Sijie Zhang

Measurement of the intersystem crossing rate in aluminum tris(8-hydroxyquinoline) and its modulation by an applied magnetic field

The rate constant for intersystem crossing in aluminum tris(8-hydroxyquinoline) was measured using the time dependence of the luminescence under high excitation intensity and modeling using a rate equation approach. Under high illumination levels intersystem crossing results in the transfer of singlets into triplets, which due to their long lifetime effectively remove molecules from participating in photoluminescence. The intersystem crossing rate was found to be ∼2.2×104 s−1 at 80 K. The presence of a magnetic field was found to increase the rate constant by ∼10% with applied fields of ∼100 mT.

Temporal mapping of photochemical reactions and molecular excited states with carbon specificity

Photochemical reactions are essential to a large number of important industrial and biological processes. A method for monitoring photochemical reaction kinetics and the dynamics of molecular excitations with spatial resolution within the active molecule would allow a rigorous exploration of the pathway and mechanism of photophysical and photochemical processes. Here we demonstrate that laser-excited muon pump-probe spin spectroscopy (photo-μSR) can temporally and spatially map these processes with a spatial resolution at the single-carbon level in a molecule with a pentacene backbone. The observed time-dependent light-induced changes of an avoided level crossing resonance demonstrate that the photochemical reactivity of a specific carbon atom is modified as a result of the presence of the excited state wavefunction. This demonstrates the sensitivity and potential of this technique in probing molecular excitations and photochemistry.

Spintronic and electronic phenomena in organic molecules measured with μSR

The use of implanted muons to probe the spin dynamics and electronic excitations in organic materials is reviewed. At first, a brief introduction to the historical context and background of the muon technique is given, followed by an outline of some of the underlying theoretical models needed to quantitatively interpret data taken on organic molecules. Caution is advised when using certain theoretical models for the interpretation of low-field spin relaxation data. The next section deals with spin dynamics in soft materials, and starts with discussing many of the key results in thin films, followed by a review of bulk measurements in three different materials classes — polymers, biologically active molecules, and small molecules. Finally, we present a detailed discussion of the density functional theory methodology when applied to μSR, and present the common issues encountered when trying to perform these calculations to support muon experiments. In particular, we discuss a method for benchmarking to mana...

Hole-exciton interaction induced high field decay of magneto-electroluminescence in Alq3-based organic light-emitting diodes at room temperature

The magnetic field effects on the electroluminescence of aluminium tris-(8-hydroxyqinoline) (Alq3) based organic light emitting diodes have been investigated by varying the electron/hole ratio in the emissive layer. Experimental results reveal that a negative high field effect in the magneto-electroluminescence (MEL) can be found in devices with very low triplet exciton concentration at room temperature. This suggests triplet-triplet annihilation cannot be used to explain the negative high field MEL in the Alq3 system. Our results suggest that hole-exciton interaction may be the origin of the negative high field MEL and also, in parallel with this interaction, there is also the more common positive high field process occurring which has been tentatively attributed to electron-exciton interactions. The competition between these different processes decides the final shape of the MEL at high fields.

Measurement of hyperfine coupling constants of muoniated radicals in small molecule semiconductors

We report the hyperfine coupling constants of muoniated radicals formed in a number of organic semiconductors, via transverse field measurements taken in the Paschen Back limit, and compare the results to avoided level crossing resonances. Five muoniated radicals are found in tetracene, despite there only being three potential non-equivalent bonding sites, and we suggest that this might be down to crystal packing effects. For 6,13-bis(triisopropylsilylethynyl) pentacene and 6,13-bis(trimethlsilylethynyl)-pentacene, we demonstrate that the transverse field data supports the previously published avoided level crossing resonances.

Understanding the role of electron and hole trions on current transport in aluminium tris(8-hydroxyquinoline) using organic magnetoresistance

The change in current through an organic light emitting diode (OLED) when it is placed in a magnetic field has been dubbed organic magnetoresistance and provides a means to understand the spin interactions that are occurring in working devices. Whilst there are a wide range of interactions that have been proposed to be the cause of the measured effects, there is still a need to identify their individual roles and in particular how they respond to an applied magnetic field. In this work, we investigate the effect of changing the balance of electron and hole injection in a simple aluminium tris(8-hydroxyqinoline) based OLED and demonstrate that the triplet polaron interaction appears to be much stronger for electrons than for holes in this material.

Ferromagnetic-organic interfacial states and their role on low voltage current injection in tris-8-hydroxyquinloline (Alq3) organic spin valves

Organic Spin Valves (OSVs) operate at small bias (<100 mV) when carrier injection should not occur due to injection barriers and in built potentials. We explore the consequences of hybrid-interface states between a ferromagnetic electrode and an organic semiconductor in OSV carrier injection. By temperature-dependent Dark Injection measurements, we observe hole trapping due to these filled states and measure a low thermal activation energy (∼100 meV) of the carrier density within OSVs. The small injection barrier is consistent with a significant interfacial potential, due to hybrid-interface state filling, overcoming the injection barrier due to the electrode work function—transport level mismatch.