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.

Example of an advanced characterization study of a complex system (thin film solar cell) that may serve as an instructive handbook to help building up the full picture of multilayer-based devices for a broad spectrum of readers and researchers.

This work presents the development of a novel chalcogenization process for the fabrication of Cu 2 ZnSn(S,Se) 4 (CZTSSe or kesterite)-based solar cells that enable the generation of sharp graded anionic compositional profiles with high S content at the top and low S content at the bottom. This is achieved through the optimization of the annealing parameters including the study of several sulfur sources with different predicted reactivities (elemental S, thiourea, SnS, and SeS 2 ). As a result, depending on the sulfur source employed, devices with superficially localized maximum sulfur content between 50 and 20% within the charge depletion zone and between 10 and 30% toward the bulk material are obtained. This complex graded structure is confirmed and characterized by combining multiwavelength depth-resolved Raman spectroscopy measurements together with in-depth Auger electron spectroscopy and X-ray fluorescence. In addition, the devices fabricated with such graded band gap absorbers are shown to be fully functional with conversion efficiencies around 9% and with improved V OC deficit values that correlate with the presence of a gradient. These results represent one step forward toward anionic band gap grading in kesterite solar cells. KEYWORDS: thin-film solar cells, kesterite, Cu 2 ZnSn(S,Se) 4 , band gap grading, V OC deficit

This work provides insights for understanding and further developing the Cu2ZnGeSe4 photovoltaic technology, and gives an example of the potential of combinatorial analysis and machine learning for the study of complex systems in materials research.

An unusual solid–liquid–vapor extended growth mechanism is reported in the synthesis reaction of Cu2ZnGeSe4 absorbers through a specific break-off experiment.