High Efficiency Cu(In,Ga)Se2 Flexible Solar Cells Fabricated by Roll-to-Roll Metallic Precursor Co-sputtering Method

Abstract

We report on a Cu(In,Ga)Se2 (CIGS) solar cell fabricated on flexible stainless steel substrate by a low cost mass production roll-to-roll process. Fabricated device has a high energy conversion efficiency of 14%, with short circuit current density (Jsc) of 36.6 mA cm-2 and open circuit voltage (Voc) of 0.55 V. A two-dimensional (2D) simulation model for CIGS solar cell design and optimization was proposed. Opto-electrical properties showed that both experimental and simulated results are consistent with each other. The photons absorber in CIGS solar cells was prepared by co-sputtering metallic precursors of In and CuGa followed by thermal annealing in Se vapor. The device chemical properties were analyzed by secondary ion mass spectrometry (SIMS) and transmission/scan electron microscopy (TEM/SEM). Indium and gallium interdiffusions were observed during the growth of film, forming a band grading in CIGS layer. Accumulation of In at the top CIGS surface, resulting in a low bandgap, was responsible for the limited output open circuit voltage. Nano-scale voids were observed in the grown CIGS layer. A model based on Kirkendal effect and interdiffusion of atoms during selenization is developed to explain the formation mechanism of these voids. Na and K incorporation as well as metallic impurities diffusion are also discussed.