Monte Carlo Simulations of Charge Transport in 2D Organic Photovoltaics

Efficient evaluation of Coulomb interactions in kinetic Monte Carlo simulations of charge transport

The application of predictive and reliable modeling techniques for the simulation of charge transport in functional materials is an essential step for the development of advanced platforms for electronics, optoelectronics, and photovoltaics. In this context, kinetic Monte Carlo (KMC) methods have emerged as a valuable tool, especially for the simulation of systems where charge transport can be described by the hopping of charge carriers across localized quantum states, as, for example, in organic semiconductor materials. The accuracy, computational efficiency, and reliability of KMC simulations of charge transport, however, crucially depend on the methods and approximations used to evaluate electrostatic interactions arising from the distribution of charges in the system. The long-range nature of Coulomb interactions and the need to simulate large model systems to capture the details of charge transport phenomena in complex devices lead, typically, to a computational bottleneck, which hampers the application of KMC methods. Here, we propose and assess computational schemes for the evaluation of electrostatic interactions in KMC simulations of charge transport based on the locality of the charge redistribution in the hopping regime. The methods outlined in this work provide an overall accuracy that outperforms typical approaches for the evaluation of electrostatic interactions in KMC simulations at a fraction of the computational cost. In addition, the computational schemes proposed allow a spatial decomposition of the evaluation of Coulomb interactions, leading to an essentially linear scaling of the computational load with the size of the system.

Enhancing Photoluminescence Quenching in Donor-Acceptor PCE11:PPCBMB Films through the Optimization of Film Microstructure

We show that a precise control of deposition speed during the fabrication of polyfullerenes and donor polymer films by convective self-assembly leads to an optimized film microstructure comprised of interconnected crystalline polymer domains comparable to molecular dimensions intercalated with similar polyfullerene domains. Moreover, in blended films, we have found a correlation between deposition speed, the resulting microstructure, and photoluminescence quenching. The latter appeared more intense for lower deposition speeds due to a more favorable structuring at the nanoscale of the two donor and acceptor systems in the resulting blend films.

Efficient, non-stochastic, Monte-Carlo-like-accurate method for the calculation of the temperature-dependent mobility in nanocrystal films

We present a new non-stochastic framework for the calculation of the temperature dependence of the mobility in nanocrystal films, that enables speed-ups of several orders of magnitude compared to conventional Monte Carlo approaches, while maintaining a similar accuracy. Our model identifies a new contribution to the reduction of the mobility with increasing temperature in these systems (conventionally attributed to interactions with phonons), that alone is sufficient to explain the observed experimental trend up to room temperature. Comparison of our results with the theoretical predictions of the hopping model and the observed temperature dependence of recent field-effect mobility measurements in nanocrystal films, provides the means to discriminate between band-like and hopping transport and a definitive answer to whether the former has been achieved in quantum dot films.

Simulating charge transport in organic semiconductors and devices: a review

Charge transport simulation can be a valuable tool to better understand, optimise and design organic transistors (OTFTs), photovoltaics (OPVs), and light-emitting diodes (OLEDs). This review presents an overview of common charge transport and device models; namely drift-diffusion, master equation, mesoscale kinetic Monte Carlo and quantum chemical Monte Carlo, and a discussion of the relative merits of each. This is followed by a review of the application of these models as applied to charge transport in organic semiconductors and devices, highlighting in particular the insights made possible by modelling. The review concludes with an outlook for charge transport modelling in organic electronics.

Thermal effect on the morphology and performance of organic photovoltaics

The morphology of organic photovoltaics (OPVs) is a significant factor in improving performance, and establishing a method for controlling morphology is necessary.

Influence of Molecular Conformations and Microstructure on the Optoelectronic Properties of Conjugated Polymers

It is increasingly obvious that the molecular conformations and the long-range arrangement that conjugated polymers can adopt under various experimental conditions in bulk, solutions or thin films, significantly impact their resulting optoelectronic properties. As a consequence, the functionalities and efficiencies of resulting organic devices, such as field-effect transistors, light-emitting diodes, or photovoltaic cells, also dramatically change due to the close structure/property relationship. A range of structure/optoelectronic properties relationships have been investigated over the last few years using various experimental and theoretical methods, and, further, interesting correlations are continuously revealed by the scientific community. In this review, we discuss the latest findings related to the structure/optoelectronic properties interrelationships that exist in organic devices fabricated with conjugated polymers in terms of charge mobility, absorption, photoluminescence, as well as photovoltaic properties.

Efficient Computational Screening of Organic Polymer Photovoltaics

There has been increasing interest in rational, computationally driven design methods for materials, including organic photovoltaics (OPVs). Our approach focuses on a screening "pipeline", using a genetic algorithm for first stage screening and multiple filtering stages for further refinement. An important step forward is to expand our diversity of candidate compounds, including both synthetic and property-based measures of diversity. For example, top monomer pairs from our screening are all donor-donor (D-D) combinations, in contrast with the typical donor-acceptor (D-A) motif used in organic photovoltaics. We also find a strong "sequence effect", in which the average HOMO-LUMO gap of tetramers changes by ∼0.2 eV as a function of monomer sequence (e.g., ABBA versus BAAB); this has rarely been explored in conjugated polymers. Beyond such optoelectronic optimization, we discuss other properties needed for high-efficiency organic solar cells, and applications of screening methods to other areas, including non-fullerene n-type materials, tandem cells, and improving charge and exciton transport.