Optimizing the Rear Side of Heterojunction Solar Cells on n-Type CZ Substrates

Hydrogenated amorphous silicon/ CZ silicon heterojunction solar cells are fabricated by plasma enhanced chemical vapour deposition using an entirely low temperature process. Rear side deposition parameters, i.e. dopant gas phase concentration, depositon time, and silane dilution in hydrogen, are varied. Cell current-voltage characteristics under illumination, evidenced that a decrease of phosphine dilution in silane results in higher opencircuit voltage, due to lower defect concentration associated to lower doping. Furthermore, layer thickness is increased either by increasing deposition time or by increasing the silane/hydrogen ratio. In both cases, higher opencircuit voltage is obtained with thicker n+ layer, due to shift of the Fermi level towards the bulk value. In the case of high silane/hydrogen ratio, the amorphous to microcrystalline fraction of the rear layer is higher, thus partly acting as a back surface field.