Investigating intrinsic silicon oxide and a lightly doped p-type layer for evolution of tail states and estimating its impact as a passivation layer in a HIT type device

Abstract

Hydrogenated amorphous silicon is well known for its various alloys and wide ranging opto-electronic properties. Hydrogenated silicon sub-oxide (aSiO:H) is one of them. The effect of boron doping on optoelectronic properties of the aSiO:H have been investigated with intrinsic and delta-doped materials. The Urbach energy of the tail and midgap defect states of the intrinsic and lightly doped(0.01% gas phase doping) materials were found to be 103, 151 meV and 8.19x1016 , 5.47x1017 cm-3 respectively. As a result of the 0.01% doping a reduction in optical gap was observed from 1.99 to 1.967 eV along with a shift in Fermi level by 0.258eV, indicating an efficient boron doping of the intrinsic silicon oxide material. The lightly doped p-type layer was used to simulate device characteristics in amorphous silicon (aSi:H) and HIT-type solar cell. In the aSi:H device the power conversion efficiency (PCE) was moderate while in a HIT-type device the PCE was 26.4%. It indicated that a good quality silicon oxide layer can be an attractive passivation layer for a HIT type device. It further indicated that defect density of the passivation layer can have a significant impact on performance of a HIT type device. The primary difference of the two passivation layers (intrinsic and delta-doped) are the difference in mid-gap and tail states. Passivation layer with a higher mid-gap state leads to poorer device performance. Therefore, it highlights the importance of using a less defective passivation layer to improve device performance.