By Bas van de Loo, Plasma and Materials Processing Group, Eindhoven University of Technology; Gijs Dingemans, ASM; Ernst Granneman, CTO, Levitech BV; Erwin Kessels, Department of Applied Physics, Eindhoven University of Technology; Gaby Janssen, Researcher and Project Manager, ECN Solar Energy; Ingrid Romijn, Coordinator for Industrial N-type Cells and Modules, ECN Solar Energy
The n-Pasha n-type silicon solar cell currently achieves an average conversion efficiency of 20.2% using a relatively simple process flow. This bifacial cell concept developed by ECN is based on homogeneously doped p+ front and n+ back surfaces. To enhance the cell efficiency, it is important to reduce the carrier recombination within the boron-diffused p+ region and at its surface. This paper addresses a novel way to tune the boron-doping profile and presents advanced surface passivation schemes. In particular, it is demonstrated that a very thin (2nm) Al2O3 interlayer improves the passivation of the boron-doped surface; the Al2O3 films were deposited in industrial atomic layer deposition (ALD) reactors (batch or spatial). Moreover, it is shown that the boron-doping profile can be improved by etching back the boron diffusion. On the basis of the results presented, it is expect that n-Pasha solar cells with 21% efficiency will soon be within reach.