Effective passivation of silicon surface by AZO films: Application in bifacial solar cells

Abstract

Aluminum rich zinc oxide (AZO) film was made on silicon substrates by spin coating using a precursor containing Al and Zn in the 20% atomic ratio. The films were annealed in air atmosphere and in hydrogen ambient. The minimum reflectance (Rλ) in air annealed (AZO:Air) samples was ~2.5% in the vicinity (±50nm) of 750nm, however, the value was marginally lower (~2%) in hydrogen annealed (AZO:H2) samples. The average Rλ over 300-1200nm range was 18.5% and 14.1% respectively. An additional XRD peak of H2Al10O16 at 2θ=51.5° corresponding to (121) reflection plane was observed in the AZO:H2 film. The AZO films were applied on the front and back surfaces of bifacial crystalline silicon (c-Si) solar cells where an improvement both in short circuit current density (Jsc) and open circuit voltage (Voc) was observed. The values of Jsc and Voc after AZO:Air were increased by 23% and ~10mV respectively w.r.t without AZO layer cells which on subsequent annealing in H2 raised to 32% and ~21mV. Jsc (AZO:Air) was used to calculate the expected enhancement in Voc which matched with the measured increase in Voc. The improvement in Jsc is due to reduced reflectance and, hence, the observed increase in Voc (AZO:Air) is attributed to it. On the other hand, increase in Voc after AZO:H2 step was much more and cannot be accounted only by Rλ, and therefore, the additional enhancement in Voc is attributed to back surface passivation that has occurred after annealing of the solar cells in hydrogen ambient. This attribution is substantiated by the internal and external quantum efficiencies (IQE and EQE) after back and front illumination of the solar cells and minority carrier lifetime measurements. The IQE data was used to calculate the diffusion length (Ln) of the base region where the value of Ln did not change after AZO:Air layer (=130μm for Cell #1) but its value substantially increased (=244μm) after the AZO:H2 treatment. This observation further reiterates our claim that the AZO film has surface passivation property. © 2013 Elsevier Ltd.