Crystals generated for perovskite solar cells
Researchers from King Abdullah University of Science and Technology (KAUST) claim to have synthetically generated homogeneous and defect-free crystals, which could be used to fast-track the commercialisation of perovskite solar cells. Their work has been published in the journal ACS Energy Letters.
“Perovskite solar cells are the fastest developing type of photovoltaic technology, with power-conversion efficiencies rising from 3.8% in 2009 to 24.2% in 2019 for single-junction devices,” said Osman Bakr, co-leader on the study. This rapid increase in performance is associated with inexpensive and simple device fabrication, which makes these solar cells commercially appealing.
The performance and stability of solar cells depend on the morphology of the perovskite thin films, which act as light-harvesting layers in the devices. As well as their low cost and easy processing, these materials are said to have impressive optical and transport properties.
Hybrid lead-based perovskites that combine a methylammonium cation with several halides, such as the anionic forms of bromine and iodine, present a narrow and tuneable optical bandgap. This bandgap nears the theoretical value required to reach the maximum conversion efficiency for a single-junction solar cell. Therefore, perovskites could become a substitute of choice for silicon-based solar materials; however, existing perovskite solar cells usually consist of polycrystalline thin films that are highly disordered and defective, which prevents devices from achieving optimal performance.
To address this issue, Bakr and colleague Omar Mohammad produced high-aspect-ratio, single-crystal films of methylammonium lead-triiodide perovskites. They achieved this by starting the crystallisation between two polymer-coated substrates that would then physically restrict crystal growth to one dimension under heating.
Compared with their polycrystalline counterparts, single-crystal perovskites display substantially lower defect density and much higher charge-carrier diffusion lengths: this is a measure of their ability to maintain light-generated electrons separate from positively charged holes and create electrical current. According to Mohammed, “We reasoned that these single crystals offer a chance for perovskite solar-cell technology to overcome these limitations and get as close as possible to the theoretical efficiency limit.”
The crystals, which exhibited a thickness of 20 µm and an area of several square millimetres, provided high-quality solar cells with a maximum power-conversion efficiency of 21.09%. These devices set a new performance record for perovskite single-crystal solar cells.
“We were pleasantly surprised by these results,” said Bakr, who had initially thought he would need to grow crystals much thinner than 20 µm to achieve this performance — a feat which would have been extremely challenging.
The researchers believe that this record efficiency highlights the potential role of single crystals in the development of perovskite-containing devices in parallel with the path taken by their polycrystalline counterparts.
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