Adhesion and Failure in Stretchable
Organic Electronic Systems
Adviser: Winston O. Soboyejo
David G. Kwabi
Mechanical and Aerospace Engineering
“The opportunity to delve into a subject that is open-ended by definition carries with it the chance to verify a hypothesis, but for me it also held the promise that at the end of every week, I’d have something remarkable to think about.”
My thesis was the culmination of a long relationship with my adviser that began with a summer internship after my sophomore year. I had approached Professor Winston Soboyejo in the hope of working on a project of his I remember hearing described simply as having to do with “flexible solar cells.” To me then, that way of putting it was general enough to warrant fantasies of making actual flexible solar cells, and then, I suppose, staring at them in utter amazement. I learned very quickly that my fancy imaginations were off the mark, and—as a first lesson in making distinctions vital to not being jaded by hours of lab work—that what research is often motivated by can be far from what it’s immediately about.
In this case, the inspiration behind Professor Soboyejo’s project was the fact that while the advent of organic electronics has made it possible to realize cheaper versions of originally silicon-based applications, the unique properties of organic materials also make it possible to modify these applications for devices that can withstand bulk stresses, such as pulling and bending. In many cases, such as for light-emitting diodes, the transition from a silicon-based device to an organic version of it already has been achieved, but the problem of developing organic electronic devices that can bend and stretch depends on solving rather involved mechanics and adhesion issues. In other words, if you can imagine a cell phone made of nano-thin layers of organic and semiconductor material sandwiched inside a sheet of rubber, you won’t be too far off from what “electronic textiles” is shooting for, and why it is no easy target.
Professor Soboyejo’s approach was simple: Study how these systems fail, and use that knowledge to propose better ones. That summer, I collaborated with a graduate student studying failure mechanisms in thin gold films bound to an elastic substrate (as this assembly is a typical component in would-be real devices), and I had a real glimpse into the depth the topic held, as well as how it elegantly combined mechanics and materials science issues. I kept tabs on the group, and by senior year, it seemed like a natural extension of my interests and past experiences to apply Professor Soboyejo’s overall research objective to looking at deformation in a model stretchable organic electronic system.
The thesis proceeded in two phases. The first involved making force measurements between typical bimaterial pairs that would comprise a flexible organic electronic device, and fitting those values to established elastic and fracture mechanics models. The objective was to extract information about how well particular layers stuck to others and propose reasons for why they did so. The second part had to do with putting together a model flexible organic solar cell (not what I had imagined in sophomore year, of course), observing damage mechanisms within it under various stress conditions, and comparing those results against the predictions from the first part of the thesis. In the time it took to get everything done, almost all of the clichés I had heard about the difficulties of engineering research—that it takes a lot of time, that you will often need help, that some machines are just evil—seem to have been amply justified.
At the same time, those other clichés about the joys and unanticipated developments of engineering research also were validated. There were conventional moments of progress, like when I was finally able, after months of tedious microscopy measurements, to back up oxygen plasma’s role in improving device adhesion with my data. But there were other moments. Sometime in the spring semester, I discovered rather strange-looking buckling patterns in my devices, which I showed to Professor Soboyejo with every expectation that he would just frown at them, and then frown at me. It turned out that the features I saw have been fairly well studied for thin film systems, and that in our case the parameters of the buckles were doubly useful, since they estimated the material properties of our metal and organic layers as well as residual stresses that were a function of how the cells were fabricated. This provided an independent check of the residual stresses I had calculated by a more mainstream method, and growing confidence in the narrative my research had begun to develop.
In the end, I believe my thesis offered plausible recommendations for research efforts in the area of stretchable organic devices, but also highlighted areas for further exploration, such as the need to optimize the materials selection process so that device performance is not sacrificed because of high adhesion, and vice versa. It was amazing to see, from a bird’s-eye perspective, that the many little pieces of work I had done did fit together into a coherent argument about a set of potential real-world issues, and doubly amazing to realize that friends of mine in other disciplines felt the same way about their theses when all was said and done.
As far as advice goes, picking an adviser and topic based on what interests you is a great idea. Having worked with Professor Soboyejo that summer after sophomore year meant I could check the box next to his name without anxiety, and throughout the process, he proved to be a truly generous, supportive, and razor-sharp mentor. I should add, however, that I also benefited a lot from interacting with the graduate students in the lab. Research being what it is, there were many times when I got stuck and asked others for help, and in all cases, I’m glad I did.
What I found most compelling about my thesis experience, then, was how surprising the entire affair was. The opportunity to delve into a subject that is open-ended by definition carries with it the chance to verify a hypothesis, but for me it also held the promise that at the end of every week, I’d have something remarkable to think about. Seeing how facile expectations—about how easy an experiment would be, or how some data would turn out—could be so comprehensively debunked, and seemingly unpromising avenues turn generative, was truly fun. From all this, though, I learned two important lessons: that I should not determine the terms of my surprises, and that while a thesis title could have the word “failure” in it, there is no such thing as a completely botched experiment.
Adhesion and Failure in Stretchable
Organic Electronic Systems
David G. Kwabi
Winston O. Soboyejo
Professor of Mechanical and Aerospace Engineering and the
Princeton Institute for the Science and Technology of Materials
“David’s experience at Princeton provides us with many examples of what it takes to complete a successful senior thesis at Princeton.”
The experience of working with Princeton seniors who have taken the time to learn about what I do is one of the rewarding aspects of my life at Princeton. I find this exciting and rewarding because the students are ready to tackle important problems, after spending some time learning about what we do in my research group. The yearlong experience also gives me just enough time to show a well-prepared Princeton student how to solve an unstructured problem through research. It is, in my own opinion, one of the most important aspects of the Princeton undergraduate experience. It also is very different from the solving of structured problems that we normally do in most of our classes in science and engineering.
David Kwabi is one of the best examples of a student who joined my research group during his sophomore year. David initially came up to me to express an interest in some of our work on solar cells and light emitting devices (LEDs) for people in the developing world. As a student from Ghana, David was aware of the challenges that people have in finding sustainable sources of electricity in the developing world. He was, therefore, interested in learning more about our work, which appeared to have the potential for future applications in the developing world. However, I wanted him to have a broader introduction to the science and technology of solar cells and LEDs, in ways that would prepare him for a future that could span the full range of applications between the developed and developing world.
With this in mind, I paired up David with two of my Ph.D. students, Wali Akande and Onobu Akogwu, who were doing their research on organic light emitting devices (OLEDs) and organic solar cells. David learned the basics on organic electronic structures from Wali and Onobu. He also spent the summers after his sophomore and junior years doing research on the effects of stretching on the electrical properties of flexible electronic structures that are relevant to future electronic textiles and photovoltaic devices. I was particularly impressed with David because he chose to attend our group meetings after the summer holidays. Hence, David was present at most of our group meetings, even during the academic year, when he had to juggle a hectic course schedule with his side interest in our research.
By the time David reached his senior year, he was already well prepared to do research. He knew how to make organic solar cells and organic light emitting devices, and was very familiar with the underlying science and literature in the field. He also was very familiar with most of the techniques that we use in our lab, and those that are used in the micro- and nano-fabrication lab in the Princeton Institute for the Science and Technology of Materials (PRISM). It was, therefore, relatively easy for us to identify potential topics that could be addressed by his senior thesis research. The challenge, however, was to define a well-posed problem that could be addressed in about eight months!
After some thought, we decided to address the problem of adhesion in rigid and flexible organic electronic structures. Essentially, we were concerned about the challenges in getting different materials to stick to each other within the layered structures that are used to make OLEDs and organic solar cells. Like most Princeton engineers, we chose to use a scientific approach to explore the adhesion between the layers. We did this with the hope that the understanding of the science would provide us with important clues on how to design the interfaces between the different layers. We therefore proposed a multiscale experimental approach that involved making adhesion measurements between bimaterial pairs that are relevant to the different interfaces in OLEDs and organic solar cells.
Having learned most of the experimental techniques in his sophomore and junior years, David was able to make molecular measurements of adhesion using atomic force microscopy. He also explored the effects of adhesion promoters, such as chromium and titanium, on the interfaces between organic layers and glass and plastic substrates that are relevant to rigid and flexible devices. David’s work revealed the weak interfaces in our structures. He also was able to identify what we need to do to make our devices more robust in the future. David’s senior thesis, along with the pioneering work of Wali and Onobu, laid the foundation for most of the work that is now being done in my research group on flexible and rigid organic solar cells and LEDs. The work also has been published in two journal papers (The Journal of Applied Physics and Materials Science and Engineering B) in which David is a coauthor.
It was especially rewarding for me to see David’s hard work acknowledged at the end of his senior year. He received PRISM’s prize for energy research by an undergraduate, as well as a very prestigious School of Engineering and Applied Science prize for senior thesis research. It was truly a proud moment for me and David’s family when his name was announced as one of the recipients of the engineering school’s prize. I looked toward his family and smiled, knowing that my job was done, and that David was now ready to face the excitement and challenges of graduate school at MIT. Later on that afternoon, David’s dad came up to thank me, and to ask why I had not encouraged David to stay at Princeton for his graduate work. I responded, quite instinctively, that we almost always send our students away to learn more from professors at other institutions.
David’s experience at Princeton provides us with many examples of what it takes to complete a successful senior thesis at Princeton. First, talk to different professors about your interests and learn more about what they do in their research groups. This is a period of exploration that requires some time. It is a period in which you must find a topic that you are excited about, and a professor that you would like to work with. Once this is done, get to know the professor’s graduate students and become a part of their group. This will give you a support structure and a network that will provide you with the knowledge that you need to do research.
At the start of your senior thesis, make sure that you have a problem that is well posed. You also should be careful to define a problem that can be completed within an eight-month period that passes very quickly. Meet regularly with your adviser, and discuss your future goals and challenges with him or her. You also should develop a careful plan for your work, and a strategy to start writing up your thesis by the end of January of your senior year. Since most of the professors, like myself, are very busy, it is helpful to submit your thesis in sections that can be corrected within a day. This makes the process of writing up your thesis a gradual one in which you can learn to write and think critically, while discussing every aspect of your work with your adviser. It is, potentially, one of the most rewarding and exciting aspects of being a student or a professor at Princeton.