High-Efficiency n-Type Silicon Solar Cells with Front Side Boron Emitter

Abstract

High-efficiency n-type PERL solar cells with a front side boron emitter passivated by ALD Al2O3 are presented within this work. For the applied PERL cell design two variations have been employed: i) different boron emitters (deep / shallow) and ii) different dielectric layers for rear side passivation (thermal grown SiO2 and PECVD SiNx). Both, thermal grown SiO2 as well as PECVD SiNx provide an effective passivation of the n-type rear surface with effective surface recombination velocities of 4 cm/s and 7 cm/s respectively. If the metalized rear side point contacts (with BSF) together with the recombination of the 1 Ω cm FZ base silicon are taken into account this results in saturation current densities of 30 fA/cm2 and 37 fA/cm2 respectively, limiting the open-circuit voltage (all recombination losses due to the front side are neglected) to 717 mV and 712 mV. The passivation of the boron emitter with ALD Al2O3 results in an emitter saturation current density as low as 11 fA/cm2. Together with the losses at the rear side as well as the front side contacts this allows for an open-circuit voltage of the applied PERL solar cell design of ~700 mV. For n-type PERL solar cells featuring a lowly doped boron emitter as well as a SiO2 passivated rear such a high open-circuit voltage (up to 703.6 mV) could be reached also at the device level, resulting in a conversion efficiency of 23.4%. Also for the PERL solar cells featuring a high surface concentration boron emitter with a PECVD SiNx passivated rear, i.e. first steps towards an industrial structure, still a high conversion efficiency of 21.8% could be achieved. All cells have been shown to be perfectly stable under illumination at 1 sun.