Nanostructured organic–inorganic heterojunction diodes as gas sensors

Abstract

Nanostructured organic–inorganic heterojunction devices were fabricated by employing n-type zinc sulphide (ZnS) grown on ITO by RF magnetron sputtering and thin film of zinc phthalocyanine (ZnPc) deposited on the top of the ZnS layer by thermal evaporation. On the top of the ZnPc layer, an Al film was coated to form an electrode. Thin films of ZnS and ZnPc were characterized by optical absorption and Field Emission Scanning Electron Microscopy (FESEM). Current–voltage (I–V) characteristics of the fabricated ITO/ZnS/ZnPc/Al hetero-structures were studied at different temperatures (30 to 90 °C) in vacuum and diode-like behaviour was observed. In the case of single layer devices ITO/ZnS/Al and ITO/ZnPc/Al diode-like behavior was not obtained. The effects of donor ammonia and acceptor oxygen gases on the electrical properties of single layer devices ITO/ZnS/Al, ITO/ZnPc/Al and ITO/ZnS/ZnPc/Al heterojunctions were studied in order to investigate the utility of these devices as gas sensors. A drastic increase in forward current of the heterojunction has been noticed in the presence of donor ammonia gas containing water vapor, while a decrease in forward current has been noticed in the presence of acceptor oxygen gas. This change in forward current (diode current) in the presence of gases (under doped conditions) has been explained by the creation of charge carriers inside the ZnPc layer and hence bending of ZnPc bands. The threshold voltage (V_th) of the junction diodes has been found to depend on the temperature and the presence of donor and acceptor gases. Our experimental results suggest that this heterojunction diode could be a novel candidate for gas sensors.