Low‐Cost Copper Zinc Tin Sulfide Counter Electrodes for High‐Efficiency Dye‐Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) are among the most promising photovoltaic devices for low-cost light-to-energy conversion with relatively high efficiency. A typical DSSC consists of three key components: a dye-sensitized semiconductor photoanode, an electrolyte with a redox couple (triiodide/iodide), and a counter electrode (CE). Upon photoexcitation, electrons generated from photoexcited dyes are injected into the conduction band of photoanode composed of TiO2 and the dyes are regenerated by redox reaction with the electrolyte. Oxidized ions (triiodide) in the electrolyte then diffuse to the CE and are finally reduced to iodide at the surface of the CE. An ideal CE should possess high electrocatalytic activity for the reduction of charge carriers in electrolyte as well as high conductivity. To date, the most commonly used CE is fluorine-doped tin oxide (FTO) glass coated with a thin layer of platinum. However, as a noble metal, the low abundance (0.0037 ppm) and high cost (US$50/gram) prevent platinum from being used for largescale manufacturing. In this context, considerable efforts have been made to replace Pt with abundant low-cost alternatives, including carbon-based materials (for example, carbon nanotubes, carbon black, and graphite), conjugated polymers, and inorganic materials as CEs. In comparison to carbon materials and polymers, inorganic compounds carry many advantageous characteristics, such as simple preparation and a diversity of materials that can be used. In recent years, a variety of binary metal oxides, metal sulfides, metal nitrides, and metal carbides have been developed as CEs. To the best of our knowledge, the use of abundant ternary or quaternary materials as potential substitutes for Pt as low-cost CEs has not yet been explored. A quaternary chalcogenide semiconductor, copper zinc tin sulfide (hereafter referred to as CZTS), is most widely known as one of the most promising photovoltaic (PV) materials, and it is widely used in thin-film solar cells. Notably, CZTS is composed of naturally abundant elements in the Earth s crust and has very low toxicity: it is environmentally friendly compared to two high-efficiency thin-film solar cells with CdTe and Cu(In1 x,Gax)S2 (CIGS) that have toxic elements (Cd) and rare metals (indium and gallium). Recently, high-efficiency thin-film solar cells have been demonstrated based on the superior PV performance of CZTS as a p-type semiconductor owing to its direct band gap of 1.5 eV and a large absorption coefficient (> 10 cm ). However, no studies have centered on the electrocatalytic activity of CZTS for use in DSSCs. Herein, we present, for the first time, that CZTS can be exploited as an effective CE material to replace expensive Pt, yielding a low-cost, highefficiency DSSCs. It is noteworthy that a power conversion efficiency (PCE) of 7.37% was achieved by a simple process of spin-coating CZTS followed by selenization. This efficiency was highly comparable to the DSSC prepared by utilizing Pt (PCE= 7.04%) as the CE under the same device configuration. We employed a solution-base synthesis approach to prepare CZTS nanocrystals. Specifically, copper, zinc, and tin precursors dissolved in oleylamine (OLA) were purified at 130 8C and heated to 225 8C in argon. Subsequently, a sulfur solution was rapidly injected and stirred at 225 8C for 1 h. The product was centrifuged to yield CZTS nanoparticles (see the Experimental Section). Figure 1a,b shows scanning transmission electron microscope (STEM) images of CZTS nanoparticles. The nanoparticle diameter was approximately (15 6) nm and the lattice constant was 0.31 nm, corresponding to the (112) plane, which was consistent with the XRD result (Supporting Information, Figure S1). It is worth noting that compared to conventional costly and low-throughput highvacuum sputtering and vapor deposition of CZTS, the ability to produce a CZTS nanocrystal dispersion (that is, a nanocrystal “ink”) that can be sprayed and coated on surface and then thermally annealed into larger-grain thin film would substantially lower the manufacturing cost and allow highthroughput solar-cell production. The CZTS ink was then either spin-coated or drop-cast onto the clean FTO glass and sintered at 540 8C for 1 h in selenium vapor. The morphologies of resulting CZTS films after sintering in Se vapor are shown in Figure 1c,d. The thickness of the CZTS layer was approximately 180 nm for the spin-coated sample and 2.3 mm for the drop-cast sample, respectively. Cracks were clearly evident on the drop-cast sample sintered in Se vapor, which are due to the stress induced during the solvent evaporation (Supporting Information, Figure S2b). The compositions of CZTS nanocrystals before and after treatment with Se vapor (selenization to yield CZTSSe) were [*] X. Xin, W. Han, J. Jung, Prof. Z. Lin School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 (USA) E-mail: zhiqun.lin@mse.gatech.edu