New OPportunities with low environmental footprint and Earth abundant absorbeRs for solAr cells – OPERA

ZnSnN2, ZnGeN2 and MgSnN2 films have been developed on different substrates by physical vapor deposition processes. Particular attention was paid to the study of the influence of synthesis conditions (bias voltage, nature of sputtering targets, addition of hydrogen in the plasma mixture, etc.) on the properties of thin films. The advanced materials characterization techniques available within the consortium, such as transmission electron microscopy or X-ray diffraction, have been widely used to describe the structure, microstructure and chemistry of the films produced. The functional properties of the different materials were evaluated by optical and electrical methods.

This project highlighted the role played by the chemical composition and the morphology of II-IV-N2 films on their functional properties. Compared to the work known before the start of this project, major advances have been obtained on the potential applications of this new class of materials. New formulations of compounds have been tested. All the results obtained within the framework of the OPERA project open up new avenues of study that should be exploited in the context of new collaborative projects.

In spite of the theoretical forecasts which suggested a most interesting inorganic material for the realization of an upper cell of a tandem cell, ZnSnN2 proved experimentally difficult to master. Despite results that remain below what had been hoped for when the project was drafted, significant progress has nevertheless been obtained within the framework of the OPERA project. Innovative avenues have been unveiled for the potential continuity of research on this material and suggest prospects for future studies which should make it possible to draw the roadmap for the application of this family of materials for photovoltaic solar energy.

In addition to the two theses defended during this project, a publication (https://doi.org/10.1021/acsaelm.1c00478) has been published on the effect of the use of a substrate bias during the ZnSnN2 film growth. This parameter makes it possible to densify the films and to limit their post-synthesis oxidation. Other works are being published on this same material as well as on ZnGeN2 and MgSnN2.

The development of “green” energy sources has attracted much intention over the last decade and is currently of a crucial importance for our societies. Because of the greenhouse effect the policies for the development of carbon free energies have become central.
Nevertheless, the terawatt-scale energy demand will remain. The electrical energy supply will therefore have to be an energy mix. Photovoltaics will be the backbone of such a renewable energy system. To fulfill the challenging price targets and to develop new markets (e.g. building integration) the design and production of solar cells working at highest efficiencies is essential.
Crystalline Si dominates the PV market (85%) and will be essential for still a very long time. However, as maximum efficiency is limited to 29.4 %, strategies must be developed to maintain its dominant role in the PV market. One way to reduce losses is to add a top cell with a higher band gap above a conventional silicon cell to create a tandem cell with potentially more than 40 % efficiency. Different ways of developing new top absorbers have been investigated but require the use of indium and gallium. Significant volatility in the price and supply of such matter over the past years has led to considerable concern given their critical roles and their use in a wide range of large scale electronic devices including solar cells. Moreover, III-V tandem cells require the use of epitaxial growth that remain both expensive and limiting for large scale realization and will therefore negatively influence the production costs even at high production capacity. Finaly, the materials used are toxic and their acceptance in society is therefore limited.
It is so important to study and develop new indium-free Earth abundant and non-toxic materials with optimized properties for the realization of innovative solar cell demonstrating affordable cost for mass production.
The OPERA project aims at developing a new kind of low-cost, indium/gallium-free, non-toxic nitride absorber and to realize first test nitride cell by using easy-to-use, up-sizeable and cost affordable production technique (sputtering). This is an ambitious goal that would be a major step forward in the field of solar energy.
Moreover, each intermediate step of the project would bring new knowledge more especially about single top nitride solar cell or related materials fundamental properties. Indeed, the family of Zn-IV-N2 alloys is promising as it could span the solar spectrum and could then replace the InGaN alloys as absorbers. Nonetheless, data about ZnSnN2 alloys remain scarce. The interest of such alloy for PV has increased the last two years, but numerous efforts remain to be done to better understand its fundamental properties.
The OPERA project gathers four laboratories with different specialties and complementary skills: the Jean Lamour Institute - CNRS - France dedicated to material science. The Institute of Electronics Microelectronics and Nanotechnology – CNRS – France with skills in material sciences and device technologies. Institut National de l’Energie Solaire – Commissariat à l’Energie Atomique et aux Energies Alternatives: a well-known French institute for solar technologies and transfer to industry. The Group of electrical engineering of Paris (GeePs), formerly LGEP, is one of the main PV laboratories in France, a founding member of the CNRS PV Photovoltaic Federation (FedPV), and an active partner of IPVF (Institut Photovoltaïque Francilien).