Annealing and doping-dependent magnetoresistance in single layer poly(3-hexyl-thiophene) organic semiconductor device

We compare the current density–voltage (J–V) and magnetoconductance (MC) response of a poly(3-hexyl-thiophene) (P3HT) device (Au/P3HT(350 nm)/Al) before and after annealing above the glass transition temperature of 150 °C under vacuum. There is a decrease of more than 3 orders of magnitude in current density due to an increase of the charge injection barriers after de-doping through annealing. An increase, approaching 1 order of magnitude, in the negative MC response after annealing can be explained by a shift in the Fermi level due to de-doping, according to the bipolaron mechanism. We successfully tune the charge injection barrier through re-doping by photo-oxidation. This leads to the charge injection and transport transitioning from unipolar to ambipolar, as the bias increases, and we model the MC response using a combination of bipolaron and triplet-polaron interaction mechanisms.     

Traps as interaction sites for hyperfine mixing: The origin of magnetoconductance in organic light-emitting diodes

The origin of magnetoconductance (MC) in organic light-emitting diodes under bipolar injection conditions was investigated using devices containing pristine Super-Yellow poly(phenylene vinylene) (SY-PPV) or SY-PPV:phenyl-C61-butyric acid methyl ester (PCBM) (x wt%) blends as the active layers. In pristine SY-PPV device, it was found that the low-field component of MC was always larger than the high-field component. Additionally, the low-field component increased and then saturated with increasing the electrical stressing time, whereas the high-field component remained unchanged. These behaviors were analyzed using empirical formula (containing a Lorentzian and a non-Lorentzian function), which suggested that the dominant mechanism in the MC response was hyperfine mixing between single and triplet polaron pairs that occurred on trap sites. The specific role of these traps, providing interaction sites for hyperfine mixing, was confirmed by controlling the lifetime of the trapped polaron-pairs states by doping the active layer with PCBM.     

Modulations in line shapes of magnetoconductance curves for diodes of pentacene:fullerene charge transfer complexes

This work investigates that the applied electric field modulates the line shapes of magnetoconductance (MC) responses in pentacene:fullerene diodes under illumination. We attribute the line shape of MC curves herein is correlated with the strength of the exchange interaction in pentacene:fullerene charge transfer (CT) complex states. Applying the reversed bias increases the built-in electric field (E_built-in) of the diodes to offset the exchange interaction of CT complex states and narrows the line shapes of MC responses. The saturation field of MC curves in the pentacene:fullerene bulk heterojunction device is 752 Oe when the device is biased at 0.4 V and decreases to 212 Oe at a reversed bias of −1.0 V. The line shape of MC curves for the diode made of pristine pentacene or fullerene as the active layer does not change with the applied bias voltage. Our results indicate the correlations of MC responses with the exchange interaction of CT complexes as modulated by the applied electric field.     

Magnetoconductance of polymer–fullerene bulk heterojunction solar cells

The organic magnetoconductance (MC) effects in poly(3-hexylthiophene): [6,6]-phenyl-C61-butyricacid methylester based bulk heterojunction solar cells were studied in dark and under illumination. The correlations between the MC and current character were revealed in this study. Results show that the dark current always exhibits a negative MC whereas a sign change in MC under illumination occurs at the bias around the open circuit voltage V_oc. We suggest that the positive MC in photocurrent is due to the field dependent conversion of singlet electron–hole pairs to triplet states and the negative MC is associated with space charge limited current with traps. Other possible mechanisms about the magnetoconductance effects are also discussed.     

Full-three dimensional quantum approach to evaluate the surface-roughness-limited magnetoresistance mobility in SNWT

We present a theoretical method to simulate magnetotransport in silicon nanowire (Si-NW) MOSFET including the effect of Surface Roughness (SR). We use a full three dimensional (3D) real-space self-consistent Poisson-Schrödinger solver based on Non Equilibrium Green’s function Formalism (NEGF) which can treat the influence of an external magnetic field on the device. By comparing magnetoconductance curves with the classical Drude formula we extract magnetoresistance (MR) mobility for nanowires with and without roughness. From the preliminary results it seems that the MR mobility is not dramatically reduced for the SR parameters considered in this work.     

Antagonistic responses between magnetoconductance and magnetoelectroluminescence in polymer light-emitting diodes

Antagonistic responses between magnetoconductance (MC) and magnetoelectroluminescence (MEL) in the polymer light-emitting diodes with an interfacial layer between Al cathode and active layer are simultaneously measured. As the interfacial layer (tetraoctylammonium bromide) is used, the significant increase in the number of injected negative polarons and the blocking of positive polarons promote the triplets-(free polaron) reaction and provide a good explanation for the reason that electroluminescence (EL) efficiency is maximal in the trap free space charge limited current regime at high bias. By fitting of MC and MEL curves using Lorentzian and non-Lorentzian empirical equations, three magnetic field dependent mechanisms, which are the intersystem crossing between singlet/triplet polaron pairs, the triplets-(free polaron) reaction, and the triplets-(trapped polaron) reaction are elucidated. The distribution of the three components is tunable by varying the applied electric field, which primarily modulates the triplets-(free polaron) reaction rate. The results pave a new route toward understanding the mechanism of organic spintronics for developing of multifunctional devices.     

The influence of the triplet exciton and charge transfer state energy alignment on organic magnetoresistance

Recently, it was discovered that the current through an organic semiconductor, sandwiched between two non-magnetic electrodes, can be changed significantly (up to 25%) by applying a small (a few millitesla) magnetic field. At present, the microscopic mechanisms underlying this so-called organic magnetoresistance (OMAR) are intensively being debated. One of the mechanisms which can successfully describe the magnetic field effects on the current in pristine organic semiconductor devices uses the reactions of triplet excitons and polarons. Here, we present a proof of concept study in which we tune these interactions in the device by deliberately doping our devices with fullerene, creating additional charge transfer states (CTS). By engineering devices with different energetic alignments of the CTS and triplet exciton, we can influence the triplet exciton density in the device. We correlate pronounced changes in the magnetic field effect magnitude and lineshape to the energy of the CTS with respect to the triplet exciton.     

Magnetoconductance response due to the triplet exciton–charge interaction in organic light-emitting diodes

The magnetoconductance (MC) effects in three organic light-emitting diodes have been measured over a range of operating current and temperatures. The peculiar roles played by the hyperfine field and triplet excitons are outlined by fitting the MC traces using the sum of two non-Lorentzian functions. The MC response evident at large magnetic fields is confirmed to be resulted from the triplet exciton–charge interaction by investigating its variation with the triplet population. The remarkable agreement between the experimental data and fit lines demonstrates that the method used in this work could help to identify the correct models for understanding the fundamental mechanism behind the magnetic field effects universally observed in organic devices.     

Theoretical investigation on magnetic field effect in organic devices with asymmetrical molecules

A mechanism of organic magnetic field effect (OMFE) based on Lorentz effect in organic light-emitting devices with asymmetrical molecules is suggested and the magnetoconductance (MC) value is calculated. By considering the collision of a positive and a negative charged polaron and exciton formation in the organic layer through a non-adiabatic dynamic process, it is found that the exciton yield can be changed by applying a magnetic field due to the Lorentz effect on the moving polarons’ phase factors. By calculating the current through the device and the MC, we obtain that the calculated MC is well consistent to some experimental observations. It is also found that the MC value is sensitive to the asymmetrical structure and the electron–phonon (e–ph) coupling of the organic material, which explains why magnetic effect in an organic semiconductor is much more apparent than its inorganic counterpart.