Efficient Generation of Model Bulk Heterojunction Morphologies for Organic Photovoltaic Device Modeling

Kinetic Monte Carlo (KMC) simulations have been previously used to model and understand a wide range of behaviors in bulk heterojunction (BHJ) organic photovoltaic devices, from fundamental mechanisms to full device performance. One particularly unique and valuable aspect of this type of modeling technique is the ability to explicitly implement models for the bicontinuous nanostructured morphology present in these devices. For this purpose, an Ising-based method for creating model BHJ morphologies has become prevalent. However, this technique can be computationally expensive, and a detailed characterization of this method has not yet been published. Here, we perform a thorough characterization of this method and describe how to efficiently generate controlled model BHJ morphologies. We show how the interaction energy affects the tortuosity of the interconnected domains and the resulting charge transport behavior in KMC simulations. We also demonstrate how to dramatically reduce calculation time by several orders of magnitude without detrimentally affecting the resulting morphologies. In the end, we propose standard conditions for generating model morphologies and introduce a new open-source software tool. These developments to the Ising method provide a strong foundation for future simulation and modeling of BHJ organic photovoltaic devices that will lead to a more detailed understanding of the important link between morphological features and device performance.Comment: Main article: 9 pages, 6 figures, Supplementary Information: 6 pages, 6 figure

Electrical Conductivity of Doped Organic Semiconductors Limited by Carrier-Carrier Interactions

High electrical conductivity is a prerequisite for improving the performance of organic semiconductors for various applications and can be achieved through molecular doping. However, often the conductivity is enhanced only up to a certain optimum doping concentration, beyond which it decreases significantly. We combine analytical work and Monte Carlo simulations to demonstrate that carrier-carrier interactions can cause this conductivity decrease and reduce the maximum conductivity by orders of magnitude, possibly in a broad range of materials. Using Monte Carlo simulations, we disentangle the effect of carrier-carrier interactions from carrier-dopant interactions. Coulomb potentials of ionized dopants are shown to decrease the conductivity, but barely influence the trend of conductivity versus doping concentration. We illustrate these findings using a doped fullerene derivative for which we can correctly estimate the carrier density at which the conductivity maximizes. We use grazing-incidence wide-angle X-ray scattering to show that the decrease of the conductivity cannot be explained by changes to the microstructure. We propose the reduction of carrier-carrier interactions as a strategy to unlock higher-conductivity organic semiconductors

Thermal receptivity of free convective flow from a heated vertical surface: linear waves

Numerical techniques are used to study the receptivity to small-amplitude thermal disturbances of the boundary layer flow of air which is induced by a heated vertical flat plate. The fully elliptic nonlinear, time-dependent Navier–Stokes and energy equations are first solved to determine the steady state boundary-layer flow, while a linearised version of the same code is used to determine the stability characteristics. In particular we investigate (i) the ultimate fate of a localised thermal disturbance placed in the region near the leading edge and (ii) the effect of small-scale surface temperature oscillations as means of understanding the stability characteristics of the boundary layer. We show that there is a favoured frequency of excitation for the time-periodic disturbance which maximises the local response in terms of the local rate of heat transfer. However the magnitude of the favoured frequency depends on precisely how far from the leading edge the local response is measured. We also find that the instability is advective in nature and that the response of the boundary layer consists of a starting transient which eventually leaves the computational domain, leaving behind the large-time time-periodic asymptotic state. Our detailed numerical results are compared with those obtained using parallel flow theory

MikeHeiber/KMC_Lattice_example: KMC_Lattice_example v1.1-beta

This object-oriented C++ software tool provides a simple demonstration of how to use the KMC_Lattice package to create a lattice kinetic Monte Carlo simulation. This example shows how to extend the base classes in the KMC_Lattice package to simulate exciton creation, diffusion, and decay in an organic semiconducting material. See the readme file for more information. Changes in v1.1-beta: This update contain a number of adjustments that now makes it simpler to construct a lattice KMC simulation updated code to work with new KMC_Lattice update, v1.1-alpha.2 corrected event queue handling so that events are chosen based on their calculated execution time instead of their calculated wait time simplified implementation of logfile output simplified pointer usage reduced the code needed to implement events removed static class variables that define the exciton properties, so that exciton properties are more flexible and can be assigned and accounted for in the Exciton_sim class when calculating the events, which allows different excitons in the simulation to have different propertie

Dynamic Monte Carlo modeling of exciton dissociation in organic donor acceptor solar cells

A general dynamic Monte Carlo model for exciton dissociation at a donor-acceptor interface that includes exciton delocalization and hot charge separation is developed to model the experimental behavior observed for the poly(3-hexylthiophene):fullerene system and predict the theoretical performance of future materials systems. The presence of delocalized excitons and the direct formation of separated charge pairs has been recently measured by transient photo-induced absorption experiments and has been proposed to facilitate charge separation. The excess energy of the exciton dissociation process has also been observed to have a strong correlation with the charge separation yield for a series of thiophene based polymer:fullerene systems, suggesting that a hot charge separation process is also occurring. Hot charge separation has been previously theorized as a cause for highly efficient charge separation. However, a detailed model for this process has not been implemented and tested. Here, both conceptual models are implemented into a dynamic Monte Carlo simulation and tested using a simple bilayer donor-acceptor system. We find that exciton delocalization can account for a significant reduction in geminate recombination when compared to the traditional, bound polaron pair model. In addition, the hot charge separation process could further reduce the geminate recombination, but only if the hot charge mobility is several orders of magnitude larger than the standard charge mobility

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

Development of a colloidal lithography method for patterning nonplanar surfaces

A colloidal lithography method has been developed for patterning nonplanar surfaces. Hexagonal noncontiguously packed (HNCP) colloidal particles 127 nm−2.7 μm in diameter were first formed at the air−water interface and then adsorbed onto a substrate coated with a layer of polymer adhesive 17 nm thick. The adhesive layer plays the critical role of securing the order of the particles against the destructive lateral capillary force generated by a thin film of water after the initial transfer of the particles from the air−water interface. The soft lithography method is robust and very simple to carry out. It is applicable to a variety of surface curvatures and for both inorganic and organic colloidal particles