Charge Carrier Dynamics of Polymer:Fullerene Blends: From Geminate to Non‐Geminate Recombination

The charge carrier dynamics of a new polymer‐fullerene blend are examined on the femtosecond to the millisecond time scale. The full time range is globally fitted using a chemical reaction rate model that includes all key processes, charge generation, energy transfer, charge separation, and recombination, over the full 12 orders of magnitude in time and a factor of 33 in light intensity. Particular attention is paid to the charge recombination processes and it is found that they are highly material specific. Comparison of the dynamics to those of a previously studied polymer:fullerene blend reveals that while for one blend the recombination dynamics are mainly controlled by geminate recombination, the charge recombination in the presently studied polymer:fullerene blend are entirely controlled by non‐geminate electron‐hole recombination. Carrier density dependence of the non‐geminate recombination rate is analyzed and a correlated disorder model of site energies is proposed to explain the observed dependency.     

Improving Perovskite Solar Cells: Insights From a Validated Device Model

To improve the efficiency of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential. Here, a device model has been developed and validated that describes the operation of PSCs and quantitatively explains the role of contacts, the electron and hole transport layers, charge generation, drift and diffusion of charge carriers and recombination. The simulation to the experimental data of vacuum‐deposited CH₃NH₃PbI₃ solar cells over multiple thicknesses has been fit and the device behavior under different operating conditions has been studied to delineate the influence of the external bias, charge‐carrier mobilities, energetic barriers for charge injection/extraction and, different recombination channels on the solar cell performance. By doing so, a unique set of material parameters and physical processes that describe these solar cells is identified. Trap‐assisted recombination at material interfaces is the dominant recombination channel limiting device performance and passivation of traps increases the power conversion efficiency (PCE) of these devices by 40%. Finally, guidelines to increase their performance have been issued and it is shown that a PCE beyond 25% is within reach.     

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

The impact of trapping on the recombination dynamics in polymer:fullerene blends is clarified using the highly ordered bulk heterojunction (BHJ) blend poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) at different weight ratios as a model system. The recombination dynamics are determined using both transient charge extraction and steady‐state techniques. The results show that both the decay of photogenerated charge and the light ideality factor at a polymer:fullerene weight ratio of 1:4 are fully consistent with 2D Langevin recombination; in the 1:1 case the recombination is seen to be affected by electron trapping. The theory of 2D Langevin recombination is extended to the case with high trap density in agreement with the observations in the 1:1 case. The recombination capture coefficients are derived both for trap‐assisted and band‐to‐band recombination and it can be seen that anisotropic charge transport reduces the capture coefficients in both cases resulting in a reduced overall recombination.     

Concerted millisecond timescale dynamics in the intrinsically disordered carboxyl terminus of γ‐tubulin induced by mutation of a conserved tyrosine residue

Tubulins are an ancient family of eukaryotic proteins characterized by an amino‐terminal globular domain and disordered carboxyl terminus. These carboxyl termini play important roles in modulating the behavior of microtubules in living cells. However, the atomic‐level basis of their function is not well understood. These regions contain multiple acidic residues and their overall charges are modulated in vivo by post‐translational modifications, for example, phosphorylation. In this study, we describe an application of NMR and computer Monte Carlo simulations to investigate how the modification of local charge alters the conformational sampling of the γ‐tubulin carboxyl terminus. We compared the dynamics of two 39‐residue polypeptides corresponding to the carboxyl‐terminus of yeast γ‐tubulin. One polypeptide comprised the wild‐type amino acid sequence while the second contained a Y > D mutation at Y11 in the polypeptide (Y445 in the full protein). This mutation introduces additional negative charge at a site that is phosphorylated in vivo and produces a phenotype with perturbed microtubule function. NMR relaxation measurements show that the Y11D mutation produces dramatic changes in the millisecond‐timescale motions of the entire polypeptide. This observation is supported by Monte Carlo simulations that—similar to NMR—predict the WT γ‐CT is largely unstructured and that the substitution of Tyr 11 with Asp causes the sampling of extended conformations that are unique to the Y11D polypeptide.     

Electric Field and Mobility Dependent First‐Order Recombination Losses in Organic Solar Cells

The origin of photocurrent losses in the power‐generating regime of organic solar cells (OSCs) remains a controversial topic, although recent literature suggests that the competition between bimolecular recombination and charge extraction determines the bias dependence of the photocurrent. Here the steady‐state recombination dynamics is studied in bulk‐heterojunction OSCs with different hole mobilities from short‐circuit to maximum power point. It is shown that in this regime, in contrast to previous transient extracted charge and absorption spectroscopy studies, first‐order recombination outweighs bimolecular recombination of photogenerated charge carriers. This study demonstrates that the first‐order losses increase with decreasing slower carrier mobility, and attributes them to either mobilization of charges trapped at the donor:acceptor interface through the Poole–Frenkel effect, and/or recombination of photogenerated and injected charges. The dependence of both first‐order and higher‐order losses on the slower carrier mobility explains why the field dependence of OSC efficiencies has historically been attributed to charge‐extraction losses.     

Role of histidine for charge regulation of unstructured peptides at interfaces and in bulk

Histidine‐rich, unstructured peptides adsorb to charged interfaces such as mineral surfaces and microbial cell membranes. At a molecular level, we investigate the adsorption mechanism as a function of pH, salt, and multivalent ions showing that (1) proton charge fluctuations are—in contrast to the majority of proteins—optimal at neutral pH, promoting electrostatic interactions with anionic surfaces through charge regulation and (2) specific zinc(II)‐histidine binding competes with protons and ensures an unusually constant charge distribution over a broad pH interval. In turn, this further enhances surface adsorption. Our analysis is based on atomistic molecular dynamics simulations, coarse grained Metropolis Monte Carlo, and classical polymer density functional theory. This multiscale modeling provides a consistent picture in good agreement with experimental data on Histatin 5, an antimicrobial salivary peptide. Biological function is discussed and we suggest that charge regulation is a significant driving force for the remarkably robust activity of histidine‐rich antimicrobial peptides. Proteins 2014; 82:657–667. © 2013 Wiley Periodicals, Inc.     

Morphology‐Limited Free Carrier Generation in Donor/Acceptor Polymer Blend Solar Cells Composed of Poly(3‐hexylthiophene) and Fluorene‐Based Copolymer

The charge generation and recombination dynamics in polymer/polymer blend solar cells composed of poly(3‐hexylthiophene) (P3HT, electron donor) and poly[2,7‐(9,9‐didodecylfluorene)‐alt‐5,5‐(4′,7′‐bis(2‐thienyl)‐2′,1′,3′‐benzothiadiazole)] (PF12TBT, electron acceptor) are studied by transient absorption measurements. In the unannealed blend film, charge carriers are efficiently generated from polymer excitons, but some of them recombine geminately. In the blend film annealed at 160 °C, on the other hand, the geminate recombination loss is suppressed and hence free carrier generation efficiency increases up to 74%. These findings suggest that P3HT and PF12TBT are intermixed within a few nanometers, resulting in impure PF12TBT and disordered P3HT domains. The geminate recombination is likely due to charge carriers generated on isolated polymer chains in the matrix of the other polymer and at the domain interface with disordered P3HT. The undesired charge loss by geminate recombination is reduced by both the purification of the PF12TBT‐rich domain and crystallization of the P3HT chains. These results show that efficient free carrier generation is not inherent to the polymer/fullerene domain interface, but is possible with polymer/polymer systems composed of crystalline donor and amorphous acceptor polymers, opening up a new potential method for the improvement of solar cell materials.     

Evolution of Hot Polaron States with a Nanosecond Lifetime in a Manganite Perovskite

Understanding and controlling the relaxation process of optically excited charge carriers in solids with strong correlations is of great interest in the quest for new strategies to exploit solar energy. Usually, optically excited electrons in a solid thermalize rapidly on a femtosecond to picosecond timescale due to interactions with other electrons and phonons. New mechanisms to slow down thermalization will thus be of great significance for efficient light energy conversion, e.g., in photovoltaic devices. Ultrafast optical pump–probe experiments in the manganite Pr_0.65Ca_0.35MnO_3, a photovoltaic, thermoelectric, and electrocatalytic material with strong polaronic correlations, reveal an ultraslow recombination dynamics on a nanosecond‐time scale. The nature of long living excitations is further elucidated by photovoltaic measurements, showing the presence of photodiffusion of excited electron–hole polaron pairs. Theoretical considerations suggest that the excited charge carriers are trapped in a hot polaron state. Escape from this state is possible via a slow dipole‐forbidden recombination process or via rare thermal fluctuations toward a conical intersection followed by a radiation‐less decay. The strong correlation between the excited polaron and the octahedral dynamics of its environment appears to be substantial for stabilizing the hot polaron.     

Morphology and Efficiency: The Case of Polymer/ZnO Solar Cells

The performance of polymer solar cells critically depends on the morphology of the interface between the donor‐ and acceptor materials that are used to create and transport charge carriers. Solar cells based on poly(3‐hexylthiophene) and ZnO were fully characterized in terms of their efficiency and three‐dimensional (3D) morphology on the nanoscale. Here, we establish a quantitative link between efficiency and morphology by using the experimental 3D morphology as direct input for a 3D optoelectronic device model. This model includes the effects of exciton diffusion and quenching; space‐charge; recombination, generation, drift and diffusion of charge carriers; and the injection/extraction of carriers at the contacts. The observed trend in internal quantum efficiency as a function of layer thickness is reproduced with a single set of parameters. Several morphological aspects that determine the internal quantum efficiency are discussed and compared to other organic solar cells. This first direct use of morphological data in an optoelectronic device model highlights the importance of morphology in solar cells.     

Extracting Microscopic Device Parameters from Transient Photocurrent Measurements of P3HT:PCBM Solar Cells

By including trap‐limited recombination in a time‐domain drift‐diffusion model of a P3HT:PCBM solar cell, experimental transient photocurrent (TPC) measurements across the current–voltage (J–V) curve in both the light and the dark can be reproduced. Using the same set of model parameters, the steady‐state current–voltage curves and charge‐extraction data are also reproduced. The model is validated by predicting the recombination rate at open circuit and comparing results with the rate as measured by the transient photovoltage (TPV) measurement technique. It is demonstrated that the model, which incorporates transport, carrier trapping, carrier de‐trapping, and recombination, is able to reproduce device dynamics correctly. The fit of the model to the experimental results is improved by varying the shape of the electron and hole density of state (DoS) functions. By discretizing the DoS in energy space and allowing its shape to vary, the TPC transients can be closely fitted and the form of the tails of the electron and hole DoS extracted. It is found that the DoS of trapped carriers can be represented by a series of Gaussians offset in energy. This is tentatively attributed to electronic disorder introduced by molecular packing of the molecules.     

Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity

Here, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance.     

Beyond Langevin Recombination: How Equilibrium Between Free Carriers and Charge Transfer States Determines the Open‐Circuit Voltage of Organic Solar Cells

Organic solar cells lag behind their inorganic counterparts in efficiency due largely to low open‐circuit voltages (V_oc). In this work, a comprehensive framework for understanding and improving the open‐circuit voltage of organic solar cells is developed based on equilibrium between charge transfer (CT) states and free carriers. It is first shown that the ubiquitous reduced Langevin recombination observed in organic solar cells implies equilibrium and then statistical mechanics is used to calculate the CT state population density at each voltage. This general result permits the quantitative assignment of V_oc losses to a combination of interfacial energetic disorder, non‐negligible CT state binding energies, large degrees of mixing, and sub‐ns recombination at the donor/acceptor interface. To quantify the impact of energetic disorder, a new temperature‐dependent CT state absorption measurement is developed. By analyzing how the apparent CT energy varies with temperature, the interfacial disorder can be directly extracted. 63–104 meV of disorder is found in five systems, contributing 75–210 mV of V_oc loss. This work provides an intuitive explanation for why qV_oc is almost always 500–700 meV below the energy of the CT state and shows how the voltage can be improved.     

The Dot and Line Method: A Long Range Correction to Coulomb Interaction in a Cylindrical Pore

The charge line (CL) method had been used in the past to represent the periodic charges in Monte Carlo simulations of ions in a cylindrical pore. In this method, there exists a possible singularity when the edge of the image line overlaps with an ion in the central cylinder. This singularity is more problematic for molecular dynamics when the force is evaluated. Molecular dynamics simulations with the CL method have not been reported in the literature. By replacing the first section of the image charge line with an image point, we show that the CL method can be improved and be applicable in the molecular dynamics simulation of electrolytes in a cylindrical geometry. The modified method is demonstrated to be effective by simulations of a high packing primitive model electrolyte, representing the state of a molten salt.     

Reduced Recombination in High Efficiency Molecular Nematic Liquid Crystalline: Fullerene Solar Cells

Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground‐state decay of the CT state is significantly slower than dissociation.     

Statistical Properties of the Number of Recombination Events in the History of a Sample of DNA Sequences

Some statistical properties of samples of DNA sequences are studied under an infinite-site neutral model with recombination. The two quantities of interest are R, the number of recombination events in the history of a sample of sequences, and RM, the number of recombination events that can be parsimoniously inferred from a sample of sequences. Formulas are derived for the mean and variance of R. In contrast to R, RM can be determined from the sample. Since no formulas are known for the mean and variance of RM, they are estimated with Monte Carlo simulations. It is found that RM is often much less than R, therefore, the number of recombination events may be greatly under-estimated in a parsimonious reconstruction of the history of a sample. The statistic RM can be used to estimate the product of the recombination rate and the population size or, if the recombination rate is known, to estimate the population size. To illustrate this, DNA sequences from the Adh region of Drosophila melanogaster are used to estimate the effective population size of this species.     

Imaging Carrier Transport Properties in Halide Perovskites using Time‐Resolved Optical Microscopy

Halide perovskites have remarkable properties for relatively crudely processed semiconductors, including large optical absorption coefficients and long charge carrier lifetimes. Thanks to such properties, these materials are now competing with established technologies for use in cost‐effective and efficient light‐harvesting and light‐emitting devices. Nevertheless, the fundamental understanding of the behavior of charge carriers in these materials—particularly on the nano‐ to microscale—has, on the whole, lagged behind empirical device performance. Such understanding is essential to control charge carriers, exploit new device structures, and push devices to their performance limits. Among other tools, optical microscopy and spectroscopic techniques have revealed rich information about charge carrier recombination and transport on important length scales. In this progress report, the contribution of time‐resolved optical microscopy techniques to the collective understanding of the photophysics of these materials is detailed. The ongoing technical developments in the field that are overcoming traditional experimental limitations in order to visualize transport properties over multiple time and length scales are discussed. Finally, strategies are proposed to combine optical microscopy with complementary techniques in order to obtain a holistic picture of local carrier photophysics in state‐of‐the‐art perovskite devices.     

Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells

Charge transport in organic photovoltaic (OPV) devices is often characterized by space‐charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi‐equilibrium energies in the disorder‐broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time‐resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time‐resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum‐power point (short‐circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.     

Structural basis of hierarchical multiple substates of a protein. I: Introduction

A computer experiment of protein dynamics is carried out, which consists of two steps: (1) A Monte Carlo simulation of thermal fluctuations in the native state of a globular protein, bovine pancreatic trypsin inhibitor; and (2) a simulation of the quick freezing of fluctuating conformations into energy minima by minimization of the energy of a number of conformations sampled in the Monte Carlo simulation. From the analysis of results of the computer experiment is obtained the following picture of protein dynamics: multiple energy minima exist in the native state, and they are distributed in clusters in the conformational space. The dynamics has a hierarchical structure which has at least two levels. In the first level, dynamics is restricted within one of the clusters of minima. In the second, transitions occur among the clusters. Local parts of a protein molecule, side chains and local main chain segments, can take multiple locally stable conformations in the native state. Many minima result from combinations of these multiple local conformations. The hierarchical structure in the dynamics comes from interactions among the local parts. Protein molecules have two types of flexibility, each associated with elastic and plastic deformations, respectively.     

What is the Binding Energy of a Charge Transfer State in an Organic Solar Cell?

The high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge‐transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature‐dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron–hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield.     

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and improving the inherent material and structural defects. By inserting a porous piezoelectric poly(vinylidenefluoride‐trifluoroethylene) layer so as to generate a converging electric field, it is possible to modulate the junction properties and consequently enhance the charge carrier behavior at the junction. This study shows that due to a reduction in the recombination and an improvement in the carrier extraction, a 38% increase in the current density along with a concomitant increase of 37% in the power conversion efficiency of flexible quantum dots solar cells can be achieved by modulating the junction properties using the piezoelectric effect.