Dynamics Of Charge-transfer States Research Articles

Observation of parallel intersystem crossing and charge transfer-state dynamics in [Fe(bpy)3]2+ from ultrafast 2D electronic spectroscopy.

Transition metal-based charge-transfer complexes represent a broad class of inorganic compounds with diverse photochemical applications. Charge-transfer complexes based on earth-abundant elements have been of increasing interest, particularly the canonical [Fe(bpy)3]2+. Photoexcitation into the singlet metal-ligand charge transfer (1MLCT) state is followed by relaxation first to the ligand-field manifold and then to the ground state. While these dynamics have been well-studied, processes within the MLCT manifold that facilitate and/or compete with relaxation have been more elusive. We applied ultrafast two-dimensional electronic spectroscopy (2DES) to disentangle the dynamics immediately following MLCT excitation of this compound. First, dynamics ascribed to relaxation out of the initially formed 1MLCT state was found to correlate with the inertial response time of the solvent. Second, the additional dimension of the 2D spectra revealed a peak consistent with a ∼20 fs 1MLCT → 3MLCT intersystem crossing process. These two observations indicate that the complex simultaneously undergoes intersystem crossing and direct conversion to ligand-field state(s). Resolution of these parallel pathways in this prototypical earth-abundant complex highlights the ability of 2DES to deconvolve the otherwise obscured excited-state dynamics of charge-transfer complexes.

Intrachain Delocalization Effect of Charge Carriers on the Charge-Transfer State Dynamics in Organic Solar Cells

We studied the charge-generation mechanism in low-bandgap polymer (P4TNTz-2F)-fullerene bulk heterojunction (BHJ) organic solar cells (OSCs) using transient absorption (TA) spectroscopy. The highly crystalline nanowire structure of P4TNTz-2F in a blend film prepared with chlorobenzene (CB) and 1,8-diiodooctane (DIO) induced more long-lived charge carriers than those in a blend film prepared with CB only. Pump-wavelength-dependent TA data revealed that the increased charge-delocalization by the intrachain ordering of P4TNTz-2F in the blend film prepared with CB/DIO is the key factor to increasing the OSC efficiency. The intrachain charge-delocalization increased the charge-transfer (CT) state lifetime and suppressed geminate recombination losses, resulting in the efficient dissociation of CT states into free carriers. Our findings provide new insights into the excited-state dynamics study of BHJ blends, which can serve as a good guide for the development of novel OSC materials.

The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation

Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the "reference" PffBT4T-2OD:PC71BM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.

Charge-transfer state dynamics in all-polymer solar cells: formation, dissociation and decoherence.

All-polymer solar cells have made substantial achievements in recent years, offering numerous unsettled subjects for mechanical researchers. In order to quantitatively study the influence of the molecular electrostatic potential on the charge generation proposed by the experimenter, we simulate the ultrafast dynamics of the charge-transfer (CT) state at the interface between two polymer chains, which are respectively regarded as the donor and acceptor in all-polymer solar cells. The formation of a stable CT state is found to be sensitive to the distance between two oppositely charged polarons and the relevant critical electrostatic potential is thus quantified, which is in good agreement with experiments. In order to get insight into the dependence of the dissociation of the CT state on the width of the interfacial layer, two quantities are calculated: one is the Coulomb capture radius between the two polarons and the other is the quantum trace distance which serves as the fingerprint of the quantum coherence between them. The dissociation of the CT state is found to take place within an ultrafast timescale for an optimum interfacial width. The classical spatial distance and the quantum trace distance manifest a converging trend, suggesting a decoherence scenario for the charge separation in all-polymer solar cells.

Directly Coupled Versus Spectator Linkers on Diimine PtII Acetylides-Change the Structure, Keep the Function?

Modification of light-harvesting units with anchoring groups for surface attachment often compromises light-harnessing properties. Herein, a series of [donor-acceptor-anchor] platinum(II) diimine (bis-)acetylides was developed in order to systematically compare the effect of conjugated versus electronically decoupled modes of attachment of protected anchoring groups on the photophysical properties of light-harvesting units. The first examples of "decoupled" phosphonate diimine PtII complexes are reported, and their properties are compared and contrasted to those of carboxylate analogues studied by a diversity of methods. Ultrafast time-resolved IR and transient absorption spectroscopy revealed that all complexes have a charge-transfer (CT) lowest excited state with lifetimes between 2 and 14 ns. Vibrational signatures and dynamics of CT states were identified; the assignment of electronic states and their vibrational origin was aided by TDDFT calculations. Ultrafast energy redistribution accompanied by structural changes was directly captured in the CT states. A significant difference between the structures of the electronic ground and CT excited states, as well as differences in the structural reorganisation in the complexes bearing directly attached or electronically decoupled anchoring groups, was discovered. This work demonstrates that decoupling of the anchoring group from the light-harvesting core by a saturated spacer is an easy approach to combine surface attachment with high reduction potential and ten times longer lifetime of the CT excited state of the light-absorbing unit, and retain electron-transfer photoreactivity essential for light-harvesting applications.

A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects.

In this Letter, we explore how the microscopic dynamics of charge-transfer (CT) excitons are influenced by the presence of an external magnetic field in disordered molecular semiconductors. This influence is driven by the dynamic interplay between the spin and spatial degrees of freedom of the electron-hole pair. To account for this interplay, we have developed a numerical framework that combines a traditional model of quantum spin dynamics with a stochastic coarse-grained model of charge transport. This combination provides a general and efficient methodology for simulating the effects of magnetic field on CT state dynamics, therefore providing a basis for revealing the microscopic origin of experimentally observed magnetic field effects. We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials.

Effect of Fluorine Substitution on the Charge Carrier Dynamics of Benzothiadiazole-Based Solar Cell Materials.

The femtosecond transient absorption (TA) characterization of a new benzothiadiazole (BT)-based donor-acceptor conjugated copolymer, poly[(2,6-dithieno[3,2-b:2',3'-d]thiophene)-alt-(4,7-di(4-octyldodecylthiopen-2-yl)-2,1,3-benzo[c][1,2,5]thiadiazole (PBT), as well as its fluorinated derivatives, PFBT and PDFBT, is carried out. Additionally, bulk heterojunction (BHJ) films consisting of the copolymers and [6,6]-phenyl-C71 -butylic acid methyl ester (PC70 BM) are examined using TA spectroscopy. Both the singlet excited state dynamics in the copolymers and the charge transfer state dynamics in the BHJs are investigated in terms of fluorination dependency; the fluorinated copolymers exhibit less singlet exciton recombination rate than the fluorine-free copolymer, and the BHJs including the fluorinated copolymers display slower monomolecular recombination than the fluorine-free analogue. Furthermore, the excitation-intensity-dependent TA dynamics of the copolymers and BHJs is investigated, revealing that, when sufficiently high excitation intensity is used to induce annihilation processes, the fluorinated copolymers and BHJs incorporating the fluorinated copolymers show more rapid TA decay ascribable to morphological enhancement. These TA spectroscopic findings are found to correlate with the device characteristics with respect to fluorinated content in the polymer solar cells. In particular, both the short-circuit current density and fill factor of BHJ solar cells correspond closely with the fast decay parameters of the BHJ films under high excitation intensity.

Charge-Transfer State Dynamics Following Hole and Electron Transfer in Organic Photovoltaic Devices.

The formation of bound electron-hole pairs, also called charge-transfer (CT) states, in organic-based photovoltaic devices is one of the dominant loss mechanisms hindering performance. Whereas CT state dynamics following electron transfer from donor to acceptor have been widely studied, there is not much known about the dynamics of bound CT states produced by hole transfer from the acceptor to the donor. In this letter, we compare the dynamics of CT states formed in the different charge-transfer pathways in a range of model systems. We show that the nature and dynamics of the generated CT states are similar in the case of electron and hole transfer. However the yield of bound and free charges is observed to be strongly dependent on the HOMOD-HOMOA and LUMOD-LUMOA energy differences of the material system. We propose a qualitative model in which the effects of static disorder and sampling of states during the relaxation determine the probability of accessing CT states favorable for charge separation.

Dynamics of charge transfer states on metal surfaces: the competition between reactivity and quenching.

The dynamics of excited states of adsorbates on surfaces caused by charge transfer is studied. Both negative and positive charge transfer processes are possible. In particular we are interested in positive charge transfer from a metal surface to molecular or atomic oxygen adsorbed on the surface. Once the negatively charged oxygen on the surface loses an electron it becomes chemically activated. The ability of this species to react depends on the quenching time or back transfer. The analysis of these processes is based on a set of diabatic potential energy surfaces each representing a different charged oxygen species. The dynamics is followed by solving the multichannel time-dependent Schrödinger equation or Liouville von Neumann equation. Due to the nonadiabatic character of these reactions large isotope effects are predicted.