Gallium-Doped Silicon for High-Efficiency Commercial Passivated Emitter and Rear Solar Cells

Czochralski-grown gallium-doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear cell (PERC) devices and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski-grown wafers in dependence of resistivity, finding effective lifetimes well into the millisecond region without any gettering or hydrogenation processing, thus demonstrating one advantage over boron-doped silicon. Second, the stability of gallium-doped PERC cells are monitored under illumination (>3000 h in some cases) and anomalous behavior is detected. While some cells are stable, others exhibit a degradation then recovery, reminiscent of light and elevated temperature-induced degradation (LeTID) observed in other silicon materials. Surprisingly, cells from one ingot exhibit LeTID-like behavior when annealed at 300 °C but near stability when not annealed, but, for another ingot, the opposite is observed. Moreover, a stabilization process typically used to mitigate boron-oxygen degradation does not influence any cells that are studied. Secondary-ion mass spectrometry of the PERC cells reveals significant concentrations of unintentionally incorporated boron in some cases. Nevertheless, even in the absence of mitigating light-induced degradation, Ga-doped silicon is still more stable than unstabilized B-doped silicon under illumination.