MIT develops ultra-thin solar cells

Solar energy is the world's most abundant source of energy and the development of efficient and stable solar cells can significantly alleviate the global energy crisis, and solar cell technology is seen as a key pillar of the clean energy transition. In the future, solar cells will play an increasingly important role in technological development and production life, not only for rooftops and solar farms, but also for powering automated aerospace machines such as aircraft and satellites.

Along with the development of manufacturing processes for semiconductor electronic components, the world has seen an extraordinary amount of research into solar cells, and a wide range of manufacturing technologies. Among these, ultra-thin cell solar cells hold unique promise in this field because they can be applied to a variety of irregular, curved or otherwise unsuitable surfaces, and can reduce material consumption and manufacturing requirements, directly reducing costs.

In a recent paper published in the latest issue of the journal Small Methods, engineers at the Massachusetts Institute of Technology (MIT) say they have developed an ultra-thin solar cell that can quickly and easily turn any surface into a power source. The solar cell, which is thinner than a human hair and adheres to a piece of fabric, weighs only a hundredth of a conventional solar panel but produces 18 times more electricity per kilogram and can be integrated into boat sails, disaster relief tents and tarps, the wings of drones and the surfaces of various buildings.

A typical rooftop solar installation in Massachusetts is about 8,000 watts," says Mayuran Saravanapavanantham, co-lead author of the article. To generate the same amount of electricity, our fabric PV only requires about 20 kg (44 lbs) to be added to the roof of a house."

The creation of ultra-thin solar cells

The MIT team behind the technology sought to build on its previous advances in materials science, and its completed an ultra-thin solar cell in 2016 that is heavy enough to sit on a soap bubble without breaking. Traditional techniques for manufacturing solar cells require vacuum chambers and expensive vapour deposition methods. This time, to scale up the technology, scientists have turned to printable nanomaterials based on e-ink to simplify the process.

Ultra-thin solar cells

In a nano-clean room, the researchers used an extrusion coater to deposit layers of nano-electronic material onto a 3 micron thick substrate, followed by screen printing to print out the electrodes and complete the solar module, followed by peeling the printed module, which is about 15 microns thick, off the plastic substrate to form an ultra-light solar device module. But this slim, freestanding solar module is difficult to handle and tears easily, making it difficult to deploy.

The researchers therefore peeled and glued the module to a fabric substrate that provided the mechanical strength needed to prevent tearing. The lightweight, flexible substrate, based on the composite material Dyneema, weighs just 13 grams per square metre and can adhere solar cells to it. By adding a layer of curing adhesive that is only a few microns thick, the solar modules can be bonded to Dyneema, resulting in an ultra-lightweight and robust solar structure.

Excellent performance and broad application prospects

This durable fabric-photovoltaic system is 50 microns thick and weighs less than 1 gram of module area (equivalent to an area density of 105 g/m2). Experimental tests have shown that the freestanding ultra-thin solar cells can produce 730 watts per kilogram, and if they are bonded to a high-strength "Power Horse" fabric, they can also achieve a specific power of 370 watts per kilogram, 18 times that of conventional solar cells. The integration of the ultra-thin modules into the composite fabric makes them mechanically flexible and these fabric-photovoltaic systems retain their performance after 500 roll-up cycles, with over 90% of their initial power generation capacity. In addition, this cell production method can be extended to produce flexible cells with larger areas.

Illustration: OPV module and separate Parylene device. A) Photograph of the completed OPV module on a PET substrate. B) Current-voltage characteristics of the control device (PET-IMI, PET-AgNW) and Parylene on the PET device before and after detachment from the PET carrier.

Ultra-thin solar cells have given impetus to the search for alternative power sources. Because these solar cells are so thin and light, they can be affixed to many different surfaces. For example, they can be integrated into boat sails to provide power at sea, adhered to tents and tarps deployed in disaster recovery operations, or applied to the wings of drones to extend their flight range. This lightweight solar technology can also be easily integrated into the built environment and may have a significant impact on the future design and construction of the building industry. In addition, these portable solar cells can be powered as wearable power structures on the go, or can be transported and rapidly deployed in remote areas to provide assistance in emergency situations.

Future challenges

The researchers say that while their solar cells are lighter and more flexible than conventional cells, they need to be encased in another material to protect them from the environment. And the carbon-based organic material used to make these cells can be altered by interacting with moisture and oxygen in the air, which could reduce the cells' performance.

Photo: Ultra-thin solar cells under test

According to Jeremiah Mwaura, a research scientist at MIT's Electronics Research Laboratory, encasing these solar cells in heavy glass, as is standard practice for traditional silicon solar cells, would minimise the value of current advances, so the team is currently developing ultra-thin packaging solutions to address the degradation of the cells from environmental impacts, which would only add a ultra-lightweight devices by a fraction of the weight.

Jeremiah Mwaura added: "We are trying to remove as much non-solar active material as possible, while still retaining the form and performance of these ultra-light and flexible solar structures. We know, for example, that the manufacturing process can be further simplified by printing releasable substrates, equivalent to the process we use to make the other layers in our devices. This will accelerate the translation of this technology to the market."

As the level of science and technology continues to develop, the discovery and use of a wide variety of new materials, technologies and energy sources will surely continue to drive the development of solar cell applications. Ultra-thin solar cells will also create greater value for society in the near future.