Synthesis of highly efficient Cu₂ZnSnS_xSe_4−x (CZTSSe) nanosheet electrocatalyst for dye-sensitized solar cells

Highlights

• A low-temperature solution-phase synthetic approach used to prepare CZTSSe nanosheets.

• Self-organized CZTSSe nanosheets employed as an efficient CE for DSSC.

• The impressive PCE can be obtained (5.73%), which is comparable to a Pt CE (5.78%).

• CZTSSe CE exhibits high reproducibility and good chemical stability in DSSC.

Abstract

Cu₂ZnSnS_xSe_4-x (CZTSSe) has been reported as a promising platinum (Pt)-free counter electrode (CE) for dye-sensitized solar cells (DSSCs) with porous or crystalline structures. However, it is still challenging to develop a low-temperature method to synthesize CZTSSe electrodes with high surface area and excellent electrocatalytic activity. Herein, we present a low-temperature solution-phase synthetic approach to prepare self-organized CZTSSe nanosheets. As-synthesized CZTSSe nanosheets deposited on fluorine-doped tin oxide (FTO) substrate and applied as CE, yield a power conversion efficiency of (5.73%), which is comparable to the value we obtained using Pt-based CE (5.78%). In addition, CZTSSe nanosheets exhibit excellent electrocatalytic performance (4.60%) on carbon paper (CP) as a Pt/FTO-free CE. This impressive performance was attributed to the high specific surface area of CZTSSe with nanosheet morphology and related excellent electrocatalytic activity. Cyclic voltammetry and electrochemical impedance spectroscopy measurements confirmed that CE made of CZTSSe nanosheets have comparable catalytic activity with respect to Pt CE due to the low charge transfer resistance at the CE/electrolyte interface. The as-prepared CZTSSe CE with high reproducibility exhibits good chemical stability in the electrolyte containing. The low-temperature process, high surface area and high electrocatalytic activity help the CZTSSe nanosheets stand out as an alternative CE electrocatalyst in DSSC.

Introduction

Fossil fuels represent over 80% of the world’s energy consumption. On the other hand, their decreasing availability, climate change and environmental pollution dictate an urgent transition towards renewable energy sources [[1], [2], [3]]. Amongst various carbon-free energy sources, solar technologies are particularly promising [1,2].

Since their invention in 1991, dye-sensitized solar cells (DSSCs) have been widely studied due to their promising features, such as cost-effective fabrication technology and eco-friendliness [[4], [5], [6]]. Considerable efforts have focused on using specially designed nanostructured materials to improve the performance and stability of the different components in DSSCs [[7], [8], [9], [10], [11]]. A typical DSSC consists of four main components: a wide bandgap semiconducting oxide, dye sensitizer molecules, an electrolyte, and a counter electrode (CE). During device operation, the CE plays an important role by collecting electrons from the external circuit and regenerating the oxidized iodide/triiodide redox couple electrolyte at the CE/electrolyte interface [[12], [13], [14]]. Thus, an ideal CE should possess the high electrocatalytic activity and exhibit excellent stability towards the electrolyte [6,15,16].

To date, CEs made of fluorine-doped tin oxide (FTO) glass coated with a thin layer of platinum (Pt) are the most widely used as conventional CE material. However, as a noble metal, limited supply and high cost are unresolved challenges that need to be addressed for the large-scale commercialization of DSSCs [17]. Pt/FTO CEs used in DSSCs accounts for more than 40% of the total device cost [[18], [19], [20]]. In addition, Pt/FTO CE is not stable in contact with some electrolytes such as cobalt complexes and polysulfide electrolytes [21]. To overcome these issues and deploy commercial DSSCs, the design/fabrication of Pt-free CEs, without reducing its electrocatalytic activity and stability is a key factor. In this regard, major efforts have focused on replacing Pt with abundant low-cost narrow bandgap p-type semiconductors [10,[22], [23], [24], [25]].

Copper chalcogenide-based compounds as p-type semiconductors are considered promising materials for use as CEs in DSSCs [17,23,[26], [27], [28]]. Among these, Earth-abundant copper-zinc-tin-chalcogenide Cu₂ZnSnS_xSe_4−x (CZTSSe) is an interesting material due to its electronic structure, bandgap, and synergetic effects of the different elements [17,21,29]. Xin et al. [17] first introduced CZTSSe film as a potential substitute for Pt in low-cost CEs for the triiodide/iodide electrolyte reduction. They employed a simple solution-phase chemical synthesis approach to prepare the Cu₂ZnSnS₄ (CZTS) nanocrystal dispersion that can be coated on the FTO substrate. The CZTS thin film is then sintered at 540 °C for 1 h in selenium (Se) vapor for selenization and yields CZTSSe. Chen et al. [21] reported the effect of the crystallization of CZTSSe film on DSSCs performance. The CZTSSe film was produced by evaporation of 40 mg of Se powder mixed with Ar in a graphite box at 550 °C for 30 min. It was found that the formation of a high-crystallinity film with large grains dramatically improved the charge transport process. Shen et al. [27] explored a screen-printing process for CZTSSe CEs. Similar to previous work, CZTSSe film was prepared by screen-printing CZTS pastes (followed by heating at 400 °C for 15 min in the air to remove the organic binders), then by post selenization (at 500 °C for 10 min) using Se vapor obtained from elemental Se pellets. Although previous work [17,21,27] demonstrated a very exciting approach for the application of CZTSSe as a CE in a DSSC, there are two crucial factors that must be considered: (i) synthesis of CZTSSe by post selenization requires high temperatures (higher than 500 °C) and special annealing environment; (ii) previous studies mainly focused on designing crystalline films that could limit the surface contact between CE and electrolyte. The major issue in these reports is the post selenization reaction of the deposited film at high temperatures that could create cracks in the film due to the significant thermal stress.

Herein we report the synthesis of CZTSSe nanosheets as efficient CEs for DSSCs. The aim of this work was to use as-synthesized CZTSSe with intrinsic Se content to remove the high-temperature selenization process and related negative impact (thermal stress). In addition, we synthesized the CZTSSe with nanosheets morphology to increase the active catalytic surface area of the CE.

The as-prepared CZTSSe nanosheets could be effectively dispersed in ethanol and the resulting ink was used to fabricate the CZTSSe thin films on FTO substrates as Pt-free CEs in DSSCs. The CZTSSe nanosheets CE exhibit excellent electrocatalytic activity and stability for triiodide/iodide electrolyte reduction with high reproducibility and a comparable PCE (5.73%) to that based on conventional Pt CEs (5.78%). On the other hand, when CZTSSe nanosheets deposited on the carbon paper (CP) as a FTO and Pt free CE, exhibits a comparable PCE (4.60%).