P. Desai

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer

Spintronics has shown a remarkable and rapid development, for example from the initial discovery of giant magnetoresistance in spin valves to their ubiquity in hard-disk read heads in a relatively short time. However, the ability to fully harness electron spin as another degree of freedom in semiconductor devices has been slower to take off. One future avenue that may expand the spintronic technology base is to take advantage of the flexibility intrinsic to organic semiconductors (OSCs), where it is possible to engineer and control their electronic properties and tailor them to obtain new device concepts. Here we show that we can control the spin polarization of extracted charge carriers from an OSC by the inclusion of a thin interfacial layer of polar material. The electric dipole moment brought about by this layer shifts the OSC highest occupied molecular orbital with respect to the Fermi energy of the ferromagnetic contact. This approach allows us full control of the spin band appropriate for charge-carrier extraction, opening up new spintronic device concepts for future exploitation.

The role of magnetic fields on the transport and efficiency of aluminum tris(8-hydroxyquinoline) based organic light emitting diodes

Magnetoresistance and efficiency measurements of aluminum tris(8-hydroxyquinoline) (Alq3) based organic light emitting diode structures have been made as a function of magnetic field and Alq3 thickness. Both positive and negative magnetoresistances can be observed depending on the thickness of the Alq3 layer, the drive voltage, and the applied field. In all devices, large increases in device efficiency are observed. We suggest that the increase in device efficiency is due to conversion of triplet states into singlets through a hyperfine scale interaction. The changes in the magnetoresistance are a result of the reduction in the triplet concentration and operate either through the reduced role of free carrier trapping at triplet states or through the reduction in triplet dissociation at the cathode interface depending on the Alq3 thickness.

The magnetic field effect on the transport and efficiency of group III tris(8-hydroxyquinoline) organic light emitting diodes

Magnetoresistance and efficiency measurements of organic light emitting diode structures based on the group III hydroxyquinolates (Mq3) have been made as a function of magnetic field and Mq3 thickness, where M=Al, Ga, and In. For all quinolates, independent of thickness, we observed very similar behavior for the efficiency of the devices, with large increases in efficiency occurring at low values of applied field, which rapidly saturate as the field is increased. The current through these devices is found to be a strong function of both the device thickness and the metal ion. For Alq3 based devices, the current changes appear to correlate strongly with the triplet population in the devices. For Gaq3 and Inq3 devices, the magnetoresistance is found not to correlate with the triplet concentration and this may be evidence that there is little energetic barrier for carrier trapping in these materials. For all materials, a further dependence of the magnetoresistance on applied field was observed, which needs c...

The effect of applied magnetic field on photocurrent generation in poly-3-hexylthiophene:[6,6]-phenyl C61-butyric acid methyl ester photovoltaic devices

The effect of a magnetic field on the photocurrent generated by a bulk heterojunction solar cell made from poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is investigated. At the operating voltage, increases in photocurrent of ~9% can be obtained at magnetic fields of less than 100 mT. This increase in photocurrent is attributed to an increase in the rate of intersystem crossing, between the singlet and triplet states, leading to a higher net efficiency of exciton dissociation. Close to the open-circuit voltage, an increase of more than two orders of magnitude in the photocurrent could be obtained under applied magnetic field.

Separating the roles of electrons and holes in the organic magnetoresistance of aluminum tris(8-hydroxyquinoline) organic light emitting diodes

Measurements of the effect of a magnetic field on the photocurrent in an aluminum tris(8-hydroxyquinolate) based organic light emitting diode have shown that it is possible to identify the contribution to the organic magnetoresistance of both electrons and holes. For holes the effect of a magnetic field is to decrease the mobility, whereas for electrons the magnetic field appears to increase the mobility. These changes are suggested to be brought about through the magnetic field dependence of the scattering of electrons and holes with excited states within the device.

Magnetoresistance in triphenyl-diamine derivative blue organic light emitting devices

Magnetoresistance measurements have been performed on thin layers of the triphenyl-diamine derivative, N,N′-diphenyl-N,N′ bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′ diamine (TPD). At low drive voltages, where the current transport is solely hole mediated, no magnetoresistance is observed. At higher drive voltages, where electron injection into the TPD is occurring, magnetoresistance is seen and the sign of the magnetoresistance depends on the current density in the device.

The importance of holes in aluminium tris-8-hydroxyquinoline (Alq3) devices with Fe and NiFe contacts

To study the dominant charge carrier polarity in aluminium tris-8-hydroxyquinoline (Alq3) based spin valves, single Alq3 layer devices with NiFe, ITO, Fe, and aluminium electrodes were fabricated and characterised by Time of Flight (ToF) and Dark Injection (DI) techniques, yielding a lower hole mobility compared to electron mobility. We compare the mobility measured by DI for the dominant carrier injected from NiFe and Fe electrodes into Alq3, to that of holes measured by ToF. This comparison leads us to conclude that the dominant charge carriers in Alq3 based spin valves with NiFe or Fe electrodes are holes.

Improved electron injection into Alq3 based devices using a thin Erq3 injection layer

The role of a thin erbium(III) tris(8-hydroxyquinoline) (Erq3) interface layer on the electron injection into aluminium(III) tris(8-hydroxyquinoline) (Alq3) based organic light emitting devices (OLEDs) has been investigated. It has been shown that the use of a 40 A interface layer can increase the efficiency of a simple Alq3 OLED with an Al cathode to a level comparable with other, well established, high-efficiency cathodes such as LiF/Al. We also show that, despite the bulk HOMO and LUMO positions for Erq3 being little different from those for Alq3, the presence of an interfacial layer makes the devices turn-on voltage almost independent of the cathode metal. This is explained by there being a vacuum level shift for Erq3 which is dependent on the work function of the cathode metal.

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.