Isotope effect in the spin response of aluminum tris(8-hydroxyquinoline) based devices

Abstract

We studied the spin response of various magnetic field effects and magneto-transport in both protonated and deuterated aluminum tris(8-hydroxyquinoline) [Alq 3 ]-based organic light emitting diodes and spin-valve devices. Both conductivity-detected magnetic resonance in diodes and magneto-resistance in spin valves show substantial isotope dependence pointing to the importance of the hyperfine interaction (HFI) in the spin response of spin ½ charge polarons in Alq 3 . In addition the low field (B<20 mT) magneto-electroluminescence (MEL) response is also isotope sensitive, showing that HFI-induced spin mixing of polaron-pairs spin sublevels dominates this response too. However, the magneto-conductance (MC) response was found to be much less sensitive to isotope exchange at low fields, in agreement with previous studies. The disparity between the isotope sensitivity of MC and MEL responses in Alq 3 indicates that the HFI in the MC response is overwhelmed by an isotope independent spin mixing mechanism. We propose that collisions of spin ½ carriers -with triplet species such as polaron pairs may be the main spin mixing mechanism in the low field MC response in Alq 3 diodes.

Hyperfine interaction and its effects on spin dynamics in organic solids

“…In organics, the isotope effect is frequently studied by replacing H with D, i.e., deuteration. 7,9,10 In cases where H is the only HFI source, the effect of deuteration on the HFI is straightforward. For organics with other nuclear spins, the effect of deuteration on the HFI depends on how much the HFI originates from the H atoms.…”

Spectrally resolved hyperfine interactions between polaron and nuclear spins in organic light emitting diodes: Magneto-electroluminescence studies

“…Using layered devices based on Bphen/MTDATA -a well-known exciplex emitter -we show that the increase in EL emission intensity I due to small applied magnetic fields of order 100 mT is markedly larger at the high-energy blue end of the EL spectrum (∆I/I ∼11%) than at the low-energy red end (∼4%). Concurrently, the widths of the magneto-EL curves increase monotonically from blue to red, revealing an increasing hyperfine coupling between polarons and nuclei and directly providing insight into the energy-dependent spatial extent and localization of polarons.Several recent experiments have shown that small applied magnetic fields B on the order of 10-100 mT can induce substantial (∼10%) changes in the total light intensity emitted by organic light-emitting diodes (OLEDs) [1][2][3][4][5][6][7][8][9][10][11][12][13]. While initially surprising in view of the fact that the polymers and small molecules used in OLEDs are primarily composed of non-magnetic atoms (H, C, N), it was quickly appreciated that hyperfine spin interactions underpinned these phenomena.…”

“…This precession leads to spin mixing between singlet and triplet polaron-pair states, which are precursors to exciton or exciplex formation in an OLED. Applied fields B suppress this hyperfineinduced mixing, altering the population balance between singlet and triplet excitons or exciplexes, which in turn modifies the electroluminescence (EL) efficiency.The detailed dependence of EL intensity on B allows direct insight into not only the rates of singlet and triplet exciton/exciplex formation, but also reveals the strength of hyperfine coupling and therefore provides a measure of the spatial extent (size) of the electron and hole polarons.In magneto-EL studies to date [1][2][3][4][5][6][7][8][9][10][11][12][13], only the total (spectrally-integrated) EL intensity was measured as a function of B. However, OLED emission spectra typically span a very broad wavelength range, reflecting the fact that excitons and exciplexes form over a wide range of energies, and with varying degrees of localization for which different hyperfine couplings may be expected.…”

“…Several recent experiments have shown that small applied magnetic fields B on the order of 10-100 mT can induce substantial (∼10%) changes in the total light intensity emitted by organic light-emitting diodes (OLEDs) [1][2][3][4][5][6][7][8][9][10][11][12][13]. While initially surprising in view of the fact that the polymers and small molecules used in OLEDs are primarily composed of non-magnetic atoms (H, C, N), it was quickly appreciated that hyperfine spin interactions underpinned these phenomena.…”

Recent progress in organic spintronics

“…Compelling evidence exists that the HFI is important for spin mixing. Clear differences could be observed in studies of magnetic field effects in deuterated versus nondeuterated organic semiconductors [28,29]. Below we also discuss a recent report on direct electrical detection of nuclear spin manipulation in an OLED [30].…”