Charge transfer efficiency at the organic/electrode interface is one of the key parameters controlling the overall efficiency of organic photovoltaics (OPVs). Modification of this interface with a redox-active organic surface modifier may enhance the charge transfer across the interface by providing a facile charge-transfer pathway between the electrode and the organic active layer. Using two functionalized perylene diimide (PDI) molecules with different linkers to a phosphonic acid (PA) anchoring group, three different PDI sub-monolayer films were deposited onto indium tin oxide (ITO) surfaces (self-assembled PDI-phenyl-PA film, self-assembled PDI-diphenyl-PA film and spin-coated PDI-phenyl-PA film) to study the relationship between molecular structure, the charge transfer rate constants at the ITO surfaces and related OPV performances. Polarized attenuated total reflectance spectroscopy (polarized-ATR) shows that PDI films deposited using self-assembly are predominately out-of-plane with average tilt angles between the molecular axis and the surface normal of 33�for self-assembled PDI-phenyl-PA films and 31�for self-assembled PDI-diphenyl-PA films, while films deposited using spin-coating are more in-plane with an average tilt angle of 45�. Potential-modulated attenuated total reflectance (PM-ATR) measurements show that the spin-coated PDI-phenyl-PA film has an apparent rate constant (ks,app) of  5�104 s-1 for the first electron reduction/oxidation process. The self-assembled PDI-phenyl-PA film has an ks,app of  1.4�104 s-1 due to its more out-of plane orientation compared to the spin-coated film, while the self-assembled PDI-diphenyl-PA film has the lowest charge transfer rate constant of 0.5�104 s-1 as a result of its longer linker group compared to the PDI-phenyl-PA molecule. P3HT/PCBM bulk-heterojunction inverted organic photovoltaics (OPVs) were also fabricated on top of the PDI films on ITO. The power conversion efficiencies are higher than the OPVs fabricated on bare ITO surfaces.

Surface modification of transparent conducting oxides with functionalized perylene diimide dyes: Characterization of orientation, charge-transfer kinetics, and related OPV performance

248th National Meeting (2014)

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