Dissociation Of Triplet Excitons Research Articles

(Invited) Conformations of Exciton Pairs Associated with Spin-Entanglement Transports during Singlet Fissions

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells. Spin-entanglement in the triplet pair state via one singlet exciton is a promising phenomenon for several energy conversion applications including quantum information science. However, direct observation of the electron spin polarization by transports of entangled spin-states has not been demonstrated. Furthermore, vibronic mechanisms of the dissociation of the triplet excitons are poorly understood. In this study, time-resolved electron paramagnetic resonance has been utilized to observe the transportations of the singlet and quintet characters generating spin-correlated correlated triplet pair (SCTP) by probing the electron spin polarization (ESP) generated in thin films of 6,13-bis(triisopropylsilylethynyl)pentacene.[1] We have clearly demonstrated that the ESP detected in resonance field positions of the individual triplet excitons are dependent on morphology and on detection delay time after laser flash to cause SF. The ESPs were clearly explained by quantum superposition[1,2,3] of singlet-triplet-quintet wavefunctions via picosecond triplet-exciton dissociation as the electron spin polarization transfer from strongly exchange-coupled singlet TT states to the weakly-coupled SCTP via spin-spin dipolar couplings with preserving conformations of the excitons. Although the coherent superposition of the spin eigenstates was not directly detected, the present interpretation of the spin correlation of the separated T+T exciton pair may pave new avenues not only for elucidating the vibronic role on the de-coupling[2,3] between the two excitons but also for scalable quantum information processing using quick T+T dissociation via one-photon excitation. References Matsuda, S.; Oyama, S.; Kobori, Y. Sci. 2020, 11, 2934-2942.Kobori, Y.; Fuki, M.; Nakamura, S.; Hasobe, T. Phys. Chem. B 2020, 124, 9411-9419.Nakamura, S.; Sakai, H.; Nagashima, H.; Fuki, M.; Onishi, K.; Khan, R.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. Phys. Chem. C 2021, 125, 18287-18296.

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions.

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells. Transport of spin-entanglement in the triplet–triplet pair state via one singlet exciton is a promising phenomenon for several energy conversion applications including quantum information science. However, direct observation of electron spin polarization by transport of entangled spin-states has not been presented. In this study, time-resolved electron paramagnetic resonance has been utilized to observe the transportation of singlet and quintet characters generating correlated triplet–triplet (T + T) exciton-pair states by probing the electron spin polarization (ESP) generated in thin films of 6,13-bis(triisopropylsilylethynyl)pentacene. We have clearly demonstrated that the ESP detected at the resonance field positions of individual triplet excitons is dependent on the morphology and on the detection delay time after laser flash to cause SF. ESP was clearly explained by quantum superposition of singlet–triplet–quintet wavefunctions via picosecond triplet-exciton dissociation as the electron spin polarization transfer from strongly exchange-coupled singlet and quintet TT states to weakly-coupled spin-correlated triplet pair states. Although the coherent superposition of spin eigenstates was not directly detected, the present interpretation of the spin correlation of the separated T + T exciton pair may pave new avenues not only for elucidating the vibronic role in the de-coupling between two excitons but also for scalable quantum information processing using quick T + T dissociation via one-photon excitation.

Investigation of micro-mechanism in thermally activated delayed fluorescence quantum well devices by utilizing organic magnetic effects

The quantum well organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence material with reverse intersystem crossing (RISC) characteristics have been fabricated in this paper. Meanwhile, the magneto-efficiency (Mη) was obtained by measuring the magneto-conductance (MC) and magneto-electroluminescence (MEL) of the devices, and we research microscopic process of devices by analyzing the characteristic curves of magnetic field effects. Surprisingly, we found that the MC curves of non-quantum-well device showed typical RISC property and the dissociation of triplet excitons by holes at room temperature, but the MC curves merely exhibited the mechanism of electron scattering by triplet excitons in quantum well device. In addition, although the MEL and Mη curves of non-quantum well device demonstrated intersystem crossing (ISC) and triplet-triplet annihilation (TTA) process, the MEL of quantum well device showed the mechanism of electron scattering by triplet excitons and the Mη only displayed the typical ISC curves. As decreasing the temperature from 300 K to 20 K, the ISC processes of quantum or non-quantum well devices increased, however, the TTA process occurred in quantum well device at 20 K and the TTA process of non-quantum well device gradually vanished at 100 K. We propose that the structure property of quantum or non-quantum well devices, temperature-dependent Forster energy transfer, and the lifetime and concentration of the triplet excitons can explain the experimental phenomena very well. Obviously, the research of magnetic effects in quantum well devices not only provides ideas for designing high efficient light-emitting devices, but also can deepen the understanding of organic magnetic effects.

Exciton dissociation and energy transfer at the quantized pentacene/lead-sulfide nanocrystal functional interface

We investigated quantized-charge and energy transfer at the pentacene/lead sulfide (PbS) interface through realization of PbS nanocrystal (NC) size-dependent quantum confinement effect, providing insights into the presence of triplet exciton dissociation and energy transfer. With PbS nanocrystals (NCs) of a small size (∼3 nm), quantized charge and energy transfer at the pentacene/PbS NC interface leads to a large photocurrent while PbS NCs of a large size (∼7 nm) causes a far reduced photocurrent. Through comparison between the normal and inverted structures, importantly, exciton dissociation and energy transfer are turned out to be sensitive to subtle structural change at the interface.

Spatially resolved extrinsic photoresponse under additional intrinsic bias photoexcitation of two terminal pentacene devices

Spatially resolved extrinsic photoresponse experiments in pentacene two terminal devices without or under additional intrinsic bias-light excitation are employed. These experiments are used to investigate the microscopic mechanism of the recently observed phenomenon of the photoresponse enhancement under additional bias-light intrinsic excitation and to preclude that this phenomenon arises from contact-related artifacts. It is found that the extrinsic photogeneration near the contacts via electric field-assisted exciton splitting and/or light-induced depletion width-reduction have a negligible contribution to the extrinsic photoresponse. Under additional bias-light intrinsic excitation, a uniform increase of the photogenerated hole density is found to take place across the whole conduction channel, without changes in the electric field distribution and in the interfacial properties of the contacts. The photoresponse enhancement by the blue bias-light becomes stronger upon increasing the red-light intensity. A nearly square root dependence of the photoresponse enhancement on the blue bias-light intensity is found. It is shown that the observed dependence of the photoresponse enhancement on the light intensities of the extrinsic and intrinsic excitation can be explained with the extrinsic photogeneration mechanism based on hole detrapping by triplet exciton dissociation.

Triplet exciton dissociation and electron extraction in graphene-templated pentacene observed with ultrafast spectroscopy

We compare the ultrafast dynamics of singlet fission and charge generation in pentacene films grown on glass and graphene. Pentacene grown on graphene is interesting because it forms large crystals with the long axis of the molecules "lying-down" (parallel to the surface). At low excitation fluence, spectra for pentacene on graphene contain triplet absorptions at 507 and 545 nm and no bleaching at 630 nm, which we show is due to the orientation of the pentacene molecules. We perform the first transient absorption anisotropy measurements on pentacene, observing negative anisotropy of the 507 and 545 nm peaks, consistent with triplet absorption. A broad feature at 853 nm, observed on both glass and graphene, is isotropic, suggesting hole absorption. At high fluence, there are additional features, whose kinetics and anisotropies are not explained by heating, that we assign to charge generation; we propose a polaron pair absorption at 614 nm. The lifetimes are shorter at high fluence for both pentacene on glass and graphene, indicative of triplet-triplet annihilation that likely enhances charge generation. The anisotropy decays more slowly for pentacene on graphene than on glass, in keeping with the smaller domain size observed via atomic force microscopy. Coherent acoustic phonons are observed for pentacene on graphene, which is a consequence of more homogeneous domains. Measuring the ultrafast dynamics of pentacene as a function of molecular orientation, fluence, and polarization provides new insight to previous spectral assignments.

Identify triplet-charge interaction in rubrene-based diodes using magneto-conductance: Coexistence of dissociation and scattering channels

Although it is known that triplet excitons can be quenched via triplet-charge interaction (TQI), it is still unclear how this process occurs in rubrene-based devices. We found that magneto-conductance (MC) can be used to probe the detailed mechanism of TQI in rubrene-based organic light-emitting diodes and observed the coexistence of negative and positive MC responses in the high-field region when holes and electrons were the dominant charged species at different interfaces adjoining the rubrene layer, respectively. Further analysis suggests that the negative MC response was originated from the dissociation of triplet excitons by holes, while the positive MC response was due to electron scattering by triplet excitons. The MC responses of the devices were examined under different injection currents and temperatures to confirm our hypothesis. This work gives significant insight into mechanisms of TQI in organic semiconductors, which will allow for the design of new and improved devices.

Singlet Fission of Non-polycyclic Aromatic Molecules in Organic Photovoltaics.

Singlet fission of thienoquinoid compounds in organic photovoltaics is demonstrated. The escalation of the thienoquinoid length of the compounds realizes a suitable packing structure and energy levels for singlet fission. The magnetic-field dependence of the photocurrent and the external quantum efficiency of the devices reveal singlet fission of the compounds and dissociation of triplet excitons into charges.

Ultrafast spectroscopic study for singlet fission

Singlet fission is a spin-allowed process that creates two triplet excitons from one photo-excited singlet exciton in organic semiconductors. This process of carrier multiplication holds the great potential to break the theoretical efficiency limit in single-junction solar cells by making better use of high-energy photons, while capturing lower-energy photons in the usual style. Photovoltaic devices based on singlet fission have achieved external quantum efficiencies in excess of 100%. In this paper, we first introduce the basic concept about singlet fission and review the history of the field briefly. Then, we report some reflent advances in the reflearch of singlet fission progress with the combination of our group’s productions. Tetracene and pentacene are chosen as typical polyacene materials for discuss. We describe how scientists make progresses in understanding the underlying physics in singlet fission process. The experimental methods of transient absorption spectra, time-resolved fluorescence spectra and time-resolved two-photon photoemission spectra render numerous results for analysis. Moreover, a survey about the debate on the direct or indirect mechanism with transient optical study is provided. It has been verified that multiexciton state intermediates in singlet fission process and the factors of energy level alignments, intermolecular interaction as well as lattice vibrations play a role in it. Last, we briefly summarize the implications of singlet fission in organic solar devices by introducing several composite architectures for singlet-fission photovoltaics. Designing efficient and cheap solar cells is the ultimate goal for understanding the intrinsic photophysics of singlet fission. To obtain high efficiencies, it is important to adapt proper materials and new organic/inorganic architectures may become a promising direction. Also, finding a way for efficient triplet exciton dissociation should be considered seriously. It is believable that these guidelines can lead to the development of cheap and efficient fission-based devices.

Electron correlation effects on exciton dissociation in the presence of an electric field in polyacetylene

Based on the Su–Schrieffer–Heeger and Pariser–Parr–Pople (SSH+PPP) model and the multiconfigurational time-dependent Hartree-Fock formalism, the dynamics of exciton dissociation in conjugated polymers systems has been simulated using a nonadiabatic molecular dynamics method. Within this approach, the appropriate spin symmetry of the electronic wavefunction is taken into account, which allows us to distinguish between singlet and triplet excited states. The different dynamic dissociation behaviors in the presence of an electric field of singlet excitons (SE) and triplet excitons (TE) due to electron correlation are emphasized. Using numerical simulations, it is found that the dissociation of TE under an applied electric field is more difficult than in the case for SE. The critical dissociation electric field strength for TE is about 3 times higher than that of SE. When the conjugated length of a polymer chain is short, SE and TE can each remain as one entity even under high electric field strengths owing to the confinement effect of the chain ends. In addition, the dependence of exciton dissociation on the Coulomb interaction strength, shielding factor and the conjugated length is discussed.

Singlet Exciton Fission Photovoltaics

Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses long- and short-wavelength donors and an acceptor and a simpler, two-layer combination of a singlet-fission donor and a long-wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared-absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption.

Triplet Exciton Dissociation in Singlet Exciton Fission Photovoltaics

Triplet exciton dissociation in singlet exciton fission devices with three classes of acceptors are characterized: fullerenes, perylene diimides, and PbS and PbSe colloidal nanocrystals. Using photocurrent spectroscopy and a magnetic field probe it is found that colloidal PbSe nanocrystals are the most promising acceptors, capable of efficient triplet exciton dissociation and long wavelength absorption.

Excitation of Charge Transfer States and Low-Driving Force Triplet Exciton Dissociation at Planar Donor/Acceptor Interfaces

Here, we investigate charge transfer at archetypal planar heterojunction solar cells based upon phthalocyanines as donors and C60 or a perylene derivative as acceptors. We demonstrate the ability t...

Dissociation processes of singlet and triplet excitons in organic photovoltaic cells

The dissociation processes of singlet and triplet excitons are studied based on fluorescent aluminum (III) 8-hydroxyquinoline (Alq3) and phosphorescent fac-tris-(2-phenylpyridine) iridium [Ir(ppy)3] molecules. We find that phosphorescent Ir(ppy)3 shows a more efficient photovoltaic response as compared to fluorescent Alq3. The short-circuit photocurrent action spectra and magnetic-field-dependent photocurrents reveal that the triplet excitons dissociate directly into free charge carriers at the metal-electrode interface while the singlet excitons experience bulk dissociation through polaron-pair states. This interface dissociation of triplet excitons forms a mechanism for phosphorescent organic materials to yield efficient photovoltaic responses.

The Influence of the Hyperfine Range Magnetic Field on Charge Transfer States at the Metal–Organic Semiconductor Interface

AbstractHyperfine range (HFR) magnetic field (H < 300 Oe) induced increase of photoconductivity and decrease of dark conductivity are observed in thin‐layer sandwich type Au‐tetrathiotetracene‐Al systems. The experimental results are interpreted in terms of HFR magnetic field influence on single‐triplet transition rate of charge transfer (CT) states at metal (Me)‐organic semiconductor (OS) interface in the framework of Merrifield's et al. hyperfine interaction model. It is suggested that in the case of photoconductivity, triplet 3CT states are initially formed by molecular triplet exciton dissociation at the Me‐OS interface, whereas in the case of dark conductivity, singlet 1CT states are initially formed in the process of non‐equilibrium hole injection from the Me electrode. The HFR magnetic field reduces the 1CT–3CT transition rate and is thus assumed to cause the observed increase of photo‐ and decrease of dark conductivity. The possible role of the inhomogeneous local magnetic field caused by the charge carriers trapped near the Me‐OS interface on the rate of 1CT–3CT transition is also briefly discussed.