Anomalous diode behavior of Cu 2 S/SnO 2 p–n junction

Cu-doped SnO 2 thin films present application as a gas sensor in H 2 S atmosphere, since the conductivity of SnO 2 is increased due to the transformation of Cu into Cu 2− x S. Based on this mechanism, a p–n Cu 2 S/SnO 2 heterojunction is proposed and the electrical transport of this device is investigated. SnO 2 thin films were obtained from the sol–gel by dip-coating technique, while Cu 2 S films were obtained from resistive evaporation. The formation of materials with low crystallinity and high disorder was analyzed by X-ray diffractograms and confirmed using optical absorption (Urbach’s energy.) The bandgaps of the materials were estimated from the Tauc plot to be 3.7 ± 0.1 eV for SnO 2 and 2.5 ± 0.1 eV for Cu 2 S. Impedance spectroscopy measurements show an accumulation of charges in the material, which possibly occurs in the depletion layer region. In addition, it shows a charge release that can be associated with the leakage current in the device. I × V measurements show a surprising behavior, opposite to that expected for a diode, with the device conducting only under reverse bias. A model has been proposed to explain this effect considering minority charge transport and interfacial barriers formed between the materials.