Electronic transitions and band offsets in C60:SubPc and C60:MgPc on MoO3 studied by modulated surface photovoltage spectroscopy

Electronic transitions at interfaces between MoO3 layers and organic layers of C-60, SubPc, MgPc, and nano-composite layers of SubPc: C-60 and MgPc:C-60 have been studied by modulated surface photovoltage (SPV) spectroscopy. For all systems, time dependent and modulated SPV signals pointed to dissociation of excitons at the MoO3/organic layer interfaces with a separation of holes towards MoO3. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps (EHL) of C-60, SubPc, and MgPc and the effective EHL of SubPc: C-60 and MgPc:C-60 were measured. The offsets between the LUMO (DEL) or HOMO (DEH) bands were obtained with high precision and amounted to 0.33 or 0.73 eV for SubPc:C-60, respectively, and to -0.33 or 0.67 eV for MgPc:C-60, respectively. Exponential tails below EHL and most pronounced sub-bandgap transitions were characterized and ascribed to disorder and transitions from HOMO bands to unoccupied defect states. (C) 2015 AIP Publishing LLC.