For tandem cell photovoltaic applications, we have investigated Cu(In,Ga)Se2 (CIGS) nanoparticles and CuGaSe2 (CGS) thin-films. Alternative structures with metal Mo and transparent conductive oxide (TCO) back contacts were also studied. The CIGS nanoparticles were synthesized by pulsed laser ablation (PLA) and treated in a Se annealing process after deposition. Topography, optical and other analysis results revealed that they show unique morphology, bandgap shift, and high absorption coefficient. Electrical characteristics of the pnjunction structures showed photovoltaic properties with Jsc of 31.7 nA/cm2 and Voc of 0.17 V in case of nanopaticles. The CGS thin-films were grown via a single step by co-evaporation. Various processes for CGS films have been proposed such as bi-layer and three stage processes [3,4]. Although the so-called three-stage process has recorded the best efficiency until now, we have prepared CGS thin-films by a single process to make sure reproducibility. In order to use CGS thin films as a top cell of tandem devices, we investigated optical bandgap and photovoltaic properties. Properties of the CGS thin-films on TCO will be compared those of the films on Mo.