Interplay of electrode geometry and bias on charge transport in organic heterojunction gas sensors

Abstract

Modulation of charge transport in an organic heterojunction sensor is a promising strategy to enhance its gas sensing properties. Herein, a heterostructure consisting of a bilayer assembly of perfluorinated copper phthalocyanine and lutetium bis-phthalocyanine is investigated for ammonia sensor development in two different electrode geometries and in a wide range of applied bias. The microstructure of the heterostructure retains the bulk electronic and structural properties of the individual components and shows granular distribution of phthalocyanine crystallites on the surface. The charge transport of the heterojunction devices depends on the electrode geometry and the applied bias. Notably, interfacial charge transport gets faster, while bulk charge transport remains constant with increasing bias in both devices. But, the small gap between the electrodes fastens the bulk and the interfacial charge transport by 75 and 5 times, respectively, compared to the big gap electrodes. The implication of faster charge transport in the small gap electrodes-based sensor is demonstrated on its ammonia sensing properties, exhibiting higher response and shorter response time. Moreover, the response of the sensor exponentially increases with increasing bias. The high sensitivity and stability of the sensor at different relative humidity make it a suitable NH3 sensor for real environment applications.