Exciton Dissociation Dynamics Research Articles

Unveiling the exciton dissociation dynamics steered by built-in electric fields in conjugated microporous polymers for photoreduction of uranium (VI) from seawater

Developing highly efficient photocatalysts based on conjugated microporous polymers (CMPs) are often impeded by the intrinsically large exciton binding energy and sluggish charge transfer kinetics that result from their vulnerable driving force. Herein, a family of pyrene-based nitrogen-implanted CMPs were constructed, where the nitrogen gradient was regulated. Accordingly, the built-in electric field endowed by the nitrogen gradient dramatically accelerates the dissociation of exciton into free carriers, thereby enhancing charge separation efficiency. As a result, PyCMP-3N generated by polymerization of 1,3,6,8-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene and 2,4,6-tris(4-bromophenyl)-1,3,5-triazine featured an optimized built-in electric field and exhibited the highest photocatalytic removal efficiency of uranium (VI) (99.5 %). Our proposed strategy not only provides inspiration for constructing the built-in electric field by controlling nitrogen concentration gradients, but also offers an in-depth understanding the crucial role of built-in electric field in exciton dissociation and charge transfer, efficiently promoting CMPs photocatalysis.

Molecular Dipole-Induced Photoredox Catalysis for Hydrogen Evolution over Self-Assembled Naphthalimide Nanoribbons.

D-π-A type 4-((9-phenylcarbazol-3-yl)ethynyl)-N-dodecyl-1,8-naphthalimide (CZNI) with a large dipole moment of 8.49 D and A-π-A type bis[(4,4'-1,8-naphthalimide)-N-dodecyl]ethyne (NINI) with a negligible dipole moment of 0.28 D, were smartly designed and synthesized to demonstrate the evidence of a molecular dipole as the dominant mechanism for controlling charge separation of organic semiconductors. In aqueous solution, these two novel naphthalimides can self-assemble to form nanoribbons (NRs) that present significantly different traces of exciton dissociation dynamics. Upon photoexcitation of NINI-NRs, no charge-separated excitons (CSEs) are formed due to the large exciton binding energy, accordingly there is no hydrogen evolution. On the contrary, in the photoexcited CZNI-NRs, the initial bound Frenkel excitons are dissociated to long-lived CSEs after undergoing ultrafast charge transfer within ca. 1.25 ps and charge separation within less than 5.0 ps. Finally, these free electrons were injected into Pt co-catalysts for reducing protons to H2 at a rate of ca. 417 μmol h-1 g-1 , correspondingly an apparent quantum efficiency of ca. 1.3 % can be achieved at 400 nm.

Molecular Dipole‐Induced Photoredox Catalysis for Hydrogen Evolution over Self‐Assembled Naphthalimide Nanoribbons

AbstractD‐π‐A type 4‐((9‐phenylcarbazol‐3‐yl)ethynyl)‐N‐dodecyl‐1,8‐naphthalimide (CZNI) with a large dipole moment of 8.49 D and A‐π‐A type bis[(4,4′‐1,8‐naphthalimide)‐N‐dodecyl]ethyne (NINI) with a negligible dipole moment of 0.28 D, were smartly designed and synthesized to demonstrate the evidence of a molecular dipole as the dominant mechanism for controlling charge separation of organic semiconductors. In aqueous solution, these two novel naphthalimides can self‐assemble to form nanoribbons (NRs) that present significantly different traces of exciton dissociation dynamics. Upon photoexcitation of NINI‐NRs, no charge‐separated excitons (CSEs) are formed due to the large exciton binding energy, accordingly there is no hydrogen evolution. On the contrary, in the photoexcited CZNI‐NRs, the initial bound Frenkel excitons are dissociated to long‐lived CSEs after undergoing ultrafast charge transfer within ca. 1.25 ps and charge separation within less than 5.0 ps. Finally, these free electrons were injected into Pt co‐catalysts for reducing protons to H2 at a rate of ca. 417 μmol h−1 g−1, correspondingly an apparent quantum efficiency of ca. 1.3 % can be achieved at 400 nm.

Unraveling the Temperature Dependence of Exciton Dissociation and Free Charge Generation in Nonfullerene Organic Solar Cells

Organic solar cells based on nonfullerene acceptor molecules show high charge generation yields with negligible driving force at the donor–acceptor (D/A) interface to drive exciton dissociation. Understanding the underlying charge generation dynamics in these material systems is crucial for further development of this technology. Herein, the acceptor exciton dissociation dynamics in these materials is studied using transient optical spectroscopy. The results show that exciton dissociation at the D/A interface takes up to ≈100 ps to complete, and this process is not significantly affected by temperature. A similar timescale for charge transfer (CT) state separation into free electrons and holes is observed at room temperature. But in contrast to the weak temperature dependence of exciton dissociation, the free charge generation rate and yield are significantly reduced at low temperature. This suggests that overcoming the Coulomb barrier for charge separation at the D/A interface is the main reason for the endothermic charge separation observed for these material systems, instead of diffusion‐limited exciton dissociation.

Kinetic Modeling of the Electric Field Dependent Exciton Quenching at the Donor–Acceptor Interface

In this work, the electric field dependence of the exciton quenching efficiency at the donor/acceptor (D/A) heterojunction was studied. The concentration of singlet excitons and charge transfer states at the D/A interface was modeled by a system of coupled differential equations with transition rates obtained from Marcus theory. We applied then the model to determine the angular dependence (expressed by a parameter, θ) of the local exciton quenching induced by the orientation of the dissociation process relative to the direction of the electric field (F). We found that the exciton diffusion to the D/A heterojunction is not homogeneous for every value of θ, but it is higher toward regions where the dissociation rate is greater. The consequence of this effect is that only small values of this parameter will effectively contribute to the average quenching efficiency. There is then a gradual increase of the quenching efficiency with F, a fact that was verified experimentally. Following this principle, we were able to fit the experimental data measure in a bulk heterojunction device. In addition, we studied the field dependence of the PL quenching in a bilayer device that presents a very useful structure to test the theory. We found that the model explains the field-induced quenching efficiency not only when F has a favorable orientation that enhances the charge transfer but also when F tends to inhibit this process. In addition, our analysis might give some hints on the degree of mixing between the donor and acceptor in the active layer in this kind of device architecture. We believe that the model may clarify the processes that influence the dynamics of exciton dissociation at D/A interfaces. It is also useful to explore the effects of temperature, energy disorder, site disorder, and exciton binding energy in the photovoltaic effect of organic solar cells. Overall, it opens the possibility to deeply understand the effect of an electric field in new D/A heterojunctions with low driving force and efficient exciton dissociation.

Exciton dissociation and charge separation at donor-acceptor interfaces from quantum-classical dynamics simulations.

In organic photovoltaic (OPV) systems, exciton dissociation and ultrafast charge separation at donor-acceptor heterojunctions both play a key role in controlling the efficiency of the conversion of excitation energy into free charge carriers. In this work, nonadiabatic dynamics simulations based on the quantum-classical Liouville equation are employed to study the real-time dynamics of exciton dissociation and charge separation at a model donor-acceptor interface. Benchmark comparisons for a variety of low dimensional donor-acceptor chain models are performed to assess the accuracy of the quantum classical dynamics technique referred to as the forward-backward trajectory solution (FBTS). Although not always quantitative, the FBTS approach offers a reasonable balance between accuracy and computational cost. The short-time dynamics of exciton dissociation in related higher-dimensional lattice models for the interface are also investigated to assess the effect of the dimensionality on the first steps in the mechanism of charge carrier generation.

Unveiling the Molecular Symmetry Dependence of Exciton Dissociation Processes in Small-Molecular Heterojunctions

Understanding the dependence of molecular symmetry on exciton (EX) dissociation could facilitate optimization of overall power conversion efficiencies (PCEs) in solution-processed small-molecule (SM) solar cells. In this work, a series of high-performance oligomer molecules DRCNnT (n = 4–8) containing different numbers of thiophene units is systematically investigated to clarify the dependence of molecular symmetry on EX dissociation dynamics. Femtosecond transient absorption spectroscopy is employed to track the evolution of photogenerated electron–hole pairs across the DRCNnT:PC71BM interfaces, and faster intermolecular charge transfer is observed in the odd-numbered (axisymmetric) molecules relative to the even-numbered (centrosymmetric) molecules. It is found that those molecules with axisymmetric structures exhibit larger local dipole (Δμge) and better interpenetrating morphology than the centrosymmetric even-numbered molecules. Furthermore, a charge separation efficiency up to ∼80% is observed, whic...

Exciton dissociation dynamics and light-driven H2 generation in colloidal 2D cadmium chalcogenide nanoplatelet heterostructures

Solar-to-H2 conversion is attracting much research attention as a potential approach to meet global renewable energy demands. Although significant advances have been made using metal-tipped colloidal cadmium chalcogenide zero-dimensional (0D) quantum dots and one-dimensional (1D) nanorod heterostructures in solar-to-H2 conversion, their efficiency may be further enhanced using an emerging class of colloidal cadmium chalcogenide nanocrystals, namely two-dimensional (2D) nanoplatelets (NPLs), because of their unique properties. In this review, we summarize the recent advances on exciton dissociation dynamics and light-driven H2 generation performance of colloidal nanoplatelet heterostructures. Following an introduction on the electronic structure of 2D NPLs, we discuss the dynamics of exciton dissociation by electron transfer to molecular acceptors. The exciton quenching dynamics of CdS NPL-Pt and CdSe NPL-Pt heterostructures are compared to highlight the effect of material properties on the relative contributions of the energy-transfer and electron-transfer pathways. Representative solar-to-H2 conversion performances of 2D NPL-metal heterostructures are discussed and compared with those of 1D nanorod-metal heterostructures. Finally, we discuss the challenges in further improving the solar-to-fuel conversion efficiencies of these systems.

Incoherent Pathways of Charge Separation in Organic and Hybrid Solar Cells.

In this work, we investigate the exciton dissociation dynamics occurring at the donor:acceptor interface in organic and hybrid blends employed in the realization of photovoltaic cells. Fundamental differences in the charge separation process are studied with the organic semiconductor polymer poly(3-hexylthiophene) (P3HT) and either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO2) acting as the acceptor. By using ultrafast broad-band transient absorption spectroscopy with few-fs temporal resolution, we observe that in both cases the incoherent formation of free charges dominates the charge generation process. From the optical response of the polymer and by tracking the excited-state absorption, we extract pivotal similarities in the incoherent energy pathways that follow the impulsive excitation. On time scales shorter than 200 fs, we observe that the two acceptors display similar dynamics in the exciton delocalization. Significant differences arise only on longer time scales with only an impact on the overall photocarrier generation efficiency.

Ultrafast Exciton Migration and Dissociation in π-Conjugated Polymers Driven by Local Nonuniform Electric Fields

Experimental verification for ultrafast charge generations is a significant advancement in recent studies of polymer solar cells, but its underlying mechanism still remains unclear. In this paper, a new mechanism of ultrafast charge generation is proposed, where a local nonuniform electric field plays a vital role. We systematically simulate the exciton dissociation dynamics along a polymer chain with electric field linearly distributed. The polymer chain can be divided into two regions according to the exciton dissociation degree, i.e., complete dissociation region and partial dissociation region. In the former, a photogenerated exciton can dissociate directly and completely into free charges. In the latter, however, a photogenerated exciton first experiences an ultrafast migration process toward the complete dissociation region and then partially dissociates with fractional free charge generation. In most of our simulations, the exciton dissociation can take place within a time scale of 1 ps, contributi...

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.

Theoretical investigation of nonthermal equilibrium exciton dynamics in GaN using hydrogen plasma model

As a basis of the study on exciton stability under a nonthermal equilibrium state, the excitation and deexcitation population fluxes and population densities of several states of the principal quantum number p are calculated using a hydrogen plasma model for various electron excitation densities and temperatures of the lattice, electron, and exciton. It is found that the balance of the excitation and deexcitation population fluxes depends on the p number. At a lower-lattice-temperature region, ladderlike deexcitation flux is dominant for low p states, while the quasi-Saha–Boltzmann relation holds for high p states. At temperatures higher than 150 K, the exciton formation and dissociation fluxes become dominant. Exciton dissociation is enhanced at temperatures higher than approximately 120 K. This process is triggered by the excitation between the states of p = 1 and 2. High- and low-order states sometimes exhibit different population flow characteristics, which reveal the exciton dissociation dynamics.

Intrinsic femtosecond charge generation dynamics in single crystal CH3NH3PbI3

Using time-resolved multi-THz we measure femtosecond charge generation, conductivity and exciton dissociation dynamics in single crystal methylammonium lead triiodide, the prototypical perovskite solar cell material.

Estimating the Magnitude of Exciton Delocalization in Regioregular P3HT

Exciton delocalization has been proposed to have a strong impact on the performance of organic solar cells. For example, large exciton delocalization estimates have promoted the theory of long-range charge transfer as a mechanism for efficient charge separation. Here, two new computational modeling techniques for analyzing femtosecond transient absorption spectroscopy experiments are developed in order to estimate the magnitude of exciton delocalization in semiconducting polymers. The developed techniques are then used to analyze previously published experimental data for regioregular poly(3-hexylthiophene) (P3HT). Based on modeling both the exciton–exciton annihilation behavior in a pure P3HT film and the exciton dissociation dynamics in a P3HT:PCBM blend film, the exciton delocalization radius in regioregular P3HT is estimated to be in the range of 1–2 nm, which is significantly smaller than estimated in a number of previous studies. These results suggest that exciton delocalization is not likely to be a significant contributing factor to efficient charge separation.

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.

Impurity effects and temperature influence on the exciton dissociation dynamics in conjugated polymers

We investigate temperature effects on exciton dissociation dynamics in conjugated polymer systems. Using a modified version of the tight-binding Su–Schrieffer–Heeger model, the dissociation is studied under the influence of impurity effects with a nonadiabatic evolution method. Our results show that temperature effects reduce the critical electric field for the exciton dissociation. In the small temperature regime, the exciton is not trapped by the impurity and it is observed to perform a random walk, a fact not observed in the absence of temperature. This letter might enlighten the description of electroluminescence yields and charge transport efficiency in organic based electronic devices.

Exciton dissociation dynamics in a conjugated polymer containing aggregate states

We study the exciton dynamics in ladder-type poly( p-phenylene) (LPPP) containing aggregate states under high electric field conditions. Time-resolved emission spectroscopy reveals a different dissociation behaviour of S 1-intrachain excitons as compared to excitons located in aggregate states. No acceleration of the decay of aggregate excitons due to field-induced dissociation is found for electric fields up to 2×10 6 V/cm. The time-integrated photoluminescence quenching does not exhibit a strong spectral dependence and is completely due to the dissociation of initially excited S 1-intrachain excitons.