GaSe Formation at the Cu(In,Ga)Se2/Mo Interface-A Novel Approach for Flexible Solar Cells by Easy Mechanical Lift-Off

Implementing photovoltaic devices based on high efficiency thin-film technologies on cheap, light-weight and flexible polymeric substrates is highly appealing to cut down costs in industrial production and to accelerate very large scale deployment of photovoltaics in the upcoming years. Lift-off processes, which allow separating active layers from primary substrates and subsequent transfer onto an alternative substrate without modifying the upstream production process and without performance losses, are an emerging alternative to direct growth on polymeric substrates. This study concerns the feasibily of direct mechanical lift-off process for high efficiency Cu(In,Ga)Se-2 (CIGS) thin film solar cells grown by coevaporation on glass/molybdenum substrates without performance losses. The study presents an in depth characterization (SEM, AFM, GIXRD, XPS) of samples leading to excellent lift-off properties. They are explained by a specific gallium rich CIGS graded interface structure according to the interfacial sequence glass/Mo/MoSez/GaxSey/Ga-rich-CIGS. The interfacial layer, attributed to GaSe, has a layered structure and out performs the molybdenum diselenide layered layer which forms spontaneously at the interface Mo/CIGS. It allows a very easy lift-off process at the interface GaSe/CIGS thanks to Van-der-Waals adhesion mechanism in GaSe. Key physical-chemical parameters are identified and analyzed. After lift-off, an efficiency of 14.3%, higher than the initial reference CIGS solar cell efficiency (13.8%) is measured.