A deep eutectic solvent (DES) called Reline has attracted great interests in photovoltaic application. Many efforts have been made to get high efficiency solar cells by electrodeposition in such a low-cost and green electrolyte. In this work, the micro-structure and electrochemical properties of Reline are investigated. The metal complex system of Reline and metal salts is deduced by FTIR and UV-Vis absorption spectroscopy analyses. The theoretical diffusion coefficients of Cu, In and Ga at 65 degrees C are calculated by Cyclic voltammetry analysis to perform a controllable Ga/(In+Ga) ratio over a large range (0 similar to 0.53) by different pulse potentiostatic methods. Based on these achievements and optimizations, the best CIGS solar cell device with a promising efficiency of 10.1% is achieved. (C) The Author(s) 2016. Published by ECS. All rights reserved.