Stanford researchers aim to boost performance, shrink cost of photovoltaic cells.
Professor Krishna Saraswat hadn’t planned to go into solar energy research. It was his students’ idea.
He had spent his career focusing on nanoelectronics and photonics, mentoring 70 doctoral students along the way. Then several of his alumni founded a solar cell startup called Solexel, and they encouraged him to get involved.
Soon, Saraswat, who is the Rickey/Nielsen Professor in the School of Engineering, was co-teaching an undergraduate course on photovoltaics that occasionally hosted guest lecturers. The message he heard from industry speakers again and again: Photovoltaic cells are expensive to manufacture, and they are inefficient to operate. In entrepreneurial jargon—the solar market is ripe for a disruptive technology.
One of his current advisees, Raisul Islam, PhD ’17, was interested in exploring new approaches for designing photovoltaic cells that could slash costs. Saraswat saw great promise in his idea.
In the traditional fabrication of solar cells, manufacturers create “p-n junctions,” a basic building block for electrical devices, by incorporating semiconducting materials of different conductivity (p and n type). When sunlight impinges on the p-n junction, it starts the flow of an electrical current.
Islam’s approach would eliminate the need for these junctions, using instead an ultrathin strip of a metal oxide as the semiconductor contact. Producing the solar cells this way is efficient and requires less heat in the manufacturing process, keeping costs lower.
Saraswat decided to apply for a research seed grant from the TomKat Center for Sustainable Energy to move the concept forward.
“I didn’t have a track record of working on solar cells,” says Saraswat. “If I go out and write a proposal to the Department of Energy, they will say, ‘This guy, for 40 years he has been teaching electronics and photonics. Why should we give him money?’”
In contrast, the TomKat Center saw the crossover in his technical expertise. He and Islam were able to make the pitch that “we’re not starting from zero. We know what photons do in semiconductors.” With the center’s ability to fund high-risk, high-reward research in the early stages, their proposal was selected during the 2013 seed grant competition.
Now two years later, Islam and Saraswat have proven their basic premise, and they are fine-tuning how the materials combine to fully validate the concept. They hope to move from the discovery phase to commercialization by the time Islam completes his doctorate, and they have begun collaborating with Solexel for practical guidance.
Not only could their technique bring down the cost of a single junction solar cell, it could also improve the efficiency of solar energy. Today, most commercial solar cells only convert about 15 to 25 percent of the energy from sunlight into electricity.
There are many reasons for this inefficiency. For starters, each semiconductor material can only absorb a certain wavelength of light. The wavelengths that are longer go unused; those that are shorter waste some of the energy.
Stacking solar cells made of complementary semiconducting materials is one solution—and part of Saraswat and Islam’s approach. If each layer of the solar cell captures a different spectrum of the sunlight and is effectively transparent to the others, it allows the solar cell to cumulatively capture more sunlight than using a single layer.
But normally stacking solar cells is tedious work. Like a seamstress hand-stitching intricate embroidery, manufacturers have to carefully grow and align different semiconductor crystals in each layer with adjoining layers of the cell for electricity to move across it while losing as little energy as possible.
Saraswat and Islam believe their technique with “junctionless” solar cells will bypass this problem altogether. With metal oxide connectors between layers, their solar cells don’t have to be painstakingly aligned. In addition, their method allows for the use of three independently operating terminals within the solar cell, instead of the usual two, which boosts performance.
Their design with transparent parts in the cell will let it absorb light from the top and bottom, enabling some unique applications near swimming pools, atriums, walkways through buildings, and other places where light is reflected. They foresee solar cells with an efficiency of greater than 50 percent.
“We are looking into the future,” says Islam.