Textured Silicon Solar cells and techniques for their manu facture utilizing metal Sources to catalyze formation of randomly distributed Surface features Such as nanoscale pyramidal and columnar Structures. These structures include dimensions Smaller than the wavelength of incident light, thereby resulting in a highly effective anti-reflective Surface. According to the invention, metal Sources present in a reactive ion etching chamber permit impurities (e.g. metal particles) to be introduced into a reactive ion etch plasma resulting in deposition of micro-masks on the Surface of a Substrate to be etched. Separate embodiments are disclosed including one in which the metal Source includes one or more metal-coated Substrates Strategically positioned rela tive to the Surface to be textured, and another in which the walls of the reaction chamber are pre-conditioned with a thin coating of metal catalyst material.

Back-contact crystalline-silicon photovoltaic solar cells and modules offer a number of advantages, including the elimination of grid shadowing losses, reduced cost through use of thinner silicon substrates, simpler module assembly, and improved aesthetics. While the existing edge tab method for interconnecting and stringing edge-connected back contact cells is acceptably straightforward and reliable, there are further gains to be exploited when you have both contact polarities on one side of the cell. In this work, we produce 'busbarless' emitter wrap-through solar cells that use 41% of the gridline silver (Ag) metallization mass compared to the edge tab design. Further, series resistance power losses are reduced by extraction of current from more places on the cell rear, leading to a fill factor improvement of about 6% (relative) on the module level. Series resistance and current-generation losses associated with large rear bondpads and busbars are eliminated. Use of thin silicon (Si) wafers is enabled because of the reduced Ag metallization mass and by interconnection with conductive adhesives leading to reduced bow. The busbarless cell design interconnected with conductive adhesives passes typical International Electrotechnical Commission damp heat and thermal cycling test.

A novel stack passivation scheme, in which plasma silicon nitride (SiN) is stacked on top of a rapid thermal Si02 (RTO) layer, is developed to attain a surface recombination velocity (S) approaching 10 cm/s at the 1.3 R-cm p-type (100) silicon surface. Such low S is achieved by the stack even when the RTO and SiN films individually yield considerably poorer surface passivation. Critical to achieving low S by the stack is the use of a short, moderate temperature anneal (in this study 730°C for 30 seconds) after film growth and deposition. This anneal is believed to enhance the release and delivery of atomic hydrogen from the SiN film to the Si-Si02 interface, thereby reducing the density of interface traps at the surface. Compatibility with this post-deposition anneal makes the stack passivation scheme attractive for cost-effective solar cell production since a similar anneal is required to fire screen-printed contacts. Application of the stack to passivated rear screen-printed solar cells has resulted in V,'s of 641 mV and 633 mV on 0.65 R-cm and 1.3 R-cm FZ Si substrates, respectively. These V, values are roughly 20 mV higher than for cells with untreated, highly recombinative back surfaces. The stack passivation has also been used to form fully screen-printed bifacial solar cells which exhibit rear-illuminated efficiency as high as 11.6% with a single layer AR coating.

An efficient antireflection coating (ARC) can enhance solar cell performance through increased light coupling. Here, we investigate solution-grown ZnO nanostructures as ARCs for Si solar cells and compare them to conventional single layer ARCs. We find that nanoscale morphology, controlled through synthetic chemistry, has a great effect on the macroscopic ARC performance. Compared with a silicon nitride (SiN) single layer ARC, ZnO nanorod arrays display a broadband reflection suppression from 400 to 1200 nm. For a tapered nanorod array with average tip diameter of 10 nm, we achieve a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC. Calculations using rigorous coupled wave analysis suggest that the tapered nanorod arrays behave like modified single layer ARCs, where the tapering leads to impedance matching between Si and air through a gradual reduction of the effective refractive index away from the surface, resulting in low reflection particularly at longer wavelengths and eliminating interference fringes through roughening of the air-ZnO interface. According to the calculations, we may further improve ARC performance by tailoring the thickness of the bottom fused ZnO layer and through better control of tip tapering.

Three potential techniques for texturing commercial multicrystalline silicon solar cells are compared on the basis of reflectance measurements. Wet acidic texturing, which would be the least costly to implement, produces a modest improvement in reflection before antireflection coating and encapsulation, whereas maskless reactive-ion etching texturing, and especially masked reactive-ion etched ‘pyramids’, generate a larger gain in absorption. After antireflection coating and encapsulation however, the differences between the methods are reduced. Short-circuit current measurements on wet acidic textured cells reveal that there is a significant additional current gain above that expected from the reduced reflection. This is attributed to both light-trapping and oblique coupling of incident light into the cell, resulting in generation closer to the junction.

We will discuss the fabrication of several diffractive optical elements (DOEs) for projects at Sandia National Laboratories, which highlight the relative importance of subwavelength surface texture in the componentsi performance. This surface texture is in addition to the larger, anisotropic DOE features that manipulate the propagating orders, and is commonly referred to as grass. Surface texture on amorphous or multi-crystalline material is readily apparent in a scanning electron micrograph and is often an unavoidable consequence of the reactive ion etch (RIE) process. Contributing factors are mask erosion, self-masking, and material non-uniformity. In this presentation, we describe and quantify the effects of-unavoidable and deliberate surface texture through several projects in progress at Sandia National Laboratories.

wavelengths and have used analytical techniques such as in situ Raman and X-ray reflectivity to investigate the luminescence mechanism and quantify the properties of the porous silicon layer. Further, their experience with ion implantation in Si lead to an investigation into alternate methods of producing Si nanostructures that visibly luminesce.

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.

We report on the role of surface profiles exhibiting comparable reflectance in random reactive ion etched textured Si solar cells. Internal quantum efficiency measurements demonstrate significant near IR absorption enhancement with peaks at /spl lambda/ /spl sim/ 1120 nm and 1050 nm. Using Fourier analysis of random surfaces, we find a majority of spatial structures in /spl sim/ 0.3-5-/spl mu/m range. This random distribution of subwavelength periodic structures leads to enhanced oblique coupling into Si through diffractive optics mechanisms. The random surfaces supporting finer features create diffraction orders propagating at larger angles enhancing near IR absorption through oblique light propagation. Random surfaces with larger features create almost vertically propagating diffraction orders resulting in little oblique coupling, and for some structures, almost no enhanced near IR absorption.

We developed a maskless plasma texturing technique for multicrystalline Si (mc-Si) cells using Reactive Ion Etching (RIE) that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to low levels. Internal quantum efficiencies higher than those on planar and wet-textured cells have been obtained, boosting cell currents and efficiencies by up to 6% on tricrystalline Si cells.

The Sandia Photovoltaic Program conducted research in crystalline-silicon solar cells between 1986 and 2000 for the U.S. Department of Energy. This period saw rapid improvements in the fundamental understanding of c-Si materials and devices, improvements in c-Si PV manufacturing and control, and a rapid expansion of c-Si PV manufacturing capacity. Crystalline-silicon technology has provided the basis for PV to emerge as a serious option for global energy needs. The c-Si cell research at Sandia examined c-Si materials, devices, processing, and process integration. This report summarizes research conducted in this program over the past 15 years.

Six different resistivities (0.32, 0.57, 1.2, 2.2, 9.1 and 20 Ω cm) were investigated to understand the dopant–defect interaction in n‐type, antimony‐doped, dendritic web silicon ribbon, and to study its response to gettering and passivation during belt furnace processing (BFP). The as‐grown lifetime was found to be a strong function of resistivity with higher resistivity displaying higher lifetime. Phosphorus gettering at 925° C/6 min raised the as‐grown lifetime of ∼1 μs in 20 Ω cm n‐web to 5.4 μs. A combination of phosphorus gettering followed by simultaneous Al gettering and SiN hydrogenation raised the 20 Ω cm n‐web lifetime to 78 μs. Unlike the as‐grown web, the processed lifetime was greater than 75 μs for all resistivities, with no clear doping dependence. This is attributed to the very effective gettering and passivation during the belt furnace processing. Front surface field (FSF) n+–n–p+ cells were fabricated by spin‐on phosphorus diffusion on the front and screen‐printed Al on the back. A lifetime value of over 100 μs was obtained in a 14.2% screen‐printed FSF n‐web solar cell fabricated on 100‐μm‐thick 20 Ω cm substrate. The screen‐printed FSF cell fabricated on (111) FZ gave an efficiency of 14.9% with a fill factor of 77.6%. These results are supported by model calculations, which revealed a maximum efficiency of ∼15% for 100‐μm‐thick planar screen‐printed FSF cells and their insensitivity to bulk lifetime above 60 μs. Copyright © 2001 John Wiley & Sons, Ltd.

A maskless plasma textu~ring technique using Reactive Ion Etching for silicon solar cells results in a very low reflectance of 5.4 % before and 3.9 % after SiN deposition. A detailed study of surface recombination and emitter properties was made, then solar cells were fabricated using the DOSS solar cell process. Different plasmadamage removal treatments are tested to optimize low lifetime solar cell efficiencies. Highest efficiencies are observed for little or no plasma-damage removal etching on mc-Si. Increased & due to the RIE texture proved superior to a single layer anti-reflection coating. This indicates that RIE texturing is a promising texturing technique, especially applicable on lower lifetime (multicrystalline) silicon. The use of non-toxic, non-corrosive SFS makes this process attractive for mass production. Increasing solar cell efficiency while maintaining a low production cost is the rimary objective of solar power research. The use of low cost materials such as silicon ribbon and multi-crystalline silicon, mc-Si, provides a cheaper means of producing crystalline silicon solar cells. In terms of cell efficiency, though, mc-Si suffers from lower charge carrier lifetimes compared to single-crystalline Si. In addition, the surface is not. easy to texture. In order to increase cell efficiency, texturing is a major factor: It allows for a lower overall reflection tlhat leads to higher short cirFig. 1. SEM picture of RUE-textured silicon. Otttcr------7--y 300 500 700 90

The goals of the Photovoltaic Manufacturing Technology project (PVMaT) are to help the US PV industry improve photovoltaic manufacturing processes and accelerate cost reductions for PV components and systems. PVMaT is in its ninth year of implementation, and subcontracts with industry have been completed from four solicitations for R&D on manufacturing process problems. We are in the second year of subcontracts for a fifth PVMaT solicitation. Based on the latest (1998) data from ten PVMaT industrial participants, the average direct manufacturing cost for these producers has been reduced by 29% - from $4.08 to $2.91 per peak watt since 1992 - and there has also been a more than five-fold increase in manufacturing capacity - from 13.1 to 73.3 MW. We believe R&D on manufacturing processes contributes significantly to expeditious reductions in PV manufacturing costs, and we identify areas for future R&D. (C) 2001 Elsevier Science B.V. All rights reserved.