Abstract
Organic photovoltaics (OPVs) have attracted increasing interest as a lightweight,
low-cost, and easy to process replacement for inorganic solar cells. Moreover, the morphology of the
OPV active layer is crucial to its performance, where a bicontinuous, interconnected, phaseseparated
morphology of pure electron donor and acceptor phases is currently believed to be
optimal. In this work, we use neutron scattering to investigate the morphology of a model OPV
conjugated polymer bulk heterojunction, poly[3-hexylthiophene] (P3HT), and surface-functionalized
fullerene 1-(3-methyloxycarbonyl) propyl(1-phenyl [6,6]) C61 (PCBM). These results show that P3HT
and PCBM form a homogeneous structure containing crystalline P3HT and an amorphous P3HT/PCBM
matrix, up to ca. 20 vol % PCBM. At 50 vol % PCBM, the samples exhibit a complex structure
containing at least P3HT crystals, PCBM crystals, and a homogeneous mixture of the two. The 20 vol
% PCBM samples exhibit behavior consistent with the onset of phase separation after 6 h of thermal
annealing at 150 C, but appear to be miscible at shorter annealing times. This suggests that the
miscibility limit of PCBM in P3HT is near 20%. Moreover, for the 50 vol % PCBM sample, the interface
roughens under thermal annealing possibly owing to the growth of PCBM crystals. These
observations suggest a different morphology than is commonly presented in the literature for
optimal bulk heterojunctions. We propose a novel “rivers and streams” morphology to describe this
system, which is consistent with these scattering results and previously reported photovoltaic
functionality of P3HT/PCBM bulk heterojunctions.
