High efficiency crystalline silicon solar cells

Abstract

Based recombination at residual defect and impurity recombination centers is identified to be the likely cause of the 20% (AM 1) efficiency barrier in the highest efficiency silicon solar cells reported to date. To reach the 20% (AM 1) efficiency, base recombination must be further reduced by either stress-free and clean fabrication techniques on high lifetime crystals or novel base structure design, such as the graded thin-base back-surface-field structure proposed and analyzed by Sah and Lindholm. To break the 20% barrier, residual base recombination losses must be eliminated and emitter recombination must be reduced. Several novel emitter designs to reduce recombination losses have been proposed and one demonstrated. These involve the reduction of emitter interface recombination losses at the non-contact surface by high quality thermal oxide and at the metal-contact/silicon-emitter interface by either a thin tunneling oxide, as demonstrated by Green, or by a polysilicon barrier layer between the metal conductor and the silicon emitter surface. An efficiency of 23.8% (AM 1) has been estimated using Neugroschel's data on emitter interface recombination velocity and dark current density of polysilicon barrier layers. Novel floating emitter or non-contact emitter solar cell transistor structures have also been proposed by Sah and Cheng to reduce emitter recombination loss for >20% efficient silicon solar cells.