Excitonic Processes Research Articles

Striking the right balance of intermolecular coupling for high-efficiency singlet fission.

Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet-triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields.

Excitonic pathway to photoinduced magnetism in colloidal nanocrystals with nonmagnetic dopants.

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. Ag+ is an emerging electronic dopant in III-V and II-VI nanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4d shell Ag+ is nonmagnetic, and the dopant-related luminescence is ascribed to decay of the conduction-band electron following transfer of the photoexcited hole to Ag+. This optical activation process and the associated modification of the electronic configuration of Ag+ remain unclear. Here, we trace a comprehensive picture of the excitonic process in Ag-doped CdSe nanocrystals and demonstrate that, in contrast to expectations, capture of the photohole leads to conversion of Ag+ to paramagnetic Ag2+. The process of exciton recombination is thus inextricably tied to photoinduced magnetism. Accordingly, we observe strong optically activated magnetism and diluted magnetic semiconductor behaviour, demonstrating that optically switchable magnetic nanomaterials can be obtained by exploiting excitonic processes involving nonmagnetic impurities.

Excitonic processes at organic heterojunctions

Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.

Room-Temperature Single-Photon Emission from Micrometer-Long Air-Suspended Carbon Nanotubes

Applications in nanophotonics and optical quantum information processing often require just one photon at a time. Although typical single-photon emitters rely on electronic states localized at the nanometer scale, the authors find that carbon nanotubes over 2 $\ensuremath{\mu}$m long exhibit photon antibunching at room temperature. Monte Carlo simulations and first-passage theory reveal that high-purity single-photon emission from this system is possible in principle, by means of efficient exciton-exciton annihilation. These results point to design strategies for single-photon sources, and provide important insight into excitonic processes in carbon nanotubes.

Unusual temperature and excitation energy dependences of impurity-trapped excitons in LiBaF3:Eu2+ crystals

Luminescence properties of impurity-trapped excitons (ITEs) in LiBaF3:Eu2+ crystals are investigated by optical and time-resolved laser-excitation spectroscopy. The three excited states of 4f65d, 4f7(6P7/2) and ITE are identified in the excitation and emission spectra. The intensity of ITEs’ emission increases unexpectedly with increasing temperature and excitation energy. The initial intensity of the ITE emission just after pulsed-laser excitation increases continuously with temperature in the temperature range 7–600K. The results are discussed in relation to radiative and nonradiative rates and excitonic processes in this system. The theoretical model that includes the ITE, 4f7(6P7/2) and 4f6(7FJ)5d(eg) states is developed to describe and support the experimental observations.

Photocatalytic Mechanism Regulation of Bismuth Oxyhalogen via Changing Atomic Assembly Method

Exciton and carrier photocatalytic processes have been proved in bismuth oxyhalogen photocatalysts. But, there are no reports about how to regulate the different mechanisms to improve photocatalytic activity for different reaction. Here, we found that the photocatalytic mechanisms could be regulated by changing the assembly method of bismuth, oxygen, and halogen atoms. Reactive oxygen species (ROS) experimentals results concluded that solid solution BiOBr0.5I0.5 showed enhanced exciton photocatalytic process, and coupling 0.5BiOBr/0.5BiOI displayed improved carrier photocatalytic proces. This work promoted the understanding about solid solution and coupling for bismuth oxyhalogen.

Effect of disorder on exciton dissociation in conjugated polymers**Project supported by the National Natural Science Foundation of China (Grant Nos. 11474218 and 11575116).

By using a multi-configurational time-dependent Hartree–Fock (MCTDHF) method for the time-dependent Schrödinger equation and a Newtonian equation of motion for lattice, we investigate the disorder effects on the dissociation process of excitons in conjugated polymer chains. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include on-site disorder, off-diagonal, electron–electron interaction, and an external electric field. Our results show that Coulomb correlation effects play an important role in determining the exciton dissociation process. The electric field required to dissociate an exciton can practically impossibly occur in a pure polymer chain, especially in the case of triplet exciton. However, when the on-site disorder effects are taken into account, this leads to a reduction in mean dissociation electric fields. As the disorder strength increases, the dissociation field decreases effectively. On the contrary, the effects of off-diagonal disorder are negative in most cases. Moreover, the dependence of exciton dissociation on the conjugated length is also discussed.

Rashba Effect in a Single Colloidal CsPbBr3 Perovskite Nanocrystal Detected by Magneto-Optical Measurements.

This study depicts the influence of the Rashba effect on the band-edge exciton processes in all-inorganic CsPbBr3 perovskite single colloidal nanocrystal (NC). The study is based on magneto-optical measurements carried out at cryogenic temperatures under various magnetic field strengths in which discrete excitonic transitions were detected by linearly and circularly polarized measurements. Interestingly, the experiments show a nonlinear energy splitting between polarized transitions versus magnetic field strength, indicating a crossover between a Rashba effect (at the lowest fields) to a Zeeman effect at fields above 4 T. We postulate that the Rashba effect emanates from a lattice distortion induced by the Cs+ motion degree of freedom or due to a surface effect in nanoscale NCs. The unusual magneto-optical properties shown here underscore the importance of the Rashba effect in the implementation of such perovskite materials in various optical and spin-based devices.

Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals.

With record efficiencies achieved in lead halide perovskite-based photovoltaics, urgency has shifted toward finding alternative materials that are stable and less toxic. Bismuth-based perovskite materials are currently one of the most promising candidates among those alternatives. However, the band structures of these materials, including the nature of the bandgaps, remain elusive due to extremely low photoluminescence quantum yield (PLQY) and scattering issues in their thin-film form. Here, we reveal the specific nature of the material's electronic transitions by realizing monodisperse colloidal nanocrystals (NCs) of hexagonal-phase Cs3Bi2X9 perovskites, which afford well-resolved PL features. Interestingly, the PL profile exhibits a dual-spectral feature at room temperature with comparable intensities, based on which we propose an exciton recombination process involving both indirect and direct transitions simultaneously-an observation further supported by temperature-dependent and density functional theory (DFT) calculations. Our findings provide experimental and theoretical insights into the nature of the bandgaps in bismuth halide materials-essential information for assessing their viability in solar cells and optoelectronics.

Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2

In two dimensional (2D) transition metal dichalcogenides, defect-related processes can significantly affect carrier dynamics and transport properties. Using femtosecond degenerate pump-probe spectroscopy, exciton capture, and release by mid-gap defects have been observed in chemical vapor deposition (CVD) grown monolayer MoSe2. The observed defect state filling shows a clear saturation at high exciton densities, from which the defect density is estimated to be around 0.5 × 1012/cm2. The exciton capture time extracted from experimental data is around ~ 1 ps, while the average fast and slow release times are 52 and 700 ps, respectively. The process of defect trapping excitons is found to exist uniquely in CVD grown samples, regardless of substrate and sample thickness. X-ray photoelectron spectroscopy measurements on CVD and exfoliated samples suggest that the oxygen-associated impurities could be responsible for the exciton trapping. Our results bring new insights to understand the role of defects in capturing and releasing excitons in 2D materials, and demonstrate an approach to estimate the defect density nondestructively, both of which will facilitate the design and application of optoelectronics devices based on CVD grown 2D transition metal dichalcogenides.

(Invited) Transient Absorption Study of Singlet Excitons in Crystalline Peropyrene Derivatives: Examination of Singlet Fission Process

Transient absorption study of singlet excitons in crystalline peropyrene derivatives: examination of singlet fission process Akihiro Furube*1, Daiki Yamanaka1, Shinichiro Yanagiya1, Hiroyuki Matsuzaki2, Kazuyuki Uchida3, Takashi Kubo3 1Tokushima Univ,. 2-1, Minamijosanjima-cho, Tokushima, 770-8506, Japan 2AIST, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan 3Osaka Univ., 1-1, Machikaneyama, Toyonaka, Osaka 560-0043, Japan Furube.akihiro@tokushima-u.ac.jp Singlet fission process in organic crystal is attracting considerable interest due to its potential application of organic photovoltaic (OPV) with the internal quantum efficiency of 200%, since one absorbed photon can produce two triplet excitons that can generate two electron-hole pairs at the interface between the organic crystal and a suitable electron donor material such as fullerene derivatives and carbon nanotubes. Peropyrene, which is one of polycyclic aromatic hydrocarbon compounds, is expected to exhibit singlet fission in the crystal form. The SF is a reaction in which the excited singlet state (S1) spilt into two triplet excited states (T1 + T1). Energy matching (S1 ~ T1 + T1) reported [1] for peropyrene molecule interests researchers for development of derivatives of this molecule to control the energy levels and crystal structure for efficient singlet fission. In the present study, two kinds of 2,9-disubstituted peropyrene derivatives, one with two phenyl groups and the other with two n-butyl groups, were newly synthesized and tested. Time-resolved transient absorption spectroscopy in the femtosecond and nanosecond timescales, time-resolved fluorescence spectroscopy, and steady-state absorption/fluorescence measurements were carried out in order to reveal the relaxation processes of the singlet excitons of these peropyrene derivatives in the crystalline and solution phases. Transient absorption spectra and kinetics of phenyl peropyrene in the crystalline state are shown in Figure 1a. Here, diffuse reflectance spectroscopy was applied [2, 3]. At 10 ps after excitation, a broad absorption band appeared around 700 nm, which showed gradual decay up to 500 ps. It should be noted that the excitation laser intensity (at 400 nm wavelength) was adjusted to be weak enough so as not to undergo any non-linear decay process such as exciton–exciton annihilation. Since the absorption peak wavelength (700 nm) is close to the one observed in solution (775 nm), the absorption band can be ascribed to the singlet exciton in crystalline state. The decay component in the picosecond timescale showed a time constant of 150 ps. The involvement of singlet fission in the decay of a singlet exciton would result in the appearance of the T1absorption band being similar to the band observed in solution around 525 nm. However, we did not observe such an absorption band in the picosecond time scale. This experimental finding implies that the energy level of the S1 state is lower than twice the energy of the T1 state in the crystalline state. In this case, singlet fission is expected to occur from highly excited singlet states (Sn). Under high-density excitation conditions, two singlet excitons collide each other to produce Sn and S0 through singlet-singlet exciton annihilation (S1 + S1 → Sn + S0). Thereafter the resulting Sn state can undergo singlet fission. Figure 1b shows the transient absorption. Indeed, a broad absorption band centered at 570 nm was observed in the transition absorption spectrum measured at 1 ns, after the disappearance of the absorption band of the S1 state around 700 nm. This spectral feature is similar to that of the T1 state observed in solution, suggesting that the T1 state is generated from the highly excited singlet state Snthrough singlet fission. Interestingly, n-butyl peropyrene crystal did not show singlet fission either in weak or strong excitation condition. Based on these experiments, it was revealed that S1exciton in phenyl peropyrene is energetically low for undergoing singlet fission, therefore requiring additional energy. Further detailed discussion is given with other experimental results. Figure.1Transient absorption spectra of phenyl peropyrene crystal power samples excited by 400 nm laser with weak (a) and strong (b) intensity, respectively. Acknowledgement: A part of this work is supported by KAKENHI (16K13669) Japan.

Perspective: Optical spectroscopy in π-conjugated polymers and how it can be used to determine multiscale polymer structures.

Exciton delocalization in conjugated polymer systems is determined by polymer conformations and packing. Since exciton delocalization determines the photoluminescent vibronic progression, optical spectroscopy provides an indirect link to polymer multiscale structures. This perspective describes our current theoretical understanding of how exciton delocalization in π-conjugated polymers determines their optical spectroscopy and further shows how exciton delocalization is related to conformational and environmental disorder. If the multiscale structures in conjugated polymer systems are known, then using first-principles modeling of excitonic processes it is possible to predict a wide-range of spectroscopic observables. We propose a reverse-engineering protocol of using these experimental observables in combination with theoretical and computational modeling to determine the multiscale polymers structures, thus establishing quantitative structure-function predictions.

The role of solvent and structure in the kinetics of the excitons in porphyrin-based hybrid solar cells

In this study, we investigated the rate of exciton formation and dissociation in TiO2/Si/porphyrins hybrid solar cells in different phases. Using different porphyrin-based donors in four sets, it was found the best hybrid architectures belong the donors containing metal and alkyl group and those exposed the solvents. The proper kinetic of the excitonic processes in these hybrid solar cells are originated from lower affinity energy and exciton binding energy of their donors. Also, donors with the lowest energy barrier of electron injection as well as the highest excitonic Bohr radius showed the excitons can be dissociated in the dye/semiconductors interface, easily. The analysis of the probable correlations between the molecular descriptors and kinetic parameters of the excitons presented more credible evidences about the solvent and metal effects on the photovoltaic processes. Finally, based on the other performance characters of these solar cells, it was concluded that maximum value of the external quantum efficiency belongs to the metalloporphyrins in the solvent media.

Is the Chemical Strategy for Imbuing "Polyene" Character in Diketopyrrolopyrrole-Based Chromophores Sufficient for Singlet Fission?

In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole (TDPP)-vinyl-based dimer. We have investigated the optical and electronic properties and have probed the photophysical dynamics using transient absorption to investigate the possibility of singlet exciton fission. These revealed extremely rapid decay to the ground state (<50 ps), which we confirm is due to intramolecular excitonic processes rather than large-scale conformational change enabled by the vinyl linker. In all cases, the main excited state appears to be "dark", suggesting rapid internal conversion into a dark 2Ag-type singlet state. We found no evidence of triplet formation in TDPP-V-TDPP under direct photoexcitation. This may be a consequence of significant singlet stabilization in the dimer, bringing it below the energy needed to form two triplets. Our studies on this model compound set valuable lessons for design of novel triplet-forming materials and highlight the need for more broadly applicable design principles.

Theoretical study on dynamic acoustic modulation of free carriers, excitons, and trions in 2D MoS2 flake

We present a self-consistent model calculation to study the influence of a radio-frequency surface acoustic wave (SAW) on the dynamic processes of photo-generated free carriers, excitons and trions in a 2D MoS2 flake with an odd number of layers. We found that the concentrations of excitons and trions decreased, while the lifetime of the free carriers increased when an external SAW field was applied. We attribute this phenomenon to the spatial separation of the electrons and holes, which are confined by a SAW-induced periodic type-II band-edge. We further studied the effect of the modulations on the spatiotemporal distributions of free carriers and excited quasiparticles by varying the amplitude and wavelength of the SAW field. We finally calculated the photoluminescent properties of the MoS2 flake under modulation of the SAW field and compared our results with those of recent experiments.

Evaluating Electronic Couplings for Excited State Charge Transfer Based on Maximum Occupation Method ΔSCF Quasi-Adiabatic States.

Electronic couplings of charge-transfer states with the ground state and localized excited states at the donor/acceptor interface are crucial parameters for controlling the dynamics of exciton dissociation and charge recombination processes in organic solar cells. Here we propose a quasi-adiabatic state approach to evaluate electronic couplings through combining maximum occupation method (mom)-ΔSCF and state diabatization schemes. Compared with time-dependent density functional theory (TDDFT) using global hybrid functional, mom-ΔSCF is superior to estimate the excitation energies of charge-transfer states; moreover it can also provide good excited electronic state for property calculation. Our approach is hence reliable to evaluate electronic couplings for excited state electron transfer processes, which is demonstrated by calculations on a typical organic photovoltaic system, oligothiophene/perylenediimide complex.

Insights into the excitonic processes in polymeric photocatalysts.

Understanding the photoexcitation processes in semiconductors is critical for the design of advanced photocatalytic materials. Nevertheless, traditional viewpoints focus on photogenerated free charge carriers, which are somehow invalid once the many-body effects are taken into account, especially for polymeric photocatalysts. Here we systematically investigate the photoexcitation processes involved in the polymer matrix of graphitic carbon nitride (g-C3N4) by combining photoluminescence spectroscopy and ultrafast transient absorption spectroscopy, validating the strong excitonic effects in the well-known photocatalyst for the first time. The identification of the robust triplet-triplet annihilation process, in which two triplet excitons collide to produce a singlet exciton, highlights an important nonradiative depopulation pathway of excited species and thereby offers potential strategies to regulate the photocatalytic activities of polymeric g-C3N4. The work establishes a new understanding of the photocatalytic mechanism in the polymeric g-C3N4 matrix, and thus paves the way for designing effective polymeric photocatalysts through excitonic engineering.

Theoretical investigation of high-efficiency organic electroluminescent material: HLCT state and hot exciton process

HLCT excited state with both high PL efficiency and a large fraction of singlet exciton generation in EL. We employed the DFT method to investigate twisting D–A molecules, and found that the twist angle had a significant effect on the CT component.

Morphologies, field-emission and ultrafast nonlinear optical behavior of pure and Ag-doped ZnO nanostructures

Pure and Ag-doped ZnO nanofilms with four morphologies, namely nanocone, nanocrystal, nanochips and nanoleaves, have been successfully synthesized by magnetron sputtering method. The composition of the Ag-doped ZnO (AZO) nanostructures was explored by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The XRD patterns revealed that the samples exhibited the hexagonal wurtzite structure with preferential orientation along the (002) crystal plane, although the c-axis as preferred orientation decreased with Ag doping. Moreover, we report the photoluminescence, field-emission (FE) and ultrafast nonlinear optical behavior of AZO films. A blue-shift was demonstrated in the UV region of photoluminescence results for the AZO nanostructures. The resulting FE showed that the superior FE properties were attributed to the better morphologies. The ultrafast time resolved optical Kerr gate experiment results showed that AZO was a good candidate for an optical Kerr gate medium due to its ultrafast response and large third-order nonlinear susceptibility. The result of time-resolved photoluminescence showed that the first decay time was about several picoseconds for films, and the slow relaxation process had the lifetime longer than hundred picoseconds. By analyzing the experimental results of the time-resolved photoluminescence experiment, we found the fast decay process could be attributed to the exciton migration and the slow decay to the recombination process of the relaxed excitons.

Carrier localization effects in GaAs1−xSbx/GaAs heterostructures

We investigated the structural and optical properties of GaAs1−xSbx/GaAs heterostructures grown by molecular beam epitaxy on GaAs (001) substrates for Sb concentration up to 12% by means of high-resolution X-ray diffraction and photoluminescence. The correlation between our structural and optical analysis revealed that compositional fluctuations induced localized states which trap carriers at low temperature. Under low excitation power, the photoluminescence (PL) spectra are composed of two competing peaks in the temperature range of 30–80 K. The lower energy peak is associated with transitions from localized states in the band-tail of the density of states while the higher energy peak corresponds to transitions from free carriers. A model based on a redistribution process of localized excitons was used to reproduce the S-shape behavior of the temperature dependent PL. Reducing the growth temperature from 500 °C to 420 °C suppressed the S-shape behavior of the PL indicating a reduction in compositional variation.