Photovoltaic Devices from Silicon Nanoparticles
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Photovoltaic Devices from Silicon Nanoparticles Christoph Rier1, Gabi Schierning2,4, Hartmut Wiggers3,4, Roland Schmechel 2,4, and Dieter Jäger1,4 1
Center for Semiconductor Technology and Optoelectronics, University of Duisburg-Essen, 47057 Duisburg, Germany 2 Faculty of Engineering, University of Duisburg-Essen, 47057 Duisburg, Germany 3 Institute of Combustion and Gas Dynamics, University of Duisburg-Essen, 47057 Duisburg, Germany 4 Center for NanoIntegration Duisburg-Essen (CeNIDE), 47057 Duisburg, Germany ABSTRACT In this paper, results of designing, fabricating and characterizing photovoltaic devices made from tailored silicon nanoparticles are shown as proof-of-principle to adopt this material into the photovoltaic sector. The silicon nanoparticles are used as active material for direct separation of the light induced charge carriers. Homo pn-junctions were constructed by silicon wafers and crystalline silicon nanoparticels, the latter doped with the opposite carrier type with respect to that of the wafers. Nanoparticels were sintered on top of the wafer by a spark plasma sintering process, maintaining the nanocrystalline character. This way, the nanoparticle layers are a combined absorbing and charge separating medium. Electrical characterization of the devices show a reproducible short-circuit current of up to 20 μA under illumination. A maximum shortcircuit current density of 6.25 μA/cm² was realized. INTRODUCTION Today photovoltaic (PV) concepts suffer either from high costs or relatively low efficiencies. Therefore two trends for improving state-of-the-art PV cells are possible: increasing efficiency at nearly same costs, or reducing costs at nearly same efficiencies. Silicon nanoparticles (Si NP) are today very interesting because they are produced at low costs on large scales and feature an implementation of roll-to-roll processes for a low cost fabrication of silicon PV cells. The used Si NP are produced by a decomposition of the precursor silane (SiH4) in a gas phase process with purity adequate for photovoltaic applications. Because this synthesis of silicon nanoparticles is cost-efficient compared to the growth of mono- or polycrystalline silicon commonly used for solar cells, the use of silicon nanoparticles as raw material might save energy and material costs. Another aspect for the use of Si NP is that they can be printed via stable dispersions, which are easily achieved from a powder of the particles, on many substrates like flexible (metal) foils. Therefore, not only the material, but also the foreseen printing process of nanoparticle dispersions is energy and material efficient in contrast to the conventional overcompensation of the wafer doping in standard silicon photovoltaics, which is complex, costly and limiting the fabrication.
Furthermore, the size dependent band gap of silicon nanoparticles [1 leads to a broadband absorption of light. This results in an easy realization of a multi-junction device just made from one material of variable sizes. Thus the aim of this work
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