Abstract
Electronic transitions at interfaces between MoO3 layers and organic layers of C60, SubPc, MgPc, and nano-composite layers of SubPc:C60 and MgPc:C60 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 C60, SubPc, and MgPc and the effective EHL of SubPc:C60 and MgPc:C60 were measured. The offsets between the LUMO (ΔEL) or HOMO (ΔEH) bands were obtained with high precision and amounted to 0.33 or 0.73 eV for SubPc:C60, respectively, and to −0.33 or 0.67 eV for MgPc:C60, 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.
Highlights
Charge separation and transport at interfaces of solar cells are of basic importance for efficient devices
Electronic transitions at interfaces between MoO3 layers and organic layers of C60, SubPc, MgPc, and nano-composite layers of SubPc:C60 and MgPc:C60 have been studied by modulated surface photovoltage (SPV) spectroscopy
The offsets between the lowest unoccupied molecular orbital (LUMO) (DEL) or highest occupied molecular orbital (HOMO) (DEH) bands were obtained with high precision and amounted to 0.33 or 0.73 eV for SubPc:C60, respectively, and to À0.33 or 0.67 eV for MgPc:C60, respectively
Summary
Charge separation and transport at interfaces of solar cells are of basic importance for efficient devices. This task becomes even more striking for organic-inorganic hybrid solar cells. Illumination of organic semiconductors results in strongly bound excitons which should dissociate for charge separation in a solar cell. The alignments of the highest occupied molecular orbitals (HOMOs) and of lowest unoccupied molecular orbitals (LUMOs) are essential for the effective dissociation of excitons in organic solar cells. It has been shown that the offsets of the HOMO and/or LUMO bands (DEH and DEL, respectively) should be slightly larger than the exciton binding energy which is of the order of 0.4 eV for organic semiconductors.
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