back surface field (PV) (BSF)
The back surface field (BSF) is used to passivate silicon solar cells and thus increase their efficiency. The technology used for this is called Passive Emitter and Rear Cell( PERC).
The rear field is an electron-reflective layer of the solar cell made of silicon nitride( Si3N4) with a higher doping, which restricts the electrons' access to the surface. In this region, the distance between the different states is very large. Because there are no intermediate states between conduction band and valence band, the space charge region corresponds to the band gap. Therefore, no or only few recombinations take place in this region. The performance of the solar cell is thus significantly increased.