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Thermal stability of passivating layers in amorphous/crystalline silicon (a-Si/c-Si) heterojunction solar cells is crucial for industrial processing and long-term device stability. Hydrogenated amorphous silicon (a-Si:H) yields outstanding surface passivation as atomic hydrogen saturates silicon dangling bonds at the a-Si/c-Si interface. Yet, a-Si surface passivation typically starts to degrade already at annealing temperatures in the range of 200 to 250 °C depending on annealing time, and optical absorption in front layers of a-Si reduces the short circuit current density. We show that oxygen incorporation into a-Si:H films enhances the thermal stability of the passivation and reduces parasitic absorption. We further show that for good passivation of the a-Si/c-Si interface, a compact material structure of the a-Si:O:H films is required where atomic hydrogen is the dominating type of diffusing hydrogen species. For plasma deposited a-Si:O:H films, oxygen incorporation of up to 10 at. % leads to an increa...

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.