Search

Hydrogenated amorphous silicon carbide (a-SiC:H) provides excellent electronic surface passivation for crystalline silicon solar cells. The hydrogen and carbon content of the passivation layers control the surface passivation depending on hydrogen bonding and annealing temperature. The carbon content c C of the amorphous layers varies depending on the methan-to-silane gas flow ratio during deposition. The electronic passivation quality exhibits best thermal stability for an optimum c C = 2.3 at.%. Annealing this sample under forming gas atmosphere up to T FG = 550°C enables excellent effective minority carrier lifetimes τ eff = 1.2 ms. Hydrogen effusion measurements relate this result to an increase in H-content with rising c C and to a simultaneous shift of the effusion peaks to higher temperatures. A higher carbon content reduces the diffusion of atomic hydrogen out of the amorphous layers. The Si-H bonding configurations in the amorphous layers, analyzed from infrared absorption spectroscopy, reveal that a-SiC:H layers with lower carbon content have a higher density. Increasing c C induces voids and microvoids in the amorphous structure, favoring the diffusion of molecular hydrogen out of the a-SiC:H layers. We show the implementation of the thermally most stable a-SiC:H as back side of an industrial silicon solar cell. Evaporated and tempered Al point contacts through the amorphous layers enable the current transport through a-SiC:H. Compared to a full-area back side metallization, the lower recombination velocity of the a-SiC:H back side enhances the open circuit voltage, demonstrating the benefit of a-SiC:H passivation for industrial crystalline silicon solar cells.

The characterization of void-related microstructure in amorphous and microcrystalline Ge:H films is reported. Various methods are applied including effusion measurements of hydrogen and of implanted helium and neon, measurements of the infrared absorption of C-H bonds due to in-diffusion of contaminants and of the stretching modes of bonded hydrogen. Several microstructure effects like interconnected voids and isolated voids and a quite different material homogeneity are detected and are found to depend on the preparation conditions. Amorphous Ge:H can be prepared with a (largely) homogeneous structure while microcrystalline Ge:H tends to consist of compact grains surrounded by more or less open voids. Enhanced substrate temperatures (Ts ≈ 250°C) favour the growth of more compact material.

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.