Performance improvements from penetrating back-surface field in a very high efficiency terrestrial thin-film crystalline silicon solar cell

Abstract

This paper analyzes the performance improvements obtainable from extending the traditionally thin back-surface-field (BSF) layer deep into the base of silicon solar cells under terrestrial solar illumination (AM1). This extended BSF cell is also known as the back-drift-field cell. About 100 silicon cells have been analyzed, each with a different emitter or base dopant impurity distribution whose selection was based on physically anticipated improvements. The four principal performance parameters (the open-circuit voltage, the short-circuit current, the fill factor, and the maximum efficiency) are computed using a program which numerically solves the six Shockley equations under AM1 solar illumination at 88.92 mW/cm², at an optimum cell thickness of 50 μm. The results show that very significant performance improvements can be realized by extending the BSF layer thickness from 2 μm (18% efficiency) to 40 μm (20% efficiency). The immunity of cell performance to recombination defect or impurity center is also improved by a factor of 2 to 3 in the recombination center density. For 20% p+/n/n+ cells with about 20-μs base lifetime, a 20-μm BSF penetration is sufficient. At this or deeper penetrations, interband Auger recombination in the emitter layer becomes the limiting factor and affects mainly the short-circuit current but not the open-circuit voltage which has saturated to about 710 mV.

Key findings

• ▸Significant performance improvements can be realized by extending the BSF layer thickness from 2 μm to 40 μm.
• ▸Immunity to recombination defects or impurity centers is improved by a factor of 2 to 3 in the recombination center density.
• ▸A 20-μm BSF penetration is sufficient for 20% p+/n/n+ cells with about 20-μs base lifetime.

Keywords

Identifiers

DOI

10.1063/1.333211

Journal

Journal of Applied Physics

Year

1984