Researchers from the UK's University of Cambridge and Diamond Light Source, working with scientists from Japan's Okinawa Institute of Science and Technology (OIST), have found that the defects which limit the efficiency of perovskites are also responsible for structural changes in the material that lead to degradation.
In their work, the researchers used a combination of techniques to mimic the process of aging under sunlight and observe changes in the materials at the nanoscale, helping them gain new insights into the materials. Their findings could accelerate the development of long-lasting, commercially available perovskite photovoltaics.
In order to achieve commercialization and fulfil their ultimate potential in realizing widespread decarbonization, perovskite solar cells will need to operate for a decade, at the very least. Researchers and manufacturers have yet to develop a perovskite device with similar stability to silicon cells.
Using a toolkit of high spatial-resolution techniques, in collaboration with the Diamond Light Source synchrotron facility and the electron Physical Sciences Imaging Centre (ePSIC) in Didcot, and the Department of Materials Science and Metallurgy in Cambridge, the scientists were able to observe the nanoscale properties of perovskite thin films and how they change over time under solar illumination.
Previous work by the team using similar techniques has deepened the understanding of the defects that cause deficiencies in the performance of perovskite photovoltaics - so-called carrier traps.
"Illuminating the perovskite films over time, simulating the aging of solar cell devices, we find that the most interesting dynamics are occurring at these nanoscopic trap clusters," said co-author Dr. Stuart Macpherson from Cambridge's Cavendish Laboratory.
"We now know that the changes we see are related to photodegradation of the films. As a result, efficiency-limiting carrier traps can now be directly linked to the equally crucial issue of solar cell longevity."
"It's pretty exciting," said co-author Dr. Tiarnan Doherty, from Cambridge's Department of Chemical Engineering and Biotechnology, and Murray Edwards College, "because it suggests that if you can address the formation of these surface traps, then you will simultaneously improve performance and the stability of the devices over time."
By tuning the chemical composition, and how the perovskite film forms, in preparing the devices, the researchers have shown that it's possible to control how many of these detrimental phases form and, by extension, how long the device will last.
"The most stable devices seem to be serendipitously lowering the density of detrimental phases through subtle compositional and structural modifications," said Doherty. "We're hoping that this paper reveals a more rational, targeted approach for doing this and achieving the highest performing devices operating with maximal stability."
The researchers are optimistic that their latest findings will bring us closer still to the first commercially available perovskite photovoltaic devices.
"Perovskite solar cells are on the cusp of commercialization, with the first production lines already producing modules," said Dr. Sam Stranks from Cambridge's Department of Chemical Engineering and Biotechnology, who led the research.
"We now understand that any residual unwanted phases - even tiny nanoscale pockets remaining from the processing of the cells - will be bad news for longevity of perovskite solar cells. The manufacturing processes therefore need to incorporate careful tuning of the structure and composition across large area to eliminate any trace of these unwanted phases - even more careful control than is widely thought for these materials. This is a great example of fundamental science directly guiding scaled manufacturing."
"It has been very satisfying to see the approaches that we've developed at OIST and Cambridge over the past several years provide direct visuals of these tiny residual unwanted phases, and how they change over time," said co-author Dr Keshav Dani of OIST's Femtosecond Spectroscopy Unit. "The hope remains that these techniques will continue to reveal the performance limiting aspects of photovoltaic devices, as we work towards studying operational devices."
"Another strength of perovskite devices is that they can be made in countries where there's no existing infrastructure for processing monocrystalline silicon," said Macpherson. "Silicon solar cells are cheap in the long term but require a substantial initial capital outlay to begin processing. But for perovskites, because they can be solution processed and printed so easily, using far less material, you remove that initial cost. They offer a viable option for low- and middle-income countries looking to transition to solar energy."
