  Organic solar cells (OSCs) containing an active layer consisting of a
nanostructured blend of a conjugated polymer like poly(3-hexylthiophene) (P3HT)
and an electron acceptor molecule have the potential of competing against
silicon-based photovoltaic panels. However, this potential is unfulfilled
primarily due to interrelated production and stability issues. The generally
employed spin coating process for fabricating organic solar cells cannot be
scaled up. Recently, He et al., have reported that the gelation of P3HT with
[6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) under sub-ambient
conditions can provide a continuous extrusion/coating based route to the
processing of organic solar cells and that increases in power conversion
efficiencies (PCEs) of the P3HT/PC60BM active layer are possible under certain
shearing and thermal histories of the P3HT/PC60BM gels. Here oscillatory and
steady torsional flows were used to investigate the gel formation dynamics of
P3HT with a recently proposed non-fullerene o-IDTBR under sub-ambient
conditions. The gel strengths defined on the basis of linear viscoelastic
material functions as determined via small-amplitude oscillatory shear were
observed to be functions of the P3HT and o-IDTBR concentrations, the solvent
used and the shearing conditions. Overall, the gels which formed upon quenching
to sub-zero temperatures were found to be stable during small-amplitude
oscillatory shear (linear viscoelastic range) but broke down even at the
relatively low shear rates associated with steady torsional flows, suggesting
that the shearing conditions used during the processing of gels of P3HT with
small molecule acceptor blends can alter the gel structure and possibly affect
the resulting active layer performance.
