Photoexcitation Relaxation Research Articles

Interplay between photoinduced charge and energy transfer in manganese doped perovskite quantum dots.

A comprehensive study on the photo-excited relaxation dynamics in semiconducting perovskite quantum dots (PQDs) is pivotal in realizing their extensive potential for optoelectronics applications. Among different competing photoinduced relaxation kinetics, energy transfer and charge transfer (CT) in PQDs need special attention, as they often influence the device efficacy, particularly with the donor-acceptor hybrid architecture. In this work, we explore a detailed investigation into photoinduced CT dynamics in mixed halide undoped CsPb(Br/Cl)3 and Mn2+ doped CsPb(Br/Cl)3 PQDs with a quinone molecule, p-benzoquinone (BQ). The energy level alignment of undoped PQDs with BQ allows an efficient CT, whereas Mn2+ doping reduces the CT efficiency, experiencing a competition between energy transfer from host to dopant and CT to BQ. The conductive atomic force microscopy measurements unveil a direct correlation with the spectroscopic studies by showing a significant improvement in the conductance of undoped PQDs in the presence of BQ, while an inappreciable change is observed for doped PQDs. A much-reduced transition voltage and barrier height in the presence of BQ further validate faster CT for undoped PQD than the doped one. Furthermore, Mn2+ doping in PQDs is observed to enhance their stability, showing better air and thermal stability compared to their undoped counterparts. These results reveal that doping strategy can regulate the CT dynamics in these PQDs and increase their stability, which will be beneficial for the development of desired optoelectronic devices with long-term stability.

Photoexcitation Control of Excitation Relaxation in Mixed‐Phase Ruddlesden‐Popper Hybrid Organic‐Inorganic Lead‐Iodide Perovskites

AbstractThe electronic states and exciton binding energies of layered Ruddlesden‐Popper (RP) metal‐halide perovskites can be tailored through changes of their chemical composition, yielding multi‐phase systems with complex energy cascades. Ultrafast photoexcitation relaxation with transfer dynamics into domains of increasing layer number has been reported for these materials. Here, ultrafast optical spectroscopy is used to report an unexpected excitation energy dependence of photoexcitation relaxation dynamics in mixed‐dimensional benzylammonium cesium lead iodide RP perovskite (BeA2CsPb2I7) thin films, which gives rise to spectrally broadband luminescence over the visible region. Using transient absorption and photoluminescence spectroscopy it is found that excitations, which are formed in the n = 2 RP‐phase after photoexcitation with ≈0.2 electron volt excess energy, transfer to higher layer number RP‐phases on unexpectedly slow timescales of tens of picoseconds. Further, it is observed that such excitations are initially optically passive. Notably, luminescence occurs under these conditions from multiple RP‐phases with optical bandgaps across the visible range, yielding broadband luminescence. The results hold potential for realization of broadband white‐light emitters and other light‐emitting devices.

Near-Infrared Emission of HgTe Nanoplatelets Tuned by Pb-Doping.

Doping the semiconductor nanocrystals is one of the most effective ways to obtain unique materials suitable for high-performance next-generation optoelectronic devices. In this study, we demonstrate a novel nanomaterial for the near-infrared spectral region. To do this, we developed a partial cation exchange reaction on the HgTe nanoplatelets, substituting Hg cations with Pb cations. Under the optimized reaction conditions and Pb precursor ratio, a photoluminescence band shifts to ~1100 nm with a quantum yield of 22%. Based on steady-state and transient optical spectroscopies, we suggest a model of photoexcitation relaxation in the HgTe:Pb nanoplatelets. We also demonstrate that the thin films of doped nanoplatelets possess superior electric properties compared to their pristine counterparts. These findings show that Pb-doped HgTe nanoplatelets are new perspective material for application in both light-emitting and light-detection devices operating in the near-infrared spectral region.

Phonon Bottleneck in Temperature-Dependent Hot Carrier Relaxation in Graphene Oxide

We report temperature-dependent ultrafast hot carrier relaxation in graphene oxide (GO). At all temperatures in the range from ∼6 to 400 K, the time evolution of the photoexcited relaxation reveals...

Effects of intrachain disorder on photoexcitation in conjugated polymer chains

The luminescence efficiency of conjugated polymers has been a central topic in the study of light emitting. The effect of disorder plays an important role in generating excitons after the conjugated polymers have been excited by photons. In this paper, by using the Su-Schriffer-Heeger model, which has been modified to include intrachain disorder and electron correlation, we investigate the effects of disorder on the photoexcitation, especially on the yield of excitons in a conjugated polymer chain. We adopt the multi-configurational time-dependent Hartree–Fock method to solve the multi-electron time-dependent Schrödinger equation and the Newtonian equation of motion for the lattice vibration. The results show that after the photoexcitation relaxation process, the products of the disordered polymer chain are qualitatively distinct from those of the pure polymer chain. While the pairs of polarons can be generated directly after the photoexcitation in a pure polymer chain, the disorder favors excitons as the products of the photoexcitation, and the yield of excitons depends crucially on the kind and strength of the disorder. Furthermore, the influences of the electron correlation and the conjugation length on the yield of excitons are also discussed. Specifically, we find that in the case of diagonal disorder, when the conjugation length is short and the diagonal disorder is weak, the excitons are mainly generated by the recombination of two lattice defects with a high yield of excitons which will be reduced as the conjugation length increases. The excitons tend to be generated directly with a low yield of excitons which is enlarged as the disorder gets stronger when the conjugation length is long or the diagonal disorder is strong. The on-site Coulomb repulsion favors the generation of excitons as well. The case of off-diagonal disorder is similar to that of diagonal disorder except that the on-site Coulomb potential favors the generation of excitons in the weak disorder regime but depresses the generation of excitons in the strong disorder regime. When both diagonal and off-diagonal disorders are considered, the yield of excitons is dominated by the off-diagonal disorder. We hope that our investigations can provide useful guidance and help for designing organic photoelectric materials and devices.

Temperature-Dependent Ultrafast Response and π-Plasmon Dynamics in Single-Walled Carbon Nanotubes.

Temperature-dependent femtosecond time-resolved carrier relaxation dynamics has been studied in thin films of single-walled carbon nanotubes. An early time evolution of the photoexcited relaxation shows evidence of superimposed transient bleaching and induced photo absorption of almost similar strengths, whereas at longer times it is governed by slow recovery of long-lived dark excitons. After about 3 ps, the signal is dictated by the slowest negative relaxation component attributed to the low-energy π-plasmons. An absorption trough near 500 fs in the ultrafast response evolves with the increasing sample temperature. This particular feature is masked by the reduced induced transmission at room temperature and above. We have estimated the electron-phonon coupling constant to be ∼0.86 from the linear temperature dependence of the slow relaxation time constant. More such studies can help advance the understanding of the intrinsic charge and energy loss mechanisms to improve the efficiency of the optoelectronic devices based on them.

The role of vibronic coupling in the electronic spectroscopy of maleimide: a multi-mode and multi-state quantum dynamics study.

The first two excitation bands below 7 eV in the electronic absorption spectrum of maleimide are investigated using a model Hamiltonian including four low-lying singlet excited states within the manifold of 24 vibrational modes. The role of non-adiabatic effects is studied and shines light on both the broad, inter-state coupling-dominated spectral band as well as the fine-structured, not-so-strong coupled band. Calculations have been performed using the Multiconfigurational Time-Dependent Hartree (MCTDH) wavepacket propagation method as well as its multilayer version (ML-MCTDH) using a quadratic vibronic coupling (QVC) Hamiltonian model where parameters are obtained from fitting adiabatic potential energy surfaces computed by ab initio methods. The quantum dynamics calculations provide information on the relaxation dynamics and the vibrational modes involved. Already with a low-order vibronic coupling model and only a few modes being considered, a quantitative agreement with the experimental spectrum is obtained. However, it is found that all modes need to be considered to get a full picture of the photo-excited relaxation dynamics of this molecule.

Effects of Replacement on the Optical Properties of Narrow Bandgap Polymers: Comparing the Difference Between Thieno[3,2-b]thiophene Units and Thiophene Units

The effects of replacement on the optical properties of diketopyrrolopyrrole(DPP)-based polymers are discussed through replacing the thieno[3,2-b]thiophene units by thiophene units. It is easy to see that the role of re-placement would lead to blue shift of steady-state absorption spectrum when thieno[3,2-b]thiophene units is replaced by thiophene units. Meanwhile, the transient absorption data indicate that the role of replacement would not change the relaxation mechanism of polymer, but is able to adjust the photo-excitation relaxation rate of polymer. Their in-tensity-dependent dynamic curves show that the exciton-exciton annihilation(EEA) would participate in the relaxation process when the polymer is in the aggregation. Through comparing the EEA data between the polymers, it is found that the role of replacement would change the spatial distribution of exciton.

Intersystem crossing in thiobases proceeds by a dark intermediate state

4-thiouracil (4TU) is studied by transient absorption spectroscopy employing sub-20 fs UV-pulses and hybrid QM(CASPT2) / MM(AMBER) computations (static and dynamic), evidencing that, along the photoexcited relaxation pathway, intersystem crossing originates from a dark intermediate state.

Picosecond Relaxation of Photoexcitations in a LiLuF4:Ce3+ Crystal

The characteristic durations of energy exchange for Ce3+ impurity centers with lattices in a LiLuF4 crystal and the characteristic lifetime of free photoexcited charge carriers are determined in time-resolved experiments using a pump–probe scheme. The absorption saturation parameters of femtosecond laser pulses with a wavelength of 263 nm are characteristic of a slow absorber that participates in absorption from the excited state.

Relaxation of photoexcitations in polaron-induced magnetic microstructures

We investigate the evolution of a photoexcitation in correlated materials over a wide range of time scales. The system studied is a one-dimensional model of a manganite with correlated electron, spin, orbital, and lattice degrees of freedom, which we relate to the three-dimensional material Pr$_{1-x}$Ca$_{x}$MnO$_3$. The ground-state phases for the entire composition range are determined and rationalized by a coarse-grained polaron model. At half-doping a pattern of antiferromagnetically coupled Zener polarons is realized. Using time-dependent density-matrix renormalization group (tDMRG), we treat the electronic quantum dynamics following the excitation. The emergence of quasiparticles is addressed, and the relaxation of the nonequilibrium quasiparticle distribution is investigated via a linearized quantum-Boltzmann equation. Our approach shows that the magnetic microstructure caused by the Zener polarons leads to an increase of the relaxation times of the excitation.

Photochemistry and Electron Transfer Kinetics in a Photocatalyst Model Assessed by Marcus Theory and Quantum Dynamics

The present computational study aims at unraveling the competitive photoinduced electron transfer (ET) kinetics in a supramolecular photocatalyst model. Detailed understanding of the fundamental processes is essential for the design of novel photocatalysts in the scope of solar energy conversion that allows unidirectional ET from a light-harvesting photosensitizer to the catalytically active site. Thus, the photophysics and the photochemistry of the bimetallic complex RuCo, [(bpy)2RuII(tpphz)CoIII(bpy)2]5+, where excitation of the ruthenium(II) moiety leads to an ET to the cobalt(III), were investigated by quantum chemical and quantum dynamical methods. Time-dependent density functional theory (TDDFT) allowed us to determine the bright singlet excitations as well as to identify the triplet states involved in the photoexcited relaxation cascades associated with charge-separation (CS) and charge-recombination (CR) processes. Diabatic potential energy surfaces were constructed for selected pairs of donor–acc...

Spectral sensitization with dyes of core–silver halide shell microsystems

We have studied spectral sensitization with anionic dyes of core–silver halide shell microsystems cores of which can be either nonsilver or silver halide compounds. Conditions under which dye sensitizers, being adsorbed on cores, remain under silver halide shells after their growing are considered. Comparison of results of sensitometric and low-temperature (T = 77 K) luminescent measurements have shown that these conditions are determined by the charge state of cations of microsystem cores. If the shell contains the same univalent component in its composition as the core does, as in the case in which the core is a silver halide compound, the anionic dye is displaced to the outer surface of the shell. If the core contains a divalent cationic component, as in the case in which the core is a nonsilver compound, the dye remains under the silver halide shell; i.e., it is overgrown by the shell. We have shown that the charge state of core cations affects the character of the core interaction with anionic dyes, which ensures differences in the spectral sensitization of core–silver halide shell microsystems, as well as differences in the dye photoexcitation relaxation in them.

Transient photocurrent responses in amorphous Zn-Sn-O thin films

In this study we characterized the transient photocurrent responses in solution-processed amorphous zinc-tin-oxide (a-ZTO) thin films measured under light illumination with a wavelength of 400 nm at different temperatures. By using the temperature-dependent photoconductivities of a-ZTO thin films, we extracted the activation energies (E ac ) of photo-excitation and dark relaxation through an extended stretched exponential analysis (SEA). The SEA was found to describe well the dark relaxation characteristics as well as the photo-excitation processes. The SEA also indicates that the dark relaxation process reveals a dispersive transient photoconductivity with a broader distribution of the E ac while the photo-excitation process shows non-dispersive characteristics. Samples exposed by light at temperatures less than 373 K possess the fast processes of photo-excitation and dark relaxation. This suggests that a fast process, for example, a generation/recombination of charged carriers related to a band-to-band transition and/or shallow/deep oxygen-vacancy (V o ) sub-gap donor states, is dominant in the case of light illumination at low temperatures of less than 373 K. The SEA indicates, however, that a much slower process due mainly to the delay of the onset of ionization/neutralization of the deep V o states by multiple-trapping is dominant for samples under light illumination at a high temperature of 373 K. Based on the experimental results, for the dark relaxation process, we conclude that the process transitions from a fast recombination of electrons through band-to-band transitions and/or shallow/deep V o donor states to a slow neutralization of the ionized V o states occurs due to enhanced carrier multiple-trapping by relatively deep V o trap states when the temperature becomes greater than 363 K. An energy band diagram of a-ZTO thin films was proposed in terms of the temperature and the E ac distribution to explain these observed results.

Efficient relaxation

A study on carrier-collection efficiency in various organic photovoltaic systems now reveals that ultrafast relaxation of photoexcitations within the manifold of charge-transfer states does not impede mobile charge carrier generation.

Spectral, luminescent, and photochemical properties of o-hydroxyazomethinespiropyrans in solid matrices

Spectral, luminescent, and photochemical properties of the novel bifunctional compounds BF1 and BF2 based on indoline spiropyrans and o-hydroxyazomethines have been studied in solid media. In PMMA films and frozen solutions, redistribution of photoexcitation relaxation channels in favor of photochemical processes leading to photochromic products in the azomethine moiety and luminescence is observed as compared with liquid solutions.

Interaction of dyes with Ag2S nanoclusters adsorbed on AgBr microcrystals

The photoexcitation relaxation is considered in a system consisting of a molecular and J-aggregated dye adsorbed on the surface of silver sulfide nanosized clusters (nanoclusters) placed on AgBr microc-rystals. It is found that the photoexcitation in the region of the dye absorption relaxes not only via the fluorescence, but also via the phosphorescence of nanoclusters. It is shown that, depending on the dye, the phosphorescence of nanoclusters is most efficiently excited in the absorption region of either molecular or J-aggregated states of dyes. It is assumed that this difference is caused by different nature of the adsorption of dyes on the nanocluster surface and by different interactions between the photoexcited states of dye molecules and J aggregates. These results can find wide applications in various fields of nanophotonics and allow one to both control the photoexcitation relaxation processes in nanocluster-dye systems and to observe new manifestations of these processes.

Effects of interchain coupling on photoexcitation in two coupled polymer chains in the presence of an electric field

Within an extended Su-Schrieffer-Heeger model including interchain interactions and the extended Hubbard model (EHM), the dynamical relaxation of photoexcitations in the presence of an external electric field is investigated using a nonadiabatic evolution method. Under the action of the interchain interactions both intrachain excitons and interchain excitons are generated after photoexcitation in two coupled polymer chains. Our results show that the field required to dissociate the excitons depends sensitively on the strength of the interchain coupling. As the interchain coupling strength increases, the dissociation field decreases. By analyzing the relation between the yield of intrachain and interchain excitons and the interchain coupling strength, we explain the dependence between the dissociation field and the strength of the interchain interactions. The theoretical results are expected to provide useful predictions concerning which polymers with properly strong interchain interactions are likely to be most suitable for use in organic solar cells.

Relaxation of Photoexcitations in Optically Excited Systems Formed by 'Weakly Amorphous' or Crystalline Si Nanoparticles Embedded Within a Wide-Gap Dielectric Matrix

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Temperature Dependence of the Photoluminescence Intensity in Si3+xN4:H Films with Amorphous Si Nanoclusters: Evidence for Two Processes Involved in the Nonradiative Relaxation of Photoexcitations

The temperature dependence of the photoluminescence intensity in Si3+xN4:H films grown on Si substrates in a PECVD process held at temperatures 100 and 380oC was analyzed versus the film stoichiomery. The film stoichiometry parameter x was varied in the range x = 0.08 to 3.7 via varying the NH3/SiH4 flow ratio (0.5 to 5). According to Raman scattering data, the films contained amorphous Si nanoclusters, whose sizes increased with x and substrate temperature. For the first time, the involvement of two different processes in the nonradiative relaxation of excitations optically generated in the system under study has been demonstrated. These processes were tentatively identified as the thermal dissociation of nanocluster-bound excitons and their nondissociative migration to nonradiative sites in neighboring Si nanoclusters. The activation energies and the frequency factors of the two processes were shown to be consistent with the quantum confinement model for the amorphous Si nanocluster related photoluminescence.