Manipulating organic semiconductor morphology with visible light
Michael Korning Sørensen b, Anders Skovbo Gertsen a, Rocco Peter Fornari a, Binbin Zhou a, Xiaotong Zhang a, Peter Uhd Jepsen a, Edoardo Stanzani a, Shinhee Yun a, Marcial Fernández Castro a, Matthias Schwartzkopf c, Alexandros Koutsioubas d, Piotr de Silva a, Moises Espindola Rodriguez b, Luise Theil Kuhn a, Jens Wenzel Andreasen a
a Technical University of Denmark, Anker Engelunds Vej, 1, Kongens Lyngby, Denmark
b FOM Technologies A/S
c Deutsches Elektronen-Synchrotron (DESY), Hamburg
d Jülich Center of Neutron Science, Lichtenberstrasse 1, Garching bei München, 85747, Germany
Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
#NewOPV - New concepts for stable non-fullerene based organic solar cells and their applications
VALÈNCIA, Spain, 2023 March 6th - 10th
Organizers: Vida Engmann, Morten Madsen and Pavel Troshin
Oral, Jens Wenzel Andreasen, presentation 304
DOI: https://doi.org/10.29363/nanoge.matsus.2023.304
Publication date: 22nd December 2022

We demonstrate a method to manipulate the morphology of thin film semiconducting polymers. The manipulation is achieved by optical excitation of the polymer during roll-to-roll slot-die coating, providing a technique that is viable for large-scale production. Along with establishing the technique, the entire knowledge chain from fundamental insight in polymer physics and structure to application is presented. By correlating X-ray and neutron scattering techniques with density functional theory and molecular dynamics simulations of solvent evaporation, we successfully determine the packing of the polymers in the ground state and excited state. The findings are coupled with measurements of dynamical physical properties and solar cell device performance to pinpoint the structure-property relationship.

Different processing was applied to match energies below and above the excitation energy levels of P3HT in solution and in thin film [1]. Specifically, either no illumination (i.e., dark), or LEDs emitting red, green, or blue light illuminated the films while being coated. It is known from density functional theory (DFT) calculations [2] that, when exciting P3HT with visible light, i.e. going from the ground state to the excited state, the electron density changes to form double bonds between neighbouring thiophene rings. This change in the bonding pattern planarizes the polymer backbone. Consequently, the final morphology is affected when exciting the polymer during coating. To probe the morphological changes caused by the four treatments, we use Grazing-Incidence Wide-Angle X- ray Scattering (GIWAXS) on the coated films. To elucidate the physical origin of the changes, DFT and solvent evaporation molecular dynamics (MD) are used to simulate the effect of excitation on single polymer chains and thin films. The full picture of morphological changes is followed by a thorough investigation of the change of physical properties in the thin film. Here, the UV-VIS absorption spectra are analysed, followed by THz spectroscopy measuring the thin-film photo- conductivity. Finally, the consequence of light treating a complete OPV device (P3HT:O-IDTBR) is discussed.

The experimental and computational results demonstrate that exciting P3HT with visible light during deposition serves as a tool for manipulating the packing behaviour of P3HT and can be used to modify the final thin film morphology. It can be considered a new processing parameter for achieving the desired performance of organic thin films. Particularly, the capability to increase the out-of-plane mobility by light treatment can be used for transistor applications where directional mobility and patterning is essential [3]. Application in OPV devices will require an optimization process for the right crystal packing and domain size of donor and acceptor constituents. For the specific combination P3HT:O- IDTBR, the domain size decreases with light treatment and still results in a decrease in power conversion efficiency. Understanding how light treatment during fabrication influences the final morphology of a film can enable major improvements for specific materials systems or other technologies in flexible electronics and photovoltaics.

In conclusion, we report a method to manipulate the morphology of P3HT thin films through illumination with visible LED light during roll-to-roll slot-die coating. Optical polymer excitation temporarily constrains large sections of the chains into a planar geometry that is the minimum of the excited state potential energy surface. This structural effect is strong enough to affect the aggregation behaviour and, thereby, the final morphology of the P3HT film. The light-treated films are less crystalline overall, display a higher degree of face-on orientation, shorter conjugation length, and a change of the unit cell dimensions with less efficient packing. Consequently, the in-plane photoconductivity decreases and the out-of-plane conductivity increases drastically with light treatment.

The authors acknowledge financial support from the H2020 European Research Council through the SEEWHI Consolidator grant, ERC-2015-CoG-681881 and from the Independent Research Fund Denmark, grant no. 7200-00001B. Danscatt for travel ex- penses during beam times: DESY proposal number 15559230912. SAXS measurements were carried out at the P03/MiNaXS beamline, PETRA III at DESY. Neu- tron beam time at MLZ, proposal-15551”. Some WAXS experiments were carried out at the cSAXS beamline, Paul Scherrer Institute, Switzerland, proposal number 20182246. Deuterated SD-P3HT was synthesized at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility (CNMS Proposal ID: CNMS2020-R-00546), Kristian Larsen for countless hours of technical support.

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