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, similar to4nm 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.