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MODELLING OF THE*DISLOCATION INFLUENCE ON ELECTRICAL PROPERTIES OF POLYCRYSTALLINE SILICON CELLS H. EL GHITANI AND S. MARTINUZZI Laboratoire de Photo6lectricit6 des Semi-conducteurs, Facult6 des Sciences et Techniques de Saint-j~rome, Universit6 d'AixMarseille III, F-13397 Marseille Cedex 13. ABSTRACT The influence of the density Ndis and recombination activity Sd of dislocations on the photocurrent Jsc' the spectral variation of Jsc and electron diffusion length Ln are computed by means of the Green's function. Sd is the surface recombination velocity of the space charge cylinder surrounding a dislocation line, assumed to be perpendicular to the illuminated surface of the cells. It is found that the value and the spectral variation in the near infrared of J,sc and that of Ln , are dependent on Ndis and Sd, specially when Ndis and Sd are higher than 104 cm- 2 and 104 cm s-1 respectively. A reasonnable agreement is obtained with experimental results. INTRODUCTION In large grained cast polycrystalline silicon, the mean grain size is actually greater than 1 mm, and the influence of grain boundaries (G.B.'s) is negligeable compared to that of intragrain defects, especially dislocations. These defects reduce the minority carrier diffusion length Ln and the photocurrent Jsc of solar cells. They also increase the dark reverse current of P-N junctions degrading the photovoltage 11,2,3,41. Several tentatives have been made to model the influence of these defects on solar cells, as the last work of Yamaguchi et al. 151, who have computed the variation of Ln as a function of density Ndis of dislocations having all the same high recombination activity. Recent publications have concluded that in polycrystalline silicon, the extended crystallographic defects have not by themselves a high recombination activity, and interactions of these defects with impurities (specially oxygen) constitutes the main source of recombination centres 16,7,81. It is needfull to consider the variations of the recombination activity and of the density of dislocations, in order to obtain a fairly good agreement, between a computed and experimental variations of Ln and Jsc" In the present work, the Green's function is used, following a method previously proposed by Halder and William 191 to modelize G.B.'s influence, in order to establish a three dimensional model which leads to a rigorous analytical expression of the deleterious effect of dislocations. A comparison is also made with experimental results. Mat. Res. Soc.Symp. Proc.Vol. 106. '1988 Materials Research Society

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THE MODEL

We consider a N+P junction with a thin N+ emitter, made in columnar polycrystalline silicon. An isolated grain is represented by figure 1. Inside the grains, sufficiently far from a G.B., the dislocation pipes are assumed to be parallel and perpendicular to the junction plane. The following assumptions have been made (1) The contribution of the emitter is negligeable. (2) The shunt and series resistance effects can be neglected. (3) The back contact is perfectly ohmic