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 (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 (EHOMO^D - E_LUMO^A), which is related to the open-circuit photopotential (V_OC). 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 E_HOMO^D - E_LUMO^A energy offset of ca. 1.1 eV for the TiOPc/C₆₀ heterojunction and 0.7 eV for the CuPc/C₆₀ 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/C₆₀ heterojunctions offer higher V_OC values than CuPc/C₆₀ heterojunctions, but with a lower intrinsic driving force for exciton dissociation (photoinduced charge transfer).