Scientists at the University of Cambridge studying perovskite materials for use in solar cells and flexible LEDs have discovered that they can be more efficient when their chemical compositions are less ordered, simplifying production processes and lowering cost.
The surprising findings are the result of a collaborative project, led by Dr. Felix Deschler and Dr. Sam Stranks.
'The discovery was a big surprise really,' said Deschler, who is now leading an Emmy-Noether research group at TU Munich. 'We do a lot of spectroscopy to explore the working mechanisms of our materials, and were wondering why these really quite chemically messy films were performing so exceptionally well.'
'It was fascinating to see how much light we could get from these materials in a scenario where we'd expect them to be quite dark,' said co-lead author Stuart MacPherson, a Ph.D. student in the Cavendish Laboratory.
'Perhaps we shouldn't be surprised considering that perovskites have re-written the rule book on performance in the presence of defects and disorder.'
The researchers discovered that their rough, multi-component alloyed preparations were actually improving the efficiency of the materials by creating lots of areas with different compositions that could trap the energized charge carriers, either from sunlight in a solar cell, or an electrical current in an LED.
'It is actually because of this crude processing and subsequent de-mixing of the chemical components that you create these valleys and mountains in energy that charges can funnel down and concentrate in,' said Sascha Feldmann, a Ph.D. student at Cambridge's Cavendish Laboratory.
'This makes them easier to extract for your solar cell, and it's more efficient to produce light from these hotspots in an LED.'
Their findings could have a huge impact on the manufacturing success of these materials.
'Companies looking to make bigger fabrication lines for perovskites have been trying to solve the problem of how to make the films more homogeneous, but now we can show them that actually a simple inkjet printing process could do a better job,' said Feldmann.
'The beauty of the study really lies in the counter-intuitive discovery that easy to make does not mean the material will be worse, but can actually be better.'
'It is now an exciting challenge to find fabrication conditions which create the optimum disorder in the materials to achieve maximum efficiency, while still retaining the structural properties needed for specific applications,' said Deschler.
'If we can learn to control the disorder even more precisely, we could expect further LED or solar cell performance improvements'and even push well beyond silicon with tailored tandem solar cells comprising two different color perovskite layers that together can harvest even more power from the sun than one layer alone,' said Dr. Sam Stranks, University Lecturer in Energy at the Cambridge Department of Chemical Engineering and Biotechnology and the Cavendish Laboratory.
Another limitation of perovskite materials is their sensitivity to moisture, so the groups are also investigating ways to improve their stability.
'There's still work to do to make them last on rooftops the way silicon can'but I'm optimistic,' said Stranks.
