Passivation Layers for Indoor Solar Cells at Low Irradiation Intensities

The passivation mechanisms and qualities of Al2O3, SiNx, SiO2 and a-Si:H(i) on p- and n-type silicon are investigated by quasi-steady-state photoluminescence measurements. This technique allows effective lifetime measurements in an extremely large injection range between 1010 cm-3 and 1017 cm-3. The measurements are discussed focusing on injections below 1012 cm-3 in order to determine the most effective passivation layer for solar cells arranged for indoor applications. Fixed negative charges in the passivation layer cause field-effect passivation due to band bending leading to either accumulation or inversion at the passivation layer/silicon interface. Accumulation causes a stable passivation quality at low level injection. Inversion leads to effective lifetime losses similar to the losses in the space charge region. On p-type silicon the most effective surface passivation at low injections is provided by Al2O3 or a-Si:H(i). The n-type silicon samples passivated with a-Si:H(i) show the best effective lifetimes. SiNx and SiO2 show lifetimes one order of magnitude below a-Si:H(i). Al2O3 on n-type is the most effective passivation at high injections around 1015 cm-3. Due to inversion losses at low level injections the passivation quality decreases more than two orders of magnitude for injections around 1010 cm-3.