Relative Photon-to-Carrier Efficiencies of Alternating Nanolayers of Zinc Phthalocyanine and C60 Multilayer Films Assessed by Time-Resolved Terahertz Spectroscopy
Okan Esenturk, Joseph S. Melinger, Paul A. Lane, Edwin J. Heilweil
Alternating multi-layer and 1:1 blended films of zinc phthalocyanine (ZnPc)and buckminsterfullerene (C60) were investigated as model active layers for solar cells by Time-Resolved Terahertz Spectroscopy (TRTS). Relative photon-to-carrier efficiencies 2 were determined from ultrafast decay dynamics of photo-generated carriers 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 multi-layer 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 C60 only film initially yields a relatively strong THz signal, which is followed by a rapid (picosecond) decay almost to its base value and result in a very low carrier density beyond few picoseconds. For a given film thickness and optical density, our data suggest that the relative photon-to-carrier efficiency of the 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-freecarrier efficiencies can be achieved by designing ultra-thin films (having layers a few nm thick) with alternating multi-layer structures to achieve the highest photon harvesting that results in efficient charge separation to the opposite layers.