Large area solar cells made from degradation-free, low resistivity gallium doped Cz wafers

Abstract

Boron doped Czochralski-wafers for industrial solar cells are negatively impacted by the fact that boron forms recombination active defects with oxygen under illumination, lowering the minority carrier lifetime of the bulk. This effect is known as light-induced degradation (LID) [1,2] and causes a voltage and current drop of the solar cells. It may be circumvented by using high-resistivity material to minimize the boron content of the wafer, but this leads to other negative effects. A different approach is the replacement of boron by gallium as dopant. In this work, low-resistivity gallium doped HiCz™ wafers produced by Confluence Solar’s proprietary process were processed into solar cells together with HiCz™ boron doped wafers. The 125x125 mm² cells were manufactured similarly to mass production conditions, employing screen printing and co-firing as the metallization technique. The solar cell results showed that both the boron and gallium doped cells produced initial efficiencies of around 18%. Various degradation experiments were conducted, showing LID on boron doped samples. This degradation saturated after 48 hours with a 6 mV decrease in VOC and an efficiency decrease of 0.3% absolute. On gallium doped samples however, no LID could be observed on a comparable timescale, the cells retained their initial efficiencies.