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Morphological and electrical control of fullerene dimerization determines organic photovoltaic stability

Authors :
Max von Delius
Rongrong Cheacharoen
Urs F. Fritze
Thomas Heumueller
Michael D. McGehee
William R. Mateker
Hans-Joachim Egelhaaf
Christoph J. Brabec
Andreas Distler
Michael Salvador
William H. Nguyen
Markus Biele
Source :
Energy & Environmental Science. 9:247-256
Publication Year :
2016
Publisher :
Royal Society of Chemistry (RSC), 2016.

Abstract

Fullerene dimerization has been linked to short circuit current (Jsc) losses in organic solar cells comprised of certain polymer–fullerene systems. We investigate several polymer–fullerene systems, which present Jsc loss to varying degrees, in order to determine under which conditions dimerization occurs. By reintroducing dimers into fresh devices, we confirm that the photo-induced dimers are indeed the origin of the Jsc loss. We find that both film morphology and electrical bias affect the photodimerization process and thus the associated loss of Jsc. In plain fullerene films, a higher degree of crystallinity can inhibit the dimerization reaction, as observed by high performance liquid chromatography (HPLC) measurements. In blend films, the amount of dimerization depends on the degree of mixing between polymer and fullerene. For highly mixed systems with very amorphous polymers, no dimerization is observed. In solar cells with pure polymer and fullerene domains, we tune the fullerene morphology from amorphous to crystalline by thermal annealing. Similar to neat fullerene films, we observe improved light stability for devices with crystalline fullerene domains. Changing the operating conditions of the investigated solar cells from Voc to Jsc also significantly reduces the amount of dimerization-related Jsc loss; HPLC analysis of the active layer shows that more dimers are formed if the cell is held at Voc instead of Jsc. The effect of bias on dimerization, as well as a clear correlation between PL quenching and reduced dimerization upon addition of small amounts of an amorphous polymer into PC60BM films, suggests a reaction mechanism via excitons.

Details

ISSN :
17545706 and 17545692
Volume :
9
Database :
OpenAIRE
Journal :
Energy & Environmental Science
Accession number :
edsair.doi...........dc703656250b5871bec39150d140a9b0
Full Text :
https://doi.org/10.1039/c5ee02912k