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University of Toronto engineers study first single crystal perovskites

Team determines diffusion length as well as mobility of electrons through the material

At team at the University of Toronto has shone new light on organolead trihalide perovskites. Using an antisolvent vapour-assisted crystallisation technique, the researchers grew large, pure perovskite crystals and studied how electrons move through the material as light is converted to electricity.

Led by Ted Sargent of The Edward S. Rogers Sr. Department of Electrical & Computer Engineering at the University of Toronto and Professor Osman Bakr of the King Abdullah University of Science and Technology (KAUST), the team used a combination of laser-based techniques to measure selected properties of the crystals, which had volumes exceeding 100 cubic millimetres.

By tracking down the rapid motion of electrons in the material, they have been able to determine the diffusion length (how far electrons can travel without getting trapped by imperfections in the material) as well as mobility (how fast the electrons can move through the material). Their work was published this week in the journal Science.

The report observing exceptionally low trap-state densities on the order of 109 to 1010 per cubic centimetre in MAPbX3 single crystals (comparable to the best photovoltaic-quality silicon) and charge carrier diffusion lengths exceeding 10µm. These results were validated with density functional theory calculations.

"Our work identifies the bar for the ultimate solar energy-harvesting potential of perovskites," says Riccardo Comin, a post-doctoral fellow with the Sargent Group. "With these materials it's been a race to try to get record efficiencies, and our results indicate that progress is slated to continue without slowing down."

In recent years, perovskite efficiency has reached just over 20 per cent, beginning to approach the present-day performance of commercial-grade silicon-based solar panels. "In their efficiency, perovskites are closely approaching conventional materials that have already been commercialised," says Valerio Adinolfi, a PhD candidate in the Sargent Group and co-first author on the paper. "They have the potential to offer further progress on reducing the cost of solar electricity in light of their convenient manufacturability from a liquid chemical precursor."

The study has implications for green energy, but may also enable innovations in lighting. A more efficient electricity-to-light conversion means perovskites could open new frontiers for energy-efficient LEDs.

Parallel work in the Sargent Group focuses on improving colloidal quantum dots. "Perovskites are great visible-light harvesters, and quantum dots are great for infrared," says Professor Sargent. "The materials are highly complementary in solar energy harvesting in view of the sun's broad visible and infrared power spectrum."

"In future, we will explore the opportunities for stacking together complementary absorbent materials," says Comin. "There are very promising prospects for combining perovskite work and quantum dot work for further boosting the efficiency."

'Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals' by Don Shi et al, appears in Science 30 January 2015: Vol. 347 no. 6221 DOI: 10.1126/science.aaa2725

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