Effect of Blend Composition and Additives on the Morphology
of PCPDTBT:PC<sub>71</sub>BM Thin Films for Organic Photovoltaics
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Abstract
The use of solvent additives in the
fabrication of bulk heterojunction polymer:fullerene solar cells allows
to boost efficiencies in several low bandgap polymeric systems. It
is known that solvent additives tune the nanometer scale morphology
of the bulk heterojunction. The full mechanism of efficiency improvement
is, however, not completely understood. In this work, we investigate
the influences of blend composition and the addition of 3 vol % 1,8-octanedithiol
(ODT) as solvent additive on polymer crystallization and both, vertical
and lateral morphologies of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta
[2,1-b;3,4-b′]dithiophene)-<i>alt</i>-4,7(2,1,3-benzothiadiazole)]
and [6,6]-phenyl C<sub>71</sub>-butyric acid methyl ester (PCPDTBT:PC<sub>71</sub>BM) blend thin films processed from chlorobenzene-based solutions.
The nanoscale morphology is probed with grazing incidence small- and
wide-angle X-ray scattering as well as X-ray reflectivity and complemented
with UV/vis spectroscopy. In PCPDTBT:PC<sub>71</sub>BM films the use
of ODT is found to lower the solubility of fullerene in the polymer
matrix and to promote polymer crystallization, both vertical and lateral
microphase separation with morphological coarsening, and formation
of a fullerene-rich topping layer. The enhanced photovoltaic performance
is explained by these findings