Fraunhofer Institute for Microstructure of Materials and Systems IMWSThe development of perovskite silicon tandem solar cells consisting of stable materials and manufactured using scalable production processes is the prerequisite for the next technological leap in the photovoltaic industry. Over a period of five years, six Fraunhofer Institutes have combined their expertise in the Fraunhofer lighthouse project “MaNiTU” in order to identify the most sustainable paths possible for the market launch of these tandem solar cells. They were able to show that high efficiency levels can be achieved with industry-oriented processes. However, high efficiencies could only be achieved with lead-containing perovskites. The researchers therefore developed suitable recycling concepts to ensure sustainability.
Fraunhofer scientists in the “MaNiTU” project produced numerous new materials with a perovskite crystal structure and compared them with known materials at cell level. It turned out that high efficiencies can only be achieved with leaded perovskites. The research team created highly efficient demonstrators, for example a perovskite silicon tandem solar cell over 100 square centimetres with screen printing metallization as well as mini modules for single and interconnected tandem solar cells. Complete life cycle analyses showed that a sustainable product is available with suitable production and recycling processes and degradation rates that are comparable to today's silicon technology. “In this project, the Fraunhofer-Gesellschaft, in collaboration with several institutes, has worked its way back to the forefront of global photovoltaics and should remain there,” declared the Fraunhofer project advisors at the closing event at the end of November 2024.
Scalable perovskite silicon solar cell with 31.6 percent efficiency
The scientists researched manufacturing processes for perovskite materials that can be implemented industrially on large surfaces. Thanks to the so-called “hybrid route”, a combination of evaporation and wet-chemical deposition, they were able to produce high-quality perovskite thin films on industrially textured silicon solar cells. The research team was thus able to achieve a fully textured perovskite silicon tandem solar cell with 31.6 percent efficiency on 1 square centimeter of cell area. “Close industrial cooperation is now the next step in establishing this future technology in Europe,” summarized Prof. Andreas Bett, Institute Director at the Fraunhofer Institute for Solar Energy Systems ISE and coordinator of the Fraunhofer lighthouse project.
No suitable lead-free perovskites for solar cells currently in sight
In addition to conventional lead-containing perovskite compounds, material development focused in particular on non-toxic, lead-free alternatives. This enabled the scientists to gain detailed insights into the stability and properties of the target materials by closely interlinking theoretical simulation, experimental material synthesis and cell production. In addition to various perovskite compounds, different synthesis routes were also considered. “In particular, the scalable, semi-continuous perovskite synthesis in powder form using spray drying is a suitable screening method for a large number of compounds and their potential synthesis. The method can also be applied to industrially relevant quantities,” explained Dr. Benedikt Schug, Head of Particle Technology at the Fraunhofer Institute for Silicate Research ISC. However, the research team was unable to realize tandem solar cells with sufficient efficiency with any of the lead-free materials predicted from theory and synthesized experimentally, as the intrinsic material qualities were not sufficiently high.
Reduction of the ecological footprint
In order to consider the entire product life cycle of the tandem solar cells, the scientists also looked at the topic of recycling and the possibilities of a closed-loop economy. They carried out a detailed assessment of the environmental impact of the production, use phase and end-of-life of the tandem solar cells and developed recycling concepts for perovskite tandem modules. “By using advanced recycling processes, a circular economy for photovoltaic systems can also be created for lead-containing perovskites and long-term energy efficiency can be guaranteed,” said Prof. Dr. Peter Dold, Head of the Fraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS, summarizing the results.
System components for contacting the perovskite sub-cell
The researchers worked on the development of industry-oriented system components and coating technologies in order to establish high-performance contact materials for electron and hole contacts in the industrial wafer format G12. One of the challenges here was the temperature sensitivity of the perovskite cell, which only allows temperatures below 100 °C during the production of the front contact system. In addition, the deposition of a transparent conductive oxide on the cell is required. For this purpose, a new process chain consisting of a combination of ALD and evaporation processes was implemented in a SALD hybrid system and supplemented by a final sputtering process. “Our goal now is to transfer the development,” explains Dr. Volker Sittinger, Head of the Diamond-Based Systems and CleanTech department at the Fraunhofer Institute for Surface Engineering and Thin Films IST. “Together with plant manufacturers and end users, we are working on transferring the new process chain from research to application.”
Evaluating the efficiency and stability of tandem solar cells
The scientists also focused on the characterization of tandem solar cells and developed methods for damage-free selective analysis of the silicon and perovskite sub-cells. Using characterization data, an opto-electrical simulation model of the tandem solar cell could be used for a comprehensive loss analysis and a practical upper efficiency limit of 39.5 percent could be determined. In addition, they further developed the microstructural analysis. At the Fraunhofer Institute for Microstructure of Materials and Systems IMWS, they evaluated low-damage focused ion beam (FIB) techniques for the preparation of industrial tandem solar cells, which can then be analyzed with high resolution in the transmission electron microscope (TEM). A special sample holder was constructed that allows the direct deposition of absorber and contact layers on TEM substrates at the project partners' premises. In addition, methods for investigating the thickness, degree of coverage and chemical bonding of self-organizing molecular monolayers were developed.
Modeling of absorber materials and material interfaces
The research team developed calculation models that accurately and efficiently describe the structural and photovoltaic properties of relevant absorber materials and their interfaces with optically transparent and electrically conductive contact materials. The scientists at the Fraunhofer Institute for Mechanics of Materials IWM developed a computational simulation workflow for this purpose that can be used not only for photovoltaics, but also for industrially interesting material issues in other technologies for the generation, conversion, storage, distribution and use of sustainable electrical energy resources - for example hydrogen.
(11.12.2024)