Nanotesla magnetoresistance in π-conjugated polymer devices

Abstract

We demonstrate submicrotesla sensitivity of organic magnetoresistance in thin-film diodes made of the conducting polymer poly(styrene sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS). The magnetoresistance sensitivity is shown to be better than 20 parts per billion (ppb). As for other conjugated polymers, magnetoresistance can be separated into two regimes of field strength: the nonmonotonic ultrasmall magnetic field effect on magnetic field scales below 2 mT, and the monotonic intermediate magnetic field effect on scales over several tens of mT. The former gives the PEDOT:PSS magnetoresistance curve a characteristic W-shaped functionality, with inverted turning points compared to those found in conventional organic light-emitting diode (OLED) devices. We succeed in resolving the ultrasmall magnetic field effect of the PEDOT:PSS layer incorporated within an OLED structure, which is responsible for an additional magnetoresistive feature on the ppm scale. Such a device shows unprecedented complexity in magnetoresistance with a total of four extrema within a field range of ±1 mT. We propose that these unique characteristics arise from spin-spin interactions in the weakly bound carrier pairs responsible for the spin-dependent recombination probed in magnetoresistance.

Direct Detection of Singlet-Triplet Interconversion in OLED Magnetoelectroluminescence with a Metal-Free Fluorescence-Phosphorescence Dual Emitter

“…The central part of the magnetoresistance response, on the other hand, narrows and develops an additional feature within AE2 mT. In prior reports, this ultrasmall magnetic-field effect has typically been seen in both (singlet) MEL and in magnetoresistance and is commonly attributed to stabilization of the spin state in the absence of the external magnetic field by weak exchange or spin-dipolar coupling [52][53][54][55][56]. Observing this response only in magnetoresistance and not in MEL implies that it cannot be directly caused by magneticfield effects on the electron-hole pair exciton precursors, but is more likely related to the transport of charges towards the recombination zone.…”

High-Field Magnetoresistance of Organic Semiconductors

“…For fields exceeding a few hundred mT the Δ -dependent high-field magnetoconductance effect as described above dominates. At lower magnetic fields, below 200 mT, additional ultra-small and intermediate magnetic field effects are observed [5,12,17,30,31]. These effects are described in…”

Temperature and current dependence of the magnetoresistive behavior of poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS)

“…As previously reported, the PEDOT:PSS device retains ohmic behavior (i.e., a resistance independent of current) at room temperature up to driving currents of 300 μA and shows nonohmic behavior for higher driving currents. 14 The critical current for the transition of ohmic to nonohmic behavior decreases with temperature, down to 30 nA at 4.2 K. This crucial fact is often overlooked in temperature-dependent studies of multilayer devices employing PEDOT:PSS as a hole injection layer, but should not come as a surprise: as in every doped conductor, the number of free charges is temperature-dependent. The absolute resistance in the ohmic regime increases with decreasing temperature, as is expected of a highly doped polymer such as PEDOT:PSS.…”

“…The magnetoresistance measurements were carried out on the same setup and under the same measurement conditions as the current-resistance characteristics. As described in our previous studies, 14 the highest accuracy in measurement is achieved by taking multiple, consecutive magnetoresistance sweeps after settling times that are on the order of minutes at least. The resulting magnetoresistance measurements at a constant driving current of I ¼ 250 μA for multiple temperatures, marked in Fig.…”

“…1,2,4,9,12,13 We have recently shown that the study of specific magnetic field effects, such as the ultrasmall magnetic field effect (<1 mT), may be hampered in these devices. 14 In heterostructures, magnetic field effects of different organic compounds can be superimposed, resulting in a difficult interpretation of results that can cause ambiguities with regard to the assignment of the physical mechanisms responsible for the observed behavior as well as their quantitative characteristics. Although a rather complex compound in itself, devices with an active layer consisting solely of the polymer poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) exhibit a well-pronounced ultrasmall magnetic field effect in conductivity, owing to relatively weak hyperfine magnetic field effects compared to other conducting materials whose hyperfine fields are one-to twoorders higher in magnitude.…”