Magnetoconductance Research Articles

Identifying the magnetoconductance responses by the induced charge transfer complex states in pentacene-based diodes

We investigate the magnetoconductance (MC) responses in photocurrent, unipolar injection, and bipolar injection regimes in pentacene-based diodes. Both photocurrent and bipolar injection contributed MC responses show large difference in MC line shape, which are attributed to triplet-polaron interaction modulated by the magnetic field dependent singlet fission and the intersystem crossing of the polaron pair, respectively. By blending 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane into pentacene, all the MC responses are suppressed but the MC response at unipolar injection regime is enhanced, which is attributed to the induced charge transfer complex states (CT complex states). This work identify the MC responses between single carrier contributed MC and exciton related MC by the induced CT complex states.

Photoinduced magnetoconductance response of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine-based diodes: Sign inversion by controlling carrier extraction

We explore the magnetoconductance (MC) response of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine-based single-layer devices. The MC effect can be observed only if the device is irradiated with light, for measurement voltages |V|<5V. A positive MC with a non-Lorentzian line shape is obtained under forward bias. Under reverse bias, however, the MC shows both positive and negative components, forming a “W” shape with dips at about ±100mT. The sign of the MC under reverse bias can be changed by controlling the carrier extraction from the anode/organic interface. We suggest that the positive MC is caused by triplet–polaron interaction and the negative MC by bipolaron formation.

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

We studied the spin response of various magnetic field effects and magnetotransport 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 magnetoresistance in spin valves show substantial isotope dependence pointing to the importance of the hyperfine interaction (HFI) in the spin response of spin \textonehalf{} charge polarons in Alq${}_{3}$. In addition, the low field ($B$ 20 mT) magnetoelectroluminescence (MEL) response is also isotope sensitive, showing that HFI-induced spin mixing of polaron-pairs spin sublevels dominates this response too. However, the magnetoconductance (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 \textonehalf{} 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.

Investigation on organic magnetoconductance based on polaron-bipolaron transition

We explore the magnetoconductance (MC) effect in an organic semiconductor device based on the magnetic field related bipolaron formation. We present the transition among spin-parallel spin-antiparallel polaron pairs and bipolarons with a group of dynamic equations. The transition rates are adjusted by the external magnetic field as well as the hyperfine interaction. By considering the mobility of polarons different from that of bipolarons, we obtain the MC in an organic semiconductor device. The theoretical calculation is well consistent to the experimental results. It is predicated that a maximum MC appears at a suitable branching ratio of bipolarons.

Non-linear transport by solitons in nanofibers of polymers in high magnetic field

Nonlinear local excitations like solitons, polarons, and bipolarons are known to be responsible for physical properties of conducting polymers. Recent experiments on nano-fibers in high electric and magnetic fields provide a further insight by demonstrating an effect of vanishing magnetoconductance (MC) in the polyacetylene (PA)—in contrast to other polymers. Here we present new experimental data and describe the theoretical model based on notion of solitons—dimerization kinks which can carry either the spin or the charge; they are allowed only in the PA with its degenerate ground state. The solitons experience a confinement force due to the interchange coupling which is erased by the electric field and disappears above critical field strength. The unbinding by tunneling allows for the transport of individual solitons, which sweeps off the spins residing at electronic intragap states associated with polarons, hence the vanishing MC.

Magnetoconductance effect in organic light-emitting devices

Organic light-emitting diode (OLED) based on tris-(8-hydroxyquinoline) aluminum(III) (Alq3) is fabricated, and its magnetoconductance (MC) effects are measured at different bias voltages. When the bias voltage is small, the OLED exhibits apparently a negative MC effect. After the bias voltage is increased, the MC value changes from negative to positive, displaying a negative-positive inversion. The MC effects in N, N'-Di(naphthalen-1-yl)-N, N' diphenyl-benzidine (NPB) and Copper phthalocyanine (CuPc) unipolar devices show that the negative MC effect in OLED comes from the CuPc layer in device. The MC effect of bipolar current can be explained using the electron-hole pair model. The MC effect of unipolar current can be attributed to the polaron-bipolaron transition in device. The positive-negative MC inversion in OLED results from the simultaneous contributions of the above two mechanisms during the variation of the injection current.

Magnetoconductance responses in organic charge-transfer-complex molecules

Blending 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), a high electron affinity organic molecule, with N,N′-bis-(1-naphyl)-N,N′-diphenyl-1,1′-biphenyl′-4,4′-diamine (NPB) creates the charge-transfer-complex molecules and modulates the magnetoconductance (MC) responses of devices by an applied electrical bias. A negative MC response (−0.60%) is observed for the device made of NPB:F4-TCNQ(30%) film as the active layer, which has a distinct characteristic to negligible MC responses in pristine NPB- and F4-TCNQ-based devices. We attribute the MC responses to the magnetic-field-dependent transport of injected charge carriers through charge-transfer-complex molecules in the NPB:F4-TCNQ system, which are varied with the F4-TCNQ blending concentrations and the injection of minority charge carriers. Results of this study demonstrate a turning point of MC responses at the bias voltage near the threshold of the bipolar injection.

Strain-dependent transport properties of the ultra-thin correlated metal, LaNiO3

We explore the electrical transport and magneto-conductance (MC) in quasi-two-dimensional strongly correlated ultra-thin films of LaNiO3 (LNO) to investigate the effect of hetero-epitaxial strain on electron–electron and electron–lattice interactions from the low to intermediate temperature range (2–170 K). The fully epitaxial 10 unit cell thick films spanning tensile strain up to ∼4% are used to investigate the effects of enhanced carrier localization driven by a combination of weak localization (WL) and electron–electron interactions at low temperatures. The MC data show the importance of the increased contribution of WL to low-temperature quantum corrections. The obtained results demonstrate that with increasing tensile strain and reduced temperature, the quantum-confined LNO system gradually evolves from the Mott into the Mott–Anderson regime.

Study of magneto-electroluminescence and magneto-conductance in polymer light-emitting electrochemical cells

Magneto-electroluminescence (MEL) and magneto-conductance (MC) have been investigated in polymer light-emitting electrochemical cells based on “super-yellow” poly-(phenylene vynilene) (SY-PPV). We measured positive MEL(B) and MC(B) responses that show a modified Lorentzian shape for B &amp;lt; 120 mT. At B &amp;lt; 2 mT, both MEL(B) and MC(B) responses show a second component, opposite in sign to the higher field response. We interpret the magnetic field response using the polaron pair model, from which we extract the hyperfine interaction constant of polarons in the SY-PPV polymer.

Positive and negative components of magnetoconductance in hole transport limited organic light-emitting diodes

Two kinds of devices using N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (NPB) and dye-doped NPB as emitting layer were fabricated to study their magnetoconductance (MC) response. The MC of the NPB devices contains a positive low-field (0&amp;lt;|B|&amp;lt;40 mT) component but a negative high-field (|B|&amp;gt;40 mT) component at low temperatures. Similar MC is presented in the dye-doped NPB devices even at room temperature. Magnetoelectroluminescence results and energy-level diagram indicate that long lifetime triplet excitons and excessive holes are in these devices. All these observations suggest that triplet exciton-hole reaction is responsible for the negative MC while positive MC is assigned to hyperfine mixing of electron-hole pairs.

Alq&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; 有机发光二极管中的正负磁电导转变

Organic light-emitting diodes (OLEDs) based on Alq3 have been fabricated. The structure of devices is ITO/CuPc/NPB/Alq3/LiF/Al. The magnetoconductance (MC) effect on the current was measured with constant voltage bias. When the thickness of Alq3 layer is 15 nm, an apparent positive-negative inversion of MC occurred during the current transition from bipolar to unipolar. When the thickness of Alq3 layer was 65 nm, the MC values increased firstly and then fell with the decreasing current during the current transition from bipolar to unipolar. However, under any measurement conditions, the MC values were always positive. The inversion of MC form positive to negative was not observed in this device. The MC effect of bipolar current can be explained using the electron-hole pair model and exciton-charge reaction model. While the MC effect of uniploar current still need further study, although it can be attributed to the polaron-bipolaron transition in device.

Probing the inter-tube transport in aligned and random multiwall carbon nanotubes

The temperature and magnetic field dependence of conductivity has been used to probe the inter-tube transport in multiwall carbon nanotubes (MWNTs). The scanning electron microscopy images show highly aligned and random distribution of MWNTs. The conductivity in aligned carbon nanotube (ACNT) and random carbon nanotube (RCNT) samples at low temperature follows T1/2 (at T &amp;lt; 8 K) and T3/4 (at T &amp;gt; 8 K) dependence in accordance with the weak localization and electron-electron (e-e) interaction model. The values of diffusion coefficient in ACNT and RCNT are 0.25 × 10−2 and 0.71 × 10−2 cm2 s−1, respectively, indicating that larger number of inter-tube junctions in later enhances the bulk transport. The positive magnetoconductance (MC) data in both samples show that the weak localization contribution is dominant. However, the saturation of MC at higher fields and lower temperatures indicate that e-e interaction is quite significant in RCNT. The T−3/4 and T−1/2 dependence of inelastic scattering length (lin) in ACNT and RCNT samples show that the inelastic e-e scattering is more important in aligned tubes.

On the role of minority carriers in the frequency dependence of organic magnetoresistance

In this work we investigate the frequency dependence of organic magnetoresistance (OMAR) both in small molecule-based (Alq3) and polymer (PPV derivative) materials, and investigate its thickness dependence. For all devices, we observed a strong decrease in magnetoconductance (MC) with increasing frequency of the ac component of the applied magnetic field. Moreover, we observed a strong reduction of the cut-off frequencies for increasing film thickness. By means of admittance spectroscopy and device simulations, we show that the cut-off frequency is related to the inverse transit time of the minority charge carriers. These observations confirm the important role of minority carriers in OMAR, and show that changes in OMAR are not only due to microscopic mechanisms, but also device physics is of significant relevance.

Magneto-conductance of π-conjugated polymer based unipolar and bipolar diodes

The magnetoconductance (MC) of unipolar and bipolar organic diodes based on active layers made of the π-conjugated polymer DOO-PPV is presented in the range of magnetic field strength, B below about 100 mT, and for various polymer exchange isotopes. The MC( B) response is composed of two main regions: (i) a “sign-reversal” region for | B| < 1–2 mT, where MC( B) reverses its sign reaching a maximum absolute value |MC| m at B = B m; and (ii) a monotonic region for | B| > 2 mT, where MC( B) monotonically increases (decreases) for bipolar (unipolar) devices with an approximate Lorentzian line shape of width, Δ B. Similar behavior has been observed for the magneto-electroluminescence in bipolar devices. We found that MC( B) in both field regions is strongly isotope dependent; in particular both B m and Δ B are larger for nuclei with larger hyperfine interaction (HFI) constant. The complete MC( B) response, including the novel sign-reversal component is explained by a model of spin-coupled pairs of either same or opposite charge polarons, in which the HFI is explicitly included in the electronic spin Hamiltonian.

Control of magnetoconductance through modifying the amount of dissociated excited states in tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes

Magnetoconductance (MC) is generally believed to be controlled by the ratio of singlet to triplet excited states. In this study, it is found that the MC magnitude of tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes decreases substantially upon the introduction of narrow band gap fluorescent dopants. Since singlet to triplet ratio of excited states keeps unchanged in doped devices, this large reduction in MC means that other underlying mechanism affects the MC. The charge carrier trapping effect is proposed here to vary the magnitude of MC. By using this trapping effect, the controlling of the total amount of dissociated electron-hole pairs and consequently the magnitude of MC are realized by changing the dopant’s concentration or band gaps.

The magneto conductance responses in polymer photovoltaic devices

This article studies the magneto conductance (MC) responses of regioregular poly(3-hexylthiophene) (P3HT)-based polymer photovoltaic devices. The net MC results of devices are the sum of the positive and negative MC effects. The positive MC effect is due to the increased population of the singlet polaron-pair (PP) states (of the high dissociation rate to charge carriers) under the magnetic field. The negative MC effect results from the decline on the triplet exciton–charge reactions to charge carriers (the mobility and concentration of triplet excitons are decreased by the magnetic field). The MC responses of P3HT-based photovoltaic devices can be manipulated by the applied magnetic field, the electrical bias, the built-in potential, and the interfacial dipole layer. An inversion in the MC response is observed at the electrical bias near the open-circuit voltage. In addition, blending of an electron acceptor material, [6,6]-phenyl C61-butyric acid methyl ester, in P3HT active layer quenches the photo-excited states at the donor–acceptor interface and results in distinct MC responses of photovoltaic cells, which probably are related to the features indicating the formation of inter-molecular charge–transfer complexes at donor–acceptor junction.

Separating photocurrent and injected current contributions to the organic magnetoresistance

The magnetoresistance in the photocurrent and injected current of an organic device has been studied. When such a device is illuminated under bias, both photo-generated charges and electrically injected carriers will contribute to the current. We used a magnetic field modulation to successfully remove the time-dependent change in the current. This was combined with a modulation of the intensity of the illumination to separately measure the effects of the photocurrent. At low bias, without illumination no magnetoconductance (MC) is observed in leakage current, while with illumination an MC is observed due to the photo-generated charges. With increasing voltage, as electrical charge injection starts, the width of the MC curves as a function of magnetic field of the photocurrent remains constant and is larger than the curve observed for the electrically injected current, even when both currents are present. This result shows that the contributions from the photocurrent and the injected current cannot be combined, but should be treated separately.

Inversion of magnetic field effects on electrical current and electroluminescence in tri-(8-hydroxyquinoline)-aluminum based light-emitting diodes

The magnetic field effects on electrical current and electroluminescence have been investigated for tri-(8-hydroxyquinoline)-aluminum based organic light emitting diodes through the transient electroluminescence method. By applying rectangular pulse to drive the devices, the influence of the degradation is dramatically decreased; and a full-range evolvement for the magnetoconductance (MC) and magnetoelectroluminescence (MEL) with the driving voltage has been obtained. The transition voltages for MC (1.6 V) from the negative to positive value and MEL (7 V) from the positive to negative value are different, which suggests that the magnetic field effect on the MC and MEL may have different mechanisms.

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.

Magnetic Field Effects inπ-Conjugated Polymer-Fullerene Blends: Evidence for Multiple Components

We studied magnetoconductance (MC) and magnetoelectroluminescence (MEL) in organic diodes from blends of pi-conjugated polymers and fullerenes at various concentrations, c. The MC response is composed of several components that depend on the applied bias voltage and c. A dominant positive low-field (LF) component, which also governs the MEL response, dramatically decreases and broadens in the blends, thus unraveling a positive high-field and negative LF components. The positive MC components are caused by electrostatically bound e-h polaron pairs in unblended devices, and charge transfer pairs in the blends, which are dominated by two different field-induced spin sublevel mixing mechanisms. In contrast, the negative LF response is due to e-e and h-h pairs; this is confirmed by studying MC in electron- and hole-unipolar devices, which lack positive MC response.