Exploring cd-free buffer layer for CZTS photocathode for solar water splitting

• Main Author: Christianto, Wibie
• Other Authors: Lydia Helena Wong
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Materials::Energy materials
• Online Access: http://hdl.handle.net/10356/74228
• Institution: Nanyang Technological University

Since the discovery of its photovoltaic effect in 1966, Cu2ZnSnS4 (CZTS) has been widely studied for solar cell applications with the highest efficiency of 9.2%. Its p-type nature, suitable band gap (Eg) of 1.5 eV and high optical absorption coefficient presents a huge potential for its application as a photocathode for photoelectrochemical solar water splitting. Theoretically, for a 1.5 eV band gap material, the maximum photocurrent achievable at 0 VRHE is 28 mAcm-2. However, the highest photocurrent recorded up to date for a CZTS photocathode is 11 mAcm-2 at 0 VRHE through the incorporation of TiO2/CdS layers. Such TiO2 and CdS layers are usually employed for a photocathode system as they will create a pn junction with CZTS, thereby generating an internal electric field, which increases the separation efficiency of photogenerated carriers. The widely used CdS buffer layer severely limits the commercial application of CZTS as it undergoes photodegradation in most electrolytes, leading to a decrease in photocurrent over time. These highlights the need to design a new buffer layer to replace CdS that is comparable in terms of performance while providing good stability. Zinc tin oxide (ZTO) is a potential buffer layer for the CZTS photocathode as it has shown to be a good interface with CZTS and such CZTS/ZTO solar cells have shown comparable efficiencies with the CZTS/CdS solar cells. In this project, a systematic study on fabricating an optimum ZTO buffer layer was conducted followed by advanced characterization using Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), UV-Vis spectroscopy, AC Hall measurement and Photoelectrochemical (PEC) measurement of its impact on the electrical properties and stability of the photocathode. Our results show that by incorporating a ZTO buffer layer, the photocurrent of CZTS photocathode has been slightly improved. Furthermore, the stability of the photocathode has also been improved as a CZTS/ZTO/Pt photocathode can retain 80% of its initial photocurrent after 1 hour of measurement time at 0 VRHE as compared to 15% of remaining photocurrent from the CZTS/CdS/Pt photocathode. However, the performance of the CZTS/ZTO/Pt is still lower than that of CZTS/CdS/Pt and further optimization can be done to improve the coverage and electrical properties of the ZTO layer.