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Issue 8, 2020
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Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

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Abstract

In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly suppressed charge trapping into electronic intra-bandgap tail states compared to other polymer/non-fullerene acceptor solar cells, attributed to lower energetic disorder. The presence of tail states is a key source of energetic loss in most organic solar cells, as charge carriers relax into these states, reducing the quasi-Fermi level splitting and therefore device VOC. DFT and Raman analyses indicate this suppression of tail state energetics disorder could be associated with a higher degree of conformational rigidity and uniformity for the Y6 acceptor. We attribute the origin of such conformational rigidity and uniformity of Y6 to the presence of the two alkyl side chains on the outer core that restricts end-group rotation by acting as a conformation locker. The resultant enhanced carrier dynamics and suppressed charge carrier trapping are proposed to be a key factor behind the high performance of this blend. Low energetic disorder is suggested to be a key factor enabling reasonably efficient charge generation in this low energy offset system. In the absence of either energetic disorder or a significant electronic energy offset, it is argued that charge separation in this system is primarily entropy driven. Nevertheless, photocurrent generation is still limited by slow hole transfer from Y6 to PM6, suggesting pathways for further efficiency improvement.

Graphical abstract: Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

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Supplementary files

Article information


Submitted
28 Apr 2020
Accepted
08 Jul 2020
First published
08 Jul 2020

This article is Open Access

Energy Environ. Sci., 2020,13, 2422-2430
Article type
Communication

Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

J. Wu, J. Lee, Y. Chin, H. Yao, H. Cha, J. Luke, J. Hou, J. Kim and J. R. Durrant, Energy Environ. Sci., 2020, 13, 2422
DOI: 10.1039/D0EE01338B

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