In Situ Synthesis of Ternary Block Copolymer/Homopolymer Blends for Organic Photovoltaics

A detailed investigation of in situ-synthesized all-conjugated block copolymer (BCP) compatibilized ternary blends containing poly(3-hexylthiophene) (P3HT) and poly{[ N, N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dibcarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (PNDIT2) as donor and acceptor polymers, respectively, is presented. Both polymers are incompatible and show strong segregation in blends, which renders compatibilization with their corresponding BCPs promising to enable nanometer-phase-separated structures suitable for excitonic devices. Here, we synthesize a ternary block copolymer/homopolymer blend system and investigate the phase behavior as a function of block copolymer molecular weight and different annealing conditions. The device performance decreases on increasing annealing temperatures. To understand this effect, morphological investigations including atomic force microscopy, high-resolution transmission electron microscopy (HR-TEM), and grazing incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS) are carried out. On comparing domain sizes of pristine and compatibilized blends obtained from GISAXS, a weak compatibilization effect appears to take place for the in situ-synthesized ternary systems. The effect of thermal annealing is most prevalent for all samples, which, for the highest annealing temperature above the melting point of PNDIT2 (310 °C), ultimately leads to a change from the face-on to edge-on orientation of PNDIT2, as seen in GIWAXS. This effect dominates and decreases all photovoltaic parameters, irrespective of whether a pristine or compatibilized blend is used.