Relative Photon-to-Carrier Efficiencies of Alternating Nanolayers of Zinc Phthalocyanine and C60 Films Assessed by Time-Resolved Terahertz Spectroscopy

Multilayer and 1:1 blended films of zinc phthalocyanine (ZnPc) and buckminsterfullerene (C-60) were investigated as model active layers for solar cells by time-resolved terahertz spectroscopy (TRTS). Relative photon-to-carrier efficiencies were determined from ultrafast decay dynamics of photogenerated carriers using 400 and 800 nm excitation for delay times up to 0.5 ns. The findings are in good agreement with reported solar-cell device measurements, and the results exhibit a near linear increase of the relative efficiencies with the interface number of multilayer films. The relative photon-to-carrier efficiencies of films composed of alternating layers with an individual layer thickness of less than 20 nm were higher than that of a 1:1 blended film. In contrast, 400 nm excitation of a C-60 only film initially yields a relatively strong THz signal that is followed by a rapid (picosecond) decay almost to its base value and results in a very low carrier density beyond a few picoseconds. For a given film thickness and optical density, our data suggest that the relative photon-to-carrier efficiency of multilayer films increases with increasing total interfacial area, emphasizing the importance of close proximity between the fullerene and phythalocyanine. These findings suggest that the highest photon-to-free-carrier efficiencies can be achieved by designing ultrathin films (having layers a few nanometers thick) with alternating multilayer structures to achieve high photon harvesting and charge separation to opposite layers.