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Semiconductor Nanomaterial for Advanced Photovoltaic Solar cells Using New concept of nanocrystal and conductive host

Final Report Summary - SNAPSUN (Semiconductor Nanomaterial for Advanced Photovoltaic Solar cells Using New concept of nanocrystal and conductive host)

Executive Summary:

The EU funded SNAPSUN project gathered four leading European research organizations (CEA, Delft University of Technology, Tyndall National Institute, Uppsala University) as well as two companies (SAFC Hitech and STMicroelectronics) with the goal of developing novel nanocomposite materials based on silicon nanocrystals for photovoltaic applications. Silicon nanostructures may lead to highly efficient multijunction solar cells, since the material bandgap can be tuned via quantum confinement effects. In the SNAPSUN project, the consortium studied low-temperature and scalable routes for fabricating nanocomposite materials, in order to enable the large scale deposition of nanocomposites on low-cost (and even flexible) substrates. More precisely, the SNAPSUN team developed new processes, based either on vacuum deposition (chemical vapour deposition, physical vapour deposition) or wet deposition (chemical synthesis followed by aerosol-assisted deposition), and succeeded in fabricating nanocomposite materials with a very high silicon nanoparticle density (up to 1E12 particles per square centimeter) and a very narrow size distribution (down to 15%), which highlights the excellent control reached over process parameters. Furthermore, the SNAPSUN consortium developed and tested novel techniques for experimentally characterizing the nanocomposite materials with nanoscale resolution, in particular an electron microscopy tomography method which provides researchers with three-dimensional images of the nanocomposites. Finally, the SNAPSUN partners fabricated first proof-of-concept devices, integrating the novel nanocomposite materials into functional solar cells, thus demonstrating the potential of this new class of materials for photovoltaic applications.

Project Context and Objectives:

SNAPSUN, a 3-year project funded by the EU's Seventh Framework Programme (FP7), aims at developping an innovative nanocomposite material for solar cell applications. Nanotechnologies show broad opportunities to reduce the cost and increase the efficiency of solar cells. Indeed, at the nanoscale level, the energy band gap depends on the nanomaterial architecture (nanoparticle size, bulk dispersion, interfaces with embedding matrix). Thus silicon nanoparticles allow the design of highly efficient photovoltaic devices, like multijunction solar cells or low-cost, optimised, thin-film solar cells. However, the usual elaboration technique of silicon nanoparticles is based on the deposition of multilayers in which excess silicon is aggregated into nanoparticles through high temperature annealing. With this conventional technique, control of nanoparticle size and bulk dispersion is difficult. Moreover, only limited host matrix materials can be considered. This prevents any knowledge-based tuning of the nanomaterial properties. The main objective of the SNAPSUN project is to develop a novel nanomaterial with reliable and tailored characteristics, by achieving accurate control over silicon nanoparticle and host matrix properties. To overcome the limitations of the conventional technique for elaboration of silicon nanoparticles, control of material structure will arise from the development of very promising processes allowing the separation of silicon nanoparticle generation and host matrix codeposition. Fully tailored silicon nanoparticles will be optimised, in terms of size and size dispersion. These silicon nanoparticles will be incorporated into a wide band gap host matrix, such as silicon carbide or transparent conductive oxides. This new architecture will allow band gap engineering through accurate structure control, together with exceptional electrical characteristics (resistivity, carrier lifetime, etc.) in order to produce high photovoltaic efficiencies above 25%.

Project Results:

In the first period of the project (June 2010 - November 2011), silicon nanoparticles were prepared using three distinct original techniques, namely chemical vapour deposition, physical vapour deposition, and chemical synthesis. In all cases, a nanoparticle mean diameter of 3 nm could be reached, and the typical size distribution presented a very small deviation, lower than 15%. Then, nanocomposite materials based on these silicon nanoparticles were fabricated, using a wide bandgap semiconductor matrix. Nanocomposites with a density of silicon nanoparticles as high as 1E12 cm-2 could be achieved, with both vacuum and atmospheric pressure processes. An advanced characterization method (transmission electron microscopy tomography) was developed for obtaining the three-dimensional distribution of silicon nanoparticles inside the nanocomposite materials prepared in the SNAPSUN consortium. This new technique was validated on chemical vapour deposited silicon nanoparticles. Both ab-initio and semi-empirical models were used for predicting various optical and electrical properties of the nanocomposite materials. Finally, a reference nanocomposite material obtained by high temperature annealing was fabricated. In the second period of the project (December 2011 - May 2013), crucial milestones have been reached, opening the way towards novel photovoltaic devices based on silicon nanoparticles and nanocomposite materials. First, the industrial upscale of laboratory-scale silicon nanoparticle chemical synthesis have been validated. Batches of nanoparticles in an upscaled volume of 1 litre have been synthesized without compromising the silicon nanoparticle quality. Second,

nanocomposite material thin films with excellent optical properties (optical absorption higher than 85% with a thickness lower than 1 μm) have been demonstrated. Third, the precise specifications of proof-of-concept photovoltaic devices integrating the novel nanocomposite materials have been

provided. Encouraging experimental results have also been obtained, since solar cells have been demonstrated with nanocomposite materials fabricated by physical vapour deposition and aerosol-assisted chemical vapour deposition.

Potential Impact:

Currently no processes are available which allow the homogeneous deposition of monodisperse nanoparticles within a semiconducting host matrix. This SNAPSUN result will allow the controlled study of diverse phenomena at the nanoscale. Regarding bandgap engineering, the SNAPSUN approach will have great impacts by improving the conventional approach of band gap engineering (used for SiGe, μc-Si) and will allow the tuning of materials to a very fine range of spectral wavelengths. Regarding novel characterisation techniques, impacts will include facilitated and more reliable experimental investigations of phenomena at the nanoscale. These tools will elucidate the 3D distribution of nanoparticles in the host matrix as well as the electronic structure on sub-nanometre level. Tailored PV building blocks will enable the tailoring of PV materials and the increase of their PV efficiency. The possibility to engineer nanocomposite layers opens the door to creating finely-tuned hetero- and multi-junction PV cell architectures. Finally, SNAPSUN processes will reduce costs, since vacuum and atmospheric pressure nanotechnologies developed in SNAPSUN are both low-temperature techniques. This makes these nanotechnologies particularly suitable for the requirements of new PV applications using multifunctional and/or inexpensive substrates (textiles, paper, light alloys, etc). Regarding modelling, at the scale of the wavelengths of the solar spectrum, new knowledge will be created through SNAPSUN’s investigation of quantum effect phenomena in nanocomposite materials. Such knowledge will enable the creation and use of numerical PV cell design tools that will shorten the time-to-innovation for nano-PV architectures. Notably, results are geared towards ushering in efficient heterojunctions, and in the longer term, multijunctions.

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