Efficiency is the key!
Silicon based photovoltaic cells have, after many decades of research and development, not reached efficiencies above 25 % in the laboratory. Significantly higher conversion efficiencies can be reached with multi-junction cells based on semiconductors like gallium and indium. However, these materials are too expensive for the use in flat-plate modules. Nanowires allow to significantly reduce material needs without compromising absorption or performance. Combining nanowires made from semiconductors with today’s silicon photovoltaic technology enables:
- Very high performance
- Efficient use of materials
- Low cost
Left: Silicon solar cell; Right: Tandem solar cell produced by stacking nanowires on a silicon bottom
The success of our project requires a coordinated effort on modeling, nanowire synthesis, processing and characterization.
We have developed Si bottom cell concepts suited as base for the nanowire top cell. Samples for evaluation have been sent to the partners. Furthermore, Fraunhofer ISE implemented a rear side light trapping concept based on a diffraction grating, leading to a bottom cell current boost of 1.2 mA/cm2 for a 280 µm thick silicon absorber compared to the planar reference.
Large area nanoimprinted structure on a 4” silicon wafer, as template for III-V nanowire growth.
We pursue the fabrication and characterization of solar cells based III-V and Si materials. Here, two solar cell elements from different materials are combined to form a single tandem cell. Starting from a standard, planar silicon cell, we aim to integrate a second cell in the form of vertical nanowires on top using selective epitaxy in silicon oxide templates. This is schematically illustrated in the top left image. A scanning electron microscope image of an actual device is shown in the black and white background image. The fabrication includes the formation of specific p and n doped segments and electrical contact formation. Material characterization is an important aspect of our work.
Illustrated in the top right inset is a photo-luminescence measurement of epitaxially grown InGaP nanowires. Our target is to increase the efficiency of photo voltaic (PV) cells in a cost competitive manner using this novel approach.
To successfully implement the key concept of the Nano-Tandem project, i.e. a III-V nanowire on Si tandem device, one important step is the characterization of electrical properties of nanowires integrated on Si. In particular, it is important to assess the PV generation at the scale of individual nanowires.
This challenge has been addressed using electron beam induced current (EBIC) mapping – a technique capable to localize the current generating region with very high resolution (down to 50 nm). InGaP nanowires grown on Si using template assisted selective epitaxy by IBM-Zurich were investigated. EBIC analyses revealed the current generation from an axial p-n junction within the InGaP NWs as illustrated in Figure 1. Read more
The profile of the generated current along the nanowire axis was used to extract material parameters, such as doping and minority carrier diffusion lengths. To improve the diffusion lengths and thus boost the conversion efficiency, a radial passivation layer is required. EBIC mapping was also applied to homogeneously doped nanowires forming a p-n junction with Si to estimate the doping level in the wires.
The tunnel diode is a critical component of a tandem junction solar cell. NW tunnel diodes with material combinations optimized for solar energy harvesting (typically involving ternary NW materials) have not been studied. We fabricated, and characterized the GaInP/InP nanowire tunnel diodes with bandgap combinations suitable for a tandem junction solar cell. Electrical measurements show that the NWs behave as tunnel diodes in both InP (bottom)/GaInP (top) and GaInP (bottom)/InP (top) configurations, exhibiting a maximum peak current density of 25 A/cm2, and maximum peak to valley current ratio of 2.5 at room temperature.
Extensive modeling has been performed in order to achieve a design that includes both, high performance and efficient use of materials.
Above, a tandem solar cell design with double junctions is shown schematically. A high bandgap III-V nanowire array is used as top cell aiming to absorb the short-wavelength light. Thicker silicon solar cell is used as bottom cell aiming to absorb long-wavelength light. The geometry of top cell is optimized by maximizing the optical generated carriers in this cell and the bandgaps of top cell material are selecting by optimizing Shockley-Queisser limit. In this design, planar ITO front side contact is used instead of conformal coating to reduce the parasite absorption. To make a planar ITO, SiO2 or polymer can be used as supporting material among nanowires. Besides this ITO layer, two layers of anti-reflection coatings are used to reduce the reflection of the solar cell. A 100 nm SiO2 front side anti-reflection coating layer and a 90 nm Si3N4 at interface of silicon and nanowire can efficiently reduce the reflection from the whole structure below 4% in average.
Further reading: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-16-A665
Life cycle assessment
Under this heading, we assess the environmental impact, safety and cost of the different manufacturing approaches of nanowire tandem solar cells. Preliminary life cycle assessment has showed that direct growth of nanowires on silicon substrate performs better in most impact categories – climate change, ecotoxicity, eutrophication – compared to nanowire growth on native substrate, peel-off and transfer to silicon. The reason is the additional process steps for the production of the III-V substrate and stamp fabrication.
Si-nanowires coated with gold have been tested and found to be non-toxic. Results on literature and lab survey showed that similar to nanoparticles, dissolution of unstable NWs is an important property that might well determine the toxicity of NWs composed of for instance Ag, Cu, Zn, Ga/As. Shorter NWs tend to be more toxic than NWs of higher aspect ratio.
Here Magnus Borgström explains, why he believes semiconductor nanowires to be one of the most promising candidates for the next generation photovoltaics.
A bit of history:
The Nano-Tandem project builds on achievements of the AMON-RA project, where InP based nanowire solar cells with efficiency of 13.8 % were successfully demonstrated.
The AMON-RA project was a collaboration project of seven partners, financed by the EU 7th Framework Programme. It finished in September 2013.
Overview and results can be found on the project´s former webpage: