Radio-wave Effect on Singlet Fission in Polycrystalline Tetracene near Zero Magnetic Field

A triplet–triplet pair is a key intermediate in singlet fission (SF), which has the potential to overcome the theoretical limit of solar cell efficiency. Here, we report a new spectroscopy to directly detect a short-lived triplet–triplet pair via the effects of radio-wave (RF) irradiation near zero magnetic field at room temperature. The fluorescence of polycrystalline powder of tetracene is reduced by RF irradiation at zero field, which is caused by a quasi-static RF field effect on spin mixing and electron-spin resonance among zero-field-splitting sublevels of the triplet–triplet pair. The curve for the quasi-static RF field effect can be reproduced numerically from that for the observed magnetophotoluminescence (MPL) effect. The simultaneous simulation of the RF and MPL effects using the density matrix formalism estimates rate constants of 1.2 × 108 and 6.0 × 108 s–1 for the fusion and dissociation, respectively, of the triplet–triplet pair

Radio-wave Effect on Singlet Fission in Polycrystalline Tetracene near Zero Magnetic Field

A triplet–triplet pair is a key intermediate in singlet fission (SF), which has the potential to overcome the theoretical limit of solar cell efficiency. Here, we report a new spectroscopy to directly detect a short-lived triplet–triplet pair via the effects of radio-wave (RF) irradiation near zero magnetic field at room temperature. The fluorescence of polycrystalline powder of tetracene is reduced by RF irradiation at zero field, which is caused by a quasi-static RF field effect on spin mixing and electron-spin resonance among zero-field-splitting sublevels of the triplet–triplet pair. The curve for the quasi-static RF field effect can be reproduced numerically from that for the observed magnetophotoluminescence (MPL) effect. The simultaneous simulation of the RF and MPL effects using the density matrix formalism estimates rate constants of 1.2 × 108 and 6.0 × 108 s–1 for the fusion and dissociation, respectively, of the triplet–triplet pair

Radiowave Effects on an Electron–Hole Pair in a Poly(3-hexylthiophene) near Zero Magnetic Field

To clarify the dynamics of an electron–hole (e–h) pair, which plays an important role in the performance of optoelectronic devices, the radiowave effect on photoconduction in a regioregular poly­(3-hexylthiophene-2,5-diyl) film near zero magnetic field was investigated. Irradiation of the radiowaves of 1–25 MHz at zero field resulted in lower photoconduction because of a quasistatic magnetic field effect on intersystem crossing and electron spin resonance among hyperfine sublevels of the e–h pair. The recombination and dissociation rate constants of the e–h pair (107 s–1) were estimated by comparison with simulations based on the e–h pair dynamics under a perturbation treatment of the radiowave as an oscillating magnetic field using a density matrix formalism. An amplitude modulation technique of the radiowave at zero field in the frequency region <50 kHz revealed slow e–h pair formation (∼105 s–1), which is a rate-determining step for carrier recombination in the film

Origin of the Delayed Fluorescence by Triplet–Triplet Annihilation in Solids with a Power Law Exponent of between −1/2 and −1

Triplet–triplet annihilation (TTA) merges low electronic energies in two molecules to high electronic energy in one molecule, which, following triplet sensitization, allows us to achieve high-efficiency conversion using low-intensity light. Although efficient TTA up-conversion in solutions has been reported, the TTA up-conversion in solid matrixes is preferred for practical applications. However, the emission decay kinetics after pulsed-light excitation is more complex in solids than in liquids, and the process underlying the different TTA kinetics has not yet been elucidated. Herein, we report that the complexity in the TTA kinetics in solid matrixes can originate from processes such as slow mixing of triplet excitons by migration, strong dependence on the initial distribution of triplet excitons, and an anisotropic random walk of excitons. We show theoretically that power-law fluorescence with an exponent of between −1/2 and −1 can originate from the slow diffusional mixing of excitons in one and three dimensions, where the initial exciton generation is uniformly distributed. By analyzing experimental data, we show that rich fluorescence kinetics observed experimentally by varying the temperature in molecular solids can be deeply understood in terms of bulk and nonbulk TTA under dispersive as well as normal diffusion

Mechanism of Intersystem Crossing of Thermally Activated Delayed Fluorescence Molecules

The spin sublevel dynamics of the excited triplet state in thermally activated delayed fluorescence (TADF) molecules have not been investigated for high-intensity organic light-emitting diode materials. Understanding the mechanism for intersystem crossing (ISC) is thus important for designing novel TADF materials. We report the first study on the ISC dynamics of the lowest excited triplet state from the lowest excited singlet state with charge-transfer (CT) character of TADF molecules with different external quantum efficiencies (EQEs) using time-resolved electron paramagnetic resonance methods. Analysis of the observed spin polarization indicates a strong correlation of the EQE with the population rate due to ISC induced by hyperfine coupling with the magnetic nuclei. It is concluded that molecules with high EQE have an extremely small energy gap between the ¹CT and ³CT states, which allows an additional ISC channel due to the hyperfine interactions

Realtime Observation of Carrier Trapping and Recombination in a Bulk Heterojunction Film by Simultaneous Optical and Electrical Detection

Since the photocharge generation efficiency has reached nearly 100% for recent organic photovoltaic film materials, clarification of loss mechanisms becomes of increasing importance. Here, we report a new time-resolved measurement method that enables simultaneous detection of the optical absorption and electric current due to photogenerated carriers in organic semiconductor films. Microsecond-order decays of both the number density (recombination) and the averaged drift mobility (trapping) of photocarriers are quantitatively observed for an identical device of a regioregular poly­[3-hexylthiophene-2,5-diyl]:(6,6)-phenyl C61 butyric acid methyl ester bulk heterojunction film. The decay feature of the time-dependent mobility is highly dispersive especially at lower temperatures. It can be explained by fast thermalization of “hot” photocarriers into lower energy sites, which is followed by thermally activated hopping. This quantitative time-resolved observation method can be a powerful tool for characterization and optimization of photovoltaic materials

Simultaneous Two-Photon Absorption to Gerade Excited Singlet States of Diphenylacetylene and Diphenylbutadiyne Using Optical-Probing Photoacoustic Spectroscopy

Simultaneous two-photon absorption to one-photon forbidden electronically excited states of diphenylacetylene (DPA) and diphenylbutadiyne (DPB) was investigated by means of highly sensitive optical-probing photoacoustic spectroscopy. The incident laser power dependencies on photoacoustic signal intensity indicate that the signals are dominated by the two-photon absorption regime. Two-photon absorption is responsible for transitions to gerade excited states based on the selection rule. The two-photon absorption bands observed in the heat action spectra were assigned with the aid of quantum chemical calculations. The relative magnitude of the two-photon absorption cross sections of DPA and DPB was estimated, and the larger two-photon absorption cross section of DPB was related to the resonance effect with the red-shifted one-photon allowed 11B1u ← 11Ag transition of DPB

Long-Distance Sequential Charge Separation at Micellar Interface Mediated by Dynamic Charge Transporter: A Magnetic Field Effect Study

Construction of photogenerated long-lived charge-separated states is crucial for light-energy conversion using organic molecules. For realization of cheap and easy-to-make long-distance electron transfer (ET) systems, we have developed a supramolecular donor­(D)–chromophore­(C)–acceptor­(A) triad utilizing a micellar interface. Alkyl viologen (A²⁺) is adsorbed on the hydrophilic interface of Triton X-100 micelle, which bears D units in the hydrophobic core. Excited triplet state of a hydrophobic flavin C entrapped in the supercage gives rise to primary ET from D, which is followed by the secondary ET from C^–• to A²⁺ to give the long-lived (>10 μs) charge-separated state with negligible yield of escaped C^–•. Analysis of magnetic field effect reveals that diffusion of C^–• from the core to the hydrophilic interface leads to long-distance ET with a low charge recombination yield of ∼20%. This novel concept of “dynamic charge transporter” has important implications for development of photon-energy conversion systems in solution phase

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketo­pyrrolo­pyrrole-linked tetra­benzo­porphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magneto­conductance effect