Phase Behavior in the Active Layer of Small Molecule Organic Photovoltaics: State Diagram of p-DTS(FBTTh2)(2):PC71BM

Niko Van den Brande, Maxime Defour, Dries Devisscher, Pieter Verstappen, Gunter Reekmans, Jan D'Haen, Peter Adriaensens, Wouter Maes, Guy Van Assche, Bruno Van Mele

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Abstract

A comprehensive study was undertaken to obtain a more fundamental understanding of the phase behavior of the p-DTS(FBTTh2)2:PC71BM system, used in small molecule organic solar cells, with a strong focus on the amorphous phase and its influence on crystallinity. Three dedicated thermal protocols were used in combination with advanced thermal analysis, solid-state NMR, and wide-angle X-ray diffraction. Rapid cooling, to avoid structure formation and gain insight in the amorphous phase, and slow cooling, to promote structure formation, were used as limiting cases to explain the intermediate behavior after device processing from solution. A complete state diagram was developed, and the glass transition (Tg)–composition relationship was determined. In the case of slow cooling and the procedure used for device processing, the rapid crystallization of p-DTS(FBTTh2)2 leads to an enrichment of the amorphous phase in PC71BM, increasing its Tg and causing vitrification of the mixed amorphous phase before crystallization when the total amount of PC71BM exceeds 70 wt %. The common processing additive 1,8-diiodooctane (DIO) was found to lead to a lower p-DTS(FBTTh2)2 crystallinity and smaller average crystal size. More importantly, it acts as a strong plasticizer, lowering Tg significantly and thus reducing the morphological stability of the p-DTS(FBTTh2)2:PC71BM mixtures.
Original languageEnglish
Pages (from-to)7566-7577
Number of pages12
JournalJournal of Physical Chemistry C
Volume124
Issue number13
DOIs
Publication statusPublished - 2 Apr 2020

Bibliographical note

Funding Information:
The authors acknowledge the financial support of the Agency for Innovation by Science and Technology (IWT) (Ph.D. fellowship to M.D.) and the Research Foundation – Flanders (FWO-Vlaanderen) (project G0B2718N and postdoctoral fellowship to P.V.). The XRD measurements were performed by Bart Ruttens in the group of Jan D’Haen at Hasselt University. This work is supported by Hasselt University and the Research Foundation Flanders (FWO-Vlaanderen; Hercules project GOH3816NAUHL).

Funding Information:
The authors acknowledge the financial support of the Agency for Innovation by Science and Technology (IWT) (Ph.D. fellowship to M.D.) and the Research Foundation - Flanders (FWO-Vlaanderen) (project G0B2718N and postdoctoral fellowship to P.V.). The XRD measurements were performed by Bart Ruttens in the group of Jan D'Haen at Hasselt University. This work is supported by Hasselt University and the Research Foundation Flanders (FWO-Vlaanderen; Hercules project GOH3816NAUHL).

Publisher Copyright:
Copyright © 2020 American Chemical Society.

Copyright:
Copyright 2020 Elsevier B.V., All rights reserved.

Keywords

  • Organic solar cells
  • Phase behaviour
  • Advanced thermal analysis
  • NMR spectroscopy
  • State diagrams

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