Charge
Separation in P3HT:SWCNT Blends Studied by
EPR: Spin Signature of the Photoinduced Charged State in SWCNT
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Abstract
Single-wall carbon nanotubes (SWCNTs)
could be employed in organic
photovoltaic (OPV) devices as a replacement or additive for currently
used fullerene derivatives, but significant research remains to explain
fundamental aspects of charge generation. Electron paramagnetic resonance
(EPR) spectroscopy, which is sensitive only to unpaired electrons,
was applied to explore charge separation in P3HT:SWCNT blends. The
EPR signal of the P3HT positive polaron increases as the concentration
of SWCNT acceptors in a photoexcited P3HT:SWCNT blend is increased,
demonstrating long-lived charge separation induced by electron transfer
from P3HT to SWCNTs. An EPR signal from reduced SWCNTs was not identified
in blends due to the free and fast-relaxing nature of unpaired SWCNT
electrons as well as spectral overlap of this EPR signal with the
signal from positive P3HT polarons. However, a weak EPR signal was
observed in chemically reduced SWNTs, and the <i>g</i> values
of this signal are close to those of C<sub>70</sub>-PCBM anion radical.
The anisotropic line shape indicates that these unpaired electrons
are not free but instead localized