Nanoscale Study of the Hole-Selective Passivating Contacts with High Thermal Budget Using C-AFM Tomography

We investigate hole-selective passivating contacts that consist of an interfacial layer of silicon oxide (SiOx) and a layer of boron-doped SiCx(p). The fabrication process of these contacts involves an annealing step at temperatures above 750 degrees C which crystallizes the initially amorphous layer and diffuses dopants across the interfacial oxide into the wafer to facilitate charge transport, but it can also disrupt the SiOx layer necessary for wafer-surface passivation. To investigate the transport mechanism of the charge carriers through the selective contact and its changes during the annealing process, we utilize various characterization methods, such as transmission electron microscopy, micro Raman spectroscopy, and conductive atomic force microscopy. Combining the latter with a sequential removal of material, we assemble a tomographic reconstruction of the crystallized layer that reveals the presence of preferential vertical transport channels.

Passivating electron contact based on highly crystalline nanostructured silicon oxide layers for silicon solar cells

Christophe Ballif, Matthieu Despeisse, Franz-Josef Haug, Quentin Thomas Jeangros, Martin Ledinsky, Philipp Friedrich Hermann Löper, Xavier Niquille, Gizem Nogay, Josua Andreas Stückelberger, Philippe Wyss

We present a novel passivating contact structure based on a nanostructured silicon-based layer. Traditional poly-Si junctions feature excellent junction characteristics but their optical absorption induces current losses when applied to the solar cell front side. Targeting enhanced transparency, the poly-Si layer is replaced with a mixed-phase silicon oxide/silicon layer. This mixed-phase layer consists of an amorphous SiOx matrix with incorporated Si filaments connecting one side of the layer to the other, and is referred to as nanocrystalline silicon oxide (nc-SiOx) layer. We investigate passivation quality, measured as saturation current density, and nanostructural changes, characterized by Raman spectroscopy and transmission electron microscopy, carefully studying the influence of annealing dwell temperature. Excellent surface passivation on n-type and also p-type wafers is shown. An optimum annealing temperature of 950 °C is found, resulting in a saturation current density of 8.8 fA cm2 and 11.0 fA cm2 for n-type and p-type wafers, respectively. Even before forming gas annealing, emitter saturation current densities of 27.9 fA cm2 (n+/n junction) and 32.0 fA cm2 (n+/p junction) are reached. Efficient current extraction is presented with specific contact resistivities of 86 mΩ cm2 on n-type wafer and 19 mΩ cm2 on p-type wafers, respectively. High-resolution transmission electron microscopy reveals that the layer stack consists of intermixed SiOx and Si phases with the Si phases being partly crystalline already in the as-deposited state. Thermal annealing at temperatures > 850 °C further promotes crystallization of the Si-rich regions. The addition of the SiOx phase enhances the thermal stability of the contact and should allow to tune the refractive index and improve transparency, while still providing efficient electrical transport through the crystalline Si phase, which extends throughout almost the entire contact.

Elsevier Science Bv2016

Silicon-Rich Silicon Carbide Hole-Selective Rear Contacts for Crystalline-Silicon-Based Solar Cells

Christophe Ballif, Matthieu Despeisse, Franz-Josef Haug, Quentin Thomas Jeangros, Philipp Friedrich Hermann Löper, Xavier Niquille, Gizem Nogay, Josua Andreas Stückelberger, Philippe Wyss

The use of passivating contacts compatible with typical homojunction thermal processes is one of the most promising approaches to realizing high-efficiency silicon solar cells. In this work, we investigate an alternative rear-passivating contact targeting facile implementation to industrial p-type solar cells. The contact structure consists of a chemically grown thin silicon oxide layer, which is capped with a boron-doped silicon-rich silicon carbide [SiCx(p)] layer and then annealed at 800–900 °C. Transmission electron microscopy reveals that the thin chemical oxide layer disappears upon thermal annealing up to 900 °C, leading to degraded surface passivation. We interpret this in terms of a chemical reaction between carbon atoms in the SiCx(p) layer and the adjacent chemical oxide layer. To prevent this reaction, an intrinsic silicon interlayer was introduced between the chemical oxide and the SiCx(p) layer. We show that this intrinsic silicon interlayer is beneficial for surface passivation. Optimized passivation is obtained with a 10-nm-thick intrinsic silicon interlayer, yielding an emitter saturation current density of 17 fA cm–2 on p-type wafers, which translates into an implied open-circuit voltage of 708 mV. The potential of the developed contact at the rear side is further investigated by realizing a proof-of-concept hybrid solar cell, featuring a heterojunction front-side contact made of intrinsic amorphous silicon and phosphorus-doped amorphous silicon. Even though the presented cells are limited by front-side reflection and front-side parasitic absorption, the obtained cell with a Voc of 694.7 mV, a FF of 79.1%, and an efficiency of 20.44% demonstrates the potential of the p+/p-wafer full-side-passivated rear-side scheme shown here.

Amer Chemical Soc2016

Passivating electron contact based on highly crystalline nanostructured silicon oxide layers for silicon solar cells

Elsevier Science Bv2016

Silicon-Rich Silicon Carbide Hole-Selective Rear Contacts for Crystalline-Silicon-Based Solar Cells

Christophe Ballif, Matthieu Despeisse, Franz-Josef Haug, Quentin Thomas Jeangros, Philipp Friedrich Hermann Löper, Xavier Niquille, Gizem Nogay, Josua Andreas Stückelberger, Philippe Wyss

Amer Chemical Soc2016