Abstract
Nanophase separation plays a critical role in the performance of donor–acceptor based organic
photovoltaic (OPV) devices. Although post-fabrication annealing is often used to enhance OPV
efficiency, the ability to exert precise control over phase separated domains and connectivity remains
elusive. In this work, we use a diblock copolymer to systematically manipulate the domain sizes of an
organic solar cell active layer at the nanoscale. More specifically, a poly(3-hexylthiophene)-bpoly(
ethylene oxide) (P3HT-b-PEO) diblock copolymer with a low polydispersity index (PDI ¼ 1.3) is
added to a binary blend of P3HT and 6,6-phenyl C61-butyric acid methyl ester (PCBM) at different
concentrations (0–20 wt%). Energy-filtered TEM (EFTEM) results suggest systematic changes of
P3HT distribution as a function of block copolymer compatibilizer concentration and thermal
annealing. X-ray scattering and microscopy techniques are used to show that prior to annealing, active
layer domain sizes do not change substantially as compatibilizer is added; however after thermal
annealing, the domain sizes are significantly reduced as the amount of P3HT-b-PEO compatibilizer
increases. The impact of compatibilizer is further rationalized through quantum density functional
theory calculations. Overall, this work demonstrates the possibility of block copolymers to
systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic
devices. If coupled with efficient charge transport and collection (through judicious choice of block
copolymer type and composition), this approach may contribute to further optimization of OPV
devices.
