Sol–gel derived hydrogen annealed ZnO:Al films for silicon solar cell application

Abstract

A study of the effect of hydrogen annealing on antireflection and surface passivation properties of sol–gel derived aluminum rich zinc oxide (AZO) films coated on silicon wafers has been done in 300–600°C temperature range. The minority carrier lifetime in silicon wafers coated with AZO films was measured using microwave photoconductive decay (μ-PCD) technique. After annealing of AZO coated silicon wafer in hydrogen ambient for 30min between 400 and 600°C the effective minority carrier lifetime τeff improved above the initial value of ∼16μs and attained a maximal value of ∼71μs for annealing at 500°C. It may be that above 400°C the molecular hydrogen gets dissociated into atomic hydrogen in presence of Al induced defects at the AZO coated silicon surface and passivates them by formation of Si–H–Al complex. The annealing in air in the same temperature range did not affect the lifetime. AZO films annealed at 500°C in hydrogen or air show high transmittance in 400–1200nm wavelength range and are suitable as AR coatings for silicon solar cells. We have applied a hydrogen annealed AZO antireflection coating on n+ front surface and an equally thick hydrogen annealed AZO coating on the p-back surface of an n+-p multi crystalline silicon solar cell. The observed improvement in Jsc, Voc values of the cell established that while AZO film on front acted as a good AR coating the AZO film on the back surface passivated the back surface effectively.