Non-destructive optical characterization of phase separation in bulk heterojunction organic photovoltaic cells

The dithiophene donor-acceptor copolymers that are bridged either with carbon (C-PCPDTBT) or silicon atoms (Si-PCPDTBT) belong to a promising family of materials for use in photoactive layers for organic photovoltaic cells (OPVs). In this work, we implement the non-destructive Spectroscopic Ellipsometry technique in the near infrared to the far ultraviolet spectral region in combination with advanced theoretical modeling to investigate the vertical distribution of the C-PCPDTBT and Si-PCPDTBT polymer and fullerene ([6,6]-phenyl C71-butyric acid methyl ester - PC70BM) phases in the blend, as well as the effect of the polymer-to-fullerene ratio on the distribution mechanism. It was found that the C-PCPDTBT:PC70BM blends have donor-enriched top regions and acceptor-enriched bottom regions, whereas the donor and acceptor phases are more homogeneously intermixed in the Si-PCPDTBT:PC70BM blends. We suggest that the chemical incompatibility of the two phases as expressed by the difference in their surface energy, may be a key element in promoting the segregation of the lower surface phase to the top region of the photoactive layer. We found that the increase of the photoactive layer thickness reduces the polymer enrichment at the cathode, producing a more homogeneous phase distribution of donor and acceptor in the bulk that leads to the increase of the OPV efficiency. © 2014 Elsevier B.V.