Investigation of doped a-Si_1-xC_x:H as a novel back contact material for CdTe solar cells

Wide band-gap, p-type doped, hydrogenated amorphous silicon-carbon alloy (a-Si_1-xC_x:H:B) layers deposited by plasma enhanced chemical vapor deposition (PECVD) under conditions that yield efficient hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells have been applied as back contacts to sputter-deposited CdTe superstrate solar cells. We report a maximum observed V_oc of 0.78 V and a best initial efficiency of ∼ 7.7 % (relative to an ∼ 12% standard cell baseline) without the introduction of Cu into the back contact region. We studied the stability of the best performing cells over a two year time period and found that although V_oc is relatively stable, the series resistance of the cells increased significantly leading to fill-factor degradation. The role of hydrogen loss from the back contact layer via diffusion into the CdTe absorber is explored as a possible cause of this degradation. In related investigations, we have found that the effect of in-diffusing H on the CdTe solar cell performance is detrimental, as observed from a brief (∼ 15 s) exposure of CdTe to a low power H₂ plasma between the treatment by CdCl₂ and the application of standard Cu/Au back contacts. This detrimental effect of H in the fabrication of the back contact layer from hydride gases, and the subsequent instability of the back contact itself, were found to be significant challenges encountered in this investigation.