Investigating the preparation parameters during the synthesis of CuInS 2 thin film photoelectrodes Yiming Tang, Yun Hau Ng*, Rose Amal* Particles and Catalysis Research Group (PartCat), School of Chemical Engineering The University of New South Wales, Sydney, Australia *Email: yh.ng@unsw.edu.au, r.amal@unsw.edu.au Abstract—Effects of electrodeposition time during the n-type CuInS 2 thin film electrodes synthesis was studied in this work. All the CuInS 2 thin film were fabricated via a single-step electrodeposition of Cu/In/S precursors on a transparent fluorine-doped tin oxide (FTO) substrate and calcined in a reducing gas atmosphere at 500 °C without using the toxic H 2 S gas for sulfurization. Thirty minute deposition time was the optimum condition based on the morphological features observed in the scanning electron microscope (SEM) images and the highest anodic photocurrent generated upon visible light illumination in the photoelectrochemical (PEC) test. The optical properties characterization revealed the CuInS 2 thin films to have a direct bandgap of 1.42 eV, which confirmed the visible light active properties in the PEC performance. Keywords—deposition time; single-step; CuInS 2 ; thin film; photoelectrochemical properties I. INTRODUCTION Ternary CuInS 2 semiconductor with a narrow direct band gap of 1.5 eV and a large absorption coefficient of more than 10 5 cm -1 is a promising candidate as a visible light active material for solar energy conversion.[1-3] Ternary chalcopyrite semiconductors can act as p-/n-type depending on types of native defects, without changing the crystal structure or introducing extrinsic elements.[4-6] Conventionally, fabrication of chalcopyrite CuInS 2 thin films employs expensive evaporation equipment for metal deposition, followed by sulfurization using H 2 S gas.[7] In this work, we fabricate CuInS 2 thin film electrode on a FTO substrate via a facile single-step electrodeposition of Cu/In/S elements followed by heat treatment in N 2 and H 2 . [8] The preparation parameters during the n-type CuInS 2 thin film electrode synthesis are investigated. Deposition duration has great impact in altering the photoelectrochemical properties of the thin film photoelectrodes. In principle, the thickness of the electrodeposited thin film is proportional to the electrodeposition time under chronoamperometric condition. A thicker film with considerable absorption generates higher photocurrent as the mass of light-active component is increased. However, accompanying a thicker film is always the development of inhomogeneously distributed particles with the tendency of surface cracks formation. [9] The formation of unevenly deposited particles and surface cracks introduce grain boundaries and charge traps for the transportation of excited electrode. Therefore, an optimum electrodeposition time is present to achieve a balance between maximized charge separation and minimized charge recombination. In this work, we studied the influence of electrodeposition time on the surface morphology, photoelectrochemical properties and optical properties of electrodeposited films. The optimum film thickness was obtained based on the characterization results. II. EXPERIMENTAL A. Synthesis of CuInS 2 thin film electrodes CuInS 2 thin film electrodes were deposited using a three electrode cell configurations. A mixture solution of 10 mM copper chloride, 10 mM indium chloride and 100 mM sodium thiosulfate were used as the electrolyte for the single-step electrodeposition process. A glass substrate coated by a fluorine-doped tin oxide (FTO) layer, an Ag/AgCl and a Pt foil were used as the working electrode, reference electrode and counter electrode in the three-electrode system. Prior to the single-step electrodeposition, the FTO-coated glass substrate was firstly ultrasonically cleaned with Milli-Q water, ethanol and acetone and then dried in air. The CuInS 2 thin film electrodes were deposited at the following deposition times 5, 15, 30, 120 min. The applied bias was set as -1.0 V for the whole deposition time at room temperature, without stirring the solution. After completion of the deposition, the films were rinsed in de-ionized water and dried in air. All the deposited films were annealed in a reducing gas atmosphere consisted of 92 % N 2 and 8% H 2 . N 2 was purged through the tube furnace for 2 h to completely remove the air prior to the heating process. The temperature was increased to 500 °C with a ramping rate of 5 °C/min and maintained for 1 h. B. Characterization of CuInS 2 thin film electrodes A scanning electron microscope (SEM, S 900 Hitachi) was used to compare the morphological features of the obtained films using different deposition time for CuInS 2 thin film electrodes synthesis. The photoelectrochemical performance was employed in a three-electrode electrochemical system: a CuInS 2 /FTO film as a working electrode, an Ag/AgCl as a reference electrode, a Pt wire as a counter electrode, and 0.25 M Na 2 S and 0.35 M Na 2 SO 3 aqueous solution as the electrolyte. PREPRESS PROOF FILE CAUSAL PRODUCTIONS 1