Morphology of organic solar cell materials - studied with X-ray scattering techniques

Michael Korning Sørensen

Research output: Book/ReportPh.D. thesis

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Abstract

The global energy demand is constantly increasing. The development of technologies to transit from fossil fuels to green electricity production is one of the biggest challenges facing humanity in this century. Organic solar cells represent a promising technology to meet this demand owing to attributes such as flexibility, low weight, environmental sustainability, and short energy payback time when mass-produced. State-of-the-art organic solar cells reach power conversion efficiencies comparable with standard silicon based solar cells. Unfortunately, such organic solar cells are obtained from small areas (less than 1 cm2) and in laboratory settings with impractical techniques for large-scale production. It is generally understood that when transferring from small scale (e.g., spin coating) to large scale roll-to-roll slot-die coating, the efficiency of the solar cell is halved, i.e., from 18 % to around 5-10 %. Additionally, the lifetime of such devices is too short (maximum 2-3 years compared to 25 years for silicon based cells). Low efficiency and short lifetime are the two key parameters scientists need to understand in-depth to push this technology further forward. Important physical properties such as power conversion efficiency, fill factor, charge carrier mobility, and optical absorption are directly affected by the morphology of the active layer. Several processing parameters and material properties affect the morphology of a bulk heterojunction. Interdisciplinary cooperation is required to fully understand the correlation between morphology and physical properties, including advanced theoretical modeling, experimental characterization, and consistency in large-scale fabrication.

The aim of this thesis was to investigate the relationship between the morphology and physical properties of roll-to-roll slot-die coated solar cells. Through various grazing incidence scattering methods, the probed morphologies were correlated either with molecular dynamics simulation or with physical properties from either the active layer or the entire solar cell. To achieve this, we collaborated with theorists and fabrication experts to elucidate the structure-property relationship based on X-ray and neutron experiments. We started by investigating the nanostructure of the individual constituents, i.e., donor and acceptor, to form the basis for probing and analyzing the drying kinetics of the bulk heterojunction. In this thesis, the acceptors O-IDTBR and EH-IDTBR and the two donors, P3HT and PffBT4T-2OD, were chosen. Through GIWAXS, the morphology of the two acceptors was mapped, and the framework to correlate the scattering data with molecular dynamics simulations was built. A method to probe drying kinetics of the two blends of P3HT:O-IDTBR and P3HT:EH-IDTBR while roll-2-roll coating at an in-house X-ray source was demonstrated. Further development of the experimental setup was designed and installed at a synchrotron to perform both in situ GIWAXS and GISAXS to probe the drying dynamics of P3HT:O-IDTBR and PffBT4T-20D while roll-2-roll slot-die coating. Based on ex situ GISAXS experiments, we suggest a method to correlate the structure-property of P3HT:O-IDTBR. Additionally, a novel method to manipulate the structure of polymers with visible light was developed and demonstrated how it can be applied in the large-scale processing of polymers relevant for both organic solar cells and flexible electronics.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages151
Publication statusPublished - 2022

Bibliographical note

This PhD project is two-thirds funded by the European Commission’s Horizon 2020 program carrying the title SEEWHI (Solar Energy Enabled for the World by High-Resolution Imaging) consolidator grant ERC-2015-CoG-681881 and one-third funded by DTU Energy. All travel expenses associated with beamtimes were founded by DanScatt.

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