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SILICON NANODOTS FOR SOLAR CELL TANDEM

Final Report Summary - NASCENT (SILICON NANODOTS FOR SOLAR CELL TANDEM)

Executive Summary:
The NASCEnT project has been aiming on the development of new Nanomaterials with New Production Technologies and the fabrication of silicon quantum dot materials for all-silicon tandem solar cells to achieve increased efficiencies. The understanding of electrical transport and recombination mechanisms in these newly developed nanomaterials already enabled us to design novel solar cell structures that aim to overcome the efficiency limits of conventional solar cell concepts. Another goal was to ensure the compatibility of the newly developed technologies with high-throughput processing leading to further cost-reduction. Within the scope of this project the novel concept of band gap-tuneable quantum dot superlattices was exploited. A quantum dot cell was developed for the combination with a wafer-based high efficiency silicon solar cell. This revolutionary progress in the solar cell evolution which has been gained in the scope of this project will lead to higher efficiencies and to on-going cost-reductions from both a short-term and a long-term perspective.
One reason for this successful cooperation was the efficient work done in the consortium including leading European institutes in the field of Si nanocrystals embedded in dielectric matrices, together with the largest independent PV R&D institute and two companies who have been working in the microelectronic and in the photovoltaic market.
The main scientific and technological achievements of the project are:
• Improvement of the absorber material quality by developing new processes to reduce electronically active defects and to achieve better nanocrystal quality and doping of the structures
• Advanced characterisation of the materials and of simple devices to achieve optimised absorber material performance
• A detailed understanding of electronic transport in Si quantum dot superlattices by the use of novel simulations and by modelling of the device structures
• Specification of adequate processes for the fabrication of devices
• Realisation of new solar cell device structures
• Complete understanding of silicon nanocrystals, from preparation to the complete device (including simulation etc.)
• Detailed cost calculations for the solar cell processes
We are convinced that the knowledge gained in this project is of high scientific interest not only for photovoltaics but also for novel photonic and charge storage devices incorporating Si nanocrystals. Moreover the computational routines developed and the obtained theoretical results are reinforcing the leading position of Europe in this field and are providing a genuine understanding of the dependence of the structural, opto-electronic and transport properties of semiconductor systems on their low- and nano-dimensionality. The development of device parts such as tunnel diodes for the material systems under investigation and the optimisation of process technologies implies a considerable profit for related technologies such as SiC power devices, charge storage and optoelectronics.
Since several members of the consortium are engaged in student education the findings of the project were immediately transferred to the “brains” of the next generation of young researchers in material science and photovoltaics.
Project Context and Objectives:
The overall objective of the project was to develop new Nanomaterials with New Production Technologies and to fabricate silicon quantum dot materials for all-silicon tandem solar cells to achieve increased efficiencies. The understanding of electrical transport and recombination mechanisms in these newly developed nanomaterials enabled us to design novel solar cell structures that help to overcome the efficiency limits of conventional solar cell concepts. Another objective was to ensure the compatibility of the newly developed technologies with high-throughput processing leading to further cost-reduction. Within the scope of this project the novel concept of band gap-tuneable quantum dot superlattices was exploited. In one approach, a quantum dot cell has been developed for the combination with a wafer-based high efficiency silicon solar cell. This approach aims at an ultra-high efficiency silicon tandem. In a possible second approach, the quantum dot solar cell can be realised on thin-film solar cell structures in order to achieve a low-cost tandem cell. This revolutionary progress in the solar cell evolution will lead to higher efficiencies for solar cells and to ongoing cost-reductions from both a short-term and a long-term perspective.
For photovoltaics (PV) to become a major player in the energy mix of the future, it will be necessary to reduce the costs of PV systems by about 70 %. This may be accomplished with only the combined efforts of higher production scales, innovative production technologies and novel materials for higher cell and module efficiencies. Higher efficiencies are of particular importance, because they do not only save costs but, just as importantly, reduce resource consumption and land use of future PV power plants. At present, cell efficiencies of silicon solar cells in large scale production lie at around 17 % and will be increased to about 20 % in the future . Considerably higher efficiencies can only be achieved by tandem solar cells. Current tandem cells may be divided into III-V multijunction solar cells and “metamorphic” a-Si/μ-Si dual junction cells. A disadvantage to large scale production of III-V solar cells is that they are toxic and cost-intensive, while micromorph tandem cells are restricted to two junctions and show degradation effects. Multilayers of silicon nanocrystal in a dielectric matrix allow for band gap-engineering by controlling the nanocrystal size and density. By this means it is possible to fabricate solar cells with an optimised band gap, and thus to realise tandem solar cells (i) with two or more junctions, (ii) consisting of non-toxic and abundant materials that are (iii) compatible with silicon processing technology. Silicon multi-junction solar cells have the potential to reach efficiencies above 30 % in the near and 40 % in the far future .
In order to reach these goals, considerable R+D work has still to be performed on semiconductor bulk materials, thin layers, nanomaterials and hetero-structures for such solar cells. These have been the topics of the project which fitted perfectoy well to the subject FP7-NMP-2009-1.2-1 – Nanotechnology for harvesting energy via photovoltaic technologies. The expected impact of the project is to enhance the efficiency of crystalline silicon solar cells and thereby to increase their cost effectiveness.
Crystalline silicon (Si) based solar cells dominate the photovoltaic market and the situation is likely to continue for several decades. Until a few years ago, technological progress in the photovoltaic industry was mainly focussed on the production technology. The PV market has been characterised in the past few years by increasing competition and a very dynamic market evolution. As a consequence, an increased interest in high efficiency solar cell concepts is observable. This can be easily understood, as with maturing production technology material costs become more dominant and higher efficiencies imply more efficient use of material. In consequence, microelectronic processes and new concepts are currently being transferred from research into industry. Another possible answer to the strategic question of high material costs is the radical reduction of material usage by thin-film technologies. Therefore thin-film polysilicon solar cells have recently emerged as a promising thin-film alternative to bulk crystalline Si. With Solid Phase Crystallisation (SPC) of amorphous Si, efficiencies of more than 10 %, matching the efficiencies of the best European micromorph mini-modules, have been achieved .
Silicon nanocrystals (NCs) offer the potential to combine the advantages of both paths of development: high efficiencies and low production costs. Therefore, they have been chosen to be the topic of this research project. Si NCs have been a subject of research for more than 15 years in microelectronics. To fabricate a Si quantum dot material, several deposition methods can be used, such as, Plasma Enhanced Chemical Vapour Deposition (PECVD). The multilayer approach, as proposed by Zacharias, showed promising results and the control of the Si NCs size by a stoichiometric diffusion barrier was achieved . The resulting materials prove the adjustability of the band gap through changing the nanocrystals’ size . In this project we could show how photo excited carriers can be extracted from the material. The two crucial effects determining this behavior, recombination and transport, have been the scientific focus of this project.
Therefore we formed a consortium with the leading European institutes in the field of Si nanocrystals embedded in dielectric matrices, together with the largest independent PV R&D institute on crystalline-silicon solar cells and two companies working for years in the microelectronic and in the photovoltaic market join forces within this consortium to realise novel all-Si wafer- and c-Si thin-film-based tandem solar cells.
The objective of this project was to realise first solar cell devices with Si NCs in a dielectric material as an absorber layer and to implement them into an all-silicon tandem solar cell. Simulations point out that the electrical transport through the super lattice is achievable with a proper crystallographic quality of the silicon nanocrystal absorber material and that silicon tandem solar cells with efficiencies exceeding 30 % are feasible with advanced device design and processing. This can be achieved by improving the crystallographic and electronic quality of the active Si nanocrystals and the embedding matrix material by implementing new and advanced modelling and characterisation methods with feedback to the material production processes. Novel device structures have been developed to additionally help to measure recombination and transport effects, which could never have been done systematically in the past. A better understanding of the relationship between the growth parameters, the electrical and optical properties of the material, and the resulting device properties have been obtained by advanced characterisation of the quantum dot material. This information was used to enhance the material quality by developing more effective post-deposition treatments (defect annealing and hydrogen passivation) for the absorber layers, and by improving the material growth processes, leading to a much lower defect density within the material.
The active participation of AZUR Space and STMicroelectronics in this project allowed the consortium to produce solar cell demonstrators and to determine the influence of the developed technologies on the cost-effectiveness of the solar cells. This ensured that there is a good chance to bring the technologies developed within the project directly into a product.
The main scientific and technological objectives of the project were:
• Improvement of the absorber material quality by developing new processes to reduce electronically active defects and to achieve better nanocrystal quality and doping of the structures
• Advanced characterisation of the materials and of simple devices, to achieve optimised absorber material performance
• A detailed understanding of electronic transport in Si quantum dot superlattices by the use of novel simulations and by modelling of the device structures
• Specification of adequate processes for the fabrication of devices
• Realisation of new solar cell device structures
• Complete understanding, from preparation to the complete device (including simulation etc.)
• Detailed cost calculations for the two different solar cell routes
The main objectives stated above have been handled in 4 different work packages (WP). Each WP was defined through its own deliverables and milestones. Thus all the objectives achievable within the project were clearly specified, as the time of their achievement. The integration of these objectives into the main objective, a Si NC photovoltaic device, has been guaranteed through intensive networking and communication.

Project Results:
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Potential Impact:
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List of Websites:
www.project-nascent.eu