High efficiency PERC and PERL silicon solar cells

The aim of this thesis was to investigate techniques for improving the energy conversion efficiency of high efficiency silicon solar cells. This aim was achieved with the development of novel PERC (Passivated Emitter and Rear Cell) and PERL (Passivated Emitter and Rear Locally-diffused) cell structures of performance markedly superior to that established by the earlier PESC (Passivated Emitter Solar Cell) cells at the start of the thesis. An additional achievement was the demonstration of substantial improvements in the cell performance under the space solar spectrum. The cell efficiency improvement was achieved by improving the surface and the bulk passivation by techniques such as TCA oxidation, inverted pyramid surface structure, rear localized metal contact design and re-optimized deep emitter. The improved light trapping scheme made another contribution to the efficiency improvement. The novelly designed PERC cell with all above high efficiency techniques demonstrated an independently confirmed efficiency of 22.3% (AM1.5, 25C). Most of the improvement came from the significantly improved open circuit voltage of up to 700 mV due to the excellent surface and bulk passivation employed in the cells. The rear metal contact pattern of the PERC cells had to be very carefully designed with local contact windows well separated apart to minimize the recombination introduced by the contacts and to minimize the series resistance caused by such contact design. The newly developed boron diffusion technique with BBr3 liquid source giving low surface damage supplied a method to further passivate the rear metal contact areas. This lead to the development of a new generation PERL cell. The PERL cells had reduced series resistance and were suitable for higher resistivity substrates due to this boron diffused rear metal contact passivation. The PERL cell demonstrated an independently confirmed efficiency of 23.3% (AM1.5, 25C). This is believed to be the highest ever achieved by a silicon solar cell. The PERL cells also demonstrated an AM0 efficiency of 20.8%. Even though they suffered a rapid degradation after high energy electron radiation damage, they still had higher performance than commercial space silicon cells after electron radiation. With minor modifications, the cells have the potential to demonstrate efficiency over 14% after 1 x 1015 e/cm2 1 MeV electron radiation damage