Geminate Electron-hole Recombination Research Articles

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends.

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

Geminate electron-hole recombination in organic photovoltaic cells. A semi-empirical theory.

We propose a semi-empirical theory which describes the geminate electron-hole separation probability in both homogeneous systems and donor-acceptor heterojunction systems applicable in organic photovoltaics. The theory is based on the results of extensive simulation calculations, which were carried out using various lattice models of the medium and different charge-carrier hopping mechanisms, over the parameter ranges typical for organic solar cells. It is found that the electron-hole separation probability can be conveniently described in terms of measurable parameters by a formula whose functional form is derived from the existing recombination theories, and which contains only one empirical parameter. For homogeneous systems, this parameter is determined by the structure of the medium and only weakly depends on the charge-carrier hopping mechanism. In the case of donor-acceptor heterojunction systems, this empirical parameter shows a simple power-law dependence on the product of the dielectric constant and inter-molecular contact distance. We also study the effect of heterojunction structure on the electron-hole separation probability and show that this probability decreases with increasing roughness of the heterojunction. By analyzing the simulation results obtained for systems under the influence of an external electric field, we find that the field effect on the electron-hole separation probability in donor-acceptor heterojunction systems is weaker than in homogeneous systems. We also describe this field effect by a convenient empirical formula.

Temperature- and Energy-Dependent Separation of Charge-Transfer States in PTB7-Based Organic Solar Cells

A decisive factor for the performance of organic bulk heterojunction solar cells is the competition between charge separation and geminate electron–hole recombination through a charge-transfer (CT) state after exciton separation across the heterointerface. By time-resolving the near-infrared emission of the high-performance low-bandgap PTB7:PC71BM material system, we selectively study the properties of CT states formed after optical excitation with a femtosecond laser pulse. We observe that the CT emission yield is higher after 705 nm excitation of the polymer rather than after 400 nm excitation of the fullerene, which can be attributed to better charge separation for excitons generated in fullerene aggregates. Additionally, the CT states are weakly bound with their emission not far red-shifted from that of the exciton. Examining the time-resolved CT emission from room temperature to 10 K, we observe changes in CT state lifetime with energy and temperature, indicative of CT state separation effectively co...

Geminate-pair dissociation yield in systems with exponential energetic disorder — A Monte Carlo study

Geminate electron–hole recombination in systems with exponential energetic disorder is studied by Monte Carlo method. The field and temperature dependencies of geminate-pair dissociation probability are calculated. It is established that the dissociation yield of carrier pairs depends mainly on the extent of carrier thermalization, which influences the Einstein relationship. The approximate limiting temperature is given by Te=ε0/2kB, where ε0 determines the decay rate of exponential distribution of localized states. For systems in approximate thermal equilibrium, at T>Te, the yield of free carriers is almost independent of disorder degree and its temperature dependence at zero field has an Arrhenius form. These simulation results are well described by classical Onsager theory. For non-equilibrium systems, at T<Te, the free-carrier yield significantly increases with growing disorder and temperature dependence of zero-field carrier yield is sub-Arrhenius one. At sufficiently low temperatures the carrier yield approaches constant value. These results are described with good accuracy by modified Onsager theory, with T replaced by Te. It is concluded that experimental investigations of geminate-carrier dissociation yield in disordered materials may be a valuable tool for verifying the Einstein relation and determining the energy distribution of localized states.

Effects of Energetic Disorder and Mobility Anisotropy on Geminate Electron-hole Recombination in the Presence of a Donor-Acceptor Heterojunction

Geminate electron-hole recombination in organic solids in the presence of a donor-acceptor heterojunction is studied by computer simulations. We analyze how the charge-pair separation probability in such systems is affected by energetic disorder of the media, anisotropy of charge-carrier mobilities, and other factors. We show that in energetically disordered systems the effect of heterojunction on the charge-pair separation probability is stronger than that in idealized systems without disorder. We also show that a mismatch between electron and hole mobilities reduces the separation probability, although in energetically disordered systems this effect is weaker compared to the case of no energetic disorder. We demonstrate that the most important factor that determines the charge-pair separation probability is the ratio of the sum of electron and hole mobilities to the rate constant of recombination reaction. We also consider systems with mobility anisotropy and calculate the electric field dependence of the charge-pair separation probability for all possible orientations of high-mobility axes in the donor and acceptor phases. We theoretically show that it is possible to increase the charge-pair separation probability by controlling the mobility anisotropy in heterojunction systems and in consequence to achieve higher efficiencies of organic photovoltaic devices.

Geminate electron-hole recombination in organic solids in the presence of a donor-acceptor heterojunction

Geminate electron-hole recombination is one of the main factors limiting the efficiency of organic solar cells. We present a systematic study of this process based on both analytical and simulation models. We determine how the charge-pair separation probability is affected by the hopping length of charge carriers, the presence of a donor-acceptor heterojunction, electron and hole mobilities, and other factors. We show that the charge-pair separation probability of an electron and a hole which are initially at the contact distance is maximized when the electron and hole mobilities are equal to each other.

Control of Electric Field Strength and Orientation at the Donor–Acceptor Interface in Organic Solar Cells

Electrical doping is essential to achieve efficient photocurrent extraction in small-molecular-weight organic solar cells. It is shown that such doping creates strong electric fields at the donor-acceptor interface that prevent geminate electron–hole recombination.

Charge carrier photogeneration and recombination in ladder-type poly(para-phenylene): Interplay between impurities and external electric field

Charge carrier generation and decay in $m$-LPPP polymer films were examined by means of femtosecond transient absorption spectroscopy in the time window of $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}--15\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. Two modes of polaron formation with distinct behavior were identified, impurity induced in the absence of an external electric field and electric field induced in pristine film. While field induced charge generation is relatively slow, occurring throughout the excited state lifetime, the rate of impurity induced charge generation is much faster and depends on excitation wavelength; it occurs on the several hundred femtosecond time scale under excitation within the main absorption band, but excitation into the red wing of the absorption band results in charge generation within less than $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. Polaron decay through geminate electron-hole recombination occurs with widely distributed lifetimes, from $\ensuremath{\sim}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ to microseconds; the polarons characterized by the shortest decay time have a redshifted absorption spectrum (as compared to more long-lived polarons) and are attributed to tightly bound polaron pairs.

Geminate pair recombination in a conjugated polymer

Upon laser excitation into the S1→S0 transition, delayed luminescence from methyl-substituted ladder-type poly-para-phenylene has been observed. It varies linearly with light intensity, decays according to a power law, IDF∝t−0.8±0.1, and its spectrum consists of an intrinsic and a defect fluorescence band as well as a phosphorescence component. Since the delayed fluorescence part can be quenched by an electric field after excitation, but not the phosphorescence, it must originate from geminate electron–hole recombination rather than from triplet–triplet annihilation.

Femtosecond studies of interfacial electron-hole recombination in aqueous CdS colloids

We report the measurements of the electron-hole recombination rate in aqueous CdS colloids on the femtosecond time scale. A fast 2-ps decay and a slower 45-ps decay have been observed, and the decay dynamics are found to be very sensitive to the excitation laser intensity. Geminate electron-hole recombination, responsible for the 45-ps decay, dominates the decay process at low excitation intensities, while nongeminate recombination, accounting for the 2-ps decay, becomes more important at high excitation intensities.

Simulation of geminate ion recombination kinetics in anisotropic media

A Monte Carlo random flights technique is presented for simulating ion-pair recombination kinetics in systems with diffusional and dielectric anisotrophy, and is applied to geminate electron—hole recombination in anthracene and naphthalene. While the importance of the anisotropy is evident in the dependence of the escape probability on the orientation of the initial interion vector, it is found to be more significant in determining the time-scale of the reaction kinetics, which can vary by a factor of up to three in the systems studied. Comparisons for pairs with the same escape probability indicate that reaction in anthracene is likely to be considerably faster than in naphthalene.

Femtosecond Studies of Photoinduced Electron Dynamics at the Liquid-Solid Interface of Aqueous CdS Colloids

We report the direct measurements of the dynamics of photoinduced electrons at the liquid-solid interface of aqueous CdS colloids on the femtosecond time scale. The observed transient absorption is attributed to electrons trapped at the liquid-solid interface. We show that electron trapping due to surface states or defect sites occurs in less than 100 fs. The trapped electrons then decay by a double exponential with time constants of 2-3 ps and 50 ps at high excitation intensities, while a single-exponential decay (50 ps) is observed at low intensities. The slow, 50-ps decay is attributed primarily to geminate electron-hole recombination, which dominates the decay dynamics at low excitation intensities

Subpicosecond transient absorption study of the UV two-photon excitation of liquid alkanes

Liquid n-alkanes (from C 5 to C 16 ), as well as selected cyclic and branched alkanes, were irradiated with 0.4-ps pulses of 10 GW/cm 2 intensity at 248.5 nm. Transient absorption signals at 497 nm were recorded in pump-and-probe experiments. The observed time profiles are attributed to the result of UV two-photon excitation. The properties of these profiles appear incompatible with the assumption of ultrafast (subpicosecond) recombination of geminate electron-ion pairs. Instead, slower time scales for geminate electron-hole recombination are determined, such as those derived from pulse radiolysis scavenger studies

Femtosecond study of geminate electron–hole recombination in neat alkanes

The first measurement of the geminate electron–hole recombination times of two neat octane isomers has been studied using femtosecond techniques. The recombination times were found to be 2.2±1.0 ps in n-octane and 470±40 fs in iso-octane, thus showing that the rates are strongly dependent on the structure of the alkane solvents.

Luminescence study of geminate recombination in glassy diphenylanthracene and polyvinylcarbazole

The decay of delayed luminescence originating from geminate electron—hole recombination in polyvinylcarbazole and amorphous 9,10-diphenylanthracene is found to obey at t −3 law for 50 μs < t < 5 ms and 78 < T < 296 K (0.7 < s < 1.3). Computer simulations show that it reflects the dispersive random walk of the electron—hole pair controlled by tunneling transitions.

Theory of radiative recombination by diffusion and tunneling in amorphous Si:H

A theory for geminate electron-hole recombination in amorphous semiconductors is given, which includes the effects of both diffusion and tunneling. The exact solution of the model is obtained neglecting the Coulomb interaction, which is included later in the prescribed diffusion approximation. It is shown that any combination of diffusion and tunneling will lead to a ${t}^{\ensuremath{-}\frac{3}{2}}$ long-time behavior for the reaction rate. The model is used to carry out an analysis of the photoluminescence decay in plasma-deposited amorphous Si:H as a function of temperature. The ${t}^{\ensuremath{-}\frac{3}{2}}$ long-time decay is observed at intermediate temperatures ($T\ensuremath{\sim}150$ K), and the calculated luminescence quenching is in good agreement with experiment. The initial thermalized pair distribution function of electron-hole separations obtained from the low-temperature ($T=8$ K) luminescence data is used to determine the Onsager photogeneration efficiency at room temperature. The calculations are in good agreement with the corresponding quantum efficiency obtained from recent xerographic measurements on the same material.

A geminate recombination model for photoluminescence decay in plasma-deposited amorphous Si:H

Abstract We develop a model of geminate electron-hole recombination, including tunnelling and diffusion, to account for photoluminescence decay in amorphous semiconductors. The model correctly predicts the shape of the decay, the luminescence quantum efficiency, and the microscopic electron mobility.