Multiscale Modelling of Organic Solar Cell Materials – morphology evolution in solution processed bulk heterojunctions

Anders Skovbo Gertsen*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

Organic solar cells are presently used for niche applications due to their semi-transparency, flexibility, low weight, and possibilities of custom designs in terms of colours and shapes, but low efficiencies of large-scale fabricated modules have hampered grid implementations. However, with their low-cost solution processing and projected energy payback times of only fractions of those of silicon modules, the technology has a great potential to reach commercial viability within few years if the efficiency and lifetime can be improved. Important properties that directly affect the performance of organic solar cells such as charge carrier mobility and optical absorption are crucially dependent on the morphology of the active layer. These morphologies are in turn affected by a multitude of processing parameters and material properties, and advanced experimental techniques such as in situ X-ray scattering need to be applied to follow the blend microstructure formation during post-deposition drying. Computational modelling is often indispensable in the interpretation of these experiments, and it can furthermore provide a crucial link between structural studies and performance characteristics of devices. This thesis aims to investigate these structure-property relationships in solution processed organic functional material systems through sequential multiscale simulations combining density functional theory, atomistic molecular dynamics simulations, and coarse-grained molecular dynamics simulations. The simulation and analysis frameworks presented represent a systematic approach to obtain morphologies and structural properties of organic functional materials at experimentally relevant conditions and to infer the electronic properties that govern their function directly from these. This has been applied to state-of-the-art materials for organic transistors and organic solar cells. opls-aa atomistic force fields and martini 3.0 coarse-grained force fields have been developed for several donor polymers and small-molecule, non-fullerene acceptors as well as for the solvents from which they are processed. The atomistic models allow simulation of the dynamics and local interactions in materials where chemically specific interactions are decisive, and the coarse-grained models give access to the spatio-temporal resolution necessary to follow the morphology evolution in solvent evaporation simulations. Coupling these with quantum chemical calculations and kinetic Monte Carlo simulations can yield valuable insight into the structure-property relationships of these materials. Furthermore, the simulations provide molecular scale resolution, which fosters an intuitive insight into the nanostructure of the materials and enables easier interpretation of advanced experiments. With the present work, a general simulation and analysis framework for solution deposition of organic materials has been established, and the outlined future extensions are believed to hold the potential to accelerate computational design of materials and processing parameters for organic solar cells.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages171
Publication statusPublished - 2020

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