The Effect of Soft Pre-Annealing of Differently Stacked Cu-Sn-Zn Precursors on the Quality of Cu 2 ZnSnSe 4 Absorbers
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The Effect of Soft Pre-Annealing of Differently Stacked Cu-Sn-Zn Precursors on the Quality of Cu2ZnSnSe4 Absorbers Monika Arasimowicz1, Maxime Thevenin2, and Phillip J. Dale 1 1 Laboratory for Energy Materials, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg 2 Laboratory for Photovoltaics, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg ABSTRACT Cu2ZnSnSe4 p-type semiconductors currently investigated for use in thin film solar cells can be synthesized by firstly depositing a metallic precursor and secondly annealing the precursor in selenium vapor. Differently stacked Cu-Sn-Zn metallic precursors were characterized after a soft annealing at 350°C under nitrogen atmosphere. For the stack where the Sn and Zn were in direct contact with sufficient Cu to form a stable alloy, a bi-layered structure consisting of Cu-Sn on the bottom and Cu-Zn on the top was formed. Contrarily, when Zn was not in direct contact with Cu, the metals diffused to form a stable alloy and the system segregates horizontally, forming a mixed columnar structure. These two types of precursors were selenized under exactly the same conditions to form kesterite absorbers for solar cell devices. Using this approach the improvement from 0.44% power conversion efficiency for the bi-layered precursor to 4.5% for the mixed precursor was achieved. INTRODUCTION Vapor phase chalcogenisation of Cu-Sn-Zn metallic precursors is a low cost and scalable method of thin film Cu2ZnSn(S,Se)4 (CZTSSe) fabrication. However different absorber properties were achieved using different stacking orders for the metals [1-3]. None of the published studies explained why some metal stacking orders are successful, whilst others are not. Figure 1 shows the existence of phases in the Cu-Sn-Zn alloy system at 250°C investigated experimentally by Chou et al.[4]. Mainly a liquid phase was observed for the composition relevant to the stoichiometry of kesterite. The existence of liquid Sn, Cu-Sn alloys (bronzes) and Cu-Zn alloys (brasses) was confirmed and no ternary compound could be found in that system.
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Figure 1. The isotherm of Cu-Sn-Zn phase equilibria at 250°C, (after Chou et al.[4]). The star indicates the stoichiometry of precursors deposited in this study. Here we consider the distribution of the metals in the precursor. Our hypothesis is that for any metal precursor stack where Sn or Zn are not in direct contact with sufficient Cu to form a stable Cu-Sn alloys and Cu-Zn alloys, they will tend to diffuse to find the Cu that they need. This alloying is slow at room temperature, but accelerated during the heating ramp of the annealing step, before the chalcogen has time to react with the precursors. As a consequence two dimensional or three dimensional metallic structures emerge before the chalcogenisation step and this could lead to different absorber quality. Using mass transport controlled electrodeposition and soft annealing different Cu-Sn-Zn alloys were synthesized. Depending on the way the layers are deposited two dimensi
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