N-type multicrystalline silicon wafers and rear junction solar cells

¹ UMR TECSEN, Université Paul Cézanne-Aix-Marseille III, 13397 Marseille Cedex 20, France
² EniTecnologie, via d'Andrea 6, Nettuno, Italy

Corresponding author: santo.martinuzzi@univ.u-3mrs.fr

Received: 8 September 2005
Accepted: 29 September 2005
Published online: 30 November 2005

Abstract

N-type silicon presents several advantages compared to p-type material, among them, the most important is the small capture cross sections of metallic impurities, which are neatly smaller. As a consequence lifetime and also diffusion length of minority carriers should be neatly higher in n-type than in p-type, for a given impurity concentration. This is of a paramount interest for multicrystalline silicon wafers, in which the impurity-extended crystallographic defects interaction governs the recombination strength of minority carriers. It is experimentally verified that in 1.2  cm raw wafers lifetimes about 200 s and diffusion lengths around 220 m are measured. These values increase strongly after gettering treatments like phosphorus diffusion or Al-Si alloying. Scan maps reveal that extended defects are poorly active, although in regions where the density of dislocations is higher than 10⁶ cm^-2. Abrupt junctions are obtained by Al-Si alloying after annealing between 850 and 900 °C, which could be used for rear junction cells. Such cells can be processed by means of similar processing steps used to make conventional p-type base cells.