…”

“…Several groups compared the PL and EL of Alq 3 , and found that both the PL and EL peaked at the same wavelength, but there was more emission in the low-energy tail of the PL compared to that of the EL [11,12]. Kalinowski et al [11] explained this difference by considering the presence of ''defect states'' in the Alq 3 without any indication of the origin of these states.…”

“…Kalinowski et al [11] explained this difference by considering the presence of ''defect states'' in the Alq 3 without any indication of the origin of these states. Alternatively, Curry and Gillin inferred that the radiative recombination of triplet excitons was responsible for a significant amount of the PL of Alq 3 , and this process was responsible for the low-energy tail seen in the PL of Alq 3 , but it was not present in the EL [12].…”

Determination of the barrier heights for electron injection in organic light emitting devices

“…This ensures that no absorption of the incident light occurs within the organic layers. The lowest-energy absorption maximum of Alq 3 is at 395 nm, 6 whereas the PSϩTDAPB blend system results in colorless layers without absorption in the visible region. For all spectra the intensity distribution of the halogen lamp was considered.…”