Rear surface effects in high efficiency silicon solar cells

Abstract

Rear surface effects in PERL solar cells can lead not only to degradation in the short circuit current and open circuit voltage, but also fill factor. Three mechanisms capable of changing the effective rear surface recombination velocity with injection level are identified, two associated with oxidised p-type surfaces, and the third with two dimensional effects associated with a rear floating junction. Each of these will degrade the fill factor if the range of junction biases corresponding to the rear surface transition, coincides with the maximum power point. Despite the identified nonidealities, PERL cells with rear floating junctions (PERF cells) have achieved record open circuit voltages for silicon solar cells, while simultaneously achieving fill factor improvements relative to standard PERL solar cells. Without optimisation, a record efficiency of 22% has been demonstrated for a cell with a rear floating junction. The results of both theoretical and experimental studies are provided.