Plasma Nanotexturing of Silicon for Photovoltaic Applications : Tailoring Plasma-Surface Interactions for Improved Light Management

Abstract : This thesis is dedicated to the study of crystalline silicon (c-Si) surface texturing at the nanoscale (nanotexturing) using capacitively coupled plasma reactive ion etching (CCP-RIE). The general objective consists in tuning the nanotextured surface properties to improve light-management in c-Si solar cells through front surface texturing. To this aim, plasma-surface interactions during etching in a SF6/O2 discharge are investigated using both single-frequency excitation and Tailored Voltage Waveforms (TVWs), i.e. a multifrequency approach triggering electrical asymmetries in the plasma.To gain a full picture of the achievable processing range, the electron heating mechanisms and ion bombardment energy on the surface are first studied. An identification of the dominant electron heating mechanisms in low pressure SF6/O2 plasma is demonstrated using TVWs as an innovative probing tool. Different electrical asymmetry effects are shown to arise depending on the dominant heating mode, which therefore affects both the ion flux and bombardment energy on the etched surface. Although a complete decoupling between ion energy and flux cannot be achieved in the investigated discharge conditions, TVWs do lead to an extended playground for SF6/O2 plasma etching of c-Si surfaces in CCP-RIE.The plasma-surface interaction mechanisms during SF6/O2 plasma etching and texturing of c-Si surfaces are then investigated. A processing window to achieve nanotextured anti-reflective c-Si surfaces (“black silicon”) at room temperature is delimited. Building on the work from the first section, the ion flux and bombardment energy on the c-Si surface are varied independently in this process window. A phenomenological model (etching yield varying with the square root of the ion energy above a threshold around 13 eV) is proposed. From this model, a direct (positive) link between the energy weighted ion fluence and the nanostructure height is identified. Importantly, the final nanostructure average width is shown to also weakly depend on the instantaneous ion flux during the process.Subsequently, anti-reflection and light scattering properties of plasma nanotextured c-Si surfaces are studied. Regarding anti-reflection, when the nanostructure average width is small compared to the wavelength (in c-Si), the nanotextured surface acts as an anti-reflective graded refractive index layer and a direct link between the nanostructure average height and the reflectance can be derived. Very low reflectance (in the order of 2% at normal incidence) on a broad wavelength range (approximately [250, 1000nm]) can be achieved, and the improved anti-reflective properties extend to high angles of incidence. Additionally, strong light scattering is shown to arise when the nanostructure average width overcomes a given threshold determined experimentally. Consequently, light is more efficiently trapped in the c-Si substrate, leading to superior absorption in the range [1000, 1200nm].The aforementioned optical properties of nanotextured c-Si surfaces are of practical interest for improved light management in c-Si photovoltaic devices. However, plasma induced damages (during plasma nanotexturing), as well as enlarged surface area, are responsible for increased carrier recombination. The contribution to recombination from plasma induced defects is shown to be mitigated when ion bombardment energy is kept low. Design rules are consequently proposed: optimized conditions for c-Si nanotexturing in SF6/O2 plasma can be achieved by maximizing the ion flux while keeping ion energy low (but above the etching threshold). These requirements are conflicting in the case of a single frequency CCP discharge, but the trade-off may be (at least partly) resolved using TVWs.
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Guillaume Fischer. Plasma Nanotexturing of Silicon for Photovoltaic Applications : Tailoring Plasma-Surface Interactions for Improved Light Management. Plasma Physics [physics.plasm-ph]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLX087⟩. ⟨tel-01982208⟩

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