A researcher tests the function of the solar cells inside the glove box. [Credit: City University of Hong Kong]
Perovskite photovoltaics have come a long way in the last 10 years, with efficiencies now surpassing 25%, and is widely known as a rising star in the solar industry. A cheap starting material and low-cost manufacturing techniques have led to predictions that the technology may be the future of commercial solar panels.
But before dethroning crystalline silicon, the material that currently dominates worldwide markets, perovskite has a number of major hurdles it must overcome. In a new study, scientists from Hong Kong and the U.S. address two of these hurdles—operational stability and leakage of lead into the environment—by applying a 2D metal–organic framework to perovskite solar cells (Nat. Nanotechnol., doi: 10.1038/s41565-020-0765-7).
Challenges to commercialization
Perovskite solar cells achieved a power conversion efficiency of 25.2% in late 2019, which approaches the top values seen in single-crystalline devices. But the area where perovskite photovoltaics severely lag behind their commercial counterparts is long-term stability. Environmental stresses like humidity, oxygen and even sunlight cause degradation of the perovskite layer, reducing the device’s lifetime to mere months instead of years.
Another concern revolves around the environmental and health hazards of lead-containing perovskites. Lead-based compounds in perovskites have a high solubility in rainwater, which makes leakage a serious issue and also harms the technology’s commercial prospects.
As an approach to addressing these problems, some research groups have investigated metal–organic frameworks (MOFs). As the name implies, this class of materials consists of rigid metal ions connected by flexible organic linkers, forming a cagelike structure. A number of research groups have looked at the potential for these materials in niche photonic applications (see “2D MOFs: A New Platform for Optics,” OPN, October 2020).
Recently, researchers have explored the use of unfunctionalized 3D MOFs in perovskite photovoltaics to enhance their performance and stability. But success has been hindered by the fact that most of these materials are insulating with low carrier mobilities and far from ideal as charge-transporting layers.
Solving the stability issue
Alex Jen Kwan-yue at the City University of Hong Kong and his colleagues decided to go in a different direction by fabricating a 2D conjugated MOF functionalized with numerous thiol groups. Tests revealed the MOF’s higher electron mobility aligns with the properties of an n-type semiconductor. They created a perovskite solar cell with the material as an electron-extraction layer sandwiched between perovskite and cathode layers.
The resulting device had a peak efficiency of 22.02%, which is among the highest values reported for this type of inverted planar solar cell. It was able to maintain 90% of its initial efficiency after 1,100 hours in a humid air and nitrogen atmosphere. In comparison, the efficiency of a solar cell without the incorporation of a 2D MOF dropped to less than 50% of its original value in the same timeframe.
"This is a very significant result which proved our MOF method is technically feasible and has the potential in commercializing the [perovskite solar cell] technology," said Jen in a press release accompanying the research.
Trapping lead
Jen and his colleagues also found that MOF used as the outer layer of the solar cell captured over 80% of the leaked lead ions from the degraded perovskite, turning them into water-insoluble solids. While other methods of dealing with lead leakage involve physical encapsulation of the device itself, the researchers argue that this type of chemical adsorption of lead is more effective and sustainable.
"We are the first team to fabricate [perovskite solar cell] devices with minimized lead leakage, good long-term stability, and high power conversion efficiency simultaneously," said Jen.