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Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.

Abstract-The decrease of minority-carrier lifetime with resistivity and with illumination level in bifacial dendritic web silicon solar cells is addressed. This variation of lifetime is shown to be consistent with the presence of a distribution of defect levels in the bandgap that arise from extended defects in the web material. The extended defects are precipitates, recently shown to be oxide precipitates, that decorate dislocation cores. It follows that the sensitivity to this background distribution of defect levels increases with doping because the Fermi level moves closer to the majority-carrier band edge. It is not necessary that the dopant atom itself, or a complex including the dopant atom, acts as a recombination center in order to explain the observed decrease in lifetime with doping density. Good agreement is obtained between calculated and measured values of short-circuit current and quantum efficiency for bifacial cells covering a range of doping density ( 6 x lOI4 to 3 x 10l6 ~m -~) and illumination level (0.001 to 1 sun), with illumination from either back or front of the cell. The implications of this approach extend to concentrator cells and to other devices in which minority-carrier lifetime is an important parameter. This includes devices made using Czochralski-grown silicon, where oxygen and oxide precipitates likewise play an important role in determining lifetime.

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