Florian Einsele

Efficiency limits of photovoltaic fluorescent collectors

This paper examines the thermodynamic limits of photovoltaic solar energy conversion by fluorescent collectors. The maximum efficiency of a fluorescent collector corresponds to the Shockley–Queisser limit for a nonconcentrating solar cell with a single bandgap energy. To achieve this efficiency, the collector requires a photonic structure at its surface that acts as an omnidirectional spectral band stop filter. The large potential of photonic structures for the efficiency enhancement of idealized as well as real fluorescent collectors is highlighted.

Analysis of sub-stoichiometric hydrogenated silicon oxide films for surface passivation of crystalline silicon solar cells

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...

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

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 resistanc...

Electronic surface passivation of crystalline silicon solar cells by a-SiC:H

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 cC 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 cC = 2.3 at.%. Annealing this sample under forming gas atmosphere up to TFG = 550°C enables excellent effective minority carrier lifetimes τeff = 1.2 ms. Hydrogen effusion measurements relate this result to an increase in H-content with rising cC 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 cC 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.