Scientific project:

Solar cells made of III-V materials present the best efficiencies among currently available technologies, up to 46% under concentration. Nevertheless, their cost is significantly higher than mainstream silicon modules. The major part of this cost, about 80% to 90%, lies in the III-V substrates necessary for the growth of monocrystalline materials with sufficient quality. We propose in this project to explore a new strategy to reuse the substrate for several consecutive growths, in order to drastically reduce its contribution to the final cost. The method consists in inserting a graphene layer on top of the substrate before the III-V growth, allowing both the fabrication of a high-quality material, and its exfoliation.

The development of these processes involves a number of different challenges. Firstly, it is necessary to define and develop the technological steps, including the growth of graphene, the growth of the III-V material, and finally the manufacturing processes to obtain efficient devices. Secondly, there is still an open question about the physical mechanisms at play during growth on the graphene-covered substrate. Several phenomena may be involved, including Van der Waals interactions across the graphene layer, but also localized nucleation phenomena in openings followed by lateral growth.

In this thesis, the first objective will be to develop a process without transferring the graphene from its original substrate towards the III-V substrate, which was done in previous works. To this end, we will take advantage of graphene synthesis on germanium, a substrate lattice matched with III-V semiconductors used for solar cells (GaAs, InGaP). This strategy without graphene transfer is compatible with large scale processing, and is expected to improve reproducibility through process simplification. This study requires determining the fabrication conditions and graphene properties compatible with the fabrication of a III-V material of sufficient quality for the production of high efficiency solar cells. It will be necessary to improve the understanding of the nucleation and coalescence mechanisms of the III-V material. The two other objectives of the thesis will be firstly to demonstrate the recycling of the substrate for several successive growths, which will require the re-preparation and characterization of the surface after exfoliation of the III-V thin film. Secondly, we aim at producing functional devices. For this objective, the candidate will take advantage of the team's expertise in thin-film bonding and solar cell fabrication processes.

Project environment:

This work will take place at the Center for Nanoscience and Nanotechnologies (C2N) in the SUNLIT team. It includes several methods of fabrication (III-V by MBE, nano-fabrication clean room processing) and characterization (luminescence, SEM, TEM). Collaboration with high level scientists in specialized techniques are expected (XPS, STM). This environment gives the PhD candidate many opportunities to tackle this project challenge and gain experience.

Profile:

The ideal candidate will have a master degree in physics, engineering, material sciences or related. Previous experience in clean-room is desirable but not essential. The candidate must show good organization skills to fabricate the target materials, using methods implying numerous parameters, in a clean room environment. A proactive approach is expected. Collaborative work being at the core of the program, communication skills are required for team working as well as regular presentation of work progress in internal meetings and international conferences.

Application:

Send CV and application letter to Amaury Delamarre (amaury.delamarre@cnrs.fr)