MOS and oxide-charge-induced (OCI) BSF solar cells

The recombination current occurring in the heavily doped regions of state-of-the-art silicon solar cells poses a basic limitation on the conversion efficiency obtainable because it limits the open-circuit voltage to maximum values of 610 to 620 mV. New structures, in which the heavily doped region in the base of a back-surface-field solar cell is eliminated are proposed. Instead, the desired high concentration of majority carriers at the back surface is obtained by Coulomb attraction utilizing an oxide formed on this surface. The origin of the Coulomb attraction is either charge in the oxide (OCI-BSF cell)or a small gate voltage which draws no power (MOS-BSF cell) applied to a metal-oxide-semiconductor structure. If a gate voltage VGis to be used it can be provided by 10 to 30 conventional cells in series which can supply VGto hundreds of cells having the structure proposed in this brief. Theoretical arguments are presented that indicate the likelihood of achieving open-circuit voltages in excess of 700 mV in these new structures and thus conversion efficiencies for silicon of about 20 percent for the AMI spectrum. The theoretical arguments are based on values of lifetime, surface recombination velocities, charge density in the oxide, etc., that have been seen experimentally.