Since the beginning of the project in May 2015 the members of the project have jointly pushed forward the research and development of the three approaches to nanowire synthesis and solar cell fabrication. The work is divided into work packages (WPs) which run in parallel and in the following a summary structured according to the sequence of work packages is given. We have structured our research during the two first years into three areas, involving different methods for the nanowire fabrication: a) Direct growth of III-V nanowire arrays on (100) silicon, b) Growth of nanowires of InP or GaAs, embedding these in polymer + transfer to silicon, and c) Aerotaxy growth of nanowires, alignment and array formation, embedding in polymer and transfer to silicon. These three thrusts are key for us to be able to make evaluations and decisions after the first 24 months for which approaches to give priority.
In WP1 we analyzed different Si emitter structures for the Si to NW tandem configuration regarding recombination, voltage potential, interconnectivity and compatibility with subsequent processing. Highly doped emitters produced by ion-implantation were identified as most the promising. They can tolerate the absence of surface passivation, are robust in further processing and allow formation of a tunnel diode at the Si to NW interface. Fraunhofer ISE developed Si bottom cell concepts suited as base for the nanowire top cell. Samples for evaluation have been sent to the partners. Furthermore, 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.
In WPs 2 to 5 where we focus on synthesis and process development of large area NW solar cell materials, progress has been made in all three areas. A key result is that GaAs nanowire PV-cells have produced world-record results, with efficiencies of 15.3% and also record high open-circuit voltages. This is a very important result since GaAs has clear advantages from a cost perspective and for the intended up-scaling based on Aerotaxy.
For the template approach with direct growth on Si substrate ternary GaInP as well as GaAsP NWs were successfully grown and characterized, showing that the method can be used to grow NWs with desired band gap for the tandem approach. 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.
Regarding growth on native substrates for peel off, we developed nano imprint lithography (NIL), metal evaporation and lift off for economically viable patterning of large areas of catalyst matrix for NW growth. Parameters affecting the pattern fidelity after growth has been assessed, and synthesis optimized for growth with and without a SiN growth mask.
A dramatic up-scaling of the Aerotaxy production technique for GaAs nanowires has been achieved at Sol Voltaics, while ternary GaAsP NWs with intentional n and p type doping has been synthesized by use of aerotaxy at ULUND, achieving material with band gap suited for tandem junction with Si.
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.
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 previously. 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.
In the EBIC measurement, comparing EBIC profiles of a working and a non-working configuration confirms a wider depletion region for the non-working configuration. The realization of NW tunnel diodes in both InP/GaInP and GaInP/InP configurations lays the foundation for the next step of realizing NW tandem junction solar cells independent of the growth order of the different materials, opening up for flexibility regarding dopant incorporation polarity.
In WP 6, we have performed simulations in order to optimize the light-harvesting.
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
In WP7 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.
Aerotaxy of nanowires, array formation, embedding in polymer and transfer to silicon