Scientists at Oxford University have developed a solvent system with reduced toxicity that can be used in the manufacture of perovskite solar cells, clearing one of the barriers to the commercialisation of a technology that promises to revolutionise the solar industry.
The researchers said perovskites - a family of materials with the crystal structure of calcium titanate - have been described as a 'wonder material' and shown to be almost as efficient as silicon in harnessing solar energy, as well as being significantly cheaper to produce.
By combining methylamine and acetonitrile, researchers have developed a clean solvent with a low boiling point and low viscosity that quickly crystallises perovskite films at room temperature and could be used to help coat large solar panels with the material.
Figure: University of Oxford - Perovskite solar cell
In recent years, perovskite-based solar cells have raced to the front of emerging photovoltaics, already competing on efficiency against well-established solar technologies such as the inorganic thin-film and multi-crystalline silicon used in solar panels around the world. Perovskites also have the shortest 'energy payback time' - the time taken for a material to save the same amount of energy that was expended in its production. It has been said that the sun supplies enough power in 90 minutes to meet the world's total energy needs for a year.
Dr Nakita Noel of Oxford's Department of Physics, lead author of the study, said, "At the moment, there are three main solvents used in the manufacture of perovskite solar cells, and they are all toxic, which means you wouldn't want to come into contact with them. Nakita, added, "Additionally, the most efficient perovskite solar cells are currently made through a process called solvent quenching - a technique that is not easily transferred from lab-scale deposition techniques to large-scale deposition techniques. While vapour deposition of these materials can overcome this problem, it will come at additional costs. One of the main selling points of this material is that it is cheap and can be easily solution-processed."
Nakita continued, "We have now developed the first clean, low-boiling-point, low-viscosity solvent for this purpose." Finally, adding "What is really exciting about this breakthrough is that largely reducing the toxicity of the solvent hasn't led to a reduction in the efficiency of the material in harnessing solar energy."
Dr Bernard Wenger, study co-author, also of Oxford's Department of Physics, said, "While we are probably still a few years from seeing perovskite-based solar panels on people's roofs, this is a big step along the way."
A low viscosity, low boiling point, clean solvent system for the rapid crystallisation of highly specular perovskite films
Nakita K. Noel | Severin N. Habisreutinger | Bernard Wenger | Matthew T. Klug | Maximilian T. Hörantner | Michael B. Johnston | Robin J. Nicholas | David T. Moore | Henry J. Snaith
Energy Environ. Sci., 2016, Advance Article
Received 14 Aug 2016 | Accepted 26 Sep 2016 | First published online 26 Sep 2016
Perovskite-based photovoltaics have, in recent years, become poised to revolutionise the solar industry. While there have been many approaches taken to the deposition of this material, one-step spin-coating remains the simplest and most widely used method in research laboratories. Although spin-coating is not recognised as the ideal manufacturing methodology, it represents a starting point from which more scalable deposition methods, such as slot-dye coating or ink-jet printing can be developed. Here, we introduce a new, low-boiling point, low viscosity solvent system that enables rapid, room temperature crystallisation of methylammonium lead triiodide perovskite films, without the use of strongly coordinating aprotic solvents. Through the use of this solvent, we produce dense, pinhole free films with uniform coverage, high specularity, and enhanced optoelectronic properties. We fabricate devices and achieve stabilised power conversion efficiencies of over 18% for films which have been annealed at 100 °C, and over 17% for films which have been dried under vacuum and have undergone no thermal processing. This deposition technique allows uniform coating on substrate areas of up to 125 cm2, showing tremendous promise for the fabrication of large area, high efficiency, solution processed devices, and represents a critical step towards industrial upscaling and large area printing of perovskite solar cells.