Abstract
Understanding how additives interact and segregate within bulk heterojunction
(BHJ) thin films is critical for exercising control over structure at multiple length
scales and delivering improvements in photovoltaic performance. The morphological
evolution of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester
(PCBM) blends that are commensurate with the size of a BHJ thin fi lm is examined
using petascale coarse-grained molecular dynamics simulations. Comparisons between
2 component and 3 component systems containing short P3HT chains as additives
undergoing thermal annealing demonstrate that the short chains alter the morphol-
ogy in apparently useful ways: They efficiently migrate to the P3HT/PCBM interface,
increasing the P3HT domain size and interfacial area. Simulation results agree with
depth pro files determined from neutron reflectometry measurements that reveal PCBM
enrichment near substrate and air interfaces, but a decrease in that PCBM enrich-
ment when a small amount of short P3HT chains are integrated into the BHJ blend.
Atomistic simulations of the P3HT/PCBM blend interfaces show a non-monotonic de-
pendence of the interfacial thickness as a function of number of repeat units in the
oligomeric P3HT additive, and the thiophene rings orient parallel to the interfacial
plane as they approach the PCBM domain. Using the nanoscale geometries of the
P3HT oligomers, LUMO and HOMO energy levels calculated by density functional
theory are found to be invariant across the donor/acceptor interface. These connec-
tions between additives, processing, and morphology at all length scales are generally
useful for eff orts to improve device performance.
