Ink printed solar cells may replace silicon

Paper-thin and inexpensive solar cells printed from ink may one day replace the cumbersome traditional solar cells made from silicon thanks to a material created at Imperial College London called ferrocene.

"Silicon cells are efficient but expensive, and we urgently need new solar energy devices to accelerate the transition to renewable energy," said Nicholas Long, a professor from the department of chemistry at Imperial College London and a co-lead author of a scientific paper on the discovery.

Although silicon has good efficiency and stability, it is relatively expensive to make and can only be manufactured into stiff panels.

The novelty of perovskite solar cells, however, offers an incredible alternative-they can be printed from inks.

The printing of a low-cost, high-efficiency, thin, lightweight, and flexible renewable energy panel could be just the technology required to push solar panels toward a higher plane of power generation. However, they have trailed behind silicon solar cells in efficiency and, more importantly, breaking down under normal environmental conditions.

That's where Imperial College London's new material comes into play. New metal-containing materials called ferrocenes could help with all these problems.

Researchers from City University of Hong Kong have added Imperial-made ferrocenes into perovskite solar cells, vastly improving their efficiency and stability. The results of which were published in the academic journal "Science."

Ferrocene is an organometallic compound – meaning any member of a class of substances containing at least one metal-to-carbon bond in which the carbon is then a part of an organic group. Considered remarkable for its stability, it is unaffected by air, water, strong bases and can be heated to 400 degrees Celsius (752 degrees Fahrenheit) without decomposing.

What makes it so incredible is its sandwich-like structure that ultimately required entirely new science to explain. Ultimately, ferrocenes are compounds with iron at their center, surrounded by sandwiching rings of carbon.

While the original discovery of ferrocene is debated, the unique structure of ferrocene was first recognized by Imperial's own Nobel Prize-winner, Professor Geoffrey Wilkinson, in 1952. Today, ferrocenes are still being researched around the world, and this recent discovery may finally bring to light its incredible functions.

"Our collaboration with colleagues in Hong Kong was beautifully serendipitous, arising after I gave a talk about new ferrocene compounds and met Dr. Zonglong Zhu from CityU, who asked me to send over some samples," continued Long. "Within a few months, the CityU team told us the results were exciting, and asked us to send more samples, beginning a research program that has resulted in perovskite devices that are both more efficient and more stable."

Perovskite forms the "light-harvesting" layer of solar cell devices. However, these devices have been less efficient at converting solar energy into electricity than silicon-based solar cells, primarily because the electrons are less mobile – less able to move from the harvesting layer to the electricity conversion layers.

Ferrocene does not have such a problem; its innate structure gives it excellent conductivity. More specifically, it allows electrons to move more easily from the perovskite layer to subsequent layers, improving the efficiency of conversion of solar energy to electricity.

Tests performed by the team at CityU and in commercial labs have shown that the efficiency of perovskite devices with an added ferrocene layer can reach up to 25%, approaching the efficiency of traditional silicon cells.

Yet this was not the only problem the ferrocene-based material solved. The team at Imperial has been experimenting with attaching different chemical groups to the carbon rings of ferrocene, and after sending the Hong Kong team several versions of these, the collaborators discovered a version that significantly improves the attachment of the perovskite layers to the rest of the device.

This attachment improved the stability of the devices, meaning they maintained more than 98% of their initial efficiency after continuously operating at maximum power for 1,500 hours.

The efficiency and stability gained due to the addition of a ferrocene layer bring these perovskite devices closer to current international standards for traditional silicon cells.

"We are the first team to successfully boost the inverted perovskite solar cell to a record-high efficiency of 25% and pass the stability test set by the International Electrotechnical Commission," said Zonglong Zhu from City University of Hong Kong.

So far, the team has patented their design and hopes to license it, eventually bringing their perovskite devices to market. In the meantime, they are continuing to experiment with different ferrocene designs to further improve the performance and stability of the devices.