Copper Indium Sulfide Photoelectrode Design for Solar Water Splitting

Abstract

There is a considerable interest in copper based chalcopyrite thin films for photoelectrochemical (PEC) water-splitting due to their high absorption coefficient and tunable band gap. Among the other chalcopyrite semiconductor materials various advantages of copper indium sulfide (CuInS2) thin films made them favorable in photovoltaic applications as well as in solar water splitting. First CuInS2 thin films can be deposited on various substrates via cost effective methods including solution-based manufacturing. Second the control of structure and the properties of CuInS2 thin films are easier than that of quaternary chalcopyrite films like copper indium gallium sulfide/selenide. At last but not least, it is possible to produce fully green manufacturing of CuInS2 thin films unlike selenium containing compounds. In this work, spray pyrolysis technique has been used to manufacture CuInS2 based electrodes. Spray pyrolysis is a facile way to produce thin films over large areas using very little amount of solution. High throughput of this technique is especially important for the deposition of rare materials like indium. More specifically, here we used only 0.75 ml of precursor solution per cm2 to build 1 micrometer thick CuInS2 films, which is the lowest solution amount reported in literature. CuInS2, CuInS2/In2S3 and ZnO-nanowire/CuInS2 heterojunctions on indium tin oxide (ITO) coated glass have been studied as the photocathode for solar water splitting. The maximum current density observed under illumination and therefore the maximum photoconversion efficiency of the CuInS2 electrodes has been affected by the precursor solution. In other words stoichiometry of the films were important to enhance the electron transfer from the CuInS2 to the conduction band of ITO. It has been observed that Cu/In and S/(Cu+In) ratios directly affected the photoelectrochemical performance. As a general trend the applied bias photon-to-current efficiency (ABPE) increased with increasing the S/(Cu+In) ratio. Besides the maximum performance of the spray pyrolyzed CuInS2 electrodes were observed for Cu/In=1.3. Although the efficiencies of the CuInS2 photocathodes reached to 6.6 %, photosensitivity of the electrodes was poor. The maximum efficiency for CuInS2 electrodes calculated using the differences between dark and illuminated current density, which is called as photo current density (Jphoto), was 0.53%. Therefore, indium sulfide (In2S3) thin films have been deposited on CuInS2 electrodes via again spray pyrolysis. The photosensitiy and the stability of the CuInS2 photocathodes were increased with In2S3 deposition. The maximum efficiency calculated using Jphoto was about 0.93% for the CuInS2/In2S3 all-spray-deposited electrodes. Finally we fabricated ZnO-nanowire/CuInS2 heterojunctions to improve the sensitivity and absorption. Nanowire arrays provided better electron transport and hence enhanced performance. The efficiency of the photocathodes increased more than five times compare to the flat electrode designs of CuInS2. The maximum efficiency of 2.8% has been observed for the ZnO-nanowire/CuInS2 heterojunction electrodes. This value is very promising to utilize them into low cost and efficient photoelectrochemical solar cells as photocathode.