Electron-Selective Contacts For N-Type Crystalline Silicon Solar Cells: A Study On An Ultrathin Zirconium Oxide Layer

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2022-9-2

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Madbouly, Loay Akmal Mostafa Kamal

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Crystalline silicon (c-Si) solar cell efficiencies have peaked at around 26%. To get closer and even go beyond the Shockley-Queisser Limit, passivation of the rear contact of c-Si solar cells is mandatory. Formation of ohmic, low-resistance and economically feasible contacts is impeded by the fermi-level pinning phenomenon in lightly doped n-type c-Si (n-Si). Over the past two decades, much research has been done to find materials that can replace the commonly used silicon dioxide (SiO2) as it showed electrical instabilities at low thicknesses in microelectronic applications. Zirconium dioxide (ZrO2) is potentially capable of succeeding SiO2 due to its high dielectric constant, large band gap, environmental stability, high service temperature, electron-selective nature, and lattice constant match with Si. It can also be easily deposited using solution-based techniques or vacuum systems. ZrO2 is also economically viable to purchase and process. To understand and analyze the interactions between ZrO2 and c-Si, experiments were conducted with different deposition techniques. While reaching the required nanoscale thickness of ZrO2 layer was challenging using solution processing techniques (such as spin coating and spray coating), vacuum-based deposition techniques (such as thermal and electron-beam evaporation) were able to deposit ZrOx layers on c-Si with a thickness of 1 nm. Following successful deposition of the 1 nm thick ZrOx passivation layer on c-Si, it was profoundly characterized by X-Ray photoelectron spectroscopy (XPS), X-Ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and spectroscopic ellipsometry (SE). Afterwards, the effect of passivation was investigated by conducting contact resistivity (ρc) measurements, using Cox-Strack method (CSM), for different thickness on n-Si wafers with different resistivities. ZrOx was deposited through e-beam evaporation at room temperature. The contact resistivity was found to be highly dependent on the ZrOx thickness. The lowest ρc obtained using only 1 nm thick ZrOx was 21 mΩ.cm2. Moreover, the electron-selective nature of ZrOx was proven by CSM as it showed ohmic behavior when it was deposited as an interlayer between n-Si and aluminum (Al). As a proof of concept, an n-Si solar cell was fabricated with 1 nm ZrOx deposited at its rear contact to elucidate the effect of ZrOx passivation on the performance of n-Si solar cell. 1 nm thick ZrOx rear-contact-passivated n-Si solar cell showed significant performance with a short circuit current density (JSC) of 33.86 mA/mm2, an open circuit voltage (VOC) of 600.0 mV, a fill factor (FF) of 79.1% and an overall efficiency (η) of 16.1%. These values successfully demonstrate the use of ZrOx with Al as an emerging electron-selective layer contact for lightly doped n-Si wafers.

Subject Keywords

Zirconium Oxide, Crystalline Silicon Solar Cells, Passivation, Contacts, Renewable Energy

URI

https://hdl.handle.net/11511/99625

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

L. A. M. K. Madbouly, “Electron-Selective Contacts For N-Type Crystalline Silicon Solar Cells: A Study On An Ultrathin Zirconium Oxide Layer,” M.S. - Master of Science, Middle East Technical University, 2022.