Influence of Stacking Order of Phthalocyanine and Fullerene Layers on the Photoexcited Carrier Dynamics in Model Organic Solar Cell

Metal phthalocyanine and fullerene are typical p-type and n-type organic semiconductors and are often used as constituents of model systems of organic photovoltaics (OPVs). The light–electricity conversion efficiency of the OPVs is influenced by many factors, and a composite structure is one of them. In the present study, time-resolved X-ray photoelectron spectroscopy has been utilized to examine the influence of the stacking order of copper phthalocyanine (CuPc) and fullerene (C60) on photoexcited carrier dynamics in layered CuPc–C60 thin-film OPVs fabricated on a rutile TiO2(110) substrate. TiO2 is a strong n-type semiconductor and is found to serve as an electron acceptor, which collects the excited electrons in both CuPc and C60 layers irrespective of their stacking order. However, a clear difference is found in the electron transfer from C60 to TiO2 in short delay times below 1 ns; an electron transfer is facilitated in CuPc/C60/TiO2 stacking, whereas the fast electron transfer is suppressed in C60/CuPc/TiO2 stacking. The insertion of the CuPc layer between C60 and TiO2 is effective to block the C60 → TiO2 electron transfer even though the CuPc layer has a monolayer thickness.