Diffusion-Limited Crystallization: A Rationale for the Thermal Stability of Non-Fullerene Solar Cells

Liyang Yu, Deping Qian, Sara Marina, Ferry A.A. Nugroho, Anirudh Sharma, Sandra Hultmark, Anna I. Hofmann, Renee Kroon, Johannes Benduhn, Detlef M. Smilgies, Koen Vandewal, Mats R. Andersson, Christoph Langhammer, Jaime Martín, Feng Gao, Christian Müller

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Abstract

Organic solar cells are thought to suffer from poor thermal stability of the active layer nanostructure, a common belief that is based on the extensive work that has been carried out on fullerene-based systems. We show that a widely studied non-fullerene acceptor, the indacenodithienothiophene-based acceptor ITIC, crystallizes in a profoundly different way as compared to fullerenes. Although fullerenes are frozen below the glass-transition temperature Tg of the photovoltaic blend, ITIC can undergo a glass-crystal transition considerably below its high Tg of ∼180 °C. Nanoscopic crystallites of a low-temperature polymorph are able to form through a diffusion-limited crystallization process. The resulting fine-grained nanostructure does not evolve further with time and hence is characterized by a high degree of thermal stability. Instead, above Tg, the low temperature polymorph melts, and micrometer-sized crystals of a high-temperature polymorph develop, enabled by more rapid diffusion and hence long-range mass transport. This leads to the same detrimental decrease in photovoltaic performance that is known to occur also in the case of fullerene-based blends. Besides explaining the superior thermal stability of non-fullerene blends at relatively high temperatures, our work introduces a new rationale for the design of bulk heterojunctions that is not based on the selection of high-Tg materials per se but diffusion-limited crystallization. The planar structure of ITIC and potentially other non-fullerene acceptors readily facilitates the desired glass-crystal transition, which constitutes a significant advantage over fullerenes, and may pave the way for truly stable organic solar cells.

Original languageEnglish
Pages (from-to)21766-21774
Number of pages9
JournalACS Applied Materials and Interfaces
Volume11
Issue number24
Early online date7 Jun 2019
DOIs
Publication statusPublished - 19 Jun 2019
Externally publishedYes

Bibliographical note

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes

Keywords

  • diffusion-limited crystallization
  • glass-transition temperature
  • non-fullerene acceptor
  • organic solar cell
  • thermally stable photovoltaics

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    Yu, L., Qian, D., Marina, S., Nugroho, F. A. A., Sharma, A., Hultmark, S., Hofmann, A. I., Kroon, R., Benduhn, J., Smilgies, D. M., Vandewal, K., Andersson, M. R., Langhammer, C., Martín, J., Gao, F., & Müller, C. (2019). Diffusion-Limited Crystallization: A Rationale for the Thermal Stability of Non-Fullerene Solar Cells. ACS Applied Materials and Interfaces, 11(24), 21766-21774. https://doi.org/10.1021/acsami.9b04554