High throughput metrology for nanowire energy harvesting devices
Energy harvesting from renewable sources (solar, heat and movement) is a prominent solution to create small amounts of electrical energy in areas of difficult access, and energy harvesting devices have much potential to address our world energy problems. Nanowire (NW) based energy harvesting systems have achieved encouraging progress, but due to nanometre (nm) dimensions of the wires and large size (m2) of the devices, they also bring challenges for testing and characterisation. Average properties of energy harvesting devices can be measured, but a quantitative link and correlation between the performance of single NWs and that of the overall device is lacking. This project aims to develop reliable and high throughput metrology for the quality control of NW energy harvesting systems.
Limited fossil fuel-based energy resources and their negative effect on the environment have resulted in enormous efforts being made over several decades to make energy supply and consumption more sustainable. Scavenging energy from renewable sources like solar, waste heat and mechanical movement is seen as a prominent solution to our world energy problems. The overall market for energy harvesting is expected to grow from USD 311 Million in 2016 to USD 646 Million by 2023. Nowadays, about 1.7 % of world electricity is provided by solar cells.
Over the past two decades, major efforts have been made to develop energy harvesting devices from macro- and microscales down to nanoscale. Due to their extremely small physical size and high surface to volume ratio, NW based energy harvesting systems, including photovoltaic solar cells, thermoelectrical and electromechanical energy nanogenerators, have gained tremendous interest and encouraging progress has been achieved. In particular, it has been confirmed that the efficiency of NW solar cells can be enhanced from 17.8 % currently to its ultimate limit of 46.7 % by means of nanophotonic engineering.
While novel designs and materials for various energy harvesting devices indeed offer many potential benefits, they also bring challenges for testing and characterisation. For example, the quantitative link and correlation between the performance of a single NW and that of the overall device is still missing. Moreover, no reliable metrology for large area NW arrays (from cm2 to several m2) with diameters between 50 nm and 1 µm is currently available. Quality control of these energy harvesting systems is therefore highly challenging, and high throughput metrology is necessary, which requires the development of traceable measurement methods and models for the characterisation of NW energy harvesters, solar cells and devices.
The overall goal of this project is the traceable measurement and characterisation of energy harvesting devices based on vertical NW. The specific objectives are:
- To develop traceable measurement methods for high throughput nanodimensional characterisation of NW energy harvesters (> 108 NWs/cm2) including 3D form (cylindrical, prismatic, pyramidal) and sidewall roughness.
- To develop traceable measurement methods for high throughput nanoelectrical characterisation of semiconductor NW solar cells using conductive Atomic Force Microscopy (AFM) for current-voltage characteristic in the current range 100 fA to 1 mA, Scanning Microwave Microscopy (SMM) for doping concentration variation (between 1015 and 1020 atoms/cm3 with an accuracy better than 10 %), and Micro-Electro-Mechanical Systems - Scanning Probe Microscopy (MEMS-SPM) for lateral resolution (< 50 nm).
- To develop and validate traceable measurement methods and models for high throughput nanomechanical characterisation of NW devices, and electromechanical energy harvesters taking into account local bending and compression of NWs including the development of a traceable MEMS-SPM (< 10 pm depth resolution) for fast simultaneous nanomechanical and electrical measurement of semiconductor and polymer piezoelectric NWs.
- To develop and validate traceable measurement techniques for thermoelectrical characterisation, based on fast areal thermal imaging, of NWs (thermal conductivity lower than 10 W/(mK) with an uncertainty < 10 %) under different scanning speeds and tip-surface contact.
- To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain, standards developing organisations (IEC TC 113 and IEC TC 82) and end users (solar cell and energy generator manufacturers).
These objectives will require large-scale approaches that are beyond the capabilities of individual National Metrology Institutes and Designated Institutes. To enhance the impact of the research, the involvement of the appropriate user community such as industry, standardisation and regulatory bodies is intended, both prior to and during methodology development.
Progress beyond the state of the art and results
High throughput nanodimensional characterisation of NW energy harvesters
A set of demonstrator NW arrays made of different materials (e.g. Si, ZnO and GaN) with diameters below 100 nm, cylindrical shape, aspect ratios between 20 and 100, densities between 108 NWs/cm2 and 109 NWs/cm2 and sidewall roughness ranging from nm to tens of nm will be developed for the first time.
A new high throughput optical measuring method, imaging scatterometry, which works by capturing a series of images of an illuminated sample at several wavelengths, will be used and further developed in this project. Large scale and reliable methods for defect characterisation will be developed, either based on optical ensemble methods on their own or by sophisticated combination with high resolution microscopy methods in a hybrid metrology approach.
The complex 3D geometry of the NW arrays, their different materials and large size set metrological requirements that are beyond state-of the art techniques. The combination of advanced AFM and Scanning Electron Microscopy (SEM) metrology together with sophisticated optical metrology tools provide unique possibilities to tackle this challenge. Access to the geometrical form and size of the nanostructures are possible locally (e.g. AFM) as well as globally (i.e. scatterometry and Mueller matrix ellipsometry - MME).
Efficient modelling capabilities based, e.g. on FEM will be developed. Thus, at the end of this project, for the first time, a high-volume metrology for reliable determination of order parameters of large arrays of coordinated nanosystems such as Si nanowire arrays will be available.
High throughput nanoelectrical characterisation of semiconductor NW solar cells
The project will go beyond the state of art by developing traceable methods to perform I-V spectroscopy on individual NW junctions in the dark and under illumination using a new conductive AFM equipped with a MEMS SPM probe and coupled to a commercial version of a contact-mode AFM. The expected relative uncertainty on the data determination is in the order of 10 %. In addition, using a technique that allows 3D nano-printing conductive tip materials, new optimised tip geometries for nanowire characterisation will be realised, that will allow the nanoelectrical characterisation of NW.
Traceable Scanning Microwave Microscope (SMM) measurements of doping concentration profile will be carried out for the first time by probing individually the top of vertical doped NWs (attached to the substrate on the bottom part) in non-destructive contact mode and under environmental conditions compatible with NW solar cell design. The target relative uncertainty on the doping concentration is below ± 10 %.
High throughput nanomechanical characterisation of NW devices
A MEMS-SPM head capable of mechanical measurements will be extended by the functionality of electrical measurements. This new conductive MEMS-SPM with 10 pm resolution and 10 mN maximum testing force allows traceable measurement of direct and converse piezoelectrical properties of NW used in motion‑to‑electricity EH nanodevices. Then, for the first time, a conductive AFM will be equipped with this new MEMS‑SPM head with conductive diamond probes and coupled to a powerful contact-mode AFM developed for piezo generation measurements.
In addition, innovative diamond AFM probes with moulded pyramid-like tips and novel 3D nano-printed conductive tips will be developed which allow stable simultaneous electric and mechanical measurements of individual NWs. An AFM probe equipped with a standardised (ISO 14577) indenter tip will, for the first time, allow to link the mechanical measurements of nanomaterials using nanomechanical AFMs with those nanomechanical measurements using traditional validated instruments.
To further increase the throughput of mechanical measurements a microprobe-based high-speed contact resonance (CR) microprobe will be developed. The rate of tracked resonance frequency samples shall be increased from 10 to > 100 kSamples/sec.
Fast areal thermal imaging of NWs
The Scanning Thermal Microscopy (SThM) device will be improved to decrease the uncertainty of thermal conductivity measurements from 20 % to 10 % in the case of flat isotropic samples and for materials with a thermal conductivity lower than 10 Wm-1K-1. This will be combined with non-contact techniques for fast imaging of local defects in active devices based on nanowire arrays, like solar cells.
Impact on industrial and other user communities
New contact-based measurement modes will enable industry to simultaneously measure dimensions, electricity, thermal properties and mechanical properties of surfaces such as elasticity, hardness, viscosity, stress, friction and adhesion and thickness of nanomaterials. Conductive AFM DC (direct current)-biased and Microwave (MW) simultaneously performed with Force Modulation AFM of various types, also combined with measurements of the electron beam induced currents (EBIC) and Cathodoluminescence (CL), has the strong potential to describe overall performance, unwanted loss mechanisms, optimal operating frequencies and aging within one device, and will certainly fit the need of upcoming industrial production. Advanced diamond probe technology will provide the robustness and longevity currently lacking from contact sensors today. The outcomes of this project will facilitate strengthening the position of the energy harvesting and storage industry in the world market and secure jobs on a sustained basis.
Impact on the metrology and scientific communities
The traceable measurement methods to be developed in this project for the quantitative determination of the key geometrical specifications of NWs with high aspect ratios will contribute to the further developments of the metrology and scientific communities in the field of nanomaterials and nanometrology. In particular, the application-oriented high-end techniques developed in this project, including fast in-line areal measurement of NW arrays, will provide a visible bridge to translate seamlessly the outputs of the metrology community into commercial applications.
Impact on relevant standards
The metrological outputs of this project in the fields of NW solar cell will be presented to standardisation committees e.g. IEC TC 113 ‘Nanotechnology for electrotechnical products and systems’ and IEC TC 82 “Solar photovoltaic energy systems” to foster the creation of new standards. Good Practice Guides will be developed and disseminated to IEC TC 47 “Semiconductor devices”, ISO TC 164 “Mechanical testing of metals”, and the German VDI/VDE-GMA Technical Committee 3.41 “Surface Measurement Technology in the Micro- and Nanometer range” followed by standardisation of the new measurement modes.
Longer-term economic, social and environmental impacts
The metrology developed within the framework of this project will contribute to quality control of newly developed devices for energy harvesting and storage, and consequently help to promote and accelerate the development and fabrication and enable new nanotechnologies for renewable energy industry. This will strengthen Europe’s response to human-induced climate change.
The high throughput metrology for quality control of innovative energy harvesting and storage devices will substantially improve the competitiveness of the European semiconductor and energy industries. The developed high throughput SPM techniques can also be applied for ultrafast quality control of ultra-precision workpieces, therefore enhancing the competitiveness of European manufacturing industry.
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