Thermal Annealing Influence on the Properties of Heterostructure Based on 2 at.%Eu Doped SnO2 and Cu1.8S

Aiming at optoelectronic applications, a heterostructure based on Eu-doped tin oxide (SnO 2 ) and copper sulfide (Cu 2− x S) is built. SnO 2 thin films doped with 2 at.% Eu were obtained by the sol–gel dip-coating and spin-coating techniques, whereas the Cu 2− x S film was obtained by resistive evaporation. Samples were prepared using three distinct thermal annealing temperatures of the SnO 2 bottom layer: 150°C, 250°C and 500°C. Transmittance and absorption spectra of the heterostructure shows high transparency in the visible to near infrared range (600–1800 nm), and considering the dominance of SnO 2 on light absorption, it was possible to evaluate the sample indirect bandgap around 3.5 eV, independently of the thermal annealing temperature. Cyclic voltammetry and impedance spectroscopy, in conjunction with calculation of the hysteresis index, show that the heterostructure presents a behavior highly capacitive, and the higher annealing temperature leads to higher capacitance at low frequencies, similar to the observed qualitative behavior of supercapacitive devices. Besides, the sample with the SnO 2 bottom layer annealed at 500°C yielded a higher current density.