Cu2SnSe3 (CTSe) thin films were fabricated by selenization of a Cu-Sn alloy, which was electrochemically co-deposited from a stirred citrate solution. Photoelectrical and optical properties of the obtained CTSe thin films were investigated by using only the electrochemical (we applied the cyclic voltammetry for determination the band gap of Cu2SnSe3 thin film semiconductor), photoelectrochemical and electrochemical impedance spectroscopy methods. New method is suggested, which allows to estimate the relationship between the etching time of secondary copper selenide phase and the photoelectrical properties (open circuit photopotential and photocurrent) of the CTSe thin films. The Cu2SnSe3 thin film with the best photo-response was obtained at a selenization temperature of 500 degrees C. A significant improvement in photoelectrical behavior was observed on the CTSe thin film after 30 s of chemical etching in a 0.75 M KCN aqueous solution with the aim to remove the residual copper selenide phase. This film has a good photoelectrical and optical properties for solar cell formation: a p-type conductivity, a band gap of 0.92 eV, a carrier concentration N-alpha = 1.45 x 10(17) cm(-3) and flat band potential E-FB of 0.297 V vs. saturated hydrogen electrode and a photocurrent density of 0.44 mA x cm(-2) at cathodic potential of -600 mV vs. saturated hydrogen electrode. A diagram of energy bands of the CTSe/electrolyte junction has been presented. (C) The Author(s) 2019. Published by ECS.