The influence of the nematic phase on the phase separation of blended organic semiconductors for photovoltaics

Differential scanning calorimetry in combination with atomic force microscopy is used to examine the phase separation of a blended nematic liquid crystalline electron-donor and crystalline perylene electron-acceptor mixture. Separate domains of donor and acceptor material are mostly retained in the blend, although a small proportion of the acceptor, increasing with increasing donor concentration, is mixed in with the donor domains. Annealing in the nematic phase allows the donor and acceptor molecules to move and generate phase-separated domains of the required size, thus enhancing the performance of bulk heterojunction photovoltaic devices based on these blends. We show that the optimum annealing temperature can be controlled by manipulation of the temperature range of the nematic phase of the donor.