A Bright Future in Photovoltaics

Carissa Eisler’s first science experiment could have been her last. When she was 7, Eisler and a friend decided to “play chemist” by mixing various noxious liquids they found in the garage. They survived without harm, but when Eisler’s parents found out, she feared her days as a scientist were over. Instead, she says, after their initial horror subsided “they had the best reaction possible: they encouraged my curiosity. From that point on, a lot of my birthday presents were science gifts—which were always used in well-ventilated areas.”  In the 19 years since then, Eisler has pursued her passion for science, earning a bachelor’s degree in chemical engineering from UCLA and working to earn a doctorate in the same subject from Caltech. One reason she chose to come to Caltech was her desire to work with Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science.

After meeting him at a visiting weekend for graduate students, Eisler says she was sold on trying to get into his group. “Not only does Harry have a great intuition for picking really interesting and impactful projects, but he also is a really warm person that people love working for,” she says. Entering the Institute as a graduate student with impressive recommendation letters, Eisler quickly earned a spot in Atwater’s lab, working on photovoltaics—the conversion of solar energy into electrical energy using semiconducting materials. She is part of a group tasked with developing an ultrahigh-efficiency solar-cell module that involves designing solar collectors, optical components, and electronics.

The current world record for efficiency belongs to a French industrial firm’s solar-cell module capable of transforming 38.9 percent the sunlight it receives into useable  electricity; Eisler and her teammates aim to build a module that pushes that number beyond 50 percent. To achieve that ambitious goal, they have chosen to totally rethink how solar cells are designed.

At the heart of all solar cells are semiconductors—materials typically made of silicon, but also arsenic, boron, or other elements that conduct electricity under certain conditions but not others. Crucially, semiconductors are subject to the photoelectric effect, in which photons—particles of light—striking a semiconducting material knock electrons free and generate an electric current, effectively transforming light energy into electrical energy. For technical reasons, panels made of simple solar cells can only efficiently convert light from a narrow range of wavelengths, such as visible light, while failing to capture others such as infrared or ultraviolet.

So to collect more energy, the panels must be made bigger. A better way to harvest solar energy is to make sure to convert the widest possible range of wavelengths, Eisler notes. “The only way you’re going to get to a very high efficiency is if you use different materials that are optimally tuned to absorb and convert different portions of the solar spectrum,” she says. “You need to have one material that’s really good at converting the blue light and another one to convert the red and so forth.

That way you can absorb the entire solar spectrum and get the highest voltage possible for every photon you convert.” Current designs based on this approach use stacked layers of materials—each tuned to different frequencies—so that the first layer might absorb ultraviolet and visible light, the second near-infrared light, and the third infrared light, increasing the overall efficiency of the cell. But this approach requires an extraordinary degree of precision in aligning the materials at an atomic level: the layers must stack like Legos atop one another or the gaps between them may propagate as a crack throughout the material and hinder the flow of electrical current. Also, because the materials are wired in a series—like holiday lights in which a single blown bulb will darken all the downstream bulbs—the cell is often limited to the efficiency of its least efficient layer.

Fine-tuning the layers to achieve the best solution is additionally complicated by the fact that the solar spectrum changes dramatically throughout the day and throughout the year, which means that what is “most efficient” is constantly changing. To tackle those problems, Eisler’s team is developing a solar cell that spreads out the different materials side by side, like lanes on a freeway.

An optical element functioning like a prism splits incoming light spatially so that each color falls only on the lane best suited to absorb it. As a result, the electrical output is always maximized no matter how the solar spectrum changes over time, making the module capable of delivering far more power than other alternatives. Moreover, the design, which also enables each lane to be wired separately, has the added advantage of eliminating the holiday-lights problem.

For her part, Eisler has been working on finding the right combination of materials and design to optimize the optical element to divide light into its component parts. Her work has garnered fellowship awards and recognition as an Everhart Lecturer, an award honoring graduate students with exemplary research and presentation skills. The second prototype she helped develop, pieced together with reflecting filters and silicone concentrators, resembles the end of a fork, but made of glass. “With each iteration, we get closer and closer to our efficiency goals,” which is crucial, she says, because project funding from the Department of Energy is contingent on meeting those short-term goals. “It’s stressful, but it’s fun.”

To help balance out the stress of the job, Eisler makes a concerted effort to encourage young scientists to be unafraid in their pursuit of difficult scientific challenges. She has served three times as a mentor in Caltech’s Summer Undergraduate Research Fellowships program, and also mentored an undergraduate student from Occidental College and served as a committee member for his senior thesis this May.

“I see of a lot of potential in these undergraduates, and it’s important to me to foster that potential,” Eisler says. “I want to make them feel that they’re supported in such a way that they can learn something entirely new and not be afraid to stumble along the way. I think failure’s a big part of the learning process.” At press time, Eisler had successfully defended her thesis and accepted an offer to join Paul Alivisatos’s group at UC Berkeley in 2016.

“The most important lesson I’ve learned from Caltech is that learning itself makes life fulfilling,” she says. “Caltech is full of passionate people, and being immersed in that culture makes every new challenge—scientifically related or otherwise—an exciting opportunity to learn something new. I hope that by continuing this enthusiasm, I can pass this joy onto the students I teach and mentor in the future.”

—Jon Nalick

Header photo by Mario de Lopez Photography