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- W2912081196 abstract "The goal of this work is to study and to develop routes toward efficiency improvement of Kesterite based solar cells. The first part of the manuscript deals with the development of a baseline process: formation mechanism of the absorber is studied according to the growth condition for both Cu2ZnSnS4 (pure sulfur absorber CZTS) and Cu2ZnSnSe4 (pure selenium absorber CZTSe) compounds. Two-step sequential process is used for synthesizing Kesterite material. The first step consists in the sputtering deposition of pure metallic precursors (elemental Cu, Zn, and Sn) and the second step consists in the annealing of precursors under selenium (for CZTSe in a semi-open reactor) or sulfur (for CZTS in an open reactor). In the case of CZTSe based solar cell, a maximum power conversion efficiency of 7.6% has been obtained using a two-step temperature profile and a closed susceptor. The best performance for a CZTS based device is 5.9%, this result has been obtained by optimizing the process temperature and sulfur vapor pressure: the higher sulfur vapor pressure the better device performance.Incorporation of Na (Sodium) and Sb (Antimony) in the pure sulfur Kesterite absorber has been tested as a first strategy to enhance performances of CZTS devices. Incorporation of Sb does not show any improvement in terms of material or device properties, whereas improved morphology is obtained by co-doping with Na and Sb. However, this improvement is not related to any effect on device properties. Thus, using Sb proved to be not beneficial for the CZTS-based solar cell. On the other hand, intentional contamination with Na is found to be beneficial particularly in terms of open circuit voltage. As a result, the device power conversion efficiency with optimized Na content is doubled (> 4.5%) compared to the reference sample without Na.The second study to increase efficiencies in Kesterite solar cells deals with the introduction of chalcogen (S/Se) gradients as the function of depth in the absorber. The aim is to obtain bandgap gradients in order to increase carrier collection length as well as decrease carrier recombination. For this purpose, two processes are developed to realize only simple grading (front or back surface gradients) which consist of sequential annealing stages (sulfurization/selenization) of precursor stacks. To obtain a front surface gradient, a sulfurization step at various temperatures and for different duration has been tested after a standard selenization process. A higher sulfurization temperature shows a higher degree of grading. A pure sulfur-based defect layer is also formed during this process, which can be removed using an HCl etching. A maximum efficiency of 3.5% is achieved with a CZTS-based device using this synthesis process. To realize back grading, variable temperature sulfurization annealing prior to a standard selenization process has been used. At a low temperature of sulfurization, good absorber morphologies are obtained but without the evidence of chalcogen gradient while using higher sulfurization temperature leads to graded absorbers but with poor morphology. Thus, the routes and limitations to realize kesterite absorber with gradient are proposed." @default.
- W2912081196 created "2019-02-21" @default.
- W2912081196 creator A5029138063 @default.
- W2912081196 date "2018-12-03" @default.
- W2912081196 modified "2023-09-27" @default.
- W2912081196 title "Pathways towards efficiency improvement of Kesterite based solar cell" @default.
- W2912081196 hasPublicationYear "2018" @default.
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