Fusing chemistry and physics to improve solar energy
By Jennifer Bornkamp
September 3, 2020
A seed of an idea took root from what had been a frustrating experiment for materials scientist Nakita Noel while in her final days as a postdoctoral researcher at Oxford University in 2017. Shortly afterwards, Noel would accept a position at Princeton where she is currently a Princeton Materials Science Postdoctoral Fellow with the Princeton Center for Complex Materials (PCCM).
As an undergraduate, Noel was hard pressed to choose between chemistry or physics, so she majored in both – earning a Bachelor of Science (with First Class Honors from the University of West Indies), followed by a PhD in condensed matter physics from the University of Oxford. “I knew I wanted to do research related to generating clean, sustainable energy because mitigating climate change and securing the world’s energy economy are among the most pressing issues of our time,” said Noel.
The unexpected experimental results that had piqued her curiosity were based on a material called perovskite. Perovskites are a class of crystalline materials that have an ABX3 crystal structure and are the most abundant naturally occurring minerals in the earth. Metal halide perovskites are a man-made subcategory of these materials and are most well-known for their use in efficient photovoltaic (PV) devices (which are used to capture solar energy). In the Oxford experiment, Dr. Noel and a graduate student had been using a molecular dopant to dope an organic material (doping is introducing small amounts of a new material into a host material to alter its original electrical and/or optical properties). “For some reason, every time we used this dopant with our standard protocol, it made our solar cells worse, but when we left out a previously key ingredient, they were great. Then I realized the dopant might be doping the perovskite as well,” said Noel.
During her first month on the Princeton campus, Noel determined that she wanted to probe deeper into the enigmatic experimental results. She needed the help of an expert on doping and found Antoine Kahn, the Stephen C. Macaleer ’63 Professor in Engineering and Applied Science and Vice Dean of the School of Engineering and Applied Sciences. She set up a meeting with Kahn and proposed her research idea. That initial conversation developed into an ongoing collaboration, with Noel regularly stopping by Kahn’s office to discuss material surfaces and device interfaces.
Perovskites are solution-processed materials that are very economical to fabricate. “Not only are they inexpensive, but they also have the performance level that is comparable to silicon (Si) photovoltaic devices, which is really the gold-standard of the field,” noted Noel. Her overarching goal is to improve the efficiency and long-term stability of perovskite devices, using two research approaches. The first is to develop a more fundamental understanding of the composition of these materials and its impact on the crystallization and optoelectronic properties of perovskite materials. Her research results (along with Princeton colleagues Nikita S. Dutta and Professor Craig Arnold) were recently published in the Journal of Physical Chemistry Letters. Noel’s second research thrust is to understand how to manipulate the chemistry of these materials at the interfaces in PV devices, and investigate charge-transfer and recombination kinetics at those interfaces. This research resulted in two publications (Wiley Online Library and ACS Energy Letters).
Noel further wanted to explore if the interfaces of metal halide perovskites could be modified, and the surfaces could be doped, would efficiency and stability of solar cells be further improved? She and research team members used solution-based approaches to dope the surface of perovskite films, achieving a twofold increase in the total conductivity throughout the film and significantly improving hole extraction in perovskite solar cells. They confirmed that charge-transfer occurs between the perovskite and dopant complex, showing that surface doping enhanced device performance and stability of perovskite solar cells. “There was a lot of back and forth with the project – but it worked — and our results were published in the first paper which unequivocally proved that it wasn’t impossible to dope these materials,” recalled Noel. “Every time I presented the results at a conference, there was a lot of push back — they were super skeptical – but then they were convinced. It was the longest slog I have ever had on a paper, but at the end it felt great.” She was first and co-corresponding author reporting the results in Energy & Environmental Science, 2019 (with Princeton colleagues Fengyu Zhang, Profs. Antoine Kahn, Craig Arnold and Barry Rand). “Noel’s drive and knowledge of the field are quite impressive, and working with her on writing this paper, and ultimately on responding to criticisms by the referees, was a very interesting and rewarding experience,” said Kahn.
As a PCCM fellow, Dr. Noel commented on her experience at Princeton: “By nature, the type of work that I do is very interdisciplinary, so I often find myself working with chemists, physicists and engineers. PCCM facilitates accessibility to more people (across disciplines) and equipment than would be normal otherwise. The IAC (Imaging and Analysis Center) has definitely been a lifesaver.” Noel’s collaborations also extend to physicists at Oxford, chemists at Georgia Tech and scientists at the National Renewable Energy Lab (NREL) and SLAC National Accelerator Lab. “In my field, interdisciplinary collaboration is very important because perovskite materials have rich chemistry that then impacts optoelectronic properties and hence, device physics. Looking at a problem from the perspective of all these different fields is the most useful way to come up with a holistic solution.” On her fellowship Dr. Noel noted “One of the really great things about this fellowship is that it gives you the independence to work on the projects that you really find interesting and ask the scientific questions that really matter to you. The research groups that I primarily work with (Arnold and Rand groups) also help in that they are quite happy for me to independently explore my crazy ideas.”
Handling criticism, communicating the science, team dynamics, and a lack of diversity in the field are challenges Noel has had to face. “It is not lost on me when I’m the only woman or the only person of colour in the room — and at some point, you get used to it — but you should not have to,” said Noel. “I am fortunate that the extent of diversity in Craig Arnold’s group is amazing. It is easier to feel completely comfortable when there already are people like you in a space.” (Arnold is the Susan Dod Brown Professor of Mechanical and Aerospace Engineering.) Noel’s advice to students considering joining a research group is to not only talk to the PI (principal investigator), but also talk with lab members to get a feeling for the group dynamic. “You need to know your lab mates want to get up in the morning and come to work, not just because of the science, but because it’s a pleasant working environment. The environment can make or break you because there are days when the research will be terrible,” she added. “When things go badly, it will be the people that get you through because it is a shared experience.”