James A. Rand

Silicon nanowire solar cells

Over the past decade, silicon nanowire solar cells have been intensively explored as potential platforms for the next-generation photovoltaic (PV) technologies with high power conversion efficiency and low production cost. This chapter discusses the details of the silicon nanowire solar cells in terms of their device structures, fabrication and characterization, electrical and optical properties benefited from the nanowire geometry. These benefits are not only expected to increase the power conversion efficiency, but also considered to reduce the requirement for the material quantity and quality, allowing for potential efficiency improvements and substantial cost reductions.

Strong broadband optical absorption in silicon nanowire films

The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found to be higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

Effects of grain boundary on impurity gettering and oxygen precipitation in polycrystalline sheet silicon

The effects of grain boundaries (GB) in polycrystalline sheet silicon on impurity gettering and oxygen precipitation were investigated by electron beam induced current (EBIC), deep level transient spectroscopy (DLTS), micro-Fourier-transform infrared spectroscopy (FTIR), and preferential etching/Normaski optical microscopy techniques. Both as-grown and thermally processed wafers were studied. A correlation between GB density and transition metal concentration was quantitatively established by combining DLTS and EBIC studies. It was found that four deep levels arising from Fe–B, Fe–Al, Cr–B, and Fei were present in the as-grown sample, and their concentrations decrease with increasing GB density. GB gettering was further verified by the presence of an EBIC image contrast halo around the GB. Preferential etching also revealed a precipitate density of 2×107 cm−2 on the GB. After processing, a clearly defined oxygen precipitate denuded zone formed around the GB with the interstitial oxygen concentration [Oi] ...

Light trapping in thin crystalline silicon solar cells

Optimally designed thin crystalline silicon solar cells (<50- mu m thick) have performance and cost advantages over conventional thick devices. The modeling and fabrication of light-trapping devices are described. Thin (30- mu m-thick) Si-film layers are formed on textured silicon substrates. The back surface of the thin Si film layers are passivated with SiO/sub 2/ coatings. Long-wavelength light is confined through total internal reflections. The analysis of fabricated devices reveals the presence of the enhanced absorption predicted by the light-trapping model. Efficiencies for these structures reach 9.4%, among the highest reported for crystalline silicon devices of this thickness.<<ETX>>

High current, thin silicon-on-ceramic solar cell

Thin-film polycrystalline silicon solar cells offer the potential to achieve 19% efficient photovoltaic power conversion. Well-designed, 20-100 micron thick, thin-film silicon solar cells can achieve high efficiency by employing light trapping and back surface passivation. Low cost is achieved by minimizing the amount of feedstock silicon required per watt of power output. Electrically insulating supporting substrates enable monolithic, series-connected submodules. A solar cell device comprised of a 20 micron thick layer of silicon grown on an insulating ceramic substrate, designed to effect light-trapping and back surface passivation, has resulted in an independently verified short circuit current of 25.8 mA/cm/sup 2/. Analysis of the spectral response of the solar cell indicates the presence of both light-trapping and back surface passivation with an effective diffusion length in excess of twice the device thickness.

Development of light-trapped, interconnected, Silicon-Film modules

AstroPower is employing Silicon-Film/sup TM/ technology toward the development of an advanced thin-silicon-based, photovoltaic module product. This module combines the design and process features of advanced thin-silicon solar cells, is light trapped, and integrated in a low-cost monolithic interconnected array. This advanced product includes the following features: (a) silicon layer grown on a low-cost substrate; (b) a nominally 50-micron thick silicon layer with minority carrier diffusion lengths exceeding 100 microns; (c) light trapping due to back-surface reflection; and (d) back surface passivation. The thin silicon layer achieves high solar cell performance and can lead to a module conversion efficiency as high as 19%. These performance design features, combined with low-cost manufacturing using relatively low-cost capital equipment, continuous processing and a low-cost substrate, will lead to high performance, low cost photovoltaic panels.

Thin silicon solar cells

The silicon-film design achieves high performance by using a dun silicon layer and incorporating light trapping. Optimally designed thin crystalline solar cells (<50 microns thick) have performance advantages over conventional thick devices. The high-performance silicon-film design employs a metallurgical barrier between the low-cost substrate and the thin silicon layer. Light trapping properties of silicon-film on ceramic solar cells are presented and analyzed. Recent advances in process development are described here.

Thin film polycrystalline silicon solar cells

Abstract A thin crystalline silicon solar cell with low back surface recombination will exhibit slightly higher efficiency than a thick silicon solar cell having similar material quality. When light trapping is added to the thin silicon solar cell, the efficiency can improve by as much as 20%. Following a summary of historical results, the predictions of the model are described including the preferred geometrical structures for light trapping. Recent experimental results are reported. The thin, light trapping technology can be extended to a monolithic interconnected device.

Non-traditional light sources for solar cell and module testing

Solar cell and module performance testing is a critical component of high volume manufacturing. Indoor testing of solar cells and modules has historically utilized filtered xenon arc lamps as a sunlight simulator. There are a number of problems with are lamps in this environment including high cost, extensive maintenance, area limitations and electrical noise. In this work the use of low-cost, reliable, quartz-tungsten-halide (QTH) lamps as an alternative light source is analyzed. This analysis is focused on evaluating spectral mismatch errors due to testing solar cells with varying spectral response. When combined with outdoor calibration standards, QTH lamps are found to introduce acceptable levels of error (<2%) for testing both single crystal and polycrystalline solar cells and modules.

Monolithically integrated silicon-film photovoltaic modules

Thin crystalline silicon layers on an insulating ceramic substrate permit the realization of a serially-interconnected monolithic array. In this paper, the authors present the device design and fabrication issues relating to such a monolithically integrated device, and the latest results are presented. A novel device process is presented that involves isolating and re-interconnecting silicon solar cell elements on an insulating ceramic substrate. Device efficiencies of 7.3% have been achieved, and are presently limited by high series resistance and chemical impurities.<<ETX>>