Recombination and resistive losses at ZnO∕a-Si:H∕c-Si interfaces in heterojunction back contacts for Si solar cells

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 RH>40 resulting in a highly doped microcrystalline (μc) Si:H layer. Such a μc-Si:H layer if deposited directly on a Si wafer yields a surface recombination velocity of S≈180cm∕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 VOC=640mV and a fill factor FF=80%. Insertion of an undoped amorphous (i) a-Si:H layer between the μc-Si:H and the wafer leads to a further decrease of S and, for the solar cells, to an increase of VOC. However, if the thickness of this intrinsic layer exceeds a threshold value of 4–5nm, 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 0.49–0.65eV, which we attribute to the valence band offset between a-Si:H and c-Si. The balance of FF losses and VOC gains determines the optimum (i) a-Si:H interlayer thickness for (i) a-Si:H∕(p) μc-Si:H double layer or (i) a-Si:H∕(p) a-Si:H∕(p) μc-Si:H triple layer back contacts.