Phase control in selenium electrodeposition with bath temperature and deposition potential

In the present work selenium films were electrodeposited onto fluorine-doped SnO₂ substrates, departing from a Se⁴⁺ aqueous electrolyte. The temperature and deposition potential were varied; the structure, morphology and band gap were studied by Atomic Force Microscopy, Raman Spectroscopy and optical absorption. The voltammetry study showed two reduction peaks that shift towards positive potentials as the deposition temperature increases from 25 to 75 °C: the first one shifts between -925 mV and -1120 mV versus Pt and the second between -1060 mV to -1250 mV versus Pt A black/greyish selenium film of a-Se and trigonal Se was deposited at -925 mV@75 °C and a deep red monoclinic Se one at -960 mV@50 °C; in the other conditions the film color indicate a mixture of the red a-Se and trigonal phases confirmed by Raman and optical spectroscopy. Trigonal phase has a calculated band gap of 1.6 eV, red and black a-Se band gap value of 1.83 eV, and monoclinic Se of 2.0 eV, respectively. From Electrochemical Impedance Spectroscopy, the films were found to be n-type, with carrier densities around 10¹⁸-10¹⁹ cm^-3, and Flat Band Potentials between 0.6-1.0 V in dependence of the deposition conditions. The results point out the feasibility of controlling the phase of Se layers using a single aqueous bath and mild temperature conditions.