-Astract- Researches on renewable energy have gaining great attention due global climate change and exhaust of fossil fuels. In particular, photovoltaic (PV) devices have attracted increasing research interest owing to practically infinite supply of energy from the sun. Among numerous PV materials, Cu(In,Ga)(Se,S)2 is one of the most strong candidates to realize the high-efficiency polycrystalline thin film solar cells. Cu(In,Ga)(Se,S)2 is a direct band gap semiconductor and its band gap energy (Eg) (1.2~1.5 eV) is tunable by adjusting the composition. The highest energy conversion efficiency of the Cu(In,Ga)(Se,S)2 thin film solar cell exceeds 20%. Recently there have been intensive researches to develop non-vacuum fabrication processes of Cu(In,Ga)(Se,S)2 thin film solar cells for further cost reduction and large-area capability. Thus, in-depth electrical characterization and analyses of the solution-processed Cu(In,Ga)(Se,S)2 thin film solar cells have surfaced as important topics in novel PV research communities. In this talk, I will introduce basic principles of PV characterizations of thin film solar cells. And then, I will discuss some of the recent results in my group, regarding the investigations of the Cu(In,Ga)(Se,S)2 thin film solar cells. Temperature-dependent current-voltage (I-V) characteristics could reveal the transport mechanism, including the dominant recombination process and influences of parasitic resistance. External quantum efficiency (EQE) and capacitance (C) measurements revealed the wavelength-dependent photocurrent generation behaviors and characteristics of carriers/traps, respectively. Investigations of the stacked absorber layers allowed us to explain the influence of the microstructures and composition of the constituent layers significantly on the transport and PV performance of the devices. Researches on fabrication of solution-processed Cu(In,Ga)(Se,S)2 thin film solar cells are still in early stage. Systematic electrical characterizations of them and comparative investigations with vacuum-processed cells will help us to suggest strategies to improve the efficiency.