Wednesday, 22 May 2019

NREL scientists demonstrate remarkable stability in perovskite solar cells

NREL's unencapsulated solar cell retained 94 percent of its starting efficiency after 1,000 hours of continuous use under ambient conditions

8 Feb 2018 | Editor

Researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have recently created an environmentally stable, high-efficiency perovskite solar cell, bringing the emerging technology a step closer to commercial deployment. The research was published in Nature Energy.

perovskites solar cell technologies have rapidly evolved into a promising technology, with the ability to convert about 23 percent of sunlight into electricity, but work is still needed to make the devices durable enough for long-term use.

NREL's unencapsulated solar cell--a cell used for testing that doesn't have a protective barrier like glass between the cell's conductive parts and the elements--retained 94 percent of its starting efficiency after 1,000 hours of continuous use under ambient conditions.

While more testing is needed to prove the cells could survive for 20 years, or more, in the field (the typical lifetime of solar panels) this study represents an important benchmark for determining that perovskite solar cells are more stable than previously thought.

The typical design of a perovskite solar cell sandwiches the perovskite between a hole transport material, a thin film of an organic molecule called spiro-OMeTAD that's doped with lithium ions and an electron transport layer made of titanium dioxide, or TiO2. This type of solar cell experiences an almost immediate 20 percent drop in efficiency and then steadily declines as it became more unstable.

The researchers theorised that replacing the layer of spiro-OMeTAD could stop the initial drop in efficiency in the cell. The lithium ions within the spiro-OMeTAD film move uncontrollably throughout the device and absorb water. The free movement of the ions and the presence of water causes the cells to degrade.

A new molecule, nicknamed EH44 and developed by Alan Sellinger at the Colorado School of Mines, was incorporated as a replacement to spiro-OMeTAD because it repels water and doesn't contain lithium.

The use of EH44 as the top layer resolved the later more gradual degradation but did not solve the initial fast decreases that were seen in the cell's efficiency. The researchers tried another approach, this time swapping the cell's bottom layer of TiO2 for one with tin oxide (SnO2).

With both EH44 and SnO2 in place, as well as stable replacements to the perovskite material and metal electrodes, the solar cell efficiency remained steady. The experiment found that the new SnO2 layer resolved the chemical makeup issues seen in the perovskite layer when deposited onto the original TiO2 film.

Funding for the research came from the U.S. Department of Energy Solar Energy Technologies Office.

NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

"During testing, we intentionally stress the cells somewhat harder than real-world applications in an effort to speed up the aging."

"A solar cell in the field only operates when the sun is out, typically. In this case, even after 1,000 straight hours of testing the cell was able to generate power the whole time."

"This study reveals how to make the devices far more stable."

"It shows us that each of the layers in the cell can play an important role in degradation, not just the active perovskite layer."

"What we are trying to do is eliminate the weakest links in the solar cell"

"Those two benefits led us to believe this material would be a better replacement."
Joseph Luther and Joseph Berry, Directed the work

Tailored interfaces of unencapsulated perovskite solar cells for >1,000 hour operational stability

Received: 18 May 2017 | Accepted: 28 November 2017 | Published online: 09 January 2018 | doi:10.1038/s41560-017-0067-y


Long-term device stability is the most pressing issue that impedes perovskite solar cell commercialization, given the achieved 22.7% efficiency. The perovskite absorber material itself has been heavily scrutinized for being prone to degradation by water, oxygen and ultraviolet light. To date, most reports characterize device stability in the absence of these extrinsic factors. Here we show that, even under the combined stresses of light (including ultraviolet light), oxygen and moisture, perovskite solar cells can retain 94% of peak efficiency despite 1,000 hours of continuous unencapsulated operation in ambient air conditions (relative humidity of 10–20%). Each interface and contact layer throughout the device stack plays an important role in the overall stability which, when appropriately modified, yields devices in which both the initial rapid decay (often termed burn-in) and the gradual slower decay are suppressed. This extensively modified device architecture and the understanding developed will lead towards durable long-term device performance.

www.nrel.gov    www.mines.edu   

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