Perovskite Solar Cells: Vacuum Process Can Lead To Market Readiness

Different production approaches in research and industry - comparative study highlights ways for mass production

Research and industry worldwide are working on the commercialization of perovskite photovoltaics. Most research laboratories focus on solvent-based manufacturing processes because they are versatile and easy to use. However, established photovoltaic companies today rely almost exclusively on vacuum processes to deposit high-quality thin films. An international consortium led by the Karlsruhe Institute of Technology (KIT) and the US Department of Energy's National Renewable Energy Laboratory (NREL, USA) has analyzed this critical discrepancy between laboratory and industry. They emphasize: Industrially proven vacuum processes could, with certain improvements, contribute to the rapid commercialization of perovskite solar cells. The results have appeared in Energy & Environmental Science (DOI: 10.1039/d3ee03273f ).

Perovskite-silicon tandem solar cells have undergone rapid development in the past ten years: Efficiencies of more than 33 percent have been achieved in research. This means they are already better than conventional silicon-based solar cells. However, it is still not ready for the market. One of the hurdles is the unresolved question of which process can best be used to produce perovskite solar cells as a mass product. Solvent-based manufacturing processes that are used in laboratories worldwide are contrasted with vapor phase deposition processes in vacuum, which are still standard today in the production of thin films in photovoltaics or in the production of organic light-emitting diodes (OLEDs).

In a current comparative study, an international consortium of academic and industrial partners led by NREL and KIT revealed major differences in the scientific approach to these production processes. Tenure-track professor Ulrich W. Paetzold from the Institute of Microstructure Technology and the Light Technology Institute of KIT explains: “98 percent of all scientific studies in 2022 were published on solvent-based processes. Vacuum-based processes that have proven themselves in industry for decades and could significantly advance the commercialization of solar cells are neglected.”

To explain: Solvent-based production uses inks in which organic and inorganic salts are dissolved in a solvent. These inks can then be deposited onto the surface of a substrate via various printing techniques. In contrast, vacuum-based manufacturing uses dry and solvent-free processes. The materials are sublimated in a vacuum with the addition of heat, i.e. transferred from the solid to the gaseous state and condensed on the substrate surface. In principle, it is also possible to combine both processes for the production of perovskite solar cells.

Laboratory efficiencies and throughput are not everything when it comes to mass production

In the study, the authors analyzed the advantages and disadvantages of both methods. The current dominance of solvent-based production in research lies in its uncomplicated handling in laboratories, which is due to the very good results in terms of efficiency under laboratory conditions and its low costs. In addition, there is the possible scalable roll-to-roll production, i.e. endless deposition between two rolls, similar to newspaper printing.

In comparison, the vacuum-based production process causes slightly higher investment costs and is currently - based on the processes used in research - still lagging behind in terms of deposition speed, i.e. production throughput. However, the authors show a variety of possible solutions and estimate that it is competitive taking into account real parameters such as electricity costs, production yield, material, decommissioning or recycling costs. Above all, the good repeatability of the deposition, the simple process control, the availability of industrial process equipment and the easy scaling of the deposition from small solar cell areas in the laboratory to application-relevant product areas make the process highly interesting for commercialization. “Vacuum-based production performs better than its reputation,” says Tobias Abzieh. It is therefore not surprising that the authors, in a first-ever published overview of commercialization activities in perovskite technology, were already able to demonstrate a lively interest in vacuum-based processes for the production of perovskite solar cells on the part of industry - despite the discrepancy in this regard on the method mainly used in research.

In order for vacuum-based processes to fully exploit their scaling effects, the manufacturing method still needs to be further improved, according to the researchers. Among other things, further research must be carried out on the quality of the separation in order to further increase the efficiency. It is also important to significantly increase the speed of deposition. “Vacuum-based manufacturing processes are not only the industry’s first choice when it comes to bringing thin-film technologies to market maturity. Our analysis also shows that the processes are competitive with solvent-based approaches,” adds David More from NREL.

As “The Research University in the Helmholtz Association,” KIT creates and imparts knowledge for society and the environment. The aim is to make significant contributions to global challenges in the fields of energy, mobility and information. To this end, around 9,800 employees work together on a broad disciplinary basis in natural sciences, engineering, economics, humanities and social sciences. KIT prepares its 22,300 students for responsible tasks in society, business and science through research-oriented university studies. The innovation activity at KIT builds the bridge between knowledge and application for social benefit, economic prosperity and the preservation of our natural resources. KIT is one of the German universities of excellence.