As carriers slowly move through a disordered energy landscape in organic semiconductors, tiny spatial variations in spin dynamics relieve spin blocking at transport bottlenecks or in the electron-hole recombination process that produces light. Large room-temperature magnetic-field effects (MFEs) ensue in the conductivity and luminescence. Sources of variable spin dynamics generate much larger MFEs if their spatial structure is correlated on the nanoscale with the energetic sites governing conductivity or luminescence such as in coevaporated organic blends within which the electron resides on one molecule and the hole on the other (an exciplex). Here, we show that exciplex recombination in blends exhibiting thermally activated delayed fluorescence produces MFEs in excess of 60% at room temperature. In addition, effects greater than 4000% can be achieved by tuning the device's current-voltage response curve by device conditioning. Both of these immense MFEs are the largest reported values for their device type at room temperature. Our theory traces this MFE and its unusual temperature dependence to changes in spin mixing between triplet exciplexes and light-emitting singlet exciplexes. In contrast, spin mixing of excitons is energetically suppressed, and thus spin mixing produces comparatively weaker MFEs in materials emitting light from excitons by affecting the precursor pairs. Demonstration of immense MFEs in common organic blends provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices without patterning the constituent materials on the nanoscale. Magnetic fields increase the power efficiency of unconditioned devices by 30% at room temperature, also showing that magnetic fields may increase the efficiency of the thermally activated delayed fluorescence process.

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We describe a regime for low-field magnetoresistance in organic semiconductors, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere. The regime is studied by the controlled addition of localized electronic spins to a material that exhibits substantial room-temperature magnetoresistance (∼ 20%). Although initially the magnetoresistance is suppressed by the doping, at intermediate doping there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current. Organic semiconductors (OSC) exhibit intriguing roomtemperature spin-dependent phenomena, including magnetic field effects on "radical pairs" [1][2][3][4][5][6][7][8][9]. Radical pairs, which are spin-carrying excitations that occupy neighboring molecules in an organic film, can consist of electron-hole pairs, electronelectron (or hole-hole) pairs, and mixed pairs consisting of a spin-1/2 polaron and a spin-1 triplet exciton. These radical pairs can undergo spin-dependent reactions, which due to the large on-site exchange energies in OSC, depend sensitively on the pair spin state. The spin-dependent behavior of radical pairs that occupy transport bottleneck sites can have a significant effect on the conductivity [9,10] and electroluminescent efficiency [6,11] of an organic device. The spin dynamics of these radical pairs is sensitive to the presence of magnetic fields, including an applied magnetic field, an exchange or dipolar field with neighboring spins (localized or mobile), and a nuclear (hyperfine) field. Low-field, room-temperature magnetoresistance in OSC with nonmagnetic electrodes, so-called "OMAR," is one consequence. Even though the exact mechanism behind OMAR is still debated [12], it is widely believed to be related to hyperfine interactions and radical pair effects as described above, at least in some materials. The effects of additional radical dopant spins on these phenomena were first studied in the context of organic solar cells [13,14]. The efficiency and short circuit current of doped regio-regular poly(3-hexylthiophene)/1-(3-(methoxycarbonyl)propyl)-1-phenyl)(6,6)C 61 (P3HT/PCBM) solar cells were maximum for a certain doping percentage (∼ 3%), which was suggested to originate from galvinoxyl spins near the PCBM side of the P3HT/PCBM boundary that interacted with electrons to decrease recombination losses by facilitating intersystem crossings to triplet states.* These authors contributed equally to this work. † michael_flatte@mailaps.org Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Here we examine, experimentally and theoretically, the influence of radical doping on the transport characteristics in...

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