Characterization of 23-percent efficient silicon solar cells

A silicon solar cell structure, PERC (passivated emitter and rear cell), has very recently demonstrated energy conversion efficiency above 23%. A number of interesting features of the PERC cell design are discussed. Rear contact design is based on a balance between the beneficial effects of small sparsely spaced contact points upon the open circuit voltage and short-circuit current of the cell and the corresponding negative effects upon cell fill factor. The noncontacted regions of the rear surface are held in weak depletion by an optically isolated but electrically connected rear Al reflector. Once bulk injection levels become appreciable, the disadvantage of this surface condition disappears. The structure incorporates a reasonably effective light-trapping scheme, although there remains scope for improvements in this area. Along with other improvements, efficiency approaching 24% seems feasible with the present cell structure. If a processing regime can be found which allows boron passivation of the contact holes or the entire rear surface without loss of the present exceptionally high bulk lifetimes, efficiencies above 24% are likely