One Step Cu-Zn-Sn Alloy Deposition As a Precursor for CZTS Absorber in Photovoltaics
Authors: Pedrazzetti Lorenzo , Marchi Claudio , Khalil Md Ibrahim , Lucotti Andrea , Le Donne Alessia , Binetti Simona , Magagnin Luca 
Published: February 2020

In recent years, research on thin film photovoltaic cells is focusing on the development of inexpensive, earth abundant and environmentally benign materials [1]. CZT(S,Se) (Cu2ZnSn(S,Se)4) thin films meet this unlike the present commercial photovoltaic thin films CIG(S,Se) (Cu(In,Ga)(S,Se)2) and CdTe. The crystal structure of CZT(S,Se) is obtained by replacing half of the indium and/or gallium in CIG(S,Se) by zinc and the other half by tin, which are both more earth-abundant materials than indium and gallium [2]. CZT(S,Se) has direct band gap of 1.0 eV (Cu2ZnSnSe4) to 1.5 eV (Cu2ZnSnS4) and a large absorption coefficient of over 104 cm-1. Up to now, many different techniques have been implemented to obtain CZTS thin films, featuring evaporation, sputtering, sol-gel deposition pulsed laser deposition, spray pyrolysis and many others. In order to reduce production costs and deployment of thin film PV-based technologies in mass scale, kesterite-based thin film solar cells technologies should be manufacturable at high throughput with low cost and it should exhibit good power conversion efficiency. Fabrication of kesterites on flexible substrate has high potential to decrease production cost due to roll-to-roll manufacturing on flexible substrate enables use of compact size deposition equipment with high throughput [3]. Recently electrodeposition has proven its utility as a solid route to obtain the CZT precursor both by plating a homogeneous Cu-Zn-Sn alloy or by subsequently depositing a stacked version of the layer; in both cases, reactive annealing is a mandatory step to introduce sulphur in the system, yielding kesterite crystals. In 2010, a one-step electrodeposition of copper-zinc-tin-sulphur precursors was described for the first time from a citrate/tartrate-based solution followed by an annealing step [4]. Although a one-step electrodeposition of the four elements would be ideal, the stoichiometric deposition of the quaternary coating is difficult to control and an additional annealing step to obtain the right ratio of the chalcogenide and to improve the crystallinity is still required. Furthermore, most researchers use thiosulfate as the sulphur source in acidic citrate/tartrate-based electrolytes, which is unstable and precipitates as sulphur. High working efficiencies have been obtained by using molybdenum coated soda lime glass as the back contact; as already said, the idea of implementing this technology at a larger production scale well suits the needing of flexible materials and low cost production techniques. In order to reduce production costs and deployment of thin film PV-based technologies in mass scale, Kesterite-based thin film solar cells technologies should be manufacturable at high throughput with low cost and it should exhibit good power conversion efficiency. Fabrication of Kesterites on flexible substrate has high potential to decrease production cost due to roll-to-roll manufacturing on flexible substrate enables use of compact size deposition equipment with high throughput. Many researchers report CZT precursor production starting from acidic baths; on the other side, even if alkaline baths for the many cited precursor deposition are known since a long time, the stability issues to which they are related make them less studied. Here we propose an alkaline electrolyte for one step deposition of the CZT precursor onto flexible molybdenum substrates. Electrochemical analyses are reported to describe the behaviour of the plating bath, as well as morphological inspection to assess the quality of the so obtained thin films. To test active layer activity, thermal annealing and reactive annealing were performed on the metallised molybdenum foil, as well as the sealing with conductive oxides via RF sputtering. After the evaporation of aluminium contacts, the efficiency of the cell was tested, yielding 0.1%. Quantum efficiency of the so obtained substrate is also reported to complete the photoactivity characterization.

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[2] W. Wang, M.T. Winkler, O.Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D.B. Mitzi,
device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency,
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