Tuning of interfacial charge transport in polyporphine/phthalocyanine heterojunctions by molecular geometry control for an efficient gas sensor

Owing to high interfacial conductivity, organic heterostructures hold great promises to augment the electrical performances of electronic devices. In this endeavor, the present work reports fabrication of novel polyporphine/ phthalocyanine heterostructures and investigates the modulation of charge transport induced by structural change of polyporphine and its implication on ammonia sensing properties. Polyporphines materials are electrosynthesized by oxidation of zinc(II) porphine monomer that corresponds to the fully unsubstituted porphyrin. At less-positive anodic potential, low conducting meso,meso-singly-linked type-1 polymer (pZnP-1) is formed in which a monomer unit stays orthogonal to its neighbors. At higher anodic potential, monomer units are fused in the 2D plane to produce beta,beta-meso-meso-beta,beta-triply-fused type-2 polymer (pZnP-2), having a pi-conjugated structure and high conductivity. Association of these polymers in organic heterojunction devices with lutetium bisphthalocyanine (LuPc2) reveals non-linear current-voltage (I-V) characteristics typical for interfacial accumulation of charges in the heterostructure for pZnP-1 and a linear I-V behavior for pZnP-2. Characterization of these heterojunctions by impedance spectroscopy further confirms the predominance of interfacial charge transport in pZnP-1/LuPc2 which is improved with increasing bias, while largely bulk charge transport independent of bias prevails in pZnP-2/LuPc2 device. Different regimes of charge transport influence ammonia-sensing properties of the devices, such that pZnP-1/LuPc2 demonstrates highly sensitive, reversible and stable response, while pZnP-2/ LuPc2 shows low and unstable response.