Impedance spectroscopy on copper phthalocyanine diodes with surface-induced molecular orientation

• Monolayer graphene serves as a hole-injecting and molecular-templating surface.
• Graphene leads to enhanced out-of-plane mobility of CuPc compared to PEDOT:PSS.
• Impedance spectroscopy establishes physical understanding of diode behaviors.

Molecular orientation and packing motif governs charge-transport property of organic semiconductor films, especially for planar small molecules. We analyze the surface-induced orientation of copper phthalocyannine (CuPc) molecules deposited on graphene or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) covered indium-tin-oxide (PEDOT:PSS/ITO). The CuPc films deposited on graphene are templated with preferential face-on stacking, whereas the molecules on PEDOT:PSS/ITO crystallize with edge-on ordering. Static current–voltage measurement and small-signal impedance spectroscopy are combined to elucidate the structural impact on the electrical response when those films are part of a rectifying diode. The graphene-templated diode shows enhanced out-of-plane hole conduction as compared to the diode with a PEDOT:PSS/ITO contact. Equivalent circuits describing charge injection and transport properties are proposed.

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