Spherical Solar Cells Soak Up Scattered Sunlight

Silicon solar cells folded into spheres hint at solar power's flexibility in even small devices

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Large-scale spherical solar cell based on monocrystalline silicon developed using a corrugated architecture.
Large-scale spherical solar cell based on monocrystalline silicon developed using a corrugated architecture.
Photo: Nazek El-Atab/KAUST

Flat solar panels still face big limitations when it comes to making the most of the available sunlight each day. A new spherical solar cell design aims to boost solar power harvesting potential from nearly every angle without requiring expensive moving parts to keep tracking the sun’s apparent movement across the sky. 

The spherical solar cell prototype designed by Saudi researchers is a tiny blue sphere that a person can easily hold in one hand like a ping pong ball. Indoor experiments with a solar simulator lamp have already shown that it can achieve between 15 percent and 100 percent more power output compared with a flat solar cell with the same ground area, depending on the background materials reflecting sunlight into the spherical solar cell The research group hopes its nature-inspired design can fare similarly well in future field tests in many different locations around the world.

“The placement and shape of the housefly’s eyes increase their angular field of view so they can see roughly 270 degrees around them in the horizontal field,” says Nazek El-Atab, a postdoctoral researcher in microsystems engineering at the King Abdullah University of Science and Technology (KAUST). “Similarly, the spherical architecture increases the ‘angular field of view’ of the solar cell, which means it can harvest sunlight from more directions.” 

To create the spherical solar cell design, El-Atab and her colleagues built upon their previous work, which demonstrated how to create thinner and more flexible solar cell designs based on a corrugated groove technique. The new work is detailed in a paper that has been submitted for review to the journal MRS Communications

Measurement setup of the spherical solar cell under a solar simulator in air and using a regular a white paper as the reflective background material.Measurement setup of the spherical solar cell under a solar simulator in air and using a regular a white paper as the reflective background material.Photo: Nazek El-Atab/KAUST

Testing with the solar simulator lamp showed that the spherical solar cell provided 24 percent more power output over a traditional flat solar cell upon immediate exposure to sunlight. That power advantage jumped to 39 percent after both types of solar cells had begun to heat up and suffered some loss in power efficiency—an indication that the spherical shape may have some advantages in dissipating heat.

The spherical solar cell also delivered about 60 percent more power output than its flat counterpart when both could collect only ​scattered sunlight under a simulated roof rather than receiving direct sunlight. Additional experiments with different reflective backgrounds—including an aluminum cup, aluminum paper, white paper, and sand—showed that the hexagonal aluminum cup background helped the spherical solar cell outperform the flat solar cell by 100 percent in terms of power output.

The Saudi team created the spherical solar cell using the monocrystalline silicon solar cells that currently account for almost 90 percent of the world’s solar power production. That choice sprang from the goal of helping to maximize the light-harvesting potential of such solar cells, along with the aim of potentially making it easier to scale up production if the design proves cost efficient.

“What surprises me is the authors have demonstrated the ultra-flexibility that can be achieved with rigid silicon solar cells using the corrugation technique in a series of articles,” says Zhe Liu, a postdoctoral researcher in solar engineering at MIT, who was not involved in the study. “I’m more excited about the ability to make spherical cells, which means you can have industrial IBC-type (interdigitated back contact) silicon solar cells cover any shapes and ‘solarize’ everywhere.”

Previous solar cell designs have fabricated tiny microscale spherical cells—sometimes made with nanowires or quantum dot cells—on top of a flat surface to help better collect both direct and scattered sunlight, says Rabab Bahabry, an assistant professor of physics at the University of Jeddah in Saudi Arabia. But the larger spherical solar cell may offer improved efficiency and coverage compared with the microsphere arrays when it comes to collecting sunlight reflected from background surfaces.

Creating the large spherical solar cell required the researchers to etch alternating grooves in 15 percent of a flat solar cell to make a pattern resembling a band of elliptical shapes connected at the middle. A CO2 laser created the appropriate pattern in a polymeric hard mask covering the solar cell and allowed a deep reactive ion etching tool to create grooves in the exposed areas of the silicon solar cell. The flex and bend in those groove areas allowed the researchers to subsequently fold the solar cell into a spherical shape. 

Dust accumulation on a spherical solar cell is limited to the silicon area with a small tilt angle.Dust accumulation on a spherical solar cell is limited to the silicon area with a small tilt angle.Image: Rabab Bahabry/University of Jeddah and KAUST

The loss of solar cell material in the areas that have been etched out reduces the overall potential solar power output. But the researchers see cost over time favoring spherical solar cells over flat solar cells in certain parts of the world because the spherical design is less prone to dust accumulation and may help dissipate heat that might otherwise reduce the solar cell’s efficiency. In addition, the spherical solar cells don’t require additional costly moving parts to continually track the sun.

Still, the spherical solar cells may not replace traditional solar cell technology at utility-scale solar power plants, says Liu at MIT. In his view, this particular spherical solar cell design could find use in more niche market applications. He noted that one of his colleagues is currently searching for a solar cell design to cover a golf ball so that it can power a tracker inside the ball. But Liu sees much promise in such ultra-flexible solar cell designs being installed in buildings, cars, or even mobile devices. 

“The application of spherical design may seem very limited, but the ability to make commercial silicon solar cells into any shapes would enable broad adaption of photovoltaic in autonomous devices, such as IoT (Internet of Things) sensors, and autonomous vehicles,” Liu says. “If we can fully power these autonomous devices with shaped photovoltaic panels, this could be a game changer.”

For future testing, Liu says he would like to see how the spherical solar cell performs in a wide array of both outdoor and indoor lighting environments at different times of day. He also wants to see how well the spherical solar cells can be integrated into certain applications that they might power. And he is curious about seeing a “quantified cost” summary of all the processing steps required to make such spherical solar cells in order to better understand the technology’s commercialization potential. 

The Saudi researchers had to manually fold and form their spherical solar cells in their latest demonstration, but they have already begun designing and developing ways to automate the process using “robotic hands” to mimic the manual folding, says Muhammad Mustafa Hussain, a professor of electrical and computer engineering at KAUST who was one of the study’s coauthors. 

Eventually, Hussain and his colleagues envision building and testing large arrays of the spherical solar cells. And they’re already working on new shapes that resemble tents or umbrellas to see if those offer any advantages. They are also integrating solar cells with the surfaces of drones that have unusual shapes.

The COVID-19 pandemic that forced the closure of research labs has delayed the Saudi group’s initial plans for outdoor testing. But Hussain says the group still plans to move forward with field trials before the end of 2020. He expects help from the KAUST alumni network in eventually testing the spherical solar cells in California, along with countries such as Bangladesh, China, India, South Korea, Germany, Spain, Brazil, Colombia, Mexico, South Africa, Australia, and New Zealand.

“We will be creating arrays of spherical cells for 100-square-foot to 1,000-square-foot areas, and will compare functionality over cost benefit with that of traditional cells,” Hussain says. “Next, we will deploy it in different geographic locations throughout the year to understand its performance and reliability.”

Editor’s note: A correction to this article was made on 16 June 2020. The sentence on indoor experiments was revised to correct an inaccurate interpretation of the power output comparison between the spherical solar cell and flat solar cell in the submitted paper.

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