21% efficient screen-printed n-type silicon wafer solar cells with implanted phosphorus front surface field

In this paper n-type PERT (Passivated Emitter and Rear Totally diffused) silicon wafer solar cells with a diffused boron rear emitter and an implanted phosphorus front surface field are investigated. A key feature of the n-PERT rear emitter cell is that it uses the same sequence of surface passivation, rear local laser ablation and screen-printing processes as a commercial p-type PERC (Passivated Emitter and Rear Contact) cell. Therefore, this cell structure is very industrially relevant as it could simplify a production line upgrade from p-type cells to n-type cells. Additionally, ion implantation provides an elegant single-side doping process that further simplifies the processing sequence of n-PERT cells. Ion implantation also provides excellent control over the doping profile via a variation of post-implant annealing time. The effect of annealing time on the implanted phosphorus surface was evaluated in this study in terms of the front surface field dopant profile and its impact on the solar cells' electrical characteristics. A shallower front surface doping profile resulted in better short wavelength response. Additionally, the performance of two different Al pastes (with and without Si content in the paste) was compared. The better-performing Al-Si paste generates a homogeneous Al-p+ region under the contacts, which reduces the recombination at the contacts. By tailoring the phosphorus front surface field profiles and by minimising the recombination during the rear Al contact formation, efficiencies of up to 21% on large area 244 cm2n-type wafers were achieved so far.