Geminate Recombination Model Research Articles

A fresh look into the time-resolved fluorescence of 8-hydroxy-1,3,6-pyrenetrisulfonate with the use of the fluorescence up-conversion technique

Abstract Steady-state and time-resolved fluorescence techniques were used to study the excited-state-proton-transfer (ESPT) process of 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) in H 2 O and D 2 O. In this contribution we use the fluorescence up-conversion technique with a time resolution of ∼100 fs to monitor the short-time components of HPTS ROH (protonated) and RO − (deprotonated) signals. The ESPT rate constant, k PT , for HPTS in H 2 O and D 2 O is rather small 10 10 s −1 and 3.3 × 10 9 s −1 , respectively. In the time-resolved fluorescence signal of the deprotonated form we find a rise-component of 2.5 ps which we assign to slow charge rearrangement as was already suggested by Spry and Fayer [Spry, D. B.; Fayer, M. D. Charge Redistribution and Photoacidity: Neutral Versus Cationic Photoacids. J. Chem. Phys . 2008 , 128, 084508-1–084508-9]. Already in the time-window of 0.2–1.2 ns, the proton geminate recombination (GR) fluorescence tail of the ROH form decays as t −α where α ≈ 3/2, as predicted by the diffusion-assisted GR model, but for much longer times ( t > 5 ns). We also found that the rotation-relaxation time of the ROH form is about τ or = 80 ps in H 2 O, shorter than previously reported, whereas in methanol solution, with much lower viscosity, it is much larger – τ or = 190 ps. We explain this large difference of τ or by counter-ion association on all the three sulfonate groups of HPTS.

Photoelectrochemical evidence of nitrogen incorporation during anodizing sputtering--deposited Al-Ta alloys.

Anodic films were grown to 20 V on sputtering-deposited Al-Ta alloys in ammonium biborate and borate buffer solutions. According to glow discharge optical emission spectroscopy, anodizing in ammonium containing solution leads to the formation of N containing anodic layers. Impedance measurements did not evidence significant differences between the dielectric properties of the anodic films as a function of the anodizing electrolyte. Photoelectrochemical investigation allowed evidencing that N incorporation induces a red-shift in the light absorption threshold of the films due to the formation of allowed localized states inside their mobility gap. The estimated Fowler threshold for the internal photoemission processes of electrons resulted to be independent of the anodizing electrolyte confirming that N incorporation does not appreciably affect the density of states distribution close to the conduction band mobility edge. The transport of photogenerated carriers has been rationalized according to the Pai-Enck model of geminate recombination.

The area reactivity model of geminate recombination

We investigate the reversible diffusion-influenced reaction of an isolated pair in the context of the area reactivity model that describes the reversible binding of a single molecule in the presence of a binding site in terms of a generalized version of the Feynman-Kac equation in two dimensions. We compute the corresponding exact Green's function in the Laplace domain for both the initially unbound and bound molecule. We discuss convolution relations that facilitate the calculation of the binding and survival probabilities. Furthermore, we calculate an exact analytical expression for the Green's function in the time domain by inverting the Laplace transform via the Bromwich contour integral and derive expressions for the binding and survival probability in the time domain as well. We numerically confirm the accuracy of the obtained expressions by propagating the generalized Feynman-Kac equation in the time domain. Our results should be useful for comparing the area reactivity model with the contact reactivity model.

Influence of bimolecular recombination on the photogeneration yield values determined by xerographic method

Xerographic method consists in monitoring of decay of the surface potential of the photoconducting layer under illumination. The charge carrier photogeneration quantum yield is determined basing on the initial potential decay rate. In this work the photogeneration quantum yield for theoretically simulated initial surface potential decay is analyzed for a case when simultaneous photogeneration and recombination occur. A model disordered organic photoconductor is considered in which the photogeneration probability is described by the Onsager model of geminate recombination and the cross-section for recombination exhibits power dependence on the electric field. Photogeneration yields calculated in the classical way and the yields calculated with assumed bimolecular recombination are compared. It is shown that for high bimolecular recombination probability correctly determining the classical xerographic method of photogeneration quantum yield is difficult.

Influence of Side Chains on Geminate and Bimolecular Recombination in Organic Solar Cells

The effects of the side chains on the loss processes in three polymer:PC[61]BM organic blends are studied by comparing transient absorption spectroscopy and solar cell measurements. It is shown that, after efficient photoinduced charge transfer, charge pairs are the only significant species present in the blend and that their recombination can be followed by monitoring the PIA kinetics. Charges are found to decay following two regimes. The subnanosecond decay of the charge signal, responsible for the losses of 55–75% of the charges, is well described by an Onsager–Braun geminate recombination model. The rates are found to be strongly affected by the presence and position of side chains. We attribute these changes to a better aggregation of the side-chain-less polymer that provides a better delocalization of the holes and a better mobility. At longer times (after about 10 ns) the charges undergo bimolecular recombination. By modeling the time and intensity dependence of this process, we measure the rate constant and find that it is similar for the three systems and slightly lower than the theoretical Langevin rate. By comparing these spectroscopic results with the light-intensity and bias dependence of the photocurrent, we conclude that bimolecular recombination does not play an important role under solar intensity. Geminate recombination is the main loss mechanism and explains the difference between all the devices, their strong bias dependence, and their generally low performance.

Probing the Inner Cavities of Hydrogels by Proton Diffusion

The reversible proton dissociation and geminate recombination of a photoacid was studied in peptide-based hydrogels over a large concentration range of the self-assembled peptide nanotube (PNT) network. 8-Hydroxypyrene-1,3,6-trisulfonate trisodium salt (HPTS) was used as the photoacid. We found that the proton transfer to the solvent rate is not affected by the presence of the fibrous structures. However, the proton geminate recombination process is affected by the PNT walls. We used the photoprotolytic reversible geminate recombination model to analyze the time-resolved emission data in the hydrogel samples. Our assumptions lead to the suggestion that the hydrogels form water nanopools of the order of 100 A.

Photoconduction in the archetype organic hole transporting material TPD

Electric field, light wavelength and excitation intensity characteristics of photoconduction have been measured in a diamine derivative (TPD) serving as a common hole-transporting material in organic light emitting diodes. Single layer TPD structures, Quartz/ITO/TPD/Al and Quartz/Al/TPD/Al, have been excited trough a quartz substrate. At wavelengths longer than ≈370 nm and at low excitation levels, the photocurrent is due to singlet exciton-induced injection of electrons at the cathodes. Good agreement with experiment is provided by solving the diffusion equation for the excitons and applying the 1D-Onsager model of geminate recombination of electrons with their image countercharge in the electrode. A bulk generated (hole) photocurrent is shown to dominate with excitation energies (hν) well in excess of 3.2 eV, i.e. λ ⩽ 350 nm, and in the whole spectral range applied for moderate and high excitation intensities.

Proton Reaction with a Mild Base in Ice Studied by Proton−Photoacid Dynamics

We have studied the kinetics of the reaction of a proton with a mild base in ice. The proton was injected to the ice crystal by a proton-transfer reaction from an electronically excited photoacid. Mild bases react with the transferred proton with a large intrinsic rate constant. We used our diffusion assisted geminate recombination model based on the Debye−Smolochowski equation with an additional term to account for the proton scavenging by the base. We find that in liquid water the measured scavenging effect is slightly smaller than expected from a diffusion controlled reaction rate constant. In ice, the scavenging effect is rather large, and the diffusion controlled reaction rate constant underestimates the measured proton scavenging effect in a wide range of concentrations and temperature. We explain the large effective proton scavenging in ice by the tendency of ice formation to concentrate the impurities to confined volumes in order to minimize the ice crystal energy.

Temperature Dependence of Excited-State Proton Transfer in Water Electrolyte Solutions and Water−Methanol Solutions

The reversible proton dissociation and geminate recombination of a photoacid is studied as a function of temperature in water electrolyte solutions and binary water-methanol mixtures, containing 0.1 and 0.2 mole fractions of methanol. 8-Hydroxypyrene-1,3,6-trisulfonate trisodium salt (HPTS) is used as the photoacid. The experimental data are analyzed by the reversible geminate recombination model. We found that the slope of the logarithm of the proton-transfer rate constant as a function of the inverse of temperature (Arrhenius plot) in the liquid phase of these samples are temperature-dependent, while in the solid phase, the slope is nearly constant. The slope of the Arrhenius plot in frozen electrolyte solution is larger than that of the water-methanol mixtures, which is about the same as in pure water. Careful examination of the time-resolved emission in ice samples shows that the fit quality using the geminate recombination model is rather poor at relatively short times. We were able to get a better fit using an inhomogeneous kinetics model assuming the proton-transfer rate consists of a distribution of rates. The model is consistent with an inhomogeneous frozen water distribution next to the photoacid.

Effect of Electrolytes on the Excited-State Proton Transfer and Geminate Recombination

Time-resolved emission and steady-state fluorescence techniques are used to study the excited-state intermolecular proton transfer from 8-hydroxypyrene-1,3,6-trisulfonate (HPTS or pyranine) to water in the presence of inert salts, NaCl and MgCl(2). At low salt concentrations, up to about 0.5 M MgCl(2) or about 0.8 M NaCl, the time-resolved emission of both the photoacid and conjugate base can be quantitatively fitted by our diffusion-assisted geminate recombination model. In this concentration range, the proton transfer and geminate recombination rate constants are almost independent of the salt concentrations whereas the proton diffusion constant decreases as the salt concentration increases. At higher salt concentrations, the proton-transfer rate constant decreases while the recombination rate constant increases slightly. For the saturated solution of MgCl(2) (about 5 M at room temperature), the steady-state emission consists of only a single band of the protonated photoacid. Careful examination of the time-resolved emission of HPTS in the presence of a large concentration of MgCl(2) shows that the quality of the fit to the geminate recombination model is rather poor and we fail to find adjustable parameters for a good quality fitting. For this large concentration range of MgCl(2) we were able to get a good fit of the experimental data with a model based on a distribution of proton-transfer rates. The model is consistent with an inhomogeneous water environment next to the excited HPTS molecule in such concentrated solutions.

Dispersive photoluminescence decay by geminate recombination in amorphous semiconductors

The photoluminescence decay in amorphous semiconductors is described by power law $t^{-delta}$ at long times. The power-law decay of photoluminescence at long times is commonly observed but recent experiments have revealed that the exponent, $delta sim 1.2-1.3$, is smaller than the value 1.5 predicted from a geminate recombination model assuming normal diffusion. Transient currents observed in the time-of-flight experiments are highly dispersive characterized by the disorder parameter $alpha$ smaller than 1. Geminate recombination rate should be influenced by the dispersive transport of charge carriers. In this paper we derive the simple relation, $delta = 1+ alpha/2 $. Not only the exponent but also the amplitude of the decay calculated in this study is consistent with measured photoluminescence in a-Si:H.

High-electric-field quantum yield roll-off in efficient europium chelates-based light-emitting diodes

The europium chelates-based light-emitting-diodes (LEDs) have been fabricated showing the maximum electroluminescence (EL) quantum efficiency (QE) up to 5%photons∕carrier at electric fields slightly below F=1MV∕cm and current density j≅0.01mA∕cm2. Their line-like emission QE drops, however, rapidly within the high drive voltage range. This roll-off effect, exceeding one order of magnitude at F≅1.6MV∕cm(j≅15mA∕cm2), is shown to be underlain by the electric field-assisted dissociation of electron-hole pair precursors of europium ion-localized emissive states. The high field dependence of EL QE fits the Onsager model of geminate recombination well. This is at variance with triplet-triplet annihilation mechanism assumed previously to reduce QE in such LEDs.

On the validity of the one-particle diffusion model of geminate recombination in micelles

Diffusion and recombination of a pair of geminate particles in a micelle is usually modeled by letting one particle act as a fixed sink in the center of the micelle while the other is diffusing with the relative diffusion coefficient. In order to test whether this simplified model is realistic we have performed a series of Monte Carlo calculations for situations where either both particles are moving or one is fixed at the micellar boundary. We find that the short time evolution, which is essentially a free diffusion, is well described by the center model. The long time behavior during the quasi-equilibrium depends on the model but can be made to agree by a rescaling of the reaction constant. This is explained as a geometrical model dependence of the reencounter probability. Essential differences exist in the transition period between the two time scales. The center model predicts that the rate of recombination has a very sharp and well defined transition from the algebraic time dependence, characteristic of free diffusion, to the exponential dependence found in the quasi-equilibrium. This behavior is not found in the other models.

Dispersive geminate recombination in a conjugated polymer

Decay of delayed fluorescence (DF) originating from geminate electron–hole pair recombination in a thin film of a conjugated polymer, a ladder-type methyl-substituted poly (para-phenylene) (MeLPPP), has been measured as a function of time at different temperatures. On the time scale of 10 −4–10 −2 s an algebraic law, DF( t)∼ t − n , is approached, where n changes from 1.1 at 83 K to about 2.7 at room temperatures. Experimental results are in a good agreement with the developed theoretical model of geminate recombination of charge carriers under the premise that the transport is dispersive.

Theory of thermoluminescence gamma dose response: The unified interaction model

We describe the development of a comprehensive theory of thermoluminescence (TL) dose response, the unified interaction model (UNIM). The UNIM is based on both radiation absorption stage and recombination stage mechanisms and can describe dose response for heavy charged particles (in the framework of the extended track interaction model – ETIM) as well as for isotropically ionising gamma rays and electrons (in the framework of the TC/LC geminate recombination model) in a unified and self-consistent conceptual and mathematical formalism. A theory of optical absorption dose response is also incorporated in the UNIM to describe the radiation absorption stage. The UNIM is applied to the dose response supralinearity characteristics of LiF:Mg,Ti and is especially and uniquely successful in explaining the ionisation density dependence of the supralinearity of composite peak 5 in TLD-100. The UNIM is demonstrated to be capable of explaining either qualitatively or quantitatively all of the major features of TL dose response with many of the variable parameters of the model strongly constrained by ancilliary optical absorption and sensitisation measurements.

Time-Resolved Study of the Inner Space of Lactose Permease

Pyranine (8-hydroxy pyrene-1,3,6-trisulfonate) is a commonly used photoacid that discharges a proton when excited to its first electronic singlet state. Follow-up of its dissociation kinetics reveals the physicochemical properties of its most immediate environment. At vanishing ionic strength the dye adsorbs to the Escherichia coli lactose permease with stoichiometry of 1:1 and an association constant of 2.5 × 10 5 M −1. The reversal of the binding at high ionic strength and the lower pK value of the bound dye imply that positive charge(s) stabilize the dye in its site. The fluorescence decay curve of the bound dye was measured by time-correlated single photon counting and the measured transient was subjected to kinetic analysis based on the geminate recombination model. The analysis indicated that the binding domain is a cleft (between 9 and 17 Å deep) characterized by low activity of water ( a (water) = 0.71), reduced diffusivity of protons, and enhanced electrostatic potential. The binding of pyranine and a substrate are not mutually exclusive; however, when the substrate is added, the dye-binding environment is better solvated. These properties, if attributed to the substrate-conducting pathway, may explain some of the forces operating on the substrate in the cavity. The reduced activities of the water strips the substrate from some of its solvation water molecules and replace them by direct interaction with the protein. In parallel, the lower dielectric constant enhances the binding of the proton to the protein, thus keeping a tight seal that prevents protons from diffusing.

Photogeneration in N-carbazolyl-substituted polysilanes and their charge-transfer complexes with 2,4,7-trinitrofluorenone

Photoconductivity studies of N-carbazolyl-substituted silane homo- and copolymers and their charge-transfer complexes with 2,4,7-trinitrofluorenone (TNF) are reported. The quantum yield of photogeneration was investigated by means of xerographic discharge method. The photogeneration yields and their dependencies on electric field were analysed in terms of the Onsager model of geminate recombination and they were compared with previously obtained results for poly(N-vinylcarbazole), pure and doped with TNF.

Analysis of electric-field assisted photogeneration in polyparacyclophane doped with 2,4,7-trinitrofluorenone

A new photoconducting polymer with extended cooperative electronics effects, poly((E,E)-[6.2]paracyclophane-1,5-diene) (PDE), doped with a low molecular weight organic acceptor, 2,4,7-trinitrofluorenone (TNF), was investigated by means of steady-state photoconductivity and xerographic discharge methods. The photogeneration quantum yield in the PDE+TNF charge transfer (CT) complex for blue light is comparable with the photogeneration yield in poly(N-vinylcarbazole) (PVK) doped with TNF. It is demonstrated that the electric-field dependence of the photogeneration quantum yield in PDE+TNF systems can be described by the Braun kinetic model of dissociation of the CT complex, and not by the Onsager model of geminate recombination, which is appropriate for PVK+TNF systems.

Mechanism of carrier photogeneration in amorphous selenium: Fast transient photoconductivity.

Studies of fast transient photoconductivity in amorphous selenium reveal two distinct transport mechanisms. The first is a short-lived one due to carrier dynamics at extended band states and possibly states near the band edges where the carriers tunnel progressively into lower states. The second is a long-lived one due to phonon assisted multiple trapping at band tails. We find the quantum efficiency in this prototypic low-mobility system independent of temperature and electric field up to the maximum applied field of 5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ V/cm, inconsistent with previous models of carrier photogeneration such as the Onsager geminate recombination model. In order to reconcile the controversial issue of the carrier photogeneration mechanism in this material, we examine in detail the processes underlying various transient photoconductivity measurements. Our analysis shows that in time-of-flight and xerographic-discharge measurements the external field may modify the extent of carrier recombination and thereby determines the carrier supply yield, whereas measurements utilizing the microstripline Auston switch configuration are better suited for investigating the intrinsic properties of the quantum efficiency. \textcopyright{} 1996 The American Physical Society.

Quantum efficiency of geminate recombination under dispersive transport conditions

The Onsager model of geminate recombination is generalized to the case of dispersive hopping transport in amorphous materials. Both thermally stimulated over-barrier and tunneling jumps are taken into account as possible modes of carrier drift and diffusion in the Coulomb electric field and external electric field. Two possible channels of recombination are considered: field-stimulated carrier drift to the Coulomb center and tunneling recombination.