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Issue 34, 2019
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Stability of push–pull small molecule donors for organic photovoltaics: spectroscopic degradation of acceptor endcaps on benzo[1,2-b:4,5-b′]dithiophene cores

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Abstract

High efficiency organic photovoltaic devices have relied on the development of new donor and acceptor materials to optimize opto-electronic properties, promote free carrier generation, and suppress recombination losses. With single junction efficiencies exceeding 15%, materials development must now target long-term stability. This work focuses on the photobleaching dynamics and degradation chemistries of a class of small molecule donors inspired by benzodithiophene terthiophene cores (BDT-3T) with rhodanine endcaps, which have demonstrated 9% efficiency in single junction devices and >11% in ternary cells. Density functional theory was used to design three additional molecules with similar synthetic pathways and opto-electronic properties by simply changing the electron accepting endcap to benzothiazoleacetonitrile, pyrazolone, or barbituric acid functional groups. This new class of semiconductors with equivalent redox properties enables systematic investigation into photobleaching dynamics under white light illumination in air. Degradation chemistries are assessed via unique spectroscopic signatures for the BDT-3T cores and the endcaps using photoelectron spectroscopies. We show that the pyrazolone undergoes significant degradation due to ring opening, resulting in complete bleaching of the chromophore. The barbituric and rhodanine endcap molecules have moderate stability, while the benzothiazoleacetonitrile group produces the most stable chromophore despite undergoing some oxidative degradation. Collectively, our results suggest the following: (i) degradation is not just dependent on redox properties; (ii) core group stability is not independent of the endcap choice; and (iii) future design of high efficiency materials must consider both photo and chemical stability of the molecule as a whole, not just individual donor or acceptor building blocks.

Graphical abstract: Stability of push–pull small molecule donors for organic photovoltaics: spectroscopic degradation of acceptor endcaps on benzo[1,2-b:4,5-b′]dithiophene cores

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Supplementary files

Article information


Submitted
12 Jun 2019
Accepted
11 Aug 2019
First published
16 Aug 2019

J. Mater. Chem. A, 2019,7, 19984-19995
Article type
Paper

Stability of push–pull small molecule donors for organic photovoltaics: spectroscopic degradation of acceptor endcaps on benzo[1,2-b:4,5-b′]dithiophene cores

K. E. Watts, T. Nguyen, B. J. Tremolet de Villers, B. Neelamraju, M. A. Anderson, W. A. Braunecker, A. J. Ferguson, R. E. Larsen, B. W. Larson, Z. R. Owczarczyk, J. R. Pfeilsticker, J. E. Pemberton and E. L. Ratcliff, J. Mater. Chem. A, 2019, 7, 19984
DOI: 10.1039/C9TA06310B

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