15.7% Efficient 10-µm-thick crystalline silicon solar cells using periodic nanostructures.

Only ten micrometer thick crystalline silicon solar cells deliver a short-circuit current of 34.5 mA cm(-2) and power conversion efficiency of 15.7%. The record performance for a crystalline silicon solar cell of such thinness is enabled by an advanced light-trapping design incorporating a 2D inverted pyramid photonic crystal and a rear dielectric/reflector stack.

Efficient light trapping in inverted nanopyramid thin crystalline silicon membranes for solar cell applications.

Thin-film crystalline silicon (c-Si) solar cells with light-trapping structures can enhance light absorption within the semiconductor absorber layer and reduce material usage. Here we demonstrate that an inverted nanopyramid light-trapping scheme for c-Si thin films, fabricated at wafer scale via a low-cost wet etching process, significantly enhances absorption within the c-Si layer. A broadband enhancement in absorptance that approaches the Yablonovitch limit (Yablonovitch, E. J. Opt. Soc. Am.1987, 72, 899-907 ) is achieved with minimal angle dependence. We also show that c-Si films less than 10 µm in thickness can achieve absorptance values comparable to that of planar c-Si wafers thicker than 300 µm, amounting to an over 30-fold reduction in material usage. Furthermore the surface area increases by a factor of only 1.7, which limits surface recombination losses in comparison with other nanostructured light-trapping schemes. These structures will not only significantly curtail both the material and processing cost of solar cells but also allow the high efficiency required to enable viable c-Si thin-film solar cells in the future.

Deconstructing energy use in microelectronics manufacturing: an experimental case study of a MEMS fabrication facility.

Semiconductors are quite energy intensive to manufacture on the basis of energy required per mass of material processed. This analysis draws on original data from a case study of the Analog Devices Micromachined Products Division MEMS fabrication facility to examine the consequence of process rate on the energy intensity of semiconductor manufacturing. We trace the impact of process rate on energy intensity at different length scales, first presenting top-down data, then results of a bottom-up study, and concluding with individual process analyses. Interestingly, while production increased by almost a factor of 2 over the course of the study, energy demand remained virtually constant. At its most efficient, 270 kWh of electricity were required per six inch wafer in the manufacture of the MEMS devices produced at the fabrication facility. In part, the large amount of energy required per unit output is a function of the preponderance of energy used by support equipment; our data show that the facility support equipment is responsible for 58% of total energy requirements.

Thermodynamic analysis of resources used in manufacturing processes.

In this study we use a thermodynamic framework to characterize the material and energy resources used in manufacturing processes. The analysis and data span a wide range of processes from "conventional" processes such as machining, casting, and injection molding, to the so-called "advanced machining" processes such as electrical discharge machining and abrasive waterjet machining, and to the vapor-phase processes used in semiconductor and nanomaterials fabrication. In all, 20 processes are analyzed. The results show that the intensity of materials and energy used per unit of mass of material processed (measured either as specific energy or exergy) has increased by at least 6 orders of magnitude over the past several decades. The increase of material/energy intensity use has been primarily a consequence of the introduction of new manufacturing processes, rather than changes in traditional technologies. This phenomenon has been driven by the desire for precise small-scale devices and product features and enabled by stable and declining material and energy prices over this period. We illustrate the relevance of thermodynamics (including exergy analysis) for all processes in spite of the fact that long-lasting focus in manufacturing has been on product quality--not necessarily energy/material conversion efficiency. We promote the use of thermodynamics tools for analysis of manufacturing processes within the context of rapidly increasing relevance of sustainable human enterprises. We confirm that exergy analysis can be used to identify where resources are lost in these processes, which is the first step in proposing and/or redesigning new more efficient processes.