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

Aiming at optoelectronic applications, a heterostructure based on Eu-doped tin oxide (SnO2) and copper sulfide (Cu2-xS) is built. SnO2 thin films doped with 2at.% Eu were obtained by the sol-gel dip-coating and spin-coating techniques, whereas the Cu2-xS film was obtained by resistive evaporation. Samples were prepared using three distinct thermal annealing temperatures of the SnO2 bottom layer: 150 degrees C, 250 degrees C and 500 degrees C. Transmittance and absorption spectra of the heterostructure shows high transparency in the visible to near infrared range (600-1800nm), and considering the dominance of SnO2 on light absorption, it was possible to evaluate the sample indirect bandgap around 3.5eV, 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 SnO2 bottom layer annealed at 500 degrees C yielded a higher current density.