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Title: Investigation of copper-kesterites for solar water splitting
Authors: Tay, Ying Fan
Keywords: Engineering::Materials::Energy materials
Issue Date: 2020
Publisher: Nanyang Technological University
Source: Tay, Y. F. (2020). Investigation of copper-kesterites for solar water splitting. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Solar water splitting is a promising method to store energy by utilizing renewable solar energy to electrolyze water and generate hydrogen as a fuel. For this vision to succeed, both photoanode and photocathode driving the water oxidation and reduction reaction respectively must be able to both split water efficiently and be able to guarantee a sufficient lifetime to make economic sense. In the field of photocathodes, copper chalcogenides has been extensively studied, with Cu(In,Ga)Se2 (CIGS) achieving up to 3.7% unbiased solar-to-hydrogen (STH) conversion efficiency in 2018. While the results are attractive, the scarcity of Indium and Galium makes the alternative Cu2ZnSnS4 (CZTS) a promising candidate due to its similar characteristics as CIGS. Extensive research in the field of photovoltaics has advanced CZTS device efficiencies up to 12.7%, providing a template for its application to water splitting. The first part of the thesis aims to explore the influence of cation substitution in CZTS photocathodes (Cd2+ replacing Zn2+ and Ag+ replacing Cu+) which alters the band positions and the carrier transport properties of CZTS. Both Ag+ and Cd2+ were chosen to replace Cu+ and Zn2+ to maintain charge neutrality and to increase the difference in ionic size between Cu+ and Zn2+ which may help reduce the amount of CuZn antisite defects. Firstly, different amounts of Ag and Cd are substituted into CZTS and fabricated through solution processed spin coating. Next, the optimum amount of substitution is determined by studying its photoelectrochemical (PEC) performance, along with advanced characterizations to investigate its band structure and carrier transport properties. The next part of the thesis focuses on investigating Indium Tin Oxide (ITO) as a charge transport layer and a protective layer. ITO is deposited between the CdS buffer layer and the Pt catalyst to increase the stability and performance of the cation substituted CZTS photocathode. For Cd2+ substitution, PEC measurements reveal that the photocurrent increases steadily with increasing Cd2+ substitution reaching a maximum when 40% of Zn is substituted with Cd2+. A 3 fold increase in carrier mobility and lifetime is observed along with a reduction in radiative recombination, which may be due to the reduction of antisite defects. Cd2+ substitution is confirmed to be beneficial to reduce bulk defects in CZTS and improve the carrier transport. However, onset potential of the Cd2+ substituted photocathode decreases when compared to pristine CZTS, which may be due to the lower bandgap and the spike like heterojunction with CdS. For Ag+ substitution, PEC performances follow a similar gaussian trend as Cd2+ substitution, with a maximum photocurrent observed at 4% Ag substitution and a maximum onset potential observed at 8% Ag substitution. Ag+ substitution is found to affect mainly the CZTS/CdS interface instead of the CZTS bulk, helping to reduce the interfacial defects and leading to an increased photocurrent and onset potential. A bilayer Cd2+ substituted CZTS (near the bulk) and Ag substituted CZTS (near the CdS interface) may reap the benefits of bulk and interface improvement from both cation substitution and could be studied in the future to push the efficiency of CZTS photocathode further. To address the issue of stability usually observed in photocathodes and possibly also reap the superior charge transport properties of a transparent conducting oxide (TCO), amorphous thin Indium Tin Oxide (ITO) is DC sputtered unto CZTS/CdS at room temperature. It is found that during photoelectrodeposition of Pt on ITO, In and Sn near the ITO surface is partially reduced, creating a new metallic interface formed between ITO and Pt. While the photocurrent of the ITO protected photocathode decreases with time during stability test (under 0VRHE with illumination), stopping the stability test and doing repeated linear sweep voltammetry (LSV) in the dark is able to recover the photocurrent to its original value. This leads to the hypothesis that the decrease in photocurrent observed initially is due to phosphate ions from the potassium phosphate electrolyte adhering on the surface of ITO and Pt, preventing efficient charge transfer instead of the photocathode degrading. Further investigations on the reason for dark linear sweep voltammetry (LSV) in removing the phosphate ions and the adhesion of phosphate on ITO may help in the design of other protective layers for photocathodes. In summary, this thesis succeeds in employing cation substitution to selectively improve the bulk and interface properties of CZTS photocathodes, enhancing photocurrent and onset potential considerably. The function of ITO as a protective charge transport layer for the photocathode is also demonstrated. This thesis utilizes cation substitution to improve bulk and interface properties of CZTS photocathode and recommends that such techniques can be employed for other defective photocathodes. Furthermore, a corrosion resistant TCO capable of forming a metallic interface with Pt is studied for the first time and shown to be able to enhance the photocurrent substantially while also preventing the degradation of the underlying CdS and CZTS.
DOI: 10.32657/10356/145524
Rights: This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
Fulltext Permission: open
Fulltext Availability: With Fulltext
Appears in Collections:MSE Theses

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