Ambient effects on fullerene/copper phthalocyanine photovoltaic interface

Effects of ambient-air exposure on the energy levels at photovoltaic interface of fullerene (C-60)/copper phthalocyanine (CuPc) were studied using ultraviolet photoemission spectroscopy. The junction prepared in ultrahigh vacuum showed flat energy levels with little vacuum level offset, while exposure to ambient air at 10(-5) Torr induced p-type doping of C-60 with energy levels bend up for 0.27 eV. The energy difference between HOMOCuPc-LUMOC60, describing the theoretical maximum open-circuit voltage, increased from 0.64 to 0.81 eV. The exposure moved the LUMOC60 away from the Fermi level, leading to reduction in carrier concentration and film conductivity.