Electroluminescence-detected magnetic resonance study of OLEDs containing a mixed N , N '-diphenyl- N , N '-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB)/tris(quinolinolate) Al (Alq 3 ) layer

Hellerich, Emily; Chen, Ying; Cai, Min; Shinar, J.

doi:10.1117/12.828559

SPIE Proceedings

2009

DOI: 10.1117/12.828559

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Electroluminescence-detected magnetic resonance study of OLEDs containing a mixed N , N '-diphenyl- N , N '-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB)/tris(quinolinolate) Al (Alq 3 ) layer

Emily Hellerich

Ying Chen

Min Cai

et al.

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Cited by 2 publications

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“…Many of the recent studies have focussed on Alq 3 -based small-molecule OLEDs. These studies will not be discussed here; for more information the reader is referred to references[45][46][47][48][49][50].…”

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Electrically and Optically Detected Electron Paramagnetic Resonance in Blue Organic Light Emitting Diodes

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<p>Organic light emitting diodes (OLEDs) are an emerging technology based on electrically conducting polymer films, with great promise for large area lighting and flexible ultra-thin displays. However, despite the rapid technological development, there is still a poor understanding of the degradation and spindependent recombination processes that take place inside an OLED. In this thesis, Electron Paramagnetic Resonance (EPR) was used to investigate these processes in blue-emitting OLEDs. A successful procedure was developed and refined for fabricating OLEDs with the structure ITO/PEDOT:PSS/emissive layer/Al/Ag, with and without the PEDOT:PSS hole-transporting layer. The organic emissive layer was either F8BT, PFO, or PVK:OXD-7:FIrpic (PB). These OLEDs were fabricated in air and with a geometry optimised for EPR experiments. Critical features for satisfactory devices were found to be a sufficiently thick organic layer and minimal exposure to the air. A compact apparatus was developed for simultaneous light output, current, and voltage measurements on the OLEDs while in an inert glove box environment. Electroluminescence and current-voltage parameters measured for these devices showed predominantly trap-controlled space-charge-limited conduction. OLEDs with PFO as the emissive layer and with a PEDOT:PSS layer were investigated with conventional, electrically-detected (ED) and optically-detected (OD) EPR techniques. EDEPR and ODEPR signals were observed at ~9.2 GHz and in the low (<50 mT) and high (~330 mT) magnetic field regimes and were found to change markedly with time during operation as the device degraded. The low field signals initially showed a composite broad quenching and superimposed narrow enhancing response centred around zero field strength. These signals were attributed to magneto-resistance (MR) and magneto-electroluminescence (MEL). Following operational ageing, a third, narrow quenching line was observed in the MR and the ratio of the initial two MR responses changed substantially. These effects are tentatively attributed to a hyperfine interaction. For both EDEPR and ODEPR, quenching high field resonances with a g-value (gyromagnetic ratio) of 2.003±0.001 were observed. The current-quenching resonance gradually diminished during operation and after 4–5 hours was replaced by a current-enhancing resonance. The appearance of this latter resonance could be explained by chemical changes in the OLED due to the diffusion of oxygen through the device from the oxygen-plasma-treated ITO. A working model is proposed which can explain this observed change as spindependent trapping and recombination at free radicals, although the model requires further experimentation to test its validity.</p>

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Electrically and Optically Detected Electron Paramagnetic Resonance in Blue Organic Light Emitting Diodes

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Electrically and Optically Detected Electron Paramagnetic Resonance in Blue Organic Light Emitting Diodes

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Electroluminescence-detected magnetic resonance study of OLEDs containing a mixed N , N '-diphenyl- N , N '-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB)/tris(quinolinolate) Al (Alq 3 ) layer

Cited by 2 publications

References 17 publications

Electrically and Optically Detected Electron Paramagnetic Resonance in Blue Organic Light Emitting Diodes

Electrically and Optically Detected Electron Paramagnetic Resonance in Blue Organic Light Emitting Diodes

Electrically and Optically Detected Electron Paramagnetic Resonance in Blue Organic Light Emitting Diodes

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