Design, characterization and fabrication of silicon solar cells for >50% efficient 6-junction tandem solar cells

A major objective for photovoltaic conversion is to develop high efficiency solar cells. Many approaches are under investigation - Multiple Junction Solar Cell, Multiple Spectrum Solar Cell, Multiple Absorption Path Solar Cell, Multiple Energy Solar Cell, and Multiple Temperature Solar Cells [1]. The Multiple Junction Solar Cell approach based on a six-junction tandem solar cell has been adopted to achieve conversion efficiency of greater than 50% in the VHESC program sponsored by DARPA [2]. In six-junction tandem solar cells, individual solar cells are stacked on one another and each solar cell absorbs the best-matched slice of the solar spectrum. Silicon is one of the cells in the tandem stacks, and absorbs photon energy of 1.42 - 1.1 eV. The role of the silicon cell is to convert 7% of the light incident on the tandem stack into electricity. Other cells in the stack contribute the balance of the electricity. Key design parameters for the silicon cells are that it should have dimensions of 2.5 × 8 mm² and it needs to transfer light with energy of less than 1.1ev to the underlying solar cells. In this paper, discussion is made of the design of the silicon cell. Minority carrier recombination at surfaces and in the volume, internal quantum efficiency, resistance losses, free carrier parasitic absorption, optical reflection, light trapping, and light absorption must be traded off against each other. PC1D modeling is used to analyze the various parameters and produce estimates of short circuit current, fill factor and open-circuit voltage of the cell [3]. In addition, characterization of solar cell by photoconductance measurement to analyze carrier recombination and emitter saturation current as well as to predict the open-circuit voltage of solar cell [4, 5] is presented. Discussion of cell fabrication process followed by I-V testing is presented. Completed solar cells were tested in ANU using an in-house fabricated current-voltage flash tester [6] under AM1.5D.