Influence of sodium-containing substrates on Kesterite CZTSSe thin films based solar cells
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Influence of sodium-containing substrates on Kesterite CZTSSe thin films based solar cells Giovanni Altamura1,3,*, Charles Roger1, Louis Grenet1, Joël Bleuse1,2, Hélène Fournier1, Simon Perraud1, Henri Mariette1,2,3 2
1 CEA Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France Institut Néel - CNRS, 25 rue des Martyrs, 38054 Grenoble Cedex 9, France 3 Joseph Fourier University, 38041 Grenoble Cedex 9, France
*Corresponding author. Tel.: +33(0)4-38-78-14-68. E-mail: [email protected] Abstract — This work deals with the influence of sodium on the properties of CZTSSe material and solar cells. For that purpose, two types of substrates are compared, one with low sodium content (borosilicate glass), the other one with higher sodium content (soda-lime glass). In each case the Na-content in the CZTSSe passing from the substrate through the Mo back contact is quantified by secondary ion mass spectroscopy analysis. Photoluminescence spectroscopy indicates that better quality material is achievable when increasing the Na-content in the CZTSSe. The material characterization results are compared to the photovoltaic properties. Index Terms — Cu2ZnSn(S1-xSex)4, CZTSSe, CZTS, CZTSe, Sodium, Kesterite, thin film, solar cell. I. INTRODUCTION The scientific community has tried to tackle the question of the sustainability of the necessary terawattscale power generation capacities. Photovoltaics is a cost-effective solar energy technology which could meet the increase in electricity demand, while avoiding the worst effects of climate change. Since the Cu2ZnSn(S1-xSex)4 (CZTSSe) semiconductor compound consists in the relatively inexpensive, earth abundant, and nontoxic elements Cu, Zn, Sn, S, and Se, a considerable effort is focused on improving its efficiency. Some of the CZTSSe characteristics are: – a bandgap predicted [1] to be between 1.0 eV (Cu2ZnSnSe4) and 1.5 eV (Cu2ZnSnS4); – an absorption coefficient larger than 104 cmí1 [1]; – a good tolerance of its crystallographic structure to shifts from the stoichiometric composition [2]. In addition, the knowledge gathered on Cu(In1-xGax)Se2 (CIGS) can be used and adapted to CZTSSe solar cells yielding efficiencies up to 11% [3]. In the case of CIGS solar cells, it is well known that the presence of sodium in the absorber layer is beneficial [4] and necessary to obtain high conversion efficiencies [5]. It mainly improves device performance through the fill factor (FF) and open circuit voltage (Voc) [6]. Although the exact action of Na on the structural and electronic properties of the absorber material is not clearly understood, defect passivation at the grain boundaries and the increase of doping are nowadays the most favored explanations [7]. This paper reports on the characterization methods of CZTSSe layer grown on borosilicate (BS) and sodalime glass (SLG) substrates, and on electrical performances of solar cells built utilizing these same layers. II. EXPERIMENTAL DETAILS CZTSSe has been synthesized on two different Mo-coated SLG (12 wt% Na2O) and BS (0.01 wt% Na2O)
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