Magnetoresistance and efficiency measurements ofAlq3-based OLEDs

Abstract

Magnetoresistance and efficiency measurements of indium tin oxide/N , NЈ-diphenyl-N , NЈ bis͑3-methylphenyl͒-͑1,1Ј-biphenyl͒-4,4Ј diamine/aluminum tris͑8-hydroxyquinoline͒/cathode organic lightemitting diode structures have been made as a function of magnetic field and cathode type. It has been found that magnetoresistance occurs only when there is light emission from the devices, which suggests that the magnetoresistance is related to exciton formation. Comparison of the effects of applied field on device efficiency and magnetoresistance shows that the magnetoresistance cannot be due to the recombination current. We suggest that the effect may be due to trapping of charge carriers at triplet excitons within the device.

Microscopic modeling of magnetic-field effects on charge transport in organic semiconductors

“…[3][4][5] However, none of them unambiguously explain all the experimental results. Although the models for OMAR are based on different processes, ranging from bipolaron formation 5 to recombination of electron-hole (e-h) pairs 3 and detrapping of charges by triplet excitons, 4 there is also a strong similarity between these mechanisms. In the proposed models an applied magnetic field alters the spin-dependent reactions between two particles, thereby changing the current through the organic devices.…”

Pulsed magnetic resonance of Alq3 OLED detected by electroluminescence

“…Nowadays, the study of organic magneto-resistance (OMAR) is of great importance in the framework of organic spintronics and it introduces a new scenario of possible applications [6,7]. Spin dependent charge recombination, spin blocking bipolaron formation and triplet-polaron scattering were proposed as possible explanations [8][9][10]. Furthermore, experimental evidence showed that trapping centers may be related to OMAR, suggesting that more mechanisms may participate to the magnetoresistance [11].…”

Traps and trions as origin of magnetoresistance in organic semiconductors

“…[15][16][17][18][19][20] The effect of a magnetic field on these reactions was already considered by Ern and Merrifield in molecular crystals. 15 Desai et al 3 were the first to qualitatively describe how this can give rise to large room-temperature MC in disordered organic semiconductors. The authors consider the triplet excitons to momentarily capture polarons in states they recently referred to as charged excitons, 21 thereby reducing the polaron transport.…”

“…One competing model proposes reactions between polaronic electron and hole pairs, 2 whereas another relies on the reaction between triplet excitons and (trapped) polarons. 3,4 Conversely, the reactions of equal charges are described by a third (bipolaron) model. 5 These models have been used to explain low-field effects (LFEs), which are visible on a relatively small field scale of a few mT, as well as high-field effects (HFEs) on much broader field scales.…”

“…3,21 Additionally, the magnetic-field dependent quenching of triplet excitons at trapped charges has been considered, which can lead to the release of these charges. 15,24 The trapping of triplet excitons themselves, which can then lead to the formation of metastable trions, 23 has as far as we know not been investigated with respect to magnetic-field effects.…”