Rear band gap grading strategies on Sn−Ge-alloyed kesterite solar cells

Kesterite solar cells are at a crossroads, and a significant breakthrough in performance is needed for this technology to stay relevant in the upcoming years. In this work, we propose to follow the proven strategy of band engineering to assist charge carrier collection taking inspiration from chalcopyrite solar cells. Using a process based on a combination of metallic precursor sputtering and chalcogen-reactive annealing, we achieve controlled cationic substitutions by partly replacing Sn by Ge, hence tailoring several rear band gap grading profiles along the absorber thickness. A complete set of results is presented, with samples ranging from pure Sn to pure Ge compounds. The formation of a rear band gap grading is determined through different characterization techniques, specifically through a combination of glow discharge optical emission and Auger spectroscopies with an advanced multiwavelength Raman spectroscopy analysis carried out at the front and back (rear) sides of the films using a lift-off process. As such, a preferential Ge enrichment toward the back of the absorber is unequivocally demonstrated in kesterite absorbers and further applied to complete devices for deliberately generating distinct rear band gap profiles, leading to an efficient back surface field that potentially enhances the carrier selectivity of the back interface. The electrical analysis of the complete devices shows a complex interplay between the benefits of band gap grading and possible Ge-related defects in the absorber. Using optimized synthesis conditions, an absolute increase in efficiency (compared to the Ge-free reference) is obtained for the record device (η > 9%) without any antireflective coating or metallic grid. This performance enhancement is mostly ascribed to the presence of a drift electric field assisting in the carrier collection while preventing back side recombination. These results confirm the possibility of generating back band gap grading in kesterite solar cells and open the way to further development of the kesterite photovoltaic technology toward higher efficiencies through tailored band gap engineering.