Characterization of molybdenum oxide and magnesium doped zinc oxide charge transport layers deposited by sputtering for heterojunction solar cells

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2017

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Bektaş, Gence

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Group IV semiconductors can easily be doped by group III and V elements to possess p and n type behavior, respectively, which enables homojunction solar cell designs. However, most of the compound semiconductors including CIGS, CdTe and perovskite can intrinsically be either n or p type. Therefore, heterojunction structures with well-aligned conduction bands between the absorber and electron transport layer (ETL) and well-aligned valence bands between the absorber and hole transport layer (HTL) are required. This thesis aims to investigate structural, optical and electrical properties of molybdenum oxide (MoOx) HTL and magnesium-doped zinc oxide (ZnxMg1-xO) ETL fabricated by reactive rf magnetron co-sputtering for heterojunction solar cells. MoO3 is a semiconductor with a band gap of 3.3 eV; however, defect states in oxygen-deficient MoOx make it act like a high work function metal. In this study, oxygen vacancies were created both by stoichiometry control during deposition and by post annealing. Defect states formation near the valance band edge and its influence on work function of the material were analyzed by X-ray photoelectron spectroscopy (XPS). Crystallographic properties of MoOx films were examined by X-ray diffraction (XRD). Refractive indices, extinctionvi coefficients and thicknesses of thin films were determined by spectroscopic ellispsometry. Finally, transmittance and reflectance spectra of MoOx thin films were measured. ZnOx is an alternative ETL to replace conventional ones due to its relatively low cost and high electrical conductivity. In this study, band gap of ZnxMg1-xO was altered by Mg doping. Optical band gap studies were conducted by Tauc plot analysis using transmission spectra. Crystal structures of the samples were measured by XRD to illustrate adverse effect of Mg doping and healing effect of introducing oxygen during deposition. Spectroscopic ellipsometry characterization was performed to determine optical constants of thin films. Finally, perovskite solar cells were fabricated using sputtered ZnOx.

Subject Keywords

Solar cells., Zinc oxide., Magnesium., Semiconductors., Molybdenum oxides.

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http://etd.lib.metu.edu.tr/upload/12620828/index.pdf
https://hdl.handle.net/11511/26341

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

G. Bektaş, “Characterization of molybdenum oxide and magnesium doped zinc oxide charge transport layers deposited by sputtering for heterojunction solar cells,” M.S. - Master of Science, Middle East Technical University, 2017.