Electronic properties of Al p+ surfaces formed by laser doping from aluminium oxide precursors: implications for PERC cell design and performance

Aluminium oxide (Al2O3) functions doubly as a high-quality surface passivation material for crystalline silicon and as an aluminium (Al) p-type precursor for laser doping. Thus, p(+) doping based on laser ablation of Al2O3 thin-films deposited on a silicon substrate is an attractively simplified process for concurrent local contact definition and aligned surface doping. A number of studies have demonstrated this process, but a careful examination of the influence of laser parameters on the electronic properties of the Al laser doped p(+) surface itself, including the influence of post-doping annealing, has yet to be presented. Such information is valuable for establishing process windows and for providing parameters by which the contact geometry can be optimized and the performance of locally Al2O3 laser doped solar cells predicted. In this work, we present accurate characterization of the electronic properties of primary importance to solar cell performance: effective surface recombination and contact resistivity. Recombination at the Al2O3 laser doped p(+) surface is found to exceed that of equivalently-doped p(+) silicon from furnace diffused boron, while contact resistivity to vacuum evaporated Al is up to two orders of magnitude less than screen-printed, fired Al. Based on this characterization, computer simulations demonstrate that with optimized rear contact geometries, an industrially relevant PERC cell can approach 21 % efficiency, and the high-performance UNSW PERL structure can exceed 24 %. (C) 2015 The Authors. Published by Elsevier Ltd.