Toward 21% Efficiency nPERT Solar Cells with Selective Back Surface Field Technique

We have demonstrated 20.83% large-area conversion efficiency with selective Back Surface Field(s-BSF) on the n-type Passivated Emitter, Rear Totally (nPERT) cell structure. Industrial 156 mm (6” inch) n-type Czochralski mono-crystalline silicon wafers were used as substrates. In this kind of cell, we employed a single-side texture on the boron emitter side. We use AlOX and SiNX dielectric stack as ARC and passivation. The unique point is on the cell's rear side, we use the phosphorous ion implantation to form the Back Surface Field (BSF). Due to the flexible on the implant process, we can choose different implant mask pattern to form the heavy doping region to achieve the selective Back Surface Field(s-BSF) structure. Finally we use the industrial screen printing technique to form the metal contact. The front (boron) side is printed with Ag/Al paste and the rear (phosphorous) side is with two printing steps; at the first step, we use high solid content and firing-through silver paste to form the contact on the heavy doping region, and second, we use the low solid content and non-firing through silver paste to connect all the metal contact formed on the first step. Finally, we use the fast firing furnace to co-fire the sample. The champion batch shows average values of 650mV open circuit voltage and 40.1 A/mm2 short circuit current density and 79.7% on the fill factor, and 20.71% efficiency.