Synthesis and physical properties of Cu(In,Ga)Se2 nanoparticles and CuGaSe2 thin-films for tandem cell photovoltaic applications

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 pn-junction structures showed photovoltaic properties with J_sc of 31.7 nA/cm² and V_oc 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. 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.