Interactions of Efficiency and Material Requirements for Terrestrial Silicon Solar Cells

Recent recognition of the influence of "heavy doping effects" on solar cell performance and the evaluation of new design options such as the "three-layer structure" including a high/low junction, front surface texturizing, and the formation of optically reflective back surfaces has led to a rethinking of approaches to the design of silicon solar cells. This rethinking has been facilitated by the application of the transport velocity transformation method which eases the analysis of muitilayer devices. Many solar cell design variations have been analyzed using this technique in order to determine optimum cell structures. The key results of the analyses are that: low resistivity material should be used, up to the point of onset of Auger recombination; the highest lifetime material available should be used; a properly designed three-layer structure permits base regions approaching in performance the ideal device, even if the back surface is completely covered by an ohmic contact; the surface recombination velocity in the front region must be controlled by passivation or similar means, but the front region itself may be thicker than in current cells; higher resistivity front regions will require more sophisticated grid metallization structures than those that are usually applied now; cells with limited minority carrier lifetimes should be very thin, requiring surface texturizing and back surface reflectors in order to achieve optimum performance. With these design features, the idealized silicon solar cell structures analyzed yield airmass 1 efflciencies between 24 percent and 26.5 percent, so that real cell efflciencies near 22 percent should be achievable.