The growth kinetics and optical confinement studies of porous Si for application in terrestrial Si solar cells as antireflection coating

The mesoporous porous-silicon (PS) layers were grown on 〈1 0 0〉, 〈1 1 0〉, and 〈1 1 1〉 oriented wafers at constant current density of 20 mA cm−2. The pore sizes and surface morphologies were measured by atomic force and scanning electron microscopes. The thickness x of the PS formed and the refractive index were measured by an ellipsometer as a function of time duration t (in min) of anodization. The x vs. t data were fitted into a power law x=atc where c is a dimensionless constant and growth kinetics was established. The growth is practically independent of orientation. This is due the reason that the growth rate is controlled largely by the availability of holes which exchange their charge with oxidizing species and desirably large concentrations of holes were available at current density of 20 mA cm−2. For a similar reason the growth of PS layer on the front surface of the n+ region of n+–p solar cells could also be done at current density of ∼20 mA cm−2 nearly at the same rate. A large concentration of holes could be injected from p region into the n+ region because the positive contact was made on the p side and thus the junction was forward biased. The PS ARC of thickness 70 nm showed increase ∼26% in the short circuit current density Jsc and 24% in efficiency of the cells. However, the improvement in the values of the open circuit voltage Voc were lower than the expected value indicating that the PS layers had enhanced recombination of minority carriers at the front surface or in the front emitter region immediately below the PS layer.