Richard O. Bell

Natural and forced convection during solution growth of CdTe by the Traveling Heater Method (THM)

Abstract Solution growth by the “traveling heater method” (THM) employs a thermal arrangement where the highest temperature occurs near the middle of the solvent zone. One of the consequences for crystal growth in the generation of convection currents which can aid material transport in the molten zone. Since the magnitude of the convection depends on the properties of solvent and crystal, it is suggested that forced convection using “acceleration rotation” (AR) might be employed because it is reproducible and amenable to control. To observe the effect of rotation speed, cycle time, growth rate and temperature on crystallinity and numbers of voids or inclusions, a series of THM experiments was performed on the crystal growth of CdTe from Te solution. These showed that AR in this system is of only modest effect (it can increase the growth rate by a factor of 2), because the natural convection currents generated under the particular experimental conditions are already very strong indeed.

Gettering and hydrogen passivation of edge‐defined film‐fed grown multicrystalline silicon solar cells by Al diffusion and forming gas anneal

This study shows for the first time that a combination of Al treatment on the back, oxide passivation on the front, and 400 °C forming gas anneal in the presence of Al, raised the double‐layer antireflection‐coated edge‐defined film‐fed grown (EFG) silicon cell efficiency from 7.8% to 14.1%. Front oxide passivation contributed an ∼0.8% increase in absolute cell efficiency, Al diffusion on the back increased the efficiency by 1.4% (absolute), and the forming gas anneal (FGA) after the Al diffusion improved the cell efficiency by an additional 4.1% (absolute). A combination of the above three steps improved the EFG cell efficiency by 6.3%, indicating that the above three effects are complimentary. Oxide passivation reduced front surface recombination velocity and Al diffusion, while FGA improved diffusion length via gettering. We propose that the large increase in cell efficiency produced by the forming gas anneal results from bulk defect passivation by atomic hydrogen generated in the processing.

Plastic deformation influence on stress generated during silicon sheet growth at high speeds

Abstract Plastic deformation processes have been studied in high speed silicon sheet growth using finite element analysis. Stress and strain rate distributions are calculated for steady-state growth of thin sheet under plane stress conditions. Predictions of the model are used to examine factors affecting residual stress and buckle formation for growth of silicon ribbon by the EFG method.

Enhancement of diffusion length in EFG ribbon solar cells under illumination

The short‐circuit current, efficiency, and minority‐carrier diffusion length of EFG ribbon silicon solar cells have been measured as a function of illumination level between 0 and 5 suns and found to increase. The faster rate of increase of the current in ribbon cells compared to CZ we attribute to the enhancement of the minority‐carrier diffusion length under illumination. A quasicontinuous deep‐trap model with a Gaussian distribution of compensated donor states is proposed to account for these observations.

Defect passivation in multicrystalline‐Si materials by plasma‐enhanced chemical vapor deposition of SiO2/SiN coatings

It is shown for the first time that plasma‐enhanced chemical vapor deposition (PECVD) passivation which involves low temperature PECVD of ∼100 A SiO2 and ∼600 A SiN followed by photoassisted anneal is very effective for both surface and bulk defect passivation in multicrystalline‐Si materials. It is found that the effective recombination lifetime increased by a factor of 2–10 depending upon the multicrystalline material. Some solar cells were fabricated using a three‐layer PECVD coating (100 A SiO2/600 A SiN/950 A SiO2), the bottom two for passivation and the top two for antireflection coating. The bulk and surface passivation effects were quantified and decoupled by a combination of internal quantum efficiency measurements and computer modeling. It was found that the PECVD passivated solar cells increased bulk lifetime from 10 to 20 μs, and decreased the surface recombination velocity from 2×105 to 5×104 cm/s.

Characterization of grain boundaries in polycrystalline solar cells using a computerized electron beam induced current system

The automatic evaluation of grain boundary activity in a polycrystalline solar cell can be carried out by interfacing a microcomputer to a scanning electron microscope operating in the electron beam induced current mode (EBIC). The grain boundary surface recombination velocity and diffusion length in the neighboring grains are obtained by digital acquisition and processing of the related EBIC profile. The technique is applied to the evaluation of hydrogen passivation of a ribbon solar cell.

Effects of transverse temperature field nonuniformity on stress in silicon sheet growth

Abstract Stress and strain rate distributions are calculated using finite elements analysis for steady-state growth of thin silicon sheet with temperature nonuniformities imposed in the transverse (sheet width) dimension. Significant reductions in residual stress are predicted to occur for the case where the sheet edge is cooled relative to its center provided plastic deformation with high creep rates is present.

Charge collection and spectral response of amorphous‐silicon solar cells

Current generation in hydrogenated amorphous silicon, a‐Si(H), was found to be predominately from the space‐charge region. The mobility‐lifetime (μτ) products were the order of 5×10−10 cm2/V for both holes and electrons. It was necessary to consider trapping and recombination of the optically generated carriers in the space charge region to interpret solar cell and spectral response data.

Modeling of the time dependence of EFG crystal growth

Abstract Solutions for the time dependent, one-dimensional diffusion equation that applied during EFG (edge-defined film-fed growth) have been developed for cases appropriate to various growth situations. The time dependence produced by changes of parameters such as pull speed, thermal environment, and height of liquid silicon in the crucible can be calculated. Included in the solution are heat transfer and capillarity. The most significant result is that, for a range of conditions, changes of pull speed, hydrostatic pressure, or temperature can lead to transient overshooting or undershooting of the ribbon thickness and of the meniscus height. This imposes constraints on the way that one goes from one steady-state condition to another without encountering a physical situation that may terminate the growth (zero ribbon thickness, zero menicus height, or supercooled melt).

Digital electron beam induced current imaging: Apparatus and analysis

The electron beam induced current (EBIC) method is a well known technique for studying charge carrier recombination at extended defects in semiconductors. In this work a new electron beam induced current apparatus is described. It employs computer automated measurement and control equipment to acquire high resolution beam induced current images using a conventional secondary electron microscope. The electron beam induced current images typically contain a matrix of 512×512 digitally measured points. They can be displayed on a workstation monitor and imaging processing can be performed. The increased current resolution of the apparatus necessitated the modification of traditional quantitative EBIC analysis methods. The modified quantitative analysis method is presented and applied to the problem of precipitates contained in a Czochralski silicon solar cell. It results in a characterization of the amount of charge carrier recombination at the defects and a calculation of the depth of the defects below the s...