Large-scale BN tunnel barriers for graphene spintronics

We have fabricated graphene spin-valve devices utilizing scalable materials made from chemical vapor deposition (CVD). Both the spin-transporting graphene and the tunnel barrier material are CVD-grown. The tunnel barrier is realized by h-BN, used either as a monolayer or bilayer and placed over the graphene. Spin transport experiments were performed using ferromagnetic contacts deposited onto the barrier. We find that spin injection is still greatly suppressed in devices with a monolayer tunneling barrier due to resistance mismatch. This is, however, not the case for devices with bilayer barriers. For those devices, a spin relaxation time of 260 ps intrinsic to the CVD graphene material is deduced. This time scale is comparable to those reported for exfoliated graphene, suggesting that this CVD approach is promising for spintronic applications which require scalable materials.Comment: 13 pages, 3 figure

Efficient Near-Infrared-Transparent Perovskite Solar Cells Enabling Direct Comparison of 4-Terminal and Monolithic Perovskite/Silicon Tandem Cells

Combining market-proven silicon solar cell technology with an efficient wide band gap top cell into a tandem device is an attractive approach to reduce the cost of photovoltaic systems. For this, perovskite solar cells are promising high-efficiency top cell candidates, but their typical device size (<0.2 cm2), is still far from standard industrial sizes. We present a1cm2 near-infrared transparent perovskite solar cell with 14.5% steady- state efficiency, as compared to 16.4% on 0.25 cm2. By mechanically stacking these cells with silicon heterojunction cells, we experimentally demonstrate a 4-terminal tandem measurement with a steady-state efficiency of 25.2%, with a 0.25 cm2 top cell. The developed top cell processing methods enable the fabrication of a 20.5% efficient and 1.43 cm2 large monolithic perovskite/silicon heterojunction tandem solar cell, featuring a rear-side textured bottom cell to increase its near-infrared spectral response. Finally, we compare both tandem configurations to identify efficiency-limiting factors and discuss the potential for further performance improvement

Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency

Tandem devices combining perovskite and silicon solar cells are promising candidates to achieve power conversion efficiencies above 30% at reasonable costs. State-of-the-art monolithic two-terminal perovskite/silicon tandem devices have so far featured silicon bottom cells that are polished on their front side to be compatible with the perovskite fabrication process. This concession leads to higher potential production costs, higher reflection losses and non-ideal light trapping. To tackle this issue, we developed a top cell deposition process that achieves the conformal growth of multiple compounds with controlled optoelectronic properties directly on the micrometre-sized pyramids of textured monocrystalline silicon. Tandem devices featuring a silicon heterojunction cell and a nanocrystalline silicon recombination junction demonstrate a certified steady-state efficiency of 25.2%. Our optical design yields a current density of 19.5 mA cm−2 thanks to the silicon pyramidal texture and suggests a path for the realization of 30% monolithic

Tunnel barriers for spin injection into graphene

This thesis describes the fabrication and characterisation of tunnel barriers on graphene for spintronic devices. Tunnel barriers are ultrathin, smooth, homogeneous and electrically insulating layers and a prerequisite for successful spintronic experiments, since they prevent the conductivity mismatch between a spin transport material and its ferromagnetic contacts. During the course of this thesis, which constitutes the first systematic graphene spintronics study in our group, we experimented with the oxides Al2O3 (grown by atomic layer deposition/ALD) and MgO (grown by physical vapour deposition/PVD) as barrier materials. A detailed description of the fabrication process with all the recipes we used is given in the thesis. Before and after the barrier fabrication, the samples were characterised with Raman spectroscopy, allowing the determination of the quality of graphene below its top layer, and with AFM measurements from which we extracted the barrier’s smoothness and homogeneity. Finally, we fabricated graphene spin valves in a non-local geometry by depositing permalloy (Ni80Fe20) contacts onto the covered graphene flakes. In order to further investigate the performance of these devices, we conducted charge and spin transport (i. e., non-local resistance and Hanle spin precession) measurements. We successfully achieved spin injection with both oxides. Compared with the literature, most of our results hint at pinhole-dominated or transparent barriers in the devices. In a novel approach, we deposited thin exfoliated layers of the semiconductor MoS2 onto graphene using an in-house-built transfer microscope, where MoS2 acted as tunnel barrier between the ferromagnetic contacts and graphene. Using three-terminal measurements on the MoS2-graphene samples, we observed large magnetic-field-dependent jumps in the MoS2 barrier resistance. Finally, we also present the first spin signals achieved with a multilayer MoS2 barrier

EPFL Library Publishing Support Fastguides

This is a set of 6 1-sheet guides, providing abridged notions, basic and advanced information about Open Access, scientific publishing, copyright, citation, etc.: #01 Open Access: the basics #02 Make your research Open #03 Creative Commons Licenses #04 Publishing agreement #05 L'exception pédagogique #06 Bien réutiliser une oeuvr