Thin film analysis reveals secrets to renewable energy

Ina Martin wants to change the world.

How? By making renewable energy sources more durable, cheaper and better.

She’s trying to tackle an energy problem that exists on a terawatt level – that’s one trillion watts. It’s a helpful unit when you’re talking about the rate at which humans are using energy worldwide. The result of Martin’s research could help create abundant, low-cost, clean energy on a global scale.

The annual potential of solar energy far exceeds the world's energy consumption. But the goal of using the sun to provide a meaningful slice of global electricity demand is far from being realized.

Martin is a Ph.D., an adjunct Professor at Case Western Reserve University in Cleveland and the Director of Operations of the Materials for Opto/Electronics Research and Education (MORE) Center. Her research focuses on stabilizing interfaces in thin film optoelectronic devices, particularly solar cells.

 “Part of my research is understanding how the chemistry of interfaces can make something that's good better,” she said.

Martin does it by working on small samples made using scalable thin film fabrication methods. Her goal is to understand the optical properties and durability of the materials used by employing thin film characterization technology.

There are several parts to a solar cell. They include the one that absorbs the light and give you charge separation. That’s called the absorber layer.

“Then you have the contacts,” she said. “The top contact is cleverly called the top contact. That would be one that faces the sun. And the bottom contact is the one that's away from the sun. So the top contact needs to be transparent. And then in between those things, in between the absorber layer and the contacts, you have interfacial layers. The interfacial layers have multiple functions, including encouraging the transmission of select charges.”

Thin films are layers of materials ranging from fractions of a nanometer to several micrometers in thickness.

Martin used a line from the movie “Shrek” to help explain the concept.

“Solar cells are like onions,” she said. “They have layers. I specifically study the layer (top contact) that allows the light to come in and get the current out.”

Her challenge, like all those who work in optoelectronics, is to achieve favorable performance (efficiency), cost and lifetime, or durability of those contacts over time. After all, it doesn’t do anyone any good to develop a solar cell technology that’s cheap and efficient but only lasts two days.

Martin’s work is focused on creating a layer of the solar cell contacts with environmentally neutral properties. She studies transparent conductive oxides (TCO). She used one, aluminum doped zinc oxide in her current study. This TCO makes up the contacts of the solar cells, the electrodes that conduct the light through the absorber and lets electricity out. She coats it with organofunctional silane molecules, which makes it more inert, and less likely to react to environmental conditions.

Aluminum doped zinc oxide is a good material to use for the contacts because it’s cheap, abundant, a good conductor, and can be deposited at low temperatures, she said. That makes it widely useful for use with absorber layers that can’t be made at higher manufacturing temperatures.

Her work has been funded by the U.S. Department of Energy and supported by Case Western Reserve’s College of Arts and Sciences. The current phase of her research is near its completion. The next step is to try and apply her concepts on a larger scale to full devices and modules.

“We've done the proof of principle work, now we want to do it on a solar cell that then gets encapsulated,” she said.

She also wants to see how encapsulating the contacts affect it because it adds another variable, which is the contact’s interaction with the material that's used to encapsulate it.

Martin’s lab is surprisingly uncluttered, though not at all pristine. But it’s well organized. Each workstation has a set of tweezers, a reference sample, and instructions on how to use the instrument. The lab hosts about 100 or so unique users a year, from within the university and other northeastern Ohio academic institutions, and from the industrial sector.

One private company, Folio Photonics, uses spectroscopic ellipsometry in the MORE Center to study materials that may go into ultra-high capacity optical storage discs. The company is developing a disc with a capacity at the terabyte scale. The recording medium exists in many layers of polymers. It must be affordable, reliable and efficient. The developers are using spectroscopic ellipsometry to test their technology. The new optical disc would revolutionize enterprise archival data storage.

Among the instruments Martin uses in the MORE Center is a HORIBA UVISEL Spectroscopic Ellipsometer. HORIBA’s ellipsometers incorporate non-rotating phase modulation and liquid crystal modulation technologies to provide sensitive and accurate ellipsometric measurement on a large spectral range, from VUV to NIR.

“Part of the strength of the ellipsometer is that it can be used on materials systems that range from simple, single layers to complex multilayers,” Martin said. “As part of a core facility, many users can access the tool to answer materials questions, and solve problems.”

The main focus of Martin’s work with the UVISEL is to determine the thickness and/or optical constants of the materials used in solar cells in order to make them more stable and efficient.

Spectroscopic ellipsometry is a surface sensitive, non-destructive, and non-intrusive optical metrology technique. It is ideal for a wide range of thin film applications, from fields such as semiconductors, solar, optoelectronics, optical and functional coatings, to surface chemistry and biotechnology.

The UVISEL provides other characterizations, including information on material properties such as anisotropy, gradient, morphology, crystallinity, chemical composition and electrical conductivity.  

Undergraduate students can be trained on the UVISEL in a half-hour. The spectroscopic ellipsometer can be used to study a single cell or a full layer of a photovoltaic cell. By housing the instrument in a core facility such as the MORE Center, hundreds of researchers across northeastern Ohio have access to this powerful analysis method.

Optoelectronics is a broad area of technology that encompasses the interaction of electricity and light. It includes photovoltaics. It also includes LED technology, where electricity goes in and light comes out.

The future for solar cell technology is bright, according to Martin.

“There's a lot of room for working on this tuning of interfacial chemistry for improving the robustness of the solar cells,” she said.

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