Structure, chemistry and electrical properties of extended defects in crystalline silicon for photovoltaics

Abstract

The electronic properties of present‐day multicrystalline silicon (mc‐Si) materials for photovoltaic applications are strongly influenced by point defects, their mutual interaction and their interaction with dislocations and grain boundaries. This paper presents results from fundamental investigations of metal impurity interaction with extended defects, namely a small‐angle grain boundary and bulk microdefects. It is shown that the distribution of copper silicide precipitates closely follows the density of bulk microdefects indicating the underlying physics of ‘good’ and ‘bad’ grains frequently observed in mc‐Si. Co‐precipitation of copper and nickel in the same samples leads to virtually the same distribution of multimetal silicide precipitates which according to light‐beam induced current measurements show the same recombination activity as single‐metal silicide particles. Transmission electron microscopy is used to show that for copper‐rich and nickel‐rich conditions two types of silicides co‐exist, i.e. Cu3Si precipitates containing a small amount of nickel and NiSi2 precipitates containing some copper. Finally, phosphorus‐diffusion gettering (PDG) is discussed as the main gettering process used in presentday silicon photovoltaics. Special emphasis is put on the effect of extended defects and their interaction with metal impurities on PDG kinetics. It is shown that different limiting processes will be simultaneously operative in mc‐Si as a result of inhomogeneous bulk defect distributions (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gettering Processes and the Role of Extended Defects

“…Notwithstanding metastable precipitate configurations (see [120] for a summary) 3d TM impurities precipitate as the phase in equilibrium with silicon according to binary phase diagrams. Marioton and Gösele [121] describe precipitation processes as a quasichemical reaction M i + P n P n+1 + αI + βV, (4.19) where P n refers to a precipitate containing n metal atoms while α and β refer to the number of silicon self-interstitials and vacancies emitted per precipitating metal atom, respectively.…”

Electrical property of iron-related defects in n-type dislocated Czochralski silicon crystal used for solar cells

“…in mc-Si which are very effective centers of electron-hole recombination especially when dislocations are decorated by metal impurities. 11) Clean dislocations generally exhibit weak recombination activity at room temperature due to relatively shallow defect bands associated with the long-strain field. 12,13) It is well known that dislocations act as very efficient nucleation sites for precipitation.…”

A DLTS Perspective on Electrically Active Defects in Plated Crystalline Silicon n+p Solar Cells

“…z E-mail: eddy.simoen@imec.be nickel will have a strong tendency to precipitate. 25 When present in a silicon p-n junction, for example as a consequence of a too aggressive nickel silicidation step, nickel precipitates will degrade the reverse dark current, 26,27 an undesirable feature for optimal solar cell performance.…”