Ternary phase behavior of P3HT-b-PEO compatibilized P3HT/PCBM films

To lower the interfacial tension and systematically control the donor-acceptor phase separation in organic photovoltaic devices, a poly(3-hexylthiophene)-block-poly(ethylene oxide) diblock copolymer compatibilizer was added into a binary blend of regioregular P3HT and fullerene derivative, 6,6-phenyl C61 butyric acid methyl ester (PCBM). We systematically examine the ternary phase behavior of spin coated films of P3HT/ P3HT-b-PEO/ PCBM before and after annealing with selected area electron diffraction, grazing-incidence X-ray diffraction, TEM, and AFM measurements. The addition of 5% P3HT-b-PEO (block molecular weights of 10kDa and 3kDa, respectively) into a 1:1 P3HT/PCBM blend reduces the sizes of P3HT-rich domains (∼18 nm) in P3HT/PCBM films by up to 40%, while the P - P stacking in P3HT (i.e. (020) crystallinity) remains nearly unchanged. Our results demonstrate that incorporation of a P3HT-based block copolymer compatibilizer with proper block composition is a convenient and effective way to manipulate donor- acceptor phase separation at the nanoscale.