Abstract
Accurate anionic control during the formation of chalcogenide solid solutions is fundamental for tuning the physicochemical properties of this class of materials. Compositional grading is the key aspect of band gap engineering and is especially valuable at the device interfaces for an optimum band alignment, for controlling interface defects and recombination and for optimizing the formation of carrier-selective contacts. However, a simple and reliable technique that allows standardizing anionic compositional profiles is currently missing for kesterites and the feasibility of achieving a compositional gradient remains a challenging task. This work aims at addressing these issues by a simple and innovative technique. It basically consists of first preparing a pure sulfide absorber with a specific thickness followed by the synthesis of a pure selenide part of complementary thickness on top of it. Specifically, the technique is applied to the synthesis of Cu2ZnSn(S,Se)4 and Cu2ZnGe(S,Se)4 kesterite absorbers, and a series of characterizations are performed to understand the anionic redistribution within the absorbers. For identical processing conditions, different Se incorporation dynamics is identified for Sn- and Ge-based kesterites, leading to a homogeneous or graded composition in depth. It is first demonstrated that for Sn-based kesterite the anionic composition can be perfectly controlled through the thicknesses ratio of the sulfide and selenide absorber parts. Then, it is demonstrated that for Ge-based kesterite an anionic (Se–S) gradient is obtained and that by adjusting the processing conditions the composition at the back side can be finely tuned. This technique represents an innovative approach that will help to improve the compositional reproducibility and determine a band gap grading strategy pathway for kesterites. Furthermore, due to its simplicity and reliability, the proposed methodology could be extended to other chalcogenide materials.
Highlights
Thin-film techniques based on different chalcogenide materials are widely explored owing to their versatility, tailorable properties, and relatively low-cost manufacturing processes, offering a high potential for optoelectronic device application.Frequently, accurate anionic and/or cationic control for the formation of chalcogenide solid solutions is fundamental to the tuning of the physicochemical properties of the compounds and is one of the main issues in the field
An innovative approach is proposed for accurately controlling the anionic compositional ratio in kesterite absorbers that consists of first preparing a pure sulfide absorber with a specific thickness followed by the synthesis of a pure selenide part of complementary thickness on top of it. We employ this methodology for the synthesis of Cu2ZnSn(S,Se)[4] (CZTSSe) and Cu2ZnGe(S,Se)[4] (CZGSSe) kesterite absorbers, and, through the use of several characterization techniques, we show that Se presents a different incorporation dynamics in each of them, leading to a homogeneous or graded in-depth composition
We demonstrate that for Snbased kesterite, the anionic composition can be perfectly regulated through the thicknesses ratio of the sulfide and selenide absorber parts and that for the Ge-based kesterite an anionic (Se−S) gradient is obtained and can be finely tuned by adjusting the processing conditions
Summary
Thin-film techniques based on different chalcogenide materials are widely explored owing to their versatility, tailorable properties, and relatively low-cost manufacturing processes, offering a high potential for optoelectronic device application.Frequently, accurate anionic and/or cationic control for the formation of chalcogenide solid solutions is fundamental to the tuning of the physicochemical properties of the compounds and is one of the main issues in the field. Thin-film techniques based on different chalcogenide materials are widely explored owing to their versatility, tailorable properties, and relatively low-cost manufacturing processes, offering a high potential for optoelectronic device application. No consensus exists on reliable processes and reproducible efficiencies, which remain extremely challenging despite ongoing efforts at the different levels of the device. This relates to the difficult control of the synthesis process of the material and to the various layers and interfaces involved in the full solar cell devices. Interfaces within the absorber itself, i.e., defects at the grain boundaries or even in-grain defects, can seriously alter the performance of the devices.[8−10]
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