CZTS absorber layer for thin film solar cells from electrodeposited metallic stacked precursors (Zn/Cu-Sn)
Graphical abstract
Introduction
Thin film solar cells have been attracted steady by many researchers during last 20 years and considered as an alternative technology to Si-based solar cells due to their low cost and almost compatible conversion efficiencies. Market share of thin film PV solar cells are around 10% where CdTe and CuIn(Ga)(Se)2 (CIGS) are dominant as absorber layers [1]. However, In, Ga and Te are not earth abundant, while on the other hand Cd and Se are both toxic elements. Alternatively, Cu2ZnSnS4 (CZTS) is a promising material due to the fact that Zn and Sn are cheap, non-toxic and earth abundant elements. Furthermore, CZTS is a p-type semiconductor with direct bandgap energy close to 1.5 eV and a large absorption coefficient over 104 cm−1 as well as theoretical conversion efficiency of 32.2% which make it suitable among all the thin film solar cells [2], [3].
Several methods have been used to prepare CZTS thin films such as sputtering, evaporation, spray pyrolysis, solution-based methods, electrodeposition method, etc. The highest conversion efficiency was achieved for sulfo-selenide thin films (CZTSSe) (η = 12.6%) by Wang et al. using hydrazine-based solution process [4]. However, in case of pure sulfide CZTS thin films, highest 9.2% power conversion efficiency was demonstrated by Kato et al. by using evaporation-annealing route [5]. Among different deposition techniques, electrodeposition is one of the promising methods for preparing CZTS absorber films because of its low cost equipment’s, large scale production and good control over the composition and morphology. Generally, electrodeposition-based process can be divided into two categories. The former is two-steps process where electrodeposition of Cu-Zn-Sn metal precursors is followed by a sulfurization process. The latter is single-step co-electrodeposition of Cu, Zn, Sn and S directly to form CZTS compound. However, it is very difficult to deposit homogenous and good crystalline CZTS films from single-step electrodeposition process due to the large reduction potential window among the elements. Electrodeposition of Cu-Zn-Sn can be done in two ways; (i) simultaneous electrodeposition of Cu-Zn-Sn from single electrolyte and (ii) sequential electrodeposition of Cu-Zn-Sn from different electrolytes with various stacking orders like Sn/Zn/Cu, Zn/Sn/Cu, Zn/Cu-Sn, etc. Again, it is difficult to obtain homogenous and compact Cu-Zn-Sn metallic precursors through simultaneous electrodeposition from single electrolyte due to the large reduction potential gaps among the metal ions. For this reason, sequential electrodeposition of Cu-Zn-Sn has been preferred by many researchers.
Several groups attempted to fabricate CZTS thin films over the past few years by electrodeposition-annealing route. In 2008, Scragg et al. first reported sequential electrodeposition of metal precursors in the order Zn/Sn/Cu on Mo/SLG substrate followed by sulfurization at 550 °C for 2 h with 0.8% conversion efficiency [6]. In 2009, Araki et al. reported CZTS thin films from electrodeposited stacked precursors followed by sulfurization at 600 °C which exhibited 0.98% conversion efficiency [7]. Again in 2009, Araki et al. reported CZTS thin films from co-electrodeposited Cu-Zn-Sn precursors followed by sulfurization at 600 °C, showed 3.16% conversion efficiency [8]. In 2009, Ennaoui et al. fabricated CZTS thin films from co-electrodeposited Cu-Zn-Sn precursors followed by sulfurization at 550 °C for 2 h and achieved 3.4% conversion efficiency [9]. In this study, they observed secondary phases such as ZnS in Zn-rich precursor and Cu2SnS3 in Zn-poor precursors. In 2010, Scragg et al. reported sequential electrodeposition of Cu/Zn/Sn/Cu on Mo coated SLG precursors followed by sulfurization at 575 °C for 2 h and achieved 3.2% conversion efficiency [10]. In 2012, Ahmed et al. showed substantial growth on conversion efficiency of CZTS thin films solar cells by electrodeposition–annealing approach [11].They have achieved 7.3% conversion efficiency of CZTS from sequential electrodeposition of metallic precursors in the order Cu/Zn/Sn and Cu/Sn/Zn which were followed by sulfurization at 585 °C for 5–15 min. Moreover, they soft-annealed the precursors before sulfurization at temperatures between 210 °C and 350 °C (300 °C was found the best) in order to obtain well mixed and homogenous CuSn and CuZn alloys. As a result, they decreased the sulfurization time from 2 h to 5–15 min with respect to previous studies. In 2013, Lin et al. showed the mechanistic aspects of soft-annealing effects of electrodeposited metallic stacked precursors with 5.6% conversion efficiency [12]. Recently, Jiang et al. demonstrated fabrication of CZTS through electrodeposition-annealing route from stacked metal precursors of Cu-Zn-Sn with 8% power conversion efficiency [13]. This is the highest conversion efficiency of CZTS by electrodeposition-annealing approach to the best of our knowledge. Very recently also an 8.2% efficient pure selenide kesterite (CZTSe) thin film solar cells through electrodeposition-annealing route have been demonstrated by Vauche [14].
In this paper, we propose a CZTS thin-film fabrication process that involves co-electrodeposition of Cu-Sn layer on Mo foil substrate [15] and subsequent Zn electrodeposition on the Cu-Sn/Mo layer from a different solution. This stacking order of Zn/Cu-Sn/Mo has been chosen in order to minimize Sn loss during sulfurization [7], [21]. Later, electrodeposited metal stacks were soft-annealed at 350 °C [12] and then sulfurized at 585 °C to obtain well-formed kesterite CZTS.
Section snippets
Experimental
Electrodeposition of precursor layers were performed galvanostically in a conventional electrochemical cell assembly at room temperature using an AMEL Potentiostat Model-549. Mo foil substrate (200 μm thick from Goodfellows) with exposed area 1 × 1 cm2 was used as a working electrode, Titanium mesh was used as a counter inert electrode. The electrolyte for the electrodeposition of Cu-Sn contains 0.026 M Copper-sulfate (CuSO4·5H2O) (Sigma Aldrich); 0.15 M Tin-sulfate (SnSO4) (Fluka), 2 M Methane
Results and discussions
Due to the high conductivity of Cu and its comparative inert nature during deposition of other metals on it, Cu has been chosen as an under-layer for the fabrication of Kesterites by most of the researchers when stacking order of precursors (Cu-Zn-Sn) are considered [18]. As a result, Zn/Sn layer or Sn/Zn layer are used to deposit on top of Cu. In order to overcome the issues of metal exchange and layer stripping during sequential electrodeposition, Mo/Cu/Sn/Zn sequence has been chosen by
Conclusions
CZTS has been successfully fabricated through electrodeposition-annealing route from electrodeposited stacked bilayer precursor (Zn/Cu-Sn/Mo). Metal stacks were adherent to the substrate with homogeneous morphology. Though Zn losses have been observed upon sulfurization, no Sn losses were noticed, which is probably connected to the stacking order of precursors. Raman spectra recorded with 785 nm laser line showed more secondary phases with respect to those observed with the 514.5 nm excitation
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