Y. S. Tsuo

Enhancement and stabilization of porous silicon photoluminescence by oxygen incorporation with a remote‐plasma treatment

We report a treatment that enhances and stabilizes the photoluminescence (PL) from porous Si films. Films prepared by anodization in a 50% HF/ethanol solution were annealed at 450 °C in vacuum, exposed to air, and then exposed to a remote‐hydrogen plasma. Infrared absorption spectroscopy revealed that the concentration of oxygen, rather than hydrogen, was increased by the processing steps, and that silicon dihydride species had been eliminated from the surface. The PL from a treated film was initially ∼30 times more intense than from the as‐etched films. The PL intensity increased with illumination time in air until a steady‐state intensity was reached.

Existence of a Pb1‐like defect center in porous silicon

We performed a detailed study of electron spin resonance (ESR) spectra of porous silicon (PS) samples at different stages of treatment and with different porosities. In addition to the commonly observed Pb0‐like dangling bond, results of curve fitting to our ESR spectra show that a Pb1‐like center, similar to the Pb1 center observed at the (100) crystalline‐Si/SiO2 interface, appears in the PS nanostructure. The ratio of the number of Pb1‐like centers to that of Pb0‐like centers is related to the PS porosity. Remote hydrogen plasma processing of the annealed PS does not change the ratio significantly, although the total numbers of Pb0‐like and Pb1‐like centers are reduced and photoluminescence efficiency is improved.

Potential applications of porous silicon in photovoltaics

Porous Si formed on crystalline Si wafers using electrochemical etching exhibits photoluminescent and electroluminescent properties. The authors report on the results of their investigation of using porous Si as surface texturing to enhance the performance of crystalline silicon photovoltaic solar cells. Unlike conventional KOH-based texture etching, which can only be used on [100]-oriented single-crystalline Si substrates, porous Si can be etched onto Si surfaces of any crystallographic orientation and onto polycrystalline or microcrystalline Si surfaces. A porous-Si-covered single-crystal Si wafer showed an integrated reflectance of only 1.4% at a wavelength of 500 nm. The reflectance of a porous-Si-covered polycrystalline Si was found to be comparable to other much more complicated texturing methods.<<ETX>>

Grain boundary and dislocation effects on the PV performance of high-purity silicon

To quantify the effects of grain size and dislocation defects on the minority charge carrier lifetime /spl tau/ and photovoltaic (PV) efficiency of silicon, the authors grew high-purity, float-zoned (FZ) ingots with a range of grain sizes from single crystalline (dislocated and dislocation-free) down to 4/spl times/10/sup -4/ cm/sup 2/. In situ ingot cooling rates of 18/spl deg/ and 89/spl deg/C min/sup -1/ were used. Bulk ingot /spl tau/ ranged from less than 30 /spl mu/s for the multicrystalline ingots to 2,500 /spl mu/s for the dislocation-free crystals. Wafers from different positions in the ingots were used for /spl tau/ measurements and the fabrication of mesa-isolated, 0.04-cm/sup 2/ diagnostic PV device structures. They found that /spl tau/ decreased to 4 /spl mu/s and normalized solar cell efficiency decreased to 0.6 for the smallest average grain areas (4/spl times/10/sup -4/ cm/sup 2/).<<ETX>>

Porous silicon gettering

We have studied a novel extrinsic gettering method that uses the large surface areas produced by a porous-silicon etch as gettering sites. The annealing step of the gettering used a high-flux solar furnace. We found that a high density of photons during annealing enhanced the impurity diffusion to the gettering sites. We used metallurgical-grade Si (MG-Si) prepared by directional solidification casting as the starting material. We propose to use porous-silicon-gettered MG-Si as a low-cost epitaxial substrate for polycrystalline silicon thin-film growth for solar cells.

Liquid phase epitaxy for thin-layer silicon PV devices

We have studied liquid phase epitaxial growth of thin‐layer silicon (≤60‐μm thick) on substrates of cast metallurgical‐grade silicon, silicon‐dip‐coated graphite, and single‐aluminum. A solar cell made using a 60‐μm‐thick silicon layer grown on a heavily‐doped single‐crystal silicon substrate has 83% of the efficiency of a control cell made using 300‐μm‐thick float‐zone silicon. In this paper, we discuss a number of issues related to thin‐layer silicon growth including choice of solvents and substrates, thin‐layer silicon material characteristics, and solar cell performances.

Photovoltaics for rural electrification in the People's Republic of China

Rapid growth in economic development, coupled with the absence of an electric grid in large areas of the rural countryside, has created a need for new energy sources both in urban centers and rural areas in China. The most critical need for rural electrification exists in northern and western China, where 80 million people had no access to grid electricity at the end of 1995. In February 1995, the US Department of Energy signed an Energy Efficiency and Renewable Energy Protocol Agreement with the Chinese State Science and Technology Commission in Beijing. Under this agreement, NREL is providing assistance to several central government and provincial government agencies in China to develop photovoltaic and photovoltaic hybrid applications for rural electrification.

New opportunities in crystalline silicon R D

To support the expected growth of the silicon solar cell industry, we believe that research and development (RD thin-layer silicon deposition methods, and more environmentally benign cell and module manufacturing processes. For each of these activities, we identify the main issues that needed to be addressed.

Ion-beam-hydrogenated amorphous silicon

A kaufman ion-beam source has been used to study the rehydrogenation and postdeposition hydrogenation of amorphous silicon. In the rehydrogenation study, hydrogen atoms were implanted into glow-dischargedeposited amorphous silicon materials in which the hydrogen content had been driven out by heating. In the posthydrogenation study, amorphous silicon samples with no hydrogen content detectable by infrared absorption and no photoconductivity were used as the starting material. These materials were deposited by thermal CVD, magnetron sputtering, or RF glow discharge.

Photoluminescence properties of porous silicon

A porous silicon (PS) layer can be produced on a crystalline silicon substrate by electrochemical or chemical etching in hydrofluoric acid (HF) solutions. There are many properties that make PS thin films interesting for photovoltaic applications, such as a possible direct band gap that can be adjusted between 1.5 and 1.9 eV, textured surfaces for light trapping, the potential for low cost and large‐area fabrication, and the possibility of tandem cell structures with Si. We report the fabrication of large area PS (up to 3‘ diameter) with quite uniform photoluminescence (PL) properties, and studies of the effects of post‐hydrogenation treatments on the intensity and stability of the PL from PS. We have observed that a remote‐plasma processing treatment can increase the PL emission intensity from PS prepared under certain conditions by 100 times or more. The emission band is narrower and centered more toward the blue for the remote‐plasma processed sample, and the PL emission intensity does not degrade in a...