Research highlights: Perovskites
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RESEARCH HIGHLIGHTS :
Perovskites
By Prachi Patel Feature Editor: Pabitra K. Nayak
Research on perovskites has progressed rapidly, with solar-cell efficiencies now at 22%, five times higher than those of the first cells reported in 2009. MRS Bulletin presents the impact of a selection of recent advances in this burgeoning field.
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n an advance that could herald one of the first commercial applications for perovskites, South Korean researchers have printed thick perovskite layers that can be used as x-ray detectors. The detectors, reported in the journal Nature (doi:10.1038/nature24032), could produce sharper medical images at lower x-ray doses. They would also be cheaper to make than today’s flat x-ray detectors. Flat detectors use one of two methods to convert x-ray energy to electrons. The indirect conversion method uses luminescent cesium iodide on a silicon photodetector array. The direct method, based on amorphous selenium photoconductors, creates sharper images and is used for low-energy mammography. But amorphous selenium cannot absorb higher-energy x-rays that are needed for general imaging. Organic–inorganic perovskites are ideal photoconductors because they are 10 times more sensitive to x-rays than amorphous selenium. The
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igh-efficiency perovskite solar cells have, to date, been made mostly with either spiro-OMeTAD or poly(triarylamine) as the holeextracting material. But both are prohibitively expensive for large-scale use. Researchers have now made stable perovskite solar cells with efficiencies of more than 20% by using low-cost copper thiocyanate (CuSCN) as the hole-transporting layer.
X-ray photons V+ ITO PI–MAPbBr3
MPC
Illustration of an allsolution-processed digital x-ray detector. ITO, indium tin oxide; PI, polyimide; MPC, MAPbI3 photoconductor; TFT, thinfilm transistor. Credit: Nature.
PI–MAPbBl3 TFT backplane ITO
challenge has been to make perovskite layers that are thick and large enough: photodetectors are hundreds of micrometers thick and 50 cm × 50 cm in size. The team from the Samsung Advanced Institute of Technology and Sungkyunkwan University made
a dense, viscous solution of CH3NH3I and PbI 2 in γ-butyrolactone and α-terpineol, and used a doctor blade to print an 830-µm-thick CH3NH3PbI3 perovskite layer. Then they made an x-ray detector array using a 10 cm × 10 cm sample of the perovskite.
CuSCN is an attractive hole-transport material since it is a cheap p-type semiconductor that efficiently transports holes. But most solvents it dissolves in can damage the underlying perovskite layer. Researchers from École Polytechnique Fédérale de Lausanne deposited CuSCN dissolved in diethyl sulfide solvent while spinning the substrate at 5000 rpm to quickly evaporate the solvent. They
also added a conductive reduced graphene oxide layer between the CuSCN and gold contact, which led to excellent operational stability under full-sun illumination at 60°C. The resulting solar cells retained more than 95% of their efficiency after 1000 hours, surpassing the stability of spiro-OMeTAD d
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