FOCUS TOPIC: Catching more light in solar cells

Prof. Dr. Christiane Becker in the clean room at HZB campus side Berlin-Adlershof. © HZB/Phil Dera

Christiane Becker uses microscopic structures to increase the amount of light captured in solar cells and is currently scaling up the technology for industrial application. “On top of everything else, there’s this spirit at HZB that we are working on the renewable energies of the future, and that is incredibly inspiring,” she relates in portrait.

At one point, it was fifty-fifty whether she would continue as a scientist after graduating or become a musician. Christiane Becker had won in youth competitions, playing the cello well enough to enter a conservatory. “But ultimately physics won out,” Becker says in hindsight, smiling wryly at one of the big advantages of working in science: “You can always do good physics, even with a sprained finger or when you’re in a bad mood.”

Fortunately, Christiane Becker rarely has cause to be in a bad mood these days. At HZB, she heads the young investigator group Nano-SIPPE, which has made enormous progress in the five years since it formed. The acronym stands for “Nanostructured Silicon for Photonic and Photovoltaic implEmentations” (a Sippe in German is a clique). Becker looks out the window of her office to the Adlershof HZB Head Office in the building opposite.

“We are constantly moving around the whole science district here,” she says – in the cleanroom on the other side of the road, for example, or working on optical experiments that span three buildings. The practical aspect of the work she does there is especially important to the 41-year-old scientist – and that aspect is one of the fundamental tasks of her group, which is researching close to real-world applications. From her desk draw, Christiane Becker takes out a wafer-thin glass plate, about the size of a sheet of notepaper, and holds it in two fingers up to her office window. In the light, the coated glass shimmers in multiple colours. “This coating on the glass,” she says, “is what we are working on.”

Optical coatings play a key role in making solar cells more efficient

If future solar cells convert sunlight into electricity better than those we have today, it will largely be thanks to optical tricks like coatings – and that is precisely the field that Christiane Becker is working in, known as light management. To put it in her words: “We texturize the absorber and that increases the efficiency.” That means that the coating they apply to the glass is given a specific microscopic 3D structure. Superfine bumps are created on the coating only nanometres apart, which increase the light incidence while also locally concentrating the light, much like a magnifying glass does when it burns with sunlight. And yet another effect is conceivable: the coating can help to shift the wavelength of the light so that the solar cell can also produce electricity from infrared light – a part of the normal daylight spectrum that has so far never been used in ordinary solar cells.

“The subject of renewable energies excited me the instant I encountered it,” Christiane Becker says. That was when she was already well into her academic career. While finishing her doctorate in Karlsruhe, she drove out to Berlin for the annual conference of the German Physical Society and, out of pure curiosity, watched a discussion on renewable energies. “It was about photovoltaics, in particular, and I thought: that was an exciting field in which to apply my knowledge!” For Becker, the discovery came at just the right time: she had been going through an internal struggle regarding her studies, looking for a meaningful application of what she had learnt – an ethical crisis that almost made her hang up her doctorate.

Thinking about it now brings a parable to her mind: “Three stonemasons are working on the streets of a city and, when asked what they are doing, the first one says, ‘I am carving stones.’ The second one says, ‘I am working to feed my family.’ And the third one says, with a twinkle in his eye, ‘I am helping to build the Cologne Cathedral!” Christiane Becker gets the very same twinkle in her eye when she thinks of her work.

Yes, there are plenty of routine chores to do every day, “but on top of everything else, there’s this spirit at HZB that we are working on the renewable energies of the future, and that is incredibly inspiring.” Since discovering the field, she has made rapid headway in her research: in 2006, she completed her doctorate; in 2007, she started as a postdoc at HZB; in 2012, she established her own young investigator group; since 2014, she has also been a professor at HTW Berlin University of Applied Sciences.

Wanted: a process for coating large glass surfaces

The task she and her team are currently undertaking is translating their knowledge into industrial production. “We coat the glass with what we call a sol-gel,” Becker explains. The nanostructures are then pressed into the gel using a kind of stamp, and the gel is then cured to make it hard. “This is a method that can be applied to relatively large glass surfaces,” Becker says. But the devil is in the detail: solar cells are so-called optoelectronic components – and an improvement in the optics can ironically damage the electronics. Becker’s group has already identified the right materials and processes for silicon solar cells. Now they are looking to increase the efficiency of tandem solar cells as well, which are made of a combination of silicon and perovskite materials.

Christiane Becker never stops experimenting, even when she gets home: her two children, nine and twelve years old, are curious about everything there is to discover. “Recently, we built a rocket that is launched by an air cushion,” she laughs. It was through home projects this that she had found her own way into science: “When I was three years old, my mother wanted to bake a cake with me. But instead of concentrating on the dough, I wanted to fix the broken clock on the oven.”

Awakening enthusiasm for research

She wants to awaken this enthusiasm in her students as well. She is already getting some of them involved in research early in their careers: “Tomorrow, a bachelor student will come by here at HZB,” Becker says. If all goes well, he will soon be working with her as a student assistant in the laboratory. “To begin with, sure, he will be doing more of the technical work here, like making nanoimprints in the cleanroom, for example.” And yet, perhaps Christiane Becker will inspire young scientists to specialise in her important field from early on in their careers, which she only discovered for herself at a later stage.

von / by Kilian Kirchgessner