Ambipolar Organic Thin Film Transistor-like Behavior of Cationic and Anionic Phthalocyanines Fabricated Using Layer-by-Layer Deposition from Aqueous Solution

Organic thin film field effect transistors of water-soluble cationic and anionic phthalocyanine derivatives were successfully prepared using the layer-by-layer deposition technique. The alternating film growth and subsequent transistor properties were studied with a variety of techniques including ellipsometry, UV−vis spectroscopy, polarized UV−vis spectroscopy, AFM, and field effect transistor measurements. A stepwise and regular deposition of copper(II) phthalocyanine tetrakis(methyl pyridinium) chloride and copper(II) phthalocyanine tetrasulfonic acid, tetrasodium salt multilayers in both pure water and 0.03 M NaCl solution was observed. Differences in linear growth were observed between pure water and 0.03 M NaCl-based solutions. Polarized UV−vis spectra indicated that the conjugated phthalocyanine ring lies almost flat on the substrate surface with a random orientation within the plane of the substrate. Unusual “ambipolar” transistor-like behavior was found for the films that can be attributed to an ion-modulated electrical conduction mechanism, which relies on the assistance of mobile ions to stabilize the oxidized or reduced species in the channel region.