Structural, Optical, and Electrical Properties of in Situ Synthesized ZnO-CuPc Nanocomposites

Nanocomposites of copper phthalocyanine (CuPc) and ZnO nanoparticles (NPs) have been grown in situ in a colloidal solution of CuPc using zinc acetate as precursor and sodium hydroxide as precipitating agent. n-Type ZnO NPs form a network with p-type CuPc by donating electrons as evidenced by various techniques. Increase in NaOH concentration produces larger ZnO nanostructures (NS) with higher aspect ratio (length/diameter) that influences the aggregation state of the CuPc. The drop-cast films of pure CuPc show stacking in both H and J aggregation modes which changes due to composite formation. A significant blue-shift in the Q-band corresponding to the J aggregated mode (similar to 30 nm) indicates a change in the aggregation state of CuPc molecules from slipped facial to cofacial stacking. A defect related emission from ZnO shows a blue-shift with a reduced intensity, confirming the formation of ZnO nanorods that are firmly attached to the CuPc in the composite. Further, current voltage (I-V) characteristics of nanocomposites exhibit rectification ratio of about 20 at 2 V which is absent in the individual constituent materials. Photoluminescence lifetime as well as I-V measurement confirms an intimate mixing of the constituent materials through the electron transfer reaction. The broadening of the Q bands in the absorption spectrum due to a change in the aggregation may increase the light harvesting capacity of the composite material for photovoltaic applications.