In September of my first semester here at Nicholas, I attended a Duke Energy Initiative panel discussion. This panel represented the future of energy technology, not just at NSOE, but throughout the Duke campus. The most remarkable technology I heard about during that session came from the mouth of Adrienne Stiff-Roberts, professor of Electrical and Computer Engineering (ECE) at the Pratt School of Engineering. She spoke that evening about the potential of organic solar cells and how we can be using this technology to invent materials such as solar cell clothing. I was electrified.
For anyone who is interested in energy, renewable energy in particular, this could be the advent of a breakthrough in how we think about energy access solutions. Matrix-assisted pulsed laser evaporation (MAPLE) is a technique that Dr. Stiff-Roberts and her team are focused on. This technique has potential applications outside of the organic solar cell, including energy storage and electrochemical reactions relating to hydrogen production. Stiff-Roberts was open to an interview after the panel (this interview has been edited and condensed for clarity):
Soli Shin: Can you give a brief bio? How did you arrive at Duke?
Stiff-Roberts: I’m from North Carolina. I went to undergraduate at Spelman College and Georgia Tech in Atlanta, Georgia. I did a dual-degree program so I studied physics at Spelman and electrical engineering at Georgia Tech. Then, I went to the University of Michigan in Ann Arbor where I got my Ph.D. in applied physics. I also got a Master’s in Electrical and Computer Engineering. After that, I came to Duke starting as a faculty member in the ECE department here.
Shin: What got you interested in MAPLE?
Stiff-Roberts: My Ph.D. research was different, I was doing inorganic compound semiconductors, working on things like indium arsenide for quantum dot materials systems and looking at them for infrared photodetection. The quantum dots have several things about them that were interesting, but they also had some disadvantages so I got interested in these colloidal quantum dots. They’re made by inorganic chemistry and they had some promising advantages in terms of being able to control these materials on the nanoscale. I started looking into how they can be embedded in a polymer matrix for a thin film because that was one approach for making devices, especially optical electronic devices so things like solar cells. I started working in MAPLE due to my interest in trying to find a deposition technique that would be good for making thin films of colloidal quantum dots embedded in a polymer matrix.
Shin: How does your team’s use of MAPLE differ from what other researchers are doing?
Stiff-Roberts: MAPLE itself is a variation of a process called pulsed laser deposition (PLD), and in that case, the target is always a solid. You have your polymer, it’s dissolved in a solid, and that is a solution but it’s still frozen so it’s a solid during the deposition. The idea is that the solvent would absorb the laser energy and not damage the polymer. What my group did that was different was instead of just having the polymer in a solvent, both absorbing ultraviolet energy, we used an infrared laser, and we did the emulsions for the target. Now that solution is actually an emulsion. We have most often an oil and water emulsion which is then frozen by liquid nitrogen during growth so that it’s still a solid target. The reason why that’s been important is that the infrared laser that we use, its energy, is resonant with the vibrational mode of OH bonds and there are plenty of OH bonds in water. So now, you have the ideal case where the water is the only thing absorbing the laser energy. The water evaporates and then whatever is in the target gets transferred from the target to the substrate. We have decoupled the laser energy from the guest material that we want to deposit which means a few things: 1) we don’t damage the material chemically, 2) we don’t damage the material structurally, in terms of the molecular weight, we also can deposit any type of material system. We just need a way to get OH bonds to absorb that infrared laser energy.
This is a photo of Stiff-Robert’s lab where her team works on MAPLE. To the left, you can see a dark grey box where the laser is housed. The round window in the center of the photo how a researcher can observe what is happening.
Shin: What has your group found out about the technique and its applications for the organic solar cell?
Stiff-Roberts: We’ve been doing emulsions primarily for organic materials, polymers, small molecules, but we’re starting to study now how to broaden that to other types of materials systems that maybe can’t tolerate water but we can introduce OH bonds in other ways. So it really made it much more versatile in terms of the types of materials we can deposit and the fact that the process works the exact same way every time even though we’re changing the materials systems.
Shin: Once the laser hits the target, does the water evaporate immediately? What’s left? It’s hard for me to visualize what this all looks like and how it could be applied to different materials, like cotton, plastic or whatever products we could think of.
Stiff-Roberts: Now, a lot of it depends on the specifics of the target so I’ll say as long as we can get a good emulsion, we can do this. If we have a bad emulsion, then the water is not really absorbing that laser energy at a high density. In which case, there is some melting so you get splashing on the target. When we have a good emulsion, it’s evaporative. The water goes straight from ice phase to the vapor phase. When it does that, there’s these oil droplets that contain the solvent and the polymer inside. Those oil droplets get carried in the water as the water evaporates. Per laser pulse, we’re scattering these droplets across the substrate. Over time, as we have repeated laser pulses, the film builds up. We actually just recently published these two papers. One explaining this process more carefully, what our emulsion chemistry is doing to these droplets, and how it impacts film properties and solar cells. Another paper was for silver nanoparticles, that addresses your question on what it looks like on different materials. We did a range of depositions on wax paper, regular paper, aluminum foil, glass and some other things. We showed that we can deposit this on a wide variety of substrates.
Shin: Let’s say 40 years from now, something like this is being thought of being integrated into clothing. In terms of cost, can your experiment be scaled up for real-world use?
Stiff-Roberts: This is basic research and I have a research grade system so my opinion about it and approach to it has always been to focus on the basic science, find the critical aspect that makes it worth the investment; to do the engineering, to scale up the system. Do I think it’s possible? Yes. You already have organic light emitting diodes (OLED) screens on your smartphones. Those are organic but they’re small molecules and they’re made by a thermal evaporation process, which has some similarities in terms of the complexity of the system. But I think any technology, when you’re talking about scaling it up, large-scale mass production, it’s going to be expensive, and it’s going to be complicated. To me, what’s most important is to find out the materials system or the material properties that this particular approach can enable, that no other approach can do.
Shin: Especially if these devices are currently out there on the market, already being sold as some sort of product.
Stiff-Roberts: Right, so I don’t think that’s the real barrier. I think what needs to happen more often than what currently happens is the sort of thing that students do at the Nicholas School where you’re doing that entire life cycle assessment. As an engineer and a researcher, I don’t necessarily think about those things. I’m thinking about what are the fundamental materials and device physics that might give me a better performance but I’m not thinking about the entire life cycle assessment, how much it costs to get the raw materials and how much does it cost to get to dispose of this at the end of life and when I do this MAPLE process, how much energy is my system actually using and how much is that going to actually cost. I teach a class on solar cells so that’s when I started thinking about these things. Even for established technologies, it was sort of difficult to find what that life cycle assessment is. I think that’s one place that would really benefit from more collaboration.
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