Triple-junction a-Si solar cells with heavily doped thin interface layers at the tunnel junctions

Triple-junction a-Si based solar cells, having a structure of SS/Ag/ZnO/n⁺/n/b/a-SiGe-i/b/p/p⁺/n⁺/n/b/a-SiGe-i/b/p/p⁺/n⁺/n/a-Si-i/p /p⁺/ITO, are fabricated at the University of Toledo using a multi-chamber, load-locked PECVD system. We studied the effect of heavily doped p⁺ and n⁺ layers deposited at the tunnel junction interfaces between the top and middle component cells and between the middle and bottom component cells on the efficiency of triple-junction solar cells. Preliminary results show that thin, ∼1nm, interface p⁺/n⁺ layers improve the solar cell efficiency while thicker interface layers, ∼4nm thick, cause the efficiency to decrease. Incorporating the improved interface layers at the tunnel junctions, as well as earlier improvements in the intrinsic layers, the p-i interface in terms of reducing the band-edge offset, and the a-SiGe component cells using bandgap-graded buffer layers, we fabricated triple-junction solar cells with 12.71% efficiency in the initial state and 10.7% stable efficiency after 1000 hours of 1-sun light soaking. Samples sent to NREL for independent measurements show 11.8% total-area (or 12.5% active-area) initial efficiency.