Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells

Kedar D. Deshmukh, Rukiya Matsidik, Shyamal K. K. Prasad, Naresh Chandrasekaran, Adam Welford, Luke A. Connal, Amelia C.Y. Liu, Eliot Gann, Lars Thomsen, Dinesh Kabra, Justin M. Hodgkiss, Michael Sommer, Christopher R. McNeill

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20 Citations (Scopus)

Abstract

Here, we systematically study the effect of fluorination on the performance of all-polymer solar cells by employing a naphthalene diimide (NDI)-based polymer acceptor with thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer with either two or four fluorine units is used to create a series of acceptor polymers with either no fluorination (PNDITPhT), bifluorination (PNDITF2T), or tetrafluorination (PNDITF4T). In blends with the donor polymer PTB7-Th, fluorination results in an increase in power conversion efficiency from 3.1 to 4.6% despite a decrease in open-circuit voltage from 0.86 V (unfluorinated) to 0.78 V (tetrafluorinated). Countering this decrease in open-circuit voltage is an increase in short-circuit current from 7.7 to 11.7 mA/cm2 as well as an increase in fill factor from 0.45 to 0.53. The origin of the improvement in performance with fluorination is explored using a combination of morphological, photophysical, and charge-transport studies. Interestingly, fluorination is found not to affect the ultrafast charge-generation kinetics, but instead is found to improve charge-collection yield subsequent to charge generation, linked to improved electron mobility and improved phase separation. Fluorination also leads to improved light absorption, with the blue-shifted absorption profile of the fluorinated polymers complementing the absorption profile of the low-band gap PTB7-Th.

Original languageEnglish
Pages (from-to)955-969
Number of pages15
JournalACS Applied Materials & Interfaces
Volume10
Issue number1
DOIs
Publication statusPublished - 10 Jan 2018

Keywords

  • all-polymer solar cells
  • fluorination
  • GIWAXS
  • morphology
  • photophysics
  • planarization
  • R-SoXS

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