Vertical junction Si cells for concentrating photovoltaics

Vertical junction Si cell has shown a potential to operate at high concentrations, mainly the result of reduced series resistance losses due to its low-current/high-voltage design, but tests and analyses have so far only shown a modest efficiency of about 20%. We perform a comprehensive optimization study and show that the conversion efficiency of vertical multi-junction (VMJ) cells can be significantly higher, close to 30% at concentrations of 1000 and higher. Reaching this efficiency requires junction dimensions that are significantly smaller than previous VMJ cells. This may require a different approach in the fabrication process, possibly by using a monolithic method rather than the wafer stacking approach. We also show that increased photoconductivity, which is usually negligible in conventional cells, produces a significant reduction in series resistance at high concentrations making it a significant contributor to the outstanding performance of the VMJ cell at high concentration.