N-type multicrystalline silicon wafers and rear junction solar cells

S Martinuzzi,O Palais,F Ferrazza,M Pasquinelli

doi:10.1051/epjap:2005085

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.

Highlights

IntroductionMulticrystalline silicon (mc-Si) wafers or ribbons are the dominant material in photovoltaic industry
Since the capture cross sections of the fast diffusers are smaller in n-type silicon, it is expected that the consequences of the impurity-defect interaction are strongly reduced, and this reduction will be enhanced after a gettering treatment because impurity concentrations decrease
The aim of this paper is to show, by means of scan maps of minority carrier diffusion lengths and lifetimes, that n-type multicrystalline silicon (mc-Si) is an excellent material, because the recombination strength of dissolved impurities and extended crystallographic defects is low, neatly smaller than in ptype mc-Si, and that the Al-Si alloy formation gives rise to acceptable p-n rear junction solar cells (RJC)

Summary

Introduction

Multicrystalline silicon (mc-Si) wafers or ribbons are the dominant material in photovoltaic industry Such materials are characterized by the presence of a high density of extended crystallographic defects, like grain boundaries, dislocations and twins, which are more or less associated with segregated impurity atoms. These impurities are metallic fast diffusers (mainly iron, chromium and copper), oxygen atoms or oxygen precipitates and it is the interaction between extended defects and impurities that determines the recombination rate of minority carriers in the wafers. Oxygen precipitation occurs certainly at dislocation cores, at dislocation clusters and at grain boundaries These precipitates can, in turn, trap metallic impurities and become recombining, as observed in p-type silicon. Their surfaces are expected to have a hole repelling positive charge, and so in n-type materials such precipitates have been found less harmful for minority carriers [6]

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