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Register a new userMorphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors
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Verner Thorsm{\\o}lle
Publication date
2009
fields
Physics
and research's language is
English
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We present a comparative study of ultrafast photo-conversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is activated and temperature-independent respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C$_{60}$-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.
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Engineering a low singlet-triplet energy gap ({Delta}EST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors, but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient =3.8x10^5 cm^-1) and a relatively large {Delta}EST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (260 {mu}s), but in aggregated films, BF2 generates intermolecular CT (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of >1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states resolves the conflicting requirements of fast radiative emission and low {Delta}EST.
We explore the possibility that hyperfine interaction causes the recently discovered organic magnetoresistance (OMAR) effect. Our study employs both experiment and theoretical modelling. An excitonic pair mechanism model based on hyperfine interaction, previously suggested by others to explain magnetic field effects in organics, is examined. Whereas this model can explain a few key aspects of the experimental data, we, however, uncover several fundamental contradictions as well. By varying the injection efficiency for minority carriers in the devices, we show experimentally that OMAR is only weakly dependent on the ratio between excitons formed and carriers injected, likely excluding any excitonic effect as the origin of OMAR.
Different from traditional semiconductors, the organic semiconductors normally possess moderate many-body interactions with respect to charge, exciton, spin and phonons. In particular, the diagonal electron-phonon couplings give rise to the spatial localization and the off-diagonal couplings refer to the delocalization. With the competition between them, the electrons are dispersive in a finite extent and unfavorable towards thermal equilibrium. In this context, the quantities from the statistical mechanics such as the entropy have to be reexamined. In order to bridge the localization-delocalization duality and the device performance in organic semiconductors, the quantum heat engine model is employed to describe the charge, exciton and spin dynamics. We adopt the adaptive time-dependent density matrix renormalization group algorithm to calculate the time evolution of the out-of-time-ordered correlator (OTOC), a quantum dynamic measurement of the entanglement entropy, in three models with two kinds of competing many-body interactions: two-bath lattice model with a single electron, Frenkel-charge transfer mixed model, and the Merrifield model for singlet fission. We respectively investigate the parameter regime that the system is in the many-body localization (MBL) phase indicated by the behavior of OTOC. It is recognized that the novel effects of coherent electron hopping, the ultrafast charge separation and the dissociation of triplet pairs are closely related to the MBL effect. Our investigation unifies the intrinsic mechanisms correlating to charge, exciton and spin into a single framework of quantum entanglement entropy, which may help clarify the complicated and diverse phenomena in organic semiconductors.
We have experimentally tested the hypothesis of free charge carrier mediated spin-transport in the small molecule organic semiconductor Alq3 at room temperature. A spin current was pumped into this material by pulsed ferromagnetic resonance of an adjacent NiFe layer, while a charge current resulting from this spin current via the inverse spin-Hall effect (ISHE) was detected in a Pt layer adjacent on the other side of the Alq3 layer, confirming a pure spin current through the Alq3 layer. Charge carrier spin states in Alq3, were then randomized by simultaneous application of electron paramagnetic resonance (EPR). No influence of the EPR excitation on the ISHE current was found, implying that spin-transport is not mediated by free charge-carriers in Alq3.
We present neutron diffraction, magnetic susceptibility and specific heat data for a single-crystal sample of the cubic (Cu3Au structure) compound Pr3In. This compound is believed to have a singlet (Gamma1) groundstate and a low-lying triplet (Gamma4) excited state. In addition, nearest-neighbor antiferromagnetic interactions are frustrated in this structure. Antiferromagnetic order occurs below T_N = 12K with propagation vector (0, 0, 0.5 +/-delta) where delta approx 1/12. The neutron diffraction results can be approximated with the following model: ferromagnetic sheets from each of the three Pr sites alternate in sign along the propagation direction with a twelve-unit-cell square-wave modulation. The three moments of the unit cell of 1 mu_B magnitude are aligned so as to sum to zero as expected for nearest-neighbor antiferromagnetic interactions on a triangle. The magnetic susceptibility indicates that in addition to the antiferromagnetic transition at 12K, there is a transition near 70K below which there is a small (0.005 mu_B) ferromagnetic moment. There is considerable field and sample dependence to these transitions. The specific heat data show almost no anomaly at TN = 12K. This may be a consequence of the induced moment in the Gamma1 singlet, but may also be a sample-dependent effect.
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