High efficiency crystalline silicon solar cells. Third technical report; final technical report

Abstract

A review of the entire research program since its inception ten years ago is given in this final report. The initial effort focused on the effects of impurities on the efficiency of silicon solar cells to provide figures of maximum allowable impurity density for efficiencies up to about 16 to 17% (AM1). Highly accurate experimental techniques (Capacitance Transient Spectroscopy) were extended to characterize the recombination properties of the residual impurities in silicon solar cell. A novel numerical simulator of solar cell was also developed, using the Circuit Technique for Semiconductor Analysis, which has provided exact theoretical design criteria on the maximum allowable impurity density. Recent effort until the end of the program has focused on the delineation of the material and device parameters which limited the silicon AM1 efficiency to below 20% and on an investigation of cell designs to break the 20% barrier. It is shown that the known and newly proposed high efficiency design criteria, if all implemented successful in one cell, could give AM1 efficiencies of 20% or higher. These include implementing a thin graded-base back-surface-field by epitaxy, minimizing emitter contact and surface or interface recombination losses using high/low emitter junctions, removing junction perimeter recombination losses, and maintaining a high base lifetime. It is concluded that the practical limitation in silicon cells with efficiency substantially higher than 20% comes from recombination of the photogenerated carriers at the residual impurity and defect recombination centers in the base. This calls for further research on the fundamental characterization of the carrier recombination properties at the chemical impurity and physical defect centers. It is further shown that only single crystalline silicon cell technology can be successful in attaining efficiencies greater than 20%.