Influence of mobility and lifetime variations on drift-field effects in silicon-junction devices

An analysis of devices with drift fields formed by an impurity gradient is carried out allowing for lifetime and mobility variations with impurity concentration. In the case of silicon n-on-p photovoltaic solar cells, a field width of about twice the diffusion length of the minority carriers maximizes the collection efficiency. For lifetimes longer than one microsecond the optimum field width is about 25 µm, a value governed by the absorption characteristics rather than the diffusion length. In most cases, increasing the concentration ratio above 3 orders of magnitude is of little or no assistance in improving the collection efficiency. It is also shown that if the constant relating lifetime to high energy-particle flux is a strong function of impurity concentration, there is little advantage in using drift-field solar cell structures to enhance radiation resistance.