Logo: University of Southern California

III-V Solar For The Masses

Rehan Kapadia has the silver bullet that could make solar energy more efficient and very affordable
By: Daniel Druhora
March 30, 2016 —


Solar panels in Shanghai, China. In 2016, China became the world's largest producer of photovoltaic power, at 43 GW installed capacity. Photo/ Yongnian Gui | Dreamstime

If Rehan Kapadia succeeds at his job, you could solar power your home for the price of a refrigerator.

How will the USC Viterbi assistant professor of electrical engineering achieve this feat in the seemingly intransigent economics that drive Moore’s law: ever smaller, ever faster, ever cheaper to end of the roadmap?

Kapadia wants to shake things up: “It starts from a fundamental question – how do we make materials more scalable and low cost but also state-of-the-art?”

Bring costs down to earth

Remember that tiny strip on your pocket calculator? For some of us, that was the most likely place to glimpse solar in action in the 80s. Today, solar panels dot the rooftops in suburbia. Fly over Germany, for instance – currently the world’s number two in use of photovoltaic (PV) panels – and you’ll see shining burgs, great rivers of energy flowing with electrons.

Solar powered calculators were introduced in the late 1970s using amorphous silicon. Photo/Wikimedia

Much of that rise is due to the falling costs of hardware, but market barriers, grid integration challenges and non-hardware “soft costs,” such as permitting, financing and customer acquisition continue to keep many households out of the sun’s reach. These fees make up roughly 64 percent of the cost of a residential system, reflecting the resistance of power companies with centuries-old technology and threatened profits. In the U.S., the Department of Energy (DOE) reported that from 2006 to 2013, the number of homes with solar grew by an astounding 1,000 percent. The DOE estimates that some 3.8 million homes will be solar powered by 2020.

Costs, however, are still all over the map. As of 2016, the installed cost of solar panels was between $7 to $9 per watt. A 5 kW system can cost an American family $25,000 to $35,000. Even with incentives and subsidies to cut the bottom line in half, a system that generates an average $73 of electricity per month could take a long time to pay for itself. Despite this, a growing legion of solar companies continues to expand.

Cheap and state-of-the-art?

The White House is determined to sustain this momentum. In his “New Strategy for American Innovation” outlined before Congress in The State of the Union Address, President Obama pledged: “the United States will double the pace at which we cut carbon pollution.” As intriguing as this sounds, Obama emphasized that technological advances and innovative solutions are still needed to increase efficiency, drive down costs and enable utilities to rely on solar for baseload power.

The White House goes solar!
Go inside to see the installation of solar panels on the roof of the residence.

To compel utilities to do this, solar has to become state-of-the-art efficient. This is where Kapadia comes in.

When Kapadia says state-of-the-art, he means the highest speed transistors, world-record solar cells and lasers made from expensive processes.

He means electronic–photonic devices – an ultra-class of materials known as III-V.

If you want them, you have to pay the premium and master extremely sophisticated manufacturing techniques. To make them, you have to grow them. 

III-V semiconductors are grown from materials drawn from groups III, such as indium and gallium and V, like phosphorus, of the periodic table (i.e. indium and gallium). These materials not only interact with light orders of magnitude more strongly than silicon – either they use less power or they allow for drastically higher clock speeds.

Historically, III-V material systems have been grown by the vapor–solid method, such as metal-organic chemical vapor deposition (MOCVD), a technique that dominated the III-V growth space for decades, due to pioneers in the III-V growth field like Daniel Dapkus, also at USC.

Kapadia co-created a new process called thin-film vapor–liquid–solid (TF-VLS) growth. He did this in collaboration with Zhibin Yu (Florida State University) and Ali Javey at UC Berkeley, working in Javey’s lab. The technique has been shown to yield high optoelectronic quality III-V thin films on nonepitaxial substrates, such as readily available metals and glass, providing an attractive route for producing large scale III-V solar panels at high efficiency and low cost.

Thin film vapor–liquid–solid (TF-VLS) growth Photo/R. Kapadia

With the III-V TF-VLS process, this cost could potentially be reduced to about 30 cents per watt.

“It’s not just solar power though, there are also many other applications we’d like to use III-V semiconductors for, but the costs pose a challenge,” said Kapadia.

The dawn of autonomous vehicles and Internet of Things, for example, represents a growth opportunity for chip companies. They’re increasingly placing a premium on massive deployment of low power processors, communications, and sensors all manufactured together from an assortment of materials, not just ubiquitous silicon.

Moore’s Law predicts these chips will be smaller, faster and cheaper. But at an atomic scale, electricity leakage and power management will bedevil chipmakers.

“Over the years, we’ve been able to shrink transistors down to 14-nanometer node commercially,” Kapadia said. “We’ll continue to 10, and possibly even down to the 5-nanometer node until we reach the physical limits of miniaturization. But this will come with technical and economical challenges.”

The silver bullet: semiconductors on arbitrary substrates

Growing a III-V semiconductor onto silicon requires tenacious experimentation. “The two materials have different lattice parameters and different coefficients of thermal expansion. So when you grow the first layers of the III-V’s on a thick Silicon wafer, they are forced to take on the lattice parameters of Silicon. This leads to strain during growth, and once the growing film becomes thick enough, defects,” Kapadia said.

At microscopic levels, this sounds like an esoteric distinction, but it matters. “In a crystal, every atom, like everything in nature, wants to be in a specific place. When you move it away, it rebels,” said Kapadia.

Instead of growing these vertically, Kapadia starts with small seeds, which he grows out laterally in carefully preselected locations. This enables high-quality semiconductor crystals to be grown, even if the substrate is amorphous (has no crystal structure) something not presently possible with the state-of-the-art growth processes.

By removing this materials constraint, this growth mode enables direct writing of single-crystalline III–V’s on glass substrates, expanding their utility for multiple applications.

III-V multi-junction concentrator solar cells on 4-inch diameter wafer. Photo/Fraunhofer ISE

Kapadia himself prefers not be “lattice-matched” in his research. The 29-year-old engineer in the Ming Hsieh Department of Electrical Engineering recently received the Air Force Office of Scientific Research Young Investigator Award to develop an innovative way to generate beams of electrons by carefully controlling how photons interact with materials. If successful, it could have far reaching impact in a diverse set of fields, from medical science, to space exploration and even defense applications.

Kapadia hopes that his research efforts will enable new classes of low-cost, high performance devices ranging from energy generation, to sensing, to information processing.

That means that soon, we could be reaching into our pockets for hyper-efficient, durable, energy-efficient smartphones. Maybe, just maybe, they could even be solar powered.


Growth on metal foils using Thin-Film-Vapor-Liquid-Solid

Kapadia's Lab for photons, electrons, and materials

U.S. Department of Energy: Solar 

Inside the White House Solar Panels