Dr Malin Johansson is a Docent (typically translated from Swedish as Associate Professor) in the Department of Chemistry – Angstrom Lab at Uppsala University, Sweden. There she researches Metal Halide Perovskite Solar Cells (MHPSCs), and in October this area of research formed a key part of a talk given by Dr Johansson at the University of Sheffield as part of the new CBE Seminar Series.
Solar cells are the semiconducting building blocks of solar panels, converting light into electricity. Solar cells can be made from various materials, such as Silicon, Cadmium Telluride, Gallium Arsenide (GaAs), conductive polymers, or Perovskites.
Perovskites are a solution processed crystalline material which have exceeded all other single-junction thin film technologies besides GaAs in the race to develop highly efficient solar cells. High efficiency means a high rate of light-electricity conversion, ensuring less light is wasted and more energy generated.
‘Perovskite’ actually refers to the cubic ABX3 crystal structure characterising this class of compounds. The aristotypical material is MAPbI3, formed from the reaction of organic Methylammonium Iodide (MAI) and inorganic Lead Iodide (PbI2) salts, and so is often referred to as a hybrid organic-inorganic perovskite.
Their low temperature processing (up to 150°C), high power to weight ratio, and excellent optoelectronic properties have driven tremendous interest in developing this material as an alternative to Silicon Solar Cells. Crystalline Silicon relies on high energy and materially wasteful processes to create relatively thick wafers of absorbing material.
Perovskites are also tuneable: the band-gap can be varied by compositional engineering and specified for the conditions – from indoors to outdoors. As Dr Johansson explains, their tunability means these materials can be used as a top-layer in tandem solar cells—devices which are basically solar cell sandwiches, with each layer targeting different portions of the solar spectrum.
Dr Johansson depicts an idyllic life growing up the daughter of a fisherman in the Swedish archipelagos, learning to navigate the reefs and skerries.
She notes ironically that in 1981, a Soviet Submarine ran aground after failing to navigate these waters. The relevance of this incident has echoes in the future as now, 41 years later and less than 100km away, huge quantities of gas have leaked from the Nord Stream Methane Pipelines following speculated explosive sabotage. Estimates put the leaks as equivalent to ~40% of Sweden’s 2020 CO2 emissions. This has not helped with soaring gas prices, making more acute the importance of decarbonising and electrifying Europe’s gas-intensive energy system and moving to greener alternatives including solar energy.
Sweden has a heady mix of nuclear, hydroelectric, and biomass fuelling its economy, complimented by a rapidly growing contribution from wind. The transport sector however remains a resistant CO2 emitter, and this incentivised Dr Johansson to collaborate with truck company SCANIA, energy companies Dalakraft and Midsummer and trailer company MTeksjo to develop solar cell covered truck-trailers directly powering the vehicle’s powertrain.
Part of the challenge with electrifying transport is that as electric vehicles become more popular, the grid could become overloaded (without smart charging and discharging interventions, which often include shifting the time a vehicle is charged at to avoid too much power being needed at once).
Solar powered trucks could mitigate this problem by alleviating the charging requirements as well as acting as portable batteries capable of discharging to the grid to provide load stability. Efficiency of these vehicles will depend upon lightweight solar panels. And here Dr Johansson’s research becomes vital.
Dr Johansson has investigated methods for fabricating large area solar cells, using scalable techniques such as slot-die coating, blade coating and sputtering to deposit the device layers of MHPSCs. One highlight of her work has been investigating the crystallisation of perovskite in alcohols. Perovskite solutions are typically synthesised using organic solvents such as Dimethyl Sulfoxide (DMSO) and toxic Dimethylformamide (DMF).
While DMSO is relatively benign on its own, it easily dissolves Pb and can permeate the skin. Both solvents coordinate with Pb ions, encouraging formation of solvent-intercalated-intermediates. After solution casting perovskite forms via loss of the coordinated solvent. This leads to more performance reducing defects due to local inhomogeneities in stoichiometry, particularly at crystal surfaces.
Instead, Dr Johansson has looked at using alcohols to dissolve the perovskite precursors. Alcohols are relatively poor solvents of Pb. As a result, Pb ions are free to coordinate with I-, favouring the homogenous crystallisation of single crystals in solution. The perovskite suspension was dried and the powder re-dissolved in DMSO and DMF. PSCs were prepared from the solution, achieving an average PCE of 17%, and champion efficiency of ~20%—not far off the current record of ~22% for MAPbI3 PSCs.
This work sets the stage for more flexible device manufacturing routes and has ramifications that could for example address the surface quality of inhomogeneous slot-coated films.
And perhaps you’ll see trucks coated in solar cells coasting down the M1 in a few years.
Adam Urwick
Adam Urwick is a PhD Researcher in New & Sustainable Photovoltaics at The University of Sheffield and holds an MEng in Materials Science & Engineering from The University of Sheffield.
The CBE Seminar Series will see speakers from around the world offer their insight to Sheffield students. The next seminar will be Professor Lorna Dougan from the University of Leeds on 25 November 2022.
More information can be found here.