The Institute

The Centre for Nanosciences and Nanotechnology (C2N) is a joint research unit between the CNRS and Université Paris-Saclay. The center develops research in the fields of material science, nanophotonics, nanoelectronics, and nanotechnologies, covering all the range from fundamental to applied sciences. The PhD thesis offered here will take place in the Sunlit Team, which develops innovative approaches for solar cells, aiming at high efficiencies, cost reduction and reliability of the photovoltaic devices.

Context

Solar photovoltaics should play a major role to fulfil the goals set for the energy transition and Paris Agreement. In the sustainable development scenarios of the International Energy Agency (IEA), the share of photovoltaics in electricity generation is expected to increase from 6% in 2017 to 22% in 2030. Nowadays, most commercial photovoltaic modules are made of silicon (Si) wafers with thicknesses of more than 150 µm, an average conversion efficiency of about 22%, and record solar cells up to 27.3%. To accelerate the energy transition, it is necessary to further increase the conversion efficiency of solar cells, and to reduce their cost.

Reducing material use is an important way to lower costs and increase efficiency. Ultrathin solar cells with thicknesses at least 10 times lower than conventional solar cells could have the unique potential to efficiently convert solar energy into electricity while enabling material savings, shorter deposition times and improved carrier collection. In 2019, we have achieved an important milestone by trapping sunlight efficiently in a GaAs solar cell as thin as 200 nm, using a nanostructured back mirror. This new architecture is based on multi-resonant absorption and led to a record efficiency of nearly 20% [1]. Recently, we have reviewed the very high potential of ultrathin solar cells in Nature Energy [2], and we have found theoretically new upper bounds for light trapping in solar cells [3]. This work opens up new perspectives towards ultrathin solar cells.

Scientific project

The goal of this internship is to develop low-cost processes to trap light in ultrathin solar cells. The team has achieved state-of-the-art results for single-junction GaAs solar cells. The aim of this project is to extend these results to silicon solar cells and to tandem thin-film/silicon solar cells.

After a training to clean-room security and processes, the candidate will develop new fabrication processes in clean room in order to study and compare two emerging concepts: (1) multi-resonant absorption that can be achieved using perfectly ordered structures (arrays) fabricated by nanoimprint lithography, and (2) directional scattering obtained using structures with correlated disorder, fabricated by self-assembled nanopatterning techniques such as polymer-blend and colloidal lithography. The performances will be analyzed by optical spectroscopy (reflection/transmission/absorption measurements) and optical simulation. The final goal of the internship will be to implement original designs to efficient solar cells.

The team

This work will be done in close collaboration between the C2N (SUNLIT Team) and the IPVF both located on Paris- Saclay campus in Palaiseau (one block away). More information on the SUNLIT Team: https://sunlit-team.eu Profile: We are looking for a high-level student in M2 with a solid knowledge in semiconductor physics and optics. Possibility to continue with a PhD grant on ultrathin, high-efficiency tandem solar cells in 2025.

Send CV and motivation letter to: stephane.collin@cnrs.fr 

References:

• -L. Chen et al., A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror. Nature Energy 4, 761-767, 2019.
• –Massiot, A. Cattoni, S. Collin. Progress and prospects for ultrathin solar cells. Nature Energy 5, 959-972, 2020.
• – Collin and M. Giteau, Upper bounds on broadband absorption, submitted (2024). Available on arXiv:2407.19559.