Description of the thesis topic

The photovoltaic (PV) sector is experiencing fast growth and becoming one of the main pillars of the energy transition. In this context, a challenging roadmap has been issued during COP21, initiated by IPVF with ten other institutes worldwide, named “30-30-30”. It aims at reaching a conversion efficiency of 30%, from modules at 30 cents per watts in 2030. A straightforward strategy relies on shifting from single junction solar cells to tandem structures, whose respective absorbers have complementary band gaps, which in principle can reach more than 40% efficiency.
Amongst the many strategic visions of the IOTA « InnOvative Tandem Architectures » project of PEPR TASE (Technologies Avancées pour les Systèmes Energétiques), one consists of combining two industrially mature PV technologies, namely Cu(In, Ga)S2 or CIGS-thin-film and crystalline-silicon, for two terminals tandem structures. The target architecture combines a Cu(In, Ga)S2 or CIGS chalcopyrite absorber with a bandgap of around 1.6 eV with a crystalline silicon (c-Si) bottom cell. However, the direct integration of a Cu(In, Ga)S2 cell on a c-Si device encounters numerous fundamental and technological issues, in particular the poor adhesion of CIGS to Si, even after deoxidation. This makes the development of appropriate interlayers essential, providing both optical and electrical coupling between the two sub-cells. Transparent conductive oxides (TCOs) appear to be particularly promising candidates for this purpose, as they not only ensure light transmission to the lower cell while serving as a transparent back contact for the CIGS cell but also provide a favorable base for CIGS growth, particularly in a bonding approach where they advantageously replace molybdenum. Most of them were already addressed within the frame of a national project ANR-PASTEL (2021-2025), involving the two laboratories concerned by the present PhD. In particular, TCOs and passivation layers have been tested for CIGSebased devices, providing valuable data for selecting the most suitable materials for tandem integration with silicon. The aim of the present PhD position is firstly to identify the most relevant oxides, TCOs, and passivation layers, for use as transparent back contacts and/or interfacial layers (i.e. inter-layer). This selection will be based not only on the intrinsic properties of the materials but above all on the optoelectronic quality of their interface with the CIGS, to minimize losses due to interface defects and band misalignments. To identify the mechanisms limiting device performance, the PhD student will correlate them with material characteristics, in terms of physico-chemical, structural, and optoelectronic properties. He will therefore carry out both optoelectronic and materials studies on structures dedicated to the production of tandem photovoltaic devices. The final objective will be to optimize CIGS cells on TCOs best suited to c-Si/CIGS 2T-based tandem solar cells. This experimental work will be supplemented by simulations designed to better understand the operation of the devices. The second part of the PhD work will be devoted to the deep investigation of the best-performing tandem devices that will be developed as part of the project.

Work Context

The partners involved in the project have complementary expertise, know-how, and laboratory facilities. All activities related to CIGS-based solar cells—ranging from thin film growth and materials characterization to optoelectronic characterization—will be conducted at IMN. The activities dealing with TCO fall within IPVF's area of expertise.
Most of the working time will be spent in Nantes and the student will be registered at Nantes Université at the Institut des Matériaux Jean Rouxel (IMN, UMR6502, Nantes). IMN is a joint CNRS – University of Nantes research unit, composed of more than 200 staff, including about 120 permanent staff and 80 PhD and post-doctoral students, bringing together physicists and chemists conducting research activities in the field of functional materials. The work will be carried out within the MIOPS team, whose activities in the field of thin-film photovoltaics based on CIGS are internationally renowned.
The PhD student's work at the IPVF will be carried out within the UMR IPVF-CNRS 9006. The IPVF (Institut Photovoltaïque d'Île-de-France) is a cutting-edge research center dedicated to innovation in the field of photovoltaics. Located on the Saclay plateau, it brings together experts from academia and industry to develop advanced solar technologies and improve the efficiency of photovoltaic cells. The IPVF conducts research into new materials, such as perovskites, and explores innovative solutions to make solar energy more competitive and sustainable. Thanks to its collaborative approach and state-of-the-art infrastructures, the institute plays a key role in the energy transition and the development of renewable energies in France and abroad.

The position is located in a sector under the protection of scientific and technical potential (PPST), and therefore requires, in accordance with the regulations, that your arrival is authorized by the competent authority of the MESR.

Constraints and risks

The recruited person will have to plan stays at the partner's laboratories.