Isotope Effect in the Magneto-Optoelectronic Response of Organic Light-Emitting Diodes Based on Donor-Acceptor Exciplexes.

The isotope effect is studied in the magneto-electroluminescence (MEL) and pulsed electrically detected magnetic resonance of organic light-emitting diodes based on thermally activated delayed fluorescence (TADF) from donor-acceptor exciplexes that are either protonated (H) or deuterated (D). It is found that at ambient temperature, the exchange of H to D has no effect on the spin-dependent current and MEL responses in the devices. However, at cryogenic temperatures, where the reverse intersystem crossing (RISC) from triplet to singlet exciplex diminishes, a pronounced isotope effect is observed. These results show that the RISC process is not governed by the hyperfine interaction as thought previously, but proceeds through spin-mixing in the triplet exciplex. The observations are corroborated by electrically detected transient spin nutation experiments that show relatively long dephasing time at ambient temperature, and interpreted in the context of a model that involves exchange and hyperfine interactions in the spin triplet exciplex. These findings deepen the understanding of the RISC process in TADF materials.

Transient Magnetic Field Effect of Photoexcitations in Donor-Acceptor Organic Semiconductors.

We report transient photoinduced absorption (t-PA) and magnetic field ( B)-dependent t-PA (t-MPA( B)) in a pristine low band gap π-conjugated copolymer composed of donor and acceptor moieties, namely, the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thi-eno[3,4- b]thiophenediyl]]) (or PTB7) used in photovoltaic applications. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in pristine PTB7 we find at short times coexistence of two primary photoexcitation species, namely, SE and triplet-triplet (TT) pair. Both species are photogenerated directly from the ground state and are spin-correlated. Although the TT pair decomposes into two separate triplet excitons (TEs) in ∼100 ps, the separated TE spins are still entangled up to ∼6 µs. At longer times, the t-MPA( B) response of the surviving TEs shows transient narrowing effect, which is attributed to a distribution of the TE size.

Giant Rashba splitting in 2D organic-inorganic halide perovskites measured by transient spectroscopies.

Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural "multiple quantum wells" that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to "Rashba splitting" close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C6H5C2H4NH3)2PbI4. The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å, which are among the highest Rashba splitting size parameters reported so far. This finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.

Transient Magnetophotoinduced Absorption Studies of Photoexcitations in π-Conjugated Donor-Acceptor Copolymers.

We have utilized a plethora of transient and steady state optical and magneto-optical spectroscopies in a broad spectral range (0.25-2.5 eV) for elucidating the primary and long-lived photoexcitations in a low band-gap π-conjugated donor-acceptor (DA) copolymer used for efficient photovoltaic solar cells. We show that both singlet excitons (SE) and intrachain triplet-triplet (TT) pairs are photogenerated in the DA-copolymer chains. From the picosecond transient magnetic field response of these species we conclude that the SE and TT spin states are coupled. The TT decomposition into two intrachain geminate triplet excitons maintains spin coherence and thus their spin entanglement lasts into the microsecond time domain.

Doping-Induced Absorption Bands in P3HT: Polarons and Bipolarons.

In this work, we focus on the formation of different kinds of charge carriers such as polarons and bipolarons upon p-type doping (oxidation) of the organic semiconductor poly(3- hexylthiophene-2,5-diyl) (P3HT). We elucidate the cyclic voltammogram during oxidation of this polymer and present spectroscopic changes upon doping in the UV/Vis/near-IR range as well as in the mid-IR range. In the low-oxidation regime, two absorption bands related to sub-gap transitions appear, one in the UV/Vis range and another one in the mid-IR range. The UV/Vis absorption gradually decreases upon further doping while the mid-IR absorption shifts to lower energy. Additionally, electron paramagnetic resonance (EPR) measurements are performed, showing an increase of the EPR signal up to a certain doping level, which significantly decreases upon further doping. Furthermore, the absorption spectra in the UV/Vis range are analyzed in relation to the morphology (crystalline vs. amorphous) by using theoretical models. Finally, the calculated charge carriers from cyclic voltammogram are linked together with optical transitions as well as with the EPR signals upon p-type doping. We stress that our results indicate the formation of polarons at low doping levels and the existence of bipolarons at high doping levels. The presented spectroscopic data are an experimental evidence of the formation of bipolarons in P3HT.

Short-lived charge-transfer excitons in organic photovoltaic cells studied by high-field magneto-photocurrent.

The main route of charge photogeneration in efficient organic photovoltaic cells based on bulk hetero-junction donor-acceptor blends involves short-lived charge-transfer excitons at the donor-acceptor interfaces. The cell efficiency is critically affected by the charge-transfer exciton recombination and dissociation processes. By measuring the magneto-photocurrent under ambient conditions at room temperature, we show here that magnetic field-induced spin-mixing among the charge-transfer exciton spin sublevels occurs in fields up to at least 8.5 Tesla. The resulting magneto-photocurrent increases at high fields showing non-saturating behaviour up to the highest applied field. We attribute the observed high-field spin-mixing mechanism to the difference in the donor-acceptor g-factors. The non-saturating magneto-photocurrent response at high field indicates that there exist charge-transfer excitons with lifetime in the sub-nanosecond time domain. The non-Lorentzian high-field magneto-photocurrent response indicates a dispersive decay mechanism that originates due to a broad distribution of charge-transfer exciton lifetimes.

The first decade of organic spintronics research.

The first decade of organic spintronics research has benefitted from the analogy and previous experience of the inorganic spintronics field, coupled with the unlimited versatility of organic materials synthesis. At the same time, the field of organic spintronics has developed into an attractive and promising field of its own, with rich physics and promising unique potential applications. We review here a set of significant milestones achieved in organic spintronic devices such as organic spin valves, bipolar spin-valves, and hybrid organic/inorganic light emitting diodes in comparison with representative inorganic spintronic devices. We also point out acute problems that need to be resolved before the young field of organic spintronics can mature.

Organic spin-valves: from unipolar to bipolar devices.

We review the recent advances in both unipolar and bipolar organic spin valves. We discuss mechanisms that dominate the spin relaxation of injected spin aligned carriers and limit their spin diffusion length. A space charge limited current model with ferromagnetic electrodes is used to describe the bipolar organic spin valve operation. For unipolar spin valves we take into account finite spin diffusion length and obtain a general expression that is valid for devices with thick organic interlayers. For bipolar spin valves where the injected spin aligned electrons and holes selectively form polaron pairs having singlet or triplet spin configurations, we obtain modified expressions for the magneto-resistance and magneto-electroluminescence and show that they are proportional to the difference in singlet-triplet generation rates and the spin injection polarization degree of the ferromagnetic electrodes.

Spin-polarized light-emitting diode based on an organic bipolar spin valve.

The spin-polarized organic light-emitting diode (spin-OLED) has been a long-sought device within the field of organic spintronics. We designed, fabricated, and studied a spin-OLED with ferromagnetic electrodes that acts as a bipolar organic spin valve (OSV), based on a deuterated derivative of poly(phenylene-vinylene) with small hyperfine interaction. In the double-injection limit, the device shows ~1% spin valve magneto-electroluminescence (MEL) response, which follows the ferromagnetic electrode coercive fields and originates from the bipolar spin-polarized space charge-limited current. In stark contrast to the response properties of homopolar OSV devices, the MEL response in the double-injection device is practically independent of bias voltage, and its temperature dependence follows that of the ferromagnetic electrode magnetization. Our findings provide a pathway for organic displays controlled by external magnetic fields.

Isotope effect in spin response of pi-conjugated polymer films and devices.

Recent advances in organic spin response include long polaron spin-coherence times measured by optically detected magnetic resonance (ODMR), substantive room-temperature magnetoelectroluminescence and magnetoconductance obtained in organic light-emitting diodes (OLEDs) and spin-polarized carrier injection from ferromagnetic electrodes in organic spin valves (OSVs). Although the hyperfine interaction (HFI) has been foreseen to have an important role in organic spin response, no clear experimental evidence has been reported so far. Using the chemical versatility advantage of the organics, we studied and compared spin responses in films, OLED and OSV devices based on pi-conjugated polymers made of protonated, H-, and deuterated, D-hydrogen having a weaker HFI strength. We demonstrate that the HFI does indeed have a crucial role in all three spin responses. OLED films based on the D-polymers show substantially narrower magneto-electroluminescence and ODMR responses, and as a result of the longer spin diffusion obtained, OSV devices based on D-polymers show a substantially larger magnetoresistance.