Titanyl phthalocyanine/C60 heterojunctions:: Band-edge offsets and photovoltaic device performance

Planar heterojunction organic photovoltaic devices have been created using oxo-titanium phthalocyanine (TiOPc) as the donor layer and fullerene (C-60) as the acceptor layer, with comparisons to devices based on copper phthalocyanine (CuPc) as the donor. TiOPC/C-60 and CuPC/C-60 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 (E-HOMO(D) - E-LUMO(A)), which is related to the open-circuit photopotential (V-OC). A small interface dipole effect was seen at the TiOPc/C-60 interface (eD approximate to 0.02 eV), whereas a significant interface dipole was observed for the CuPC/C-60 interface (eD approximate to 0.3 eV). On the basis of the work presented here and previously reported electrochemical and UPS/XPS studies, we estimate an E-HOMO(D) - E-LUMO(A) energy offset of ca. 1.1 eV for the TiOPc/C-60 heterojunction and 0.7 eV for the CuPC/C-60 heterojunction. Maximum V-OC 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 V-OC, at room temperature, was achieved. TiOPc/C-60 heterojunctions offer higher V-OC values than CuPC/C-60 heterojunctions, but with a lower intrinsic driving force for exciton dissociation (photoinduced charge transfer).