Influences of the surface treatments of Cu(In,Ga)(Se,S)(2) (CIGSSe) thin films, which are KCN, HCl, or thiuorea treatments, were investigated by time-resolved photoluminescence (TRPL) and temperature dependent current density-voltage (J-V) characteristics of their solar cells. It is demonstrated that the KCN treatment optimized under 1 wt% leads to the significant increase in conversion efficiency (eta) up to 19.21%. On the other hand, the eta of the CIGSSe solar cells is in ranges of 13.70-15.51% and 9.86-10.70% with the HCl treatments (0.3-0.7 mol/L), and thiuorea treatments (0.5-1.5 mol/L), respectively, which are lower than 16.66% that of the reference solar cell without the surface treatment. According to TRPL measurements, the quality of near-surface CIGSSe is improved with the KCN treatment (1 wt%) owing to enhanced TRPL lifetimes, whereas that is deteriorated with the HCl and thiuorea treatments due to decreased TRPL lifetimes. In addition, according to the temperature-dependent J-V measurement, the interface recombination of the CIGSSe solar cell is decreased with the KCN treatment, while that of the CIGSSe solar cells is increased with the HCl and thiuorea treatments. Ultimately, 19.21%-efficient CIGSSe solar cell with the KCN treatment (1 wt%) at room temperature with the increased V-oc of 0.692 V was obtained, which is around 15.3% relatively higher eta than that of the solar cell without the surface treatment. (C) 2017 Elsevier B.V. All rights reserved.