THEORETICAL-STUDY OF THE INFLUENCE OF DOPING CONCENTRATION ON THE PERFORMANCE OF POLYCRYSTALLINE SILICON SOLAR-CELLS

The influence of the base doping concentration (N(A)) on the effective diffusion length of minority carriers (L(n)*) and the parameters of polycrystalline silicon solar cells with fibrous grain is investigated theoretically utilizing our recently proposed grain-boundary recombination model together with the heavy doping effects. The effects of the grain size, the grain-boundary states density, and the Auger recombination processes on the above-said parameters are also studied. It has been shown that, for very small grain sizes (d << 100-mu-m), the grain-boundary recombination process is always more effective than the bulk and the Auger recombination processes, provided the base doping level is less than 10(19) cm-3. It is found that if the grain size is much larger than the bulk diffusion length of minority carriers, L(n)* always decreases with increasing N(A), whatever the base doping level be. It is also found that at small values of the grain size and for N(A) > 3 x 10(15) cm-3, the contribution of the space-charge recombination component of the dark current is greater than that of the diffusion component. It is predicted that the effect of the emitter side on the performance of the solar cell cannot be neglected if N(A) > 2 x 10(17) cm-3. It is demonstrated that the optimum base doping level (for the maximum conversion efficiency) increases with increasing grain size and attains its corresponding value of monocrystalline silicon. It is also demonstrated that the experimentally observed values of maximum power output cannot be matched with the computed values, unless we consider the grain boundary shunting effects for N(A) greater-than-or-equal-to 10(16) cm-3.