Recombination and Resistive Losses at ZnO/a-Si:H/c-Si Interfaces in Heterojunction Back Contacts for Silicon Solar Cells

Einsele, F.; Rostan, P. J.; Schubert, M. B.; Rau, U.

doi:10.1063/1.2803749

Journal Article

Recombination and Resistive Losses at ZnO/a-Si:H/c-Si Interfaces in Heterojunction Back Contacts for Silicon Solar Cells

Einsele, F.FZJ* ; Rostan, P. J.FZJ* ; Schubert, M. B.FZJ* ; Rau, U.FZJ*

2007
American Institute of Physics Melville, NY

Journal of applied physics 102, 094507 (2007) [10.1063/1.2803749]

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Please use a persistent id in citations: http://hdl.handle.net/2128/17130 doi:10.1063/1.2803749

Abstract: We investigate resistive losses at p-type crystalline Si/hydrogen passivated Si:H/ZnO:Al heterojunction back contacts for high efficiency silicon solar cells. A low tunneling resistance for the (p-type) Si:H/(n-type) ZnO part of the junction requires deposition of Si:H with a high hydrogen dilution rate R-H>40 resulting in a highly doped microcrystalline (mu c) Si:H layer. Such a mu c-Si:H layer if deposited directly on a Si wafer yields a surface recombination velocity of S approximate to 180 cm/s. Using the same layer as part of a (p-type) c-Si/Si:H/ZnO:Al back contact in a solar cell results in an open circuit voltage V-OC=640 mV and a fill factor FF=80%. Insertion of an undoped amorphous (i) a-Si:H layer between the mu c-Si:H and the wafer leads to a further decrease of S and, for the solar cells, to an increase of V-OC. However, if the thickness of this intrinsic layer exceeds a threshold value of 4-5 nm, resistive losses degrade the fill factor FF of the solar cells. Temperature dependent measurements of the contact resistance unveil activation energies in a range of 0.49-0.65 eV, which we attribute to the valence band offset between a-Si:H and c-Si. The balance of FF losses and V-OC gains determines the optimum (i) a-Si:H interlayer thickness for (i) a-Si:H/(p) mu c-Si:H double layer or (i) a-Si:H/(p) a-Si:H/(p) mu c-Si:H triple layer back contacts.(C) 2007 American Institute of Physics.

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