CZTS absorber layer for thin film solar cells from electrodeposited metallic stacked precursors (Zn/Cu-Sn)

Highlights

• CZTS absorber layer was fabricated by electrodeposition—annealing route from stacked bilayer precursor (Zn/Cu-Sn).

• Different characterization techniques have ensured the well formed Kesterite CZTS along the film thickness also.

• Two different excitation wavelengths of laser lines (514.5 and 785 nm) have been used for the Raman characterization of the films.

• No significant Sn loss is observed in CZTS films after the sulfurization of the stacked bilayer precursors.

• Photoluminescence spectroscopy reveals the PL peak of CZTS at 1.15 eV at low temperature (15 K).

Abstract

In the present work, Kesterite-Cu₂ZnSnS₄ (CZTS) thin films were successfully synthesized from stacked bilayer precursor (Zn/Cu-Sn) through electrodeposition-annealing route. Adherent and homogeneous Cu-poor, Zn-rich stacked metal Cu-Zn-Sn precursors with different compositions were sequentially electrodeposited, in the order of Zn/Cu-Sn onto Mo foil substrates. Subsequently, stacked layers were soft annealed at 350 °C for 20 min in flowing N₂ atmosphere in order to improve intermixing of the elements. Then, sulfurization was completed at 585 °C for 15 min in elemental sulfur environment in a quartz tube furnace with N₂ atmosphere. Morphological, compositional and structural properties of the films were investigated using SEM, EDS and XRD methods. Raman spectroscopy with two different excitation lines (514.5 and 785 nm), has been carried out on the sulfurized films in order to fully characterize the CZTS phase. Higher excitation wavelength showed more secondary phases, but with low intensities. Glow discharge optical emission spectroscopy (GDOES) has also been performed on films showing well formed Kesterite CZTS along the film thickness as compositions of the elements do not change along the thickness. In order to investigate the electronic structure of the CZTS, Photoluminescence (PL) spectroscopy has been carried out on the films, whose results matched up with the literatures.

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.