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Ultrafast Transient Spectroscopy of Polymer/Fullerene Blends for Organic Photovoltaic Applications

Department of Physics & Astronomy, University of Utah, Salt Lake City, UT 84112, USA
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Author to whom correspondence should be addressed.
Current address: School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.
Materials 2013, 6(3), 897-910; https://doi.org/10.3390/ma6030897
Received: 21 December 2012 / Revised: 4 February 2013 / Accepted: 25 February 2013 / Published: 6 March 2013
(This article belongs to the Special Issue Photovoltaic Materials)
We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) (RR-P3HT) (donors-D) and fullerene (PCBM) (acceptor-A) in an unprecedented broad spectral range of 0.25 to 2.5 eV. In D-A blends with maximum domain separation, such as RR-P3HT/PCBM, with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense up to ~1 ns. Instead, there is a build-up of charge-transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D and A domains at a later time (>1 ns). This “two-step” charge photogeneration process may be typical in organic bulk heterojunction cells. We also report the effect of adding spin 1/2 radicals, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of RR-P3HT/PCBM blend. The addition of Galvinoxyl radicals to the blend reduces the geminate recombination rate of photogenerated CT excitons. In addition, the photoexcitation dynamics in a new D-A blend of RR-P3HT/Indene C60 trisadduct (ICTA) has been studied and compared with the dynamics in RR-P3HT/PCBM. View Full-Text
Keywords: transient spectroscopy; organic photovoltaic; donor-acceptor blends; charge-transfer excitons transient spectroscopy; organic photovoltaic; donor-acceptor blends; charge-transfer excitons
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Singh, S.; Vardeny, Z.V. Ultrafast Transient Spectroscopy of Polymer/Fullerene Blends for Organic Photovoltaic Applications. Materials 2013, 6, 897-910.

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