There are a range of different methods to generate a nanostructured surface on silicon (Si) but the most cost effective and optically interesting is the metal assisted wet chemical etching (MACE) (Koynov et al 2006 Appl. Phys. Lett. 88 203107). MACE of Si is a controllable, room-temperature wet-chemical technique that uses a thin layer of metal to etch the surface of Si, leaving behind various nano- and micro-scale surface features or 'black silicon'. MACE-fabricated nanowires (NWs) provide improved antireflection and light trapping functionality (Toor et al 2016 Nanoscale 8 15448-66) compared with the traditional 'iso-texturing' (Campbell and Green 1987 J. Appl. Phys. 62 243-9). The resulting lower reflection and improved light trapping can lead to higher short circuit currents in NW solar cells (Toor et al 2011 Appl. Phys. Lett. 99 103501). In addition, NW cells can have higher fill factors and voltages than traditionally processed cells, thus leading to increased solar cell efficiencies (Cabrera et al 2013 IEEE J. Photovolt. 3 102-7). MACE NW processing also has synergy with next generation Si solar cell designs, such as thin epitaxial-Si and passivated emitter rear contact (Toor et al 2016 Nanoscale 8 15448-66). While several companies have begun manufacturing black Si, and many more are researching these techniques, much of the work has not been published in traditional journals and is publicly available only through conference proceedings and patent publications, which makes learning the field challenging. There have been three specialized review articles published recently on certain aspects of MACE or black Si, but do not present a full review that would benefit the industry (Liu et al 2014 Energy Environ. Sci. 7 3223-63; Yusufoglu et al 2015 IEEE J. Photovolt. 5 320-8; Huang et al 2011 Adv. Mater. 23 285-308). In this feature article, we review the chemistry of MACE and explore how changing parameters in the wet etch process effects the resulting texture on the Si surface. Then we review efforts to increase the uniformity and reproducibility of the MACE process, which is critical for commercializing the black Si technology.

Metal-assisted catalyzed etching (MACE) of silicon (Si) is a controllable, room-temperature wet-chemical technique that uses a thin layer of metal to etch the surface of Si, leaving behind various nano- and micro-scale surface features, including nanowires (NWs), that can be tuned to achieve various useful engineering goals, in particular with respect to Si solar cells. In this review, we introduce the science and technology of MACE from the literature, and provide an in-depth analysis of MACE to enhance Si solar cells, including the outlook for commercial applications of this technology.

ACKNOWLEDGMENTSI would like to thank my advisor Professor Fatima Toor for her productive feedback of my work and her support and encouragement of my project. I am grateful to have had the opportunity to work in her research group. I would also like to thank my coworker Wenqi Duan for all of her help and support in the fabrication process of my project.iii ABSTRACT The fabrication of low cost, high efficiency solar cells is imperative in competing with existing energy technologies. Many research groups have explored using III-V materials and thin-film technologies to create high efficiency cells; however, the materials and manufacturing processes are very costly as compared to monocrystalline silicon (Si) solar cells. Since commercial Si solar cells typically have efficiencies in the range of 17-19%, techniques such as surface texturing, depositing a surface-passivating film, and creating multi-junction Si cells are used to improve the efficiency without significantly increasing the manufacturing costs. This research focused on two of these techniques: (1) a tandem junction solar cell comprised of a thin-film perovskite top cell and a wafer-based Si bottom cell, and (2) Si solar cells with single-and double-layer silicon nitride (SiNx) anti-reflection coatings (ARC). The perovskite/Si tandem junction cell was modeled using a Matlab analytical program. The model took in material properties such as doping concentrations, diffusion coefficients, and band gap energy and calculated the photocurrents, voltages, and efficiencies of the cells individually and in the tandem configuration. A planar Si bottom cell, a cell with a SiNx coating, or a nanostructured black silicon (bSi) cell can be modeled in either an n-terminal or series-connected configuration with the perovskite top cell. By optimizing the bottom and top cell parameters, a tandem cell with an efficiency of 31.78% was reached. Next, planar Si solar cells were fabricated, and the effects of single-and doublelayer SiNx films deposited on the cells were explored. Silicon nitride was sputtered onto planar Si samples, and the refractive index and thicknesses of the films were measured iv using ellipsometry. A range of refractive indices can be reached by adjusting the gas flow rate ratios of nitrogen (N2) and argon (Ar) in the system. The refractive index and thickness of the film affect where the minimum of the reflection curve is located. For Si, the optimum refractive index of a single-layer passivation film is 1.85 with a thickness of 80nm so that the minimum reflection is at 600nm, which is where the photon flux is maximized. However, using a double-layer film of SiNx, the Si solar cell performance is further improved due to surface passivation and lowered surface reflectivity. A bottom layer film with a higher refractive index passivates the Si cell and reduces surface reflectivity, while the top layer film with a smaller refractive index further reduces the surface reflectivity. The refractive indices and thicknesses of the double-layer films were varied, an...

The goal of this study was to enhance transgene expression after adenoviral-mediated gene transfer to the carotid artery. We used an adenoviral vector with a transgene that expresses beta-galactosidase, driven by the human cytomegalovirus (CMV) promoter/enhancer. The CMV promoter drives constitutive expression, and response elements within the enhancer allow inducible expression through binding of active transcription factors, such as cAMP response element binding protein (CREB) and nuclear factor kappa B (NFkappaB). Rings of rabbit carotid artery were incubated ex vivo with a replication-deficient adenovirus that expresses beta-galactosidase (AdCMV-betagal). Virus was removed from the medium, and forskolin or phorbol-12-myristate-13-acetate (PMA), which can induce activation of CREB or NFkappaB, respectively, were added to the medium. Pyrrolidine dithiocarbamate (PDTC) was used to inhibit activation of NFkappaB. Following incubation for 24 hours, beta-galactosidase activity was assessed by chemiluminescent reporter assay. Forskolin and PMA enhanced transgene expression in the carotid artery. Activity increased from 56+/-13 mU/mg protein (mean+/-SE) in rings of carotid treated with virus alone (10(9) pfu) to 159+/-23 mU/mg protein (P<0.05) in rings treated with forskolin, and to 189+/-40 mU/mg protein (P<0.05) in rings treated with PMA. Phorbol didecanoate, an inactive phorbol, did not affect expression of beta-galactosidase. After pre-incubation with PDTC prior to PMA, expression of beta-galactosidase was less than in rings incubated with PMA alone (29+/-11, P<0.05). Histochemical staining of carotid artery for beta-galactosidase demonstrated enhanced endothelial expression following administration of PMA. These findings suggest that expression after gene transfer to the carotid artery using an adenoviral vector with the CMV promoter/enhancer may be enhanced by PMA and forskolin, perhaps by activation of transcription factors.

No abstract