Understanding and improving solid-state polymer/C₇₀-fullerene bulk-heterojunction solar cells using ternary porphyrin blends

Solid-state heterojunction solar cells formed from binary blends of conjugated polymers and fullerenes provide a promising class of organic devices. We demonstrate that ternary blends incorporating porphyrins can be of similar morphology to the binary mixture and retain full device functionality. They allow, for the first time, the construction of efficient devices containing less than 10% polymer. By analyzing the absorption spectra as a function of concentration, we determine the proportion of light absorbed by each individual component in both binary and ternary mixtures. This analysis reveals that the majority of the light is absorbed by the fullerene in 1:4 polymer/C₆₀-fullerene blends, with over 50% of the photocurrent produced under AM 1.5 conditions occurring subsequent to C₆₀-fullerene absorption. This result provides for a consistent understanding of the origin of primary charge separation in general polymer/C₆₀-fullerene blends, polymer/C₇₀-fullerene blends, and polymer-fullerene dyad molecules. Porphyrins are demonstrated to add broad-band character to the device and may be used to tune for particular wavelengths; they also are shown to initiate primary charge separation through electron-transfer to the fullerene. Finally, addition of the porphyrin is shown to increase the internal quantum efficiency following polymer absorption from ca. 60 to 80%