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NREL Research into Perovskites Yields New Findings

March 28, 2016

Researchers at the National Renewable Energy Laboratory have been examining ways to increase the efficiency and stability of solar cells based on hybrid organic-inorganic perovskite absorbers. Two recently published papers show the results of photoemission spectroscopy and time-resolved optical spectroscopy, which quantify the energetics and carrier dynamics that occur at interfaces between the perovskite absorber and transport layers.

The first, Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films, was published in The Journal of Physical Chemistry Letters and was written by Philip Schulz, Anne-Marie Dowgiallo, Mengjin Yang, Kai Zhu, Jeffrey Blackburn, and Joseph Berry. The second, Efficient Charge Extraction and Slow Recombination in Organic-Inorganic Perovskites Capped with Semiconducting Single-Walled Carbon Nanotubes, appeared in the journal Energy & Environmental Science and was authored by Rachelle Ihly, Noah Stanton, Obadiah Reid, Andrew Ferguson, Anne-Marie Dowgiallo, Mengjin Yang, Philip Schulz, Kai Zhu, Joseph Berry, and Jeffrey Blackburn.

Solar cells that incorporate perovskite are relative newcomers to the photovoltaic scene. And although their efficiency has climbed rapidly to above 20 percent, much about their operations remains poorly understood. The two new research efforts used a perovskite thin film with a methylammonium lead iodide absorber layer and a semiconducting single-walled carbon nanotube (SWCNT) contact.

Previous research revealed that carbon nanotubes can extract holes from a perovskite solar cell. Photons absorbed in the active layer of a solar cell create electrons and holes, and separating and collecting these charge carriers creates electricity. The longer the carriers remain separated, the less likely they are to recombine and the more efficient the solar device can be.

For the research published in The Journal of Physical Chemistry Letters, scientists paired the perovskite film with SWCNT layers of varying thicknesses. They discovered a unique interfacial alignment process as electrons moved from the perovskite to the nanotube layer. The result of the research could potentially explain how to stabilize the perovskite surface, improve hole extraction, and reduce charge recombination, leading to a more resilient device.

The research in Energy & Environmental Science reported on what the scientists found when they used spectroscopy to track electrons and holes over time. They discovered that nanotubes were able to rapidly extract holes from the perovskite. The recombination of electrons and holes occurred in hundreds of microseconds, which represents a significantly long period of time for charge carriers to remain apart and be collected to generate electricity.

The interaction of SWCNT contacts with perovskites remains an active area of research, and NREL scientists expect to publish several more papers on this topic within the next year.

—Wayne Hicks