High efficiency tandem solar cells based on III-V nanowires on silicon – HETONAN

In the past 10 years, the PV industry has drastically reduced the fabrication cost of modules which now tends to stabilize. The module represents in 2013 only 22% of the final cost of a rooftop installation [Verlinden 2013]. It means that further cost reduction for PV installation mainly relies on better module efficiency.
HETONAN propose to develop a tandem solar cell structure able to convert solar spectra very efficiently by combining a planar Si bottom cell with a III-V cell based on nanowires (NWs) on top of it. The planar Si solar cell will be used as a substrate for the growth of p-n junction core-shell III-V NWs of higher bandgap than Si. The two subcells will be connected in series by a low-resistance tunnel junction elaborated in the planar Si structure. Two different types of III-V semiconductor NWs with the optimal bandgap value (1.7 eV), GaAs0.7P0.3 and Ga0.73Al0.27As, will be grown by MBE using self-catalyzed VLS method.
This HETONAN project involves some of the key player of academic research in photovoltaics (INL, coordinator, IEF/PASud, LPN, IMEP) and one industrial partner (SILSEF). In previous studies, they have already demonstrated a number of key building blocks necessary for the tandem cell implementation. The objective is now to investigate the practical feasibility to realise demonstrators and to determine the potential of this technology. To reach this ambitious objective, partners of the project will have to overcome the following bottlenecks:
- Growth of defect-free vertical NWs on nanostructured Si substrate
- Surface passivation of NWs
- Doping control of core shell NWs
- Optical and electrical coupling between top and bottom cells using tunnel junction, current matching and prevention of shunts and shortcuts between NWs by an adequate implementation technology.
The project requires, in addition to the management and valorization, 5 main tasks: the electro-optic simulations for the NWs array optimization, the fabrication of silicon bottom cell including tunnel junction, surface nanostructuration, core-shell NWs growth and tandem solar cell implementation. Accurate characterizations of materials and devices will be carried on and will also feed data base for simulation improvement.