Photo-carrier dynamics of blended and multi-layered films of zinc phthalocyanine and C60 measured by time-resolved terahertz spectroscopy

We present a study of photo-induced carrier dynamics in organic thin films that contain zinc phthalocyanine (ZnPc) and buckminsterfullerene (C₆₀) investigated by ultrafast time resolved terahertz spectroscopy (TRTS). We compare two classes of films: 1) blend films of ZnPc and C₆₀ prepared by co-evaporation, and 2) superlattice films from alternating neat layers of ZnPc and C₆₀, where the individual layer varies in thickness between 2 nm and 40 nm. These films are model structures for the charge generation layers in organic photovoltaics, and their photo-carrier dynamics on a femtosecond to nanosecond timescale is of high interest. Using TRTS, which employs an optical pump pulse and a time delayed terahertz (THz) probe pulse, we find evidence for a short-lived charge transfer state of C₆₀ that decays within several picoseconds of excitation. Both blend and superlattice films have a long-lived (>80 picoseconds) THz absorption that is dependent on the structure and composition of the films. The optimum composition for maximum photo-carrier generation efficiency is a 1:1 blend of ZnPc and C₆₀. Amongst the layered films, we find that the photo-carrier efficiency increases as the individual layer thickness decreases, with the film having ultrathin alternating 2 nm thick layers exhibiting the strongest THz absorption. A much stronger THz absorption signal was obtained for the 2 nm layered film than for the best blend film.