Epitaxial Silicon Solar Cells Research Articles

Chemical-vapor deposited large-area multicrystalline epitaxial silicon solar cells on upgraded metallurgical silicon/graphite substrates

Very large-area (200 cm 2) thin multicrystalline silicon sheets having large grain size were produced directly on graphite by unidirectional solidification of acid-leached and gettered metallurgical silicon powder. The metallurgical silicon powder was acid treated by refluxing in aqua regia for several days and then gettered in air with P 2O 5 at 1050°C for 3–5 days. After cleaning and drying the acid-treated and gettered silicon, it was uniformly spread on top of a graphite boat, Radio-frequency (RF) heated to melt and then unidirectionally solidified to form a polycrystalline substrate. This resulted in an n + substrate having a resistivity of about 0.03 Ω cm. P–N junction solar cells were formed by the epitaxial deposition of n- and p-layers using the hydrogen reduction of trichlorosilane. Front grid contact was formed by vacuum evaporation of Ti and Ag. Solar cell characterization results are very promising. This substrate preparation technique is cost-effective, produces low-cost flat rectangular and mechanically strong multicrystalline silicon substrates, suitable for the continuous large-volume production of solar cells.

Alternative contact designs for thin epitaxial silicon solar cells

Two major opportunities for increasing the performance of crystalline silicon solar cells involve reducing their thickness and reducing the losses associated with their front metallic grid contacts. Front grid contacted thin epitaxial silicon solar cells based on the growth of crystalline silicon films on a substrate or superstrate have been reported for many years, as have wafer-based solar cells with alternative contact approaches. Integrating these two concepts into a single device presents an opportunity for simultaneously reducing two major loss mechanisms associated with crystalline silicon solar cells. The opportunities that exist and challenges that must be overcome in order to realize such a device are described in this paper. The design space is defined and explored by considering a wide range of possible approaches. A specific approach was chosen and used to design, grow, and fabricate a proof-of-concept thin epitaxial silicon solar cell with an embedded semiconductor grid as an alternative to a conventional front metallic grid. The work presented here has resulted in a thin epitaxial silicon solar cell with a 7·8% designated area conversion efficiency, well isolated contacts, negligible series resistive power loss, and less than 1% shading of the designated area. Copyright © 1999 John Wiley & Sons, Ltd.

Measurement of minority carrier diffusion length from spectral response of thin film polycrystalline silicon solar cells

A simple model of epitaxial silicon solar cells has been used to explain their observed spectral response. This model has been applied to a series of boron-doped substrates covered by epilayers of various thicknesses and resistivities. It is concluded that the minority carrier diffusion length L n in the thin epilayer can be determined by analyzing the long wavelength spectral response with the help of this model. It is also found that the short-circuit current density J sc and L n depend more on the epilayer defects induced by the defects of the heavily doped substrates than on epilayer thickness or resistivity.

Effects of heat treatment on epitaxial silicon solar cells on metallurgical silicon substrates

Metallurgical-grade silicon partially purified by aqua regia extraction was unidirectionally solidified on graphite to yield a large-area large-grain substrate. The concentration of impurities in the grains and at grain boundaries of the silicon substrate and the impurity distribution along the length of the substrate were measured semiquantitatively by the secondary ion mass spectrometry (SIMS) technique. Epitaxial silicon layers were deposited onto the metallurgical silicon substrate by the thermal reduction of trichlorosilane. The concentration of impurities in the epitaxial layer was determined by the SIMS technique. On heat treatment a significant amount of the metallic impurities in the substrate and the epitaxial layer were found to diffuse to the grain boundaries. By utilizing grain boundaries as a sink for metallic impurities, large-area (above 30 cm 2) solar cells with an air mass one efficiency of about 9% were prepared by depositing a p-n junction structure onto the metallurgical silicon substrate. The solar cells were characterized by dark current-voltage, illuminated current-voltage, spectral response and minority carrier diffusion length measurements.

Development of High Efficiency Epitaxial Silicon Solar Cells

The optimum fabrication conditions for npp+ and n+ npp+ types epitaxial silicon solar cells are described. Epitaxial p-base and n-surface npp+ and n+ -diffused n+ npp+ solar cell structures were grown on both (111) and (100) oriented single crystalline silicon substrates. All the cells have no A.R. coating. The optimum conditions for high efficiency cells are following; resistivity of the p-epitaxial region is the range of 0.1–0.5 ohm-cm and the thickness of it is about 30µm or below, and the n+ npp+ structure is the important condition. Maximum efficiency of 10.0% has been obtained for both the npp+ cell whose resistivity of the p-epitaxial layer was 0.5 ohm-cm and the n+ npp+ cell with 3 ohm-cm.