Germany's Fraunhofer Institute for Solar Energy Systems ISE has shared results from its five-year lighthouse project MaNiTU. The project, involving six Fraunhofer institutes, worked to identify the most sustainable paths for the market launch of perovskite-silicon tandem solar cells.
The researchers produced new materials with perovskite crystal structures and compared them with existing materials at the cell level, concluding that high efficiencies can only be achieved with lead perovskites. They then fabricated highly efficient demonstrators, such as a perovskite silicon tandem solar cell of more than 100 sq cm with screen-printed metallization.
The project also included the development of a scalable perovskite-silicon tandem solar cell that achieved a 31.6% power conversion efficiency, first announced in September. The Fraunhofer researchers used a combination of vapor deposition and wet-chemical deposition to ensure an even deposition of the perovskite layer on the textured silicon surface. “Close industrial cooperation is the next step in establishing this future technology in Europe,” said Professor Andreas Bett, coordinator of the project.
Another strand of the project saw researchers evaluate the efficiency and stability of tandem solar cells. They used characterization data and an opto-electrical simulation model to carry out a comprehensive loss analysis on the tandem solar cell and determined a practical upper limit of 39.5% efficiency.
Elsewhere in the project, a research team investigated the use of non-toxic, lead-free alternatives in perovskites but were unable to produce tandem solar cells of sufficient efficiency from any of the lead-free materials they theoretically and experimentally analyzed.
Meanwhile, the Fraunhofer Institute for Microstructure of Materials and Systems IMWS evaluated low-energy focused ion beam techniques for the preparation of industrial tandem solar cells, which were then analyzed in high resolution using a transmission electron microscope (TEM). A special sample holder was constructed to allow the direct deposition of absorber and contact layers on TEM substrates, while methods were developed to investigate the thickness, degree of coverage and chemical bonding of self-organized molecular monolayers.
Fraunhofer researchers also carried out assessment of the environmental impacts of production, using phase and end-of-life of the tandem solar cells to develop recycling concepts for perovskite tandem modules. They concluded that by using advanced recycling processes, it is possible to create a circular economy for photovoltaic systems with lead perovskites.
The project also consisted of a research team developing calculation models to describe the structural and photovoltaic properties of relevant absorber materials and their interfaces with optically transparent and electrically conductive contact materials. Scientists at the Fraunhofer Institute for Mechanics of Materials IWM developed a computational simulation workflow that they say can be used for both photovoltaics and material issues on the industrial level in other technologies such as hydrogen.
