Influence of air exposure duration and a-Si capping layer thickness on the performance of p-BaSi2/n-Si heterojunction solar cells

Fabrication of p-BaSi2(20nm)/n-Si heterojunction solar cells was performed with different a-Si capping layer thicknesses (d a-Si) and varying air exposure durations (t air) prior to the formation of a 70-nm-thick indium-tin-oxide electrode. The conversion efficiencies (η) reached approximately 4.7% regardless of t air (varying from 12–150 h) for solar cells with d a-Si = 5 nm. In contrast, η increased from 5.3 to 6.6% with increasing t air for those with d a-Si = 2 nm, in contrast to our prediction. For this sample, the reverse saturation current density (J 0) and diode ideality factor decreased with t air, resulting in the enhancement of η. The effects of the variation of d a-Si (0.7, 2, 3, and 5 nm) upon the solar cell performance were examined while keeping t air = 150 h. The η reached a maximum of 9.0% when d a-Si was 3 nm, wherein the open-circuit voltage and fill factor also reached a maximum. The series resistance, shunt resistance, and J 0 exhibited a tendency to decrease as d a-Si increased. These results demonstrate that a moderate oxidation of BaSi2 is a very effective means to enhance the η of BaSi2 solar cells.