Study of electrical properties of oxidized porous silicon for back surface passivation of silicon solar cells

Back surface passivation becomes a key issue for the silicon solar cells made with thin wafers. The high surface recombination due to the metal contacts can be lowered by reducing the back contact area and forming local back surface field (LBSF) in conjunction with the passivation with dielectric layer. About 3×10-7m thick porous silicon (PS) layer with pore diameter mostly of 1×10-8–5×10-8m was formed by chemical etching of silicon using the acidic solution containing hydrofluoric acid (HF), nitric acid (HNO3) and De-ionized water in the volume ratio 1:3:5 at 298K for which etching time was kept constant for 360s. Electrical properties of oxidized PS was studied through the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of the metal–insulator–semiconductor (MIS) device in which the oxidized PS was used as an insulating layer and the results were further analyzed. The C–V curves of all the studies MIS devices showed the negative flatband voltage varying from -2 to -5V, confirming that the oxidized layer of PS has fixed positive charge.