Earth-Abundant Se/CZTSe Tandem Solar Cells: From Proof of Concept to Real-World Operation

Scientific domain and context

The transition to renewable energy is one of the most urgent challenges facing society, and solar photovoltaics (PV) represent one of the most scalable and sustainable solutions. After many years of development, commercial crystalline silicon (c-Si) solar cells are approaching their theoretical efficiency limit of around 29%¹. To surpass this limit, tandem solar cells, which combine a wide-bandgap absorber stacked on top of c-Si, have emerged as a promising solution. These architectures offer a pathway towards efficiencies beyond 40%². Currently, the most advanced c-Si-based tandem devices rely on lead-halide perovskite (PSK) top absorber. While these materials demonstrate remarkable performance, concerns remain regarding their long-term stability, as well as the toxicity associated with the lead (Pb). Therefore, identifying scalable, environmentally friendly, wide-bandgap materials that are compatible with tandem integration remains a major challenge for the PV community.  Beyond silicon-based tandems, fully thin-film tandem technologies offer additional advantages, including reduced material consumption, lower processing temperatures, lightweight and potentially flexible modules. The development of fully thin-film tandem architectures is therefore an attractive pathway for various PV applications. In this context, selenium (Se) has recently emerged as a promising wide-bandgap absorber for top cell application. It has a bandgap of around 1.8–1.9 eV, good air stability, a low material cost and compatibility with low-temperature processing. Initial proof-of-concept demonstrations using c-Si bottom cells, including a monolithic growth developed by DTU university³ and a mechanically stacked approach developed by our group⁴, have confirmed the feasibility of Se top cells in tandem-based architectures. In parallel, kesterite absorbers such as Cu₂ZnSn(S,Se)₄ (CZTSSe) have emerged as promising earth-abundant candidates for bottom-cell applications in all-thin-film tandems. Their bandgap can be tuned from 1.0 to 1.5 eV through compositional engineering, providing valuable flexibility for current matching with wide-bandgap top cells. Significant progress has been achieved in recent years through the development of cost-effective solution-based and molecular ink processing routes, enabling CZTSSe devices to reach efficiencies above 15%⁵.

Objectives

This PhD project aims to develop a proof of concept for all-thin-film Se/CZTSSe tandem solar cells and to establish advanced characterization under realistic operating conditions. The project will be structured around three main axes:

1) Development of tandem-compatible sub-cells: development of semi-transparent, wide-bandgap Se top cells and narrow-bandgap CZTSSe bottom cells optimized for tandem operation.

2) Tandem integration strategies: demonstration of Se/CZTSSe tandem solar cells through mechanically stacked configurations, followed by two-terminal architectures via direct monolithic growth and/or low-temperature bonding.

3) Advanced characterization under real-conditions: Implementation of climate-relevant characterization, including temperature-dependent measurements, extraction of temperature coefficients, and reliability assessment under controlled environmental conditions.

References

1- Green, et al. Prog. Photovolt. Res. Appl. 33, 795–810 (2025).

2- Green, Nat. Energy 1, 15015 (2016).

3- Nielsen et al. PRX Energy 3, 013013 (2024)

4- Tamin et al. JNPV 2024

5-  Arguijo et al Nat Energy 11, 194–208 (2026).

The PhD candidate will be supervised by Charif Tamin (Associate Professor, INSA Lyon) and Mohamed Amara (Senior Researcher, CNRS) within the Light Engineering and Conversion (i-Lum) team at the Institute of Nanotechnology of Lyon (INL). The experimental work will rely on the 1,500 m² NanoLyon technological platform, including 825 m² of ISO5–ISO7 cleanroom facilities. The platform provides state-of-the-art tools for thin-film deposition, device processing, and optoelectronic characterization. Real-world characterization will be performed using a recently acquired climatic chamber enabling advanced photovoltaic measurements under controlled temperature, humidity, and illumination conditions. The project will involve collaborations with European partners, with potential opportunities for research visits and scientific exchanges.

Master’s degree (M2 or equivalent) in materials science, physics, or a related field, with a solid background in semiconductor materials, optoelectronics, or nanotechnology, and a clear interest in experimental research. Official funding is subject to the successful selection of the candidate through the EEA doctoral school competitive admission procedure.