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Nanowire based Tandem Solar Cells

Periodic Reporting for period 3 - Nano-Tandem (Nanowire based Tandem Solar Cells)

Reporting period: 2018-05-01 to 2019-04-30

Silicon based photovoltaic cells are the dominant technology for terrestrial solar energy conversion with the best devices today measuring 25 % in the laboratory. Significantly higher conversion efficiencies up to 38.8 % are so far only reached with multi-junction cells based on III-V semiconductors. However, these materials are too expensive for the use in flat-plat modules on the earth, while for satellite applications they are of high value. NWs allow to significantly reduce material needs without compromising absorption or performance. Combining III-V nanowires (NWs) with today’s silicon photovoltaic technology offers the potential to reach at the same time very high performance devices, efficient use of materials and low cost.
During the Nano-Tandem project, three manufacturing concepts are evaluated against each other. After the second year of the project, the three different approaches for NW based tandem junction solar cells were assessed and a choice was made on which of these to push further in the remaining time of the project. Consequently, one of the approaches was stopped.
The most important impact of the Nano-Tandem project is to present a path for significant performance improvements of photovoltaic modules. Starting from today’s silicon PV technology, the Nano-Tandem technology allows increasing the efficiency through integration of a single NW pn-junction, and at a later stage possibly even a dual NW pn-junction. Efficiency is the most important factor in reducing the cost of solar electricity, as all area related costs are reduced by the overall power output of a PV module. Therefore, efficiency improvements of solar cells are a key to reach competitiveness with conventional energy sources based on fossil fuels or nuclear power. Furthermore, PV modules with higher efficiency allow harvesting of more energy from a constrained area, like a roof, and therefore having a higher value for the customer. Different approaches towards the implementation of “Building-integrated photovoltaics – BIPV” will constitute important steps towards our future “electricity-based society”, and will be an enabler for communication, lighting as well as electrical vehicles. High-tech products like the Nano-Tandem cell will help the European industry to have a unique differentiator compared to silicon PV modules, which have become a commodity product.
In WP1 we analyzed different Si emitter structures for the Si to NW tandem configuration. Based on these results, highly doped emitters with very deep (ca. 3 µm) diffusions of boron and phosphorus were produced. These bottom cell samples were used to successfully grow III-V nanowires in a sacrificial approach on top and to demonstrate an electrical interconnection between silicon and the nanowires.
In WP2 where we focus on synthesis and process development of large area NW solar cell materials progress has been made in all three areas. After the second year, nanowire growth and peel-off was abandoned in favor of using template assisted growth for 2T device geometry and Aerotaxy for 4T device geometry. MOVPE was chosen as a model system for what can be achieved with respective technique due to its high throughput and maturity.
In WP3, we focused on the up-scaling of the nanoimprint processes. As starting point for the upscaling, we had to fabricate new master structures. These new master structures were defined using interference lithography on a substrate size of 150 x 150 mm². We then successfully replicated this pattern on full area 4’’ silicon wafers for the direct growth on silicon approach. The inverse pattern, necessary for the catalyst driven growth on III/V substrates, was also replicated on 4’’ full area.
In WP4 the structural, optical and electrical properties of NWs grown by MOVPE (both, catalyst-assisted and template assisted selective epitaxy approaches) and by aerotaxy techniques have been analyzed and feedback has been provided to WP2 for growth optimization. In particular, EBIC microscopy applied to n-InGaP/ p+-Si TASE wires allowed to extract the InGaP doping level. Low temperature I-V measurements in these structures have evidenced negative differential resistance proving the formation of a tunnel diode.
In WP5 We have successfully adapted the membrane formation methods originally developed for only Aerotaxy to also work with MOVPE nanowires, enabling scalability and focus on fewer approaches in the project. We reproducibly achieve alignment and orientation near 100%. Using this approach, we turned the membranes into cells and achieved an impressively high open circuit voltage near 1V.
In WP6 we modelled the electro-optical properties of p-i-n junction NW array solar cells. We have optimized the geometry of tandem NW solar cells where we also predicted optimal materials combinations. The optimization was focussed on the maximum collection of photons. We considered both a tandem cell consisting of subcells within NWs, as well as a tandem cell with one of the subcells being the silicon substrate.
In WP7 a lab-scale life cycle assessment has been performed for the production of 1kWh of electricity using nanowire tandem solar panels, using data on raw material and energy use, cell and module manufacturing that are at industrial scale.
Based on our project activities we have produced 37 peer-review publications and 115 conference contributions. Moreover, we disseminated our activities for policymakers, investors and the general public.
In the following a summary of the progress beyond state of the art and expected potential impact is given without any specific attention to WPs.
As a consequence of the project we are now able to produce silicon solar cells specifically made for the application of combining these in a tandem junction with NWs. Since high efficiency III-V NW solar cells have well defined geometrical properties for optimal light absorption, high-resolution patterning techniques are required to obtain a sufficient pattern definition over large areas. The sun constantly moves during the day, which is why the efficiency of NW array solar cells under tilt is interesting. We have recently found that the efficiency remains constant or even increases for angles of tilt until approximately 45 degrees. Even at 60 degrees, the efficiency is 95% of the un-tilted efficiency. Template assisted NW growth has been developed for NWs with band gap corresponding to band gaps used in tandem configuration with Si at high performance. In parallel development to the template and MOVPE grown nanowires, aerotaxy based NWs offer integration on Si in a path to very low cost high efficiency solar cells. Great progress has been made in the alignment into highly ideal arrays over large areas of GaAs nanowires produced by Aerotaxy.
Electro-optical modeling shows advantage of using a top GaP transparent layer, which advances the knowledge of axial materials composition for optimal energy harvesting beyond the state of the art, which might be implemented by industry and result in more efficient solar cells. Life cycle assessment of the use of nanomaterials in solar/photovoltaic technologies has been reviewed. Differences in life cycle assessment methodologies from the reviewed studies and challenges related to up-scaling have been identified and valuable knowledge has been extracted to build on the state of the art.
A black (i.e. absorbing all light), high quality GaAs 2” NW membrane bonded on a glass carrier.
Examining nanowires by scanning electron microscopy. Photographer: M. Risedal, Lund University