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

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.

Advanced Architectures and Processing for High-Efficiency Silicon Heterojunction Solar Cells

Jonas Geissbühler

Crystalline silicon solar cells currently represent the largest part of the photovoltaic market. In response to the demand for higher efficiency devices, silicon heterojunction technology, which merges a crystalline silicon wafer with thin amorphous silicon (a-Si:H) films enabling the achievement of passivating contacts, may be considered as one of the most promising approaches for the next generation of industrial solar cells. These cells demonstrate record energy conversion efficiency enabled by excellent surface passivation leading to open-circuit voltages close to the theoretical limit. Despite the remarkable electronic properties of a-Si:H, its narrow bandgap induces significant parasitic light absorption. The aim of this thesis is to mitigate this loss through novel cell architectures and new fabrication processes. The outcome of this work is four-fold: First, a new silicon heterojunction solar cell structure is investigated that decouples the optical and electrical properties of the window layers through localized front contacts. By using shadow masking for the film patterning, we demonstrate the feasibility of the new cell structure under the condition that a sufficiently high contact coverage be maintained. Second, for the patterning of a-Si:H layers, we develop a hydrogen plasma etching technique carried out in a PECVD reactor. In particular, we show that a critical thickness of the intrinsic a-Si:H layer is needed to provide sufficient shielding of the amorphous/ crystalline interface to ensure good passivation. This technique enables the patterning of a-Si:H layers with nanometric accuracy while preserving the surface passivation. A third important outcome of this thesis is the replacement of the front metallization¿usually made by screen-printing of a silver paste¿with a copper front grid formed by electrodeposition. To ensure sufficient adhesion of the metallic fingers, a first process based on a double electrodeposition of a nickel/copper stack is presented. Important improvements in light management are realized with this metallization scheme, reducing the finger width from 70 um to 15 um which leads to a short-circuit gain of 1.1 mA cm2. In a second step, a simplified process based on a copper seed layer is proposed, further improving the metallization adhesion and making this technique compatible with chemically sensitive transparent conductive oxides such as zinc-oxide-based materials. The fourth outcome of this work is the replacement of the p-doped a-Si:H hole collector with a highly transparent molybdenum oxide (MoOx) film. We show that this material is an efficient hole collector if the post-deposition processes are carried out below 130 °C. From an optical point of view, this layer enables an improvement equivalent to 0.8 mA cm2 of the cell¿s response in the blue part of the spectrum. Nevertheless, this gain is reduced by the modification of the MoOx film induced during the TCO sputtering process, which leads to broadband parasitic light absorption. Finally, by merging this MoOx hole collector with the electrodeposited copper front metallization, remarkable fill factors of 80.3% are obtained leading to a certified efficiency of 22.5% for a 4 cm2 solar cell.

EPFL2015

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

Full-Area Passivating Contacts with High and Low Thermal Budgets: Solutions for High Efficiency c-Si Solar Cells

Gizem Nogay

Today more than 90% of the global PV market is covered by c-Si solar cells which are limited by recombination losses at the metal-semiconductor interface. This recombination path can be avoided by separating the metal from the c-Si wafer by introducing a buffer layer that passivates electronically the wafer surface while still allowing the flow of one type of carrier. Such structures are called carrier selective passivating contacts. In the scope of this thesis, carrier selective passivating contacts with low and high thermal budgets are investigated with the purpose of achieving a good understanding of the junction formation, the working principle, limiting factors, and integration requirements. Silicon heterojunction (SHJ) solar cells are studied as representatives for solar cells processed with low thermal budget. Solar cells with interfacial oxide contacts are investigated as the cell concept for high thermal budgets. To be industrially relevant, contact formation of the latter is studied with p-type wafers. The carrier selective passivating contacts based on microcrystalline silicon (mc-Si:H) are studied for SHJ solar cells with low thermal budget. It is demonstrated that mc-Si:H layers improve the optical and electrical performance of the SHJ solar cells compared to cells with purely amorphous contacts. However, as the layer crystallinity increases with thickness, a trade¿off between short circuit current density and fill factor is encountered. By combining a MoOx front hole-contact and a mc-Si:H(n) rear electron-contact, this trade¿off is avoided. Applying an electrodeposited copper front metallization, a short circuit current density of 38.9 mA/cm2 and a fill factor of to 80% are obtained leading to an efficiency of 22.5%. The passivating hole contacts with high thermal budget are developed using a chemically grown oxide (SiOx) and boron doped silicon-carbide which is deposited under Si-rich conditions by plasma enhanced chemical vapour deposition. It is observed that introducing an undoped Si inter¿layer between SiOx and SiCx(p) is necessary to prevent chemical reaction between carbon atoms in the SiCx(p) and the adjacent SiOx, and consequently to obtain wellpassivated SiOx/c-Si interface. With this contact structure, an implied open circuit voltage value of 718 mV and a specific contact resistivity value of 17m cm2 are attained. The contact structure is tested at device level by realizing proof-of-concept hybrid solar cells that feature a high thermal budget rear hole-contact and a low thermal budget SHJ front electron-contact. With this concept, the fill factor of 81.8% is attained. In an effort to improve the hole-contact property further, in-situ incorporation of fluorine into the boron-doped hole-contact is explored. It is observed that in the as-deposited state, fluorine is evenly distributed within the contact layer of SiCx:F(p). Upon annealing, fluorine diffuses toward the c-Si wafer and accumulates at the interface where it reduces the defect states. With this contact structure, implied open circuit voltage values up to 735mV are achieved. The potential of the fluorinated hole-contact is investigated at device level by realizing both sides-contacted co-processed solar. For this purpose in-situ phosphorous doped SiCx(n) layers were developed as a front contact. Planar, both sides-contacted p-type solar cells attain an impressive fill factor value of 84% and an open circuit voltage of 727 mV.

EPFL2018