Titanyl Phthalocyanine/C₆₀ Heterojunctions: Band-Edge Offsets and Photovoltaic Device Performance

Abstract

Planar heterojunction organic photovoltaic devices have been created using oxo-titanium phthalocyanine (TiOPc) as the donor layer and fullerene (C60) as the acceptor layer, with comparisons to devices based on copper phthalocyanine (CuPc) as the donor. TiOPc/C60 and CuPc/C60 heterojunctions were first characterized by a combination of UV-photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to estimate the frontier orbital energy offset (EDHOMO − EALUMO), which is related to the open-circuit photopotential (VOC). A small interface dipole effect was seen at the TiOPc/C60 interface (eD ≈ 0.02 eV), whereas a significant interface dipole was observed for the CuPc/C60 interface (eD ≈ 0.3 eV). On the basis of the work presented here and previously reported electrochemical and UPS/XPS studies, we estimate an EDHOMO − EALUMO energy offset of ca. 1.1 eV for the TiOPc/C60 heterojunction and 0.7 eV for the CuPc/C60 heterojunction. Maximum VOC values observed at room temperature for corresponding planar heterojunction photovoltaic devices were 0.3−0.4 V lower than the energy offset potentials, even at high light intensities, where the maximum VOC, at room temperature, was achieved. TiOPc/C60 heterojunctions offer higher VOC values than CuPc/C60 heterojunctions, but with a lower intrinsic driving force for exciton dissociation (photoinduced charge transfer).