Analysis of Optical and Morphological Properties of Aluminium Induced Texture Glass Superstrates

Texturing the glass surface is a promising method for improving the light trapping properties of superstrate thin-film silicon solar cells, as it enables thinner absorber layers and, possibly, higher cell efficiencies. In this paper we present the optical and morphological properties of borosilicate glass superstrates textured with the aluminium induced texture (AIT) method. High haze values are achieved without any reduction in the total optical transmission of the glass sheets after the AIT process. Scanning electron microscope and atomic force microscope (AFM) measurements reveal a laterally uniform surface morphology of the AIT texture. We demonstrate that the surface roughness and thus the transmission haze can be controlled by adjusting the AIT process parameters. From the AFM images, we extract histograms of the local height and angle distributions of the texture. Samples with a wide angle distribution are shown to produce the highest optical haze. The results of this analysis provide a better understanding of the correlation between the AIT process parameters and the resulting surface morphology. This analysis is further extended to an amorphous silicon pin solar cell deposited onto the textured glass substrate. (C) 2012 The Japan Society of Applied Physics

Nanocrystalline Low-Refractive Magnesium Fluoride Films Deposited by Reactive Magnetron Sputtering: Optical and Structural Properties

Stefan Mertin, Paul Muralt, Silviu Cosmin Sandu, Jean-Louis Scartezzini

In this work, we study MgF2 thin-film synthesis by reactive pulsed DC magnetron sputtering from a metallic magnesium target in a gas mixture of argon, oxygen, and carbon tetrafluoride (CF4). Nanocrystalline films on silicon and glass substrates with excellent properties for optical application are achieved. The plasma discharge is analyzed with a differentially pumped mass spectrometer before and during the deposition process. Without breaking the vacuum, monochromatic photoelectron spectroscopy (XPS) is performed for in situ determination of the atomic C and O concentration. Film microstructure, topography, and thickness are investigated by electron microscopy (SEM and TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The optical constants n and k are determined by spectroscopic ellipsometry and spectrophotometry: a consistent parametric fit of the ellipsometric angles and spectral transmittance and reflectance based on three Lorentz oscillators to determine n and k is achieved for a wide spectral range (300–2 300 nm). At 550 nm, a refractive index of 1.382 and near-zero absorption is obtained, which is in excellent agreement with n = 1.383 of polycrystalline MgF2. The measured light reflection at 760 nm is reduced by 3% for a quarter-wave nanocrystalline MgF2 coating on glass compared to the uncoated glass substrate.

Wiley-Blackwell2015

Advances in the characterization of nanowire photovoltaic devices

Dmitry Mikulik

III-V nanowires (NWs) have a great potential for solar energy applications due to their diameter-dependent optical properties, which may enhance absorption of light. In addition, core-shell radial p-i-n structures, in which the direction of light absorption is orthogonal to the carrier collection, can provide efficient carrier collection. The main goal of this thesis is the experimental study of the challenges of NW-based solar cells, related to materials and device fabrication. In the first part of the thesis, we present an analysis of where the electrical losses can be originated. By applying an equivalent circuit analysis approach, we classified them into three main groups: (i) the non-uniformity of NWs which may result in a reduction of the parallel resistance, (i) potential barriers originated at the different materials interfaces in the solar cell structure may result in an increase of the series resistance or addition of a second diode and (iii) surface recombination resulting in the reduction of the open-circuit voltage. In this thesis, we propose separate strategies to characterize and tackle these factors. The electric scheme of a NW-based solar cell consists of an ensemble of p-n junctions connected in parallel. We show how conductive-probe atomic force microscopy, C-AFM, is an essential tool for the characterization and optimization of these parallel-connected NW devices. We demonstrate topography and current mapping of the NW arrays, combined with current-voltage (IV) measurements of the individual NW junctions from the ensemble. Our results provide discussion elements on some of the factors limiting the performance of a NW-based solar cell, such as uniformity and photosensitivity of the individual NW p-n junctions within the array, and thereby a path for their improvement. Besides parallel losses due to uniformity issues, barriers in the carrier collection through the various heterointerfaces composing the device is discussed. To analyze it, we illuminate GaAs NW-based solar cells at different levels of light intensity and extract IV characteristics. This analysis helps to separately study the NW p-n junction response and the series resistance. The high series resistance of the NW-ensemble device can be attributed to the following interfaces: 1) GaAs-ITO, forming a photoactive Schottky diode, which suppresses the p-n junction at high concentrations of light, and 2) Si-GaAs heterojunction, disturbing the flow of majority carriers. Finally, the characterization of surface passivation in high-aspect-ratio nano/micro structures is addressed by electrochemical impedance spectroscopy (EIS). The method is applied to Si micropillars, as a proof-of-concept prior to the application to III-V nanowires. We tested structures passivated by a dielectric layer. The effect of different surface treatments on the interface state density were quantified by the analysis of the capacitance-voltage and conductance-voltage characteristics. This method allows the electrical measurements on rough vertical surfaces, which would otherwise suffer from high gate leakage currents if tested using solid-state metal-insulator-semiconductor scheme. The results and characterization methods, demonstrated in this work, contribute to the overall efforts of the scientific community on how to reveal the main engineering challenges in NW-based solar cells. It thus paves the way to approach the fundamental conversion efficiencies predicted by theory.

EPFL2018

Ultrafast Transverse Modulation of Free Electrons by Interaction with Shaped Optical Fields

Francesco Barantani, Fabrizio Carbone, Simone Gargiulo, Ido Kaminer, Veronica Leccese, Ivan Madan, Alexey Sapozhnik, Phoebe Marie Tengdin, Giovanni Maria Vanacore

Spatiotemporal electron-beam shaping is a bold frontier of electron microscopy. Over the past decade, shaping methods evolved from static phase plates to low-speed electrostatic and magnetostatic displays. Recently, a swift change of paradigm utilizing light to control free electrons has emerged. Here, we experimentally demonstrate arbitrary transverse modulation of electron beams without complicated electron-optics elements or material nanostructures, but rather using shaped light beams. On-demand spatial modulation of electron wavepackets is obtained via inelastic interaction with transversely shaped ultrafast light fields controlled by an external spatial light modulator. We illustrate this method for the cases of Hermite-Gaussian and Laguerre-Gaussian modulation and discuss their use in enhancing microscope sensitivity. Our approach dramatically widens the range of patterns that can be imprinted on the electron profile and greatly facilitates tailored electron-beam shaping.

AMER CHEMICAL SOC2022

Advances in the characterization of nanowire photovoltaic devices

Dmitry Mikulik

EPFL2018