Studying Charge Transport in Semiconducting Polymers with Linda Peteanu

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interview with a CFN User and Collaborator

January 30, 2024

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Linda Peteanu at CFN (Jessica Rotkiewicz/Brookhaven National Laboratory)

Linda Peteanu is a professor and the former head of the department of chemistry at Carnegie Mellon University (CMU). Her research focuses on the study of charge transport in polymer molecule chains using “transient photophysics,” an approach that measures the transient, or short-lived, light-induced responses of electronic properties in semiconducting polymers. She conducts this work, in part, at the Center for Functional Nanomaterials (CFN), a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Brookhaven National Laboratory. CFN is one of the five DOE Nanoscale Science Research Centers (NSRCs).

What initially drew you to the DOE NSRCs?

I became aware of the DOE user facilities because I had a friend who was working at the Center for Integrated Nanotechnologies (CINT), another DOE NSRC, which is jointly run by Los Alamos National Laboratory and Sandia National Laboratory. I did experiments there, and I also became a user at the Center for Nanoscale Materials (CNM) at Argonne National Laboratory. I served on user committees and proposal committees at both places. I became very interested in taking advantage of these sophisticated facilities that are staffed by very knowledgeable scientists.

At a seminar, I met chemical physicist Matt Sfeir, now a professor at the Graduate Center at the City University of New York, but who was working at CFN at the time. Matt was a member of CFN’s Electronic Nanomaterials Group and told me about his work into the photophysical properties of different nanomaterials. Later, I met my current CFN collaborator Mircea Cotlet, who leads CFN’s Advanced Optical Spectroscopy and Microscopy Facility, when he was working at CINT and I was a user there. That is often how it works: You meet people, and you find out about their facilities. It was about 10 years ago that I first came to CFN.

Once I started using these facilities, it was clear to me that this could be an ongoing collaboration. Because Carnegie Mellon is in Pittsburgh, Brookhaven is better situated for me geographically than CINT.

Tell us about your research.

I’m a physical chemist and optical spectroscopist. My group has a fundamental interest in what happens to the electronic and charge transport properties of conjugated semiconducting polymers in response to light. We study them when they are aggregated into larger structures, when they are isolated, and when they are in thin-film form. We want to understand how the molecules behave when they are manipulated in different ways, such as by altering the structure of the molecule or its morphology in the solid state. Of course, these parameters are not completely independent of one another, which makes for a complex and interesting problem.

In my lab at CMU, we use optical microscopy to study materials that are made by synthetic chemists with whom we collaborate. We also partner with scientists who do other forms of spectroscopy, such as X-ray scattering, to get more detailed structural information about the molecules and their thin film morphologies that we can relate to important functional properties, such as emissivity and energy and charge transport.

Ultimately, we are interested in finding semiconducting polymers and ideal processing conditions that could enhance applications such as flexible monitors or displays, solar cells, organic-based lighting, etc.

What tools at CFN do you use in your research?

We most often use the ultrafast transient absorption spectroscopy system that Mircea has in his lab. This system uses ultrafast pulses of light to study processes that take place at extremely short time scales, from femtoseconds to nanoseconds. It is a pump-probe system: A sample is optically “pumped” using ultra-short laser pulses, which excites the electrons in the sample, while “probed” with a weak laser that is affected by the excitation and is used to monitor the pump-induced changes.

Mircea is also developing some additional capabilities that we are interested in, such as coupling magnetic measurements and electric measurements to that system and seeing how that affects charge separation and transport in the sample. Overall, his group wants to expand how a sample can be perturbed so that users can study their samples in multiple ways during a single experimental session.

You were at CFN very recently for an experiment. What were you studying?

In this visit, we have been investigating a semiconducting polymer called poly(3-hexylthiophene), or P3HT. It was one of the first polymers developed for studying the feasibility of polymer-based transistors and solar cells, and other similar applications. We’re interested in how different solvation environments help or hinder charge transfer in P3HT. Though this polymer has been studied extensively in thin-film form, we think that studying it in the solution phase, as isolated chains or in the form of nanoaggregate suspensions, is a simpler way to monitor the initial charges separation event. This could help us understand how the polarity of the local environment—the dipole moments of the solution molecules—and aggregation affect charge separation and energy transport.

We've also been working with Mircea on a technique that he is currently developing, which is looking at the effect of circularly polarized light in these materials. I have a collaborator in my department at CMU, Professor Kevin Noonan, who is making helical polymers; they are long semiconducting polymers that are based on DNA-type structures. Their absorption and emission spectra are sensitive to the polarization of the incoming light. We’re interested in studying this as a route toward better displays, making sensors, and other applications.

We’ve been working with Mircea to determine whether the strength of this chiral response—or, in other words, the extent of helicity of the molecule—changes as the molecule resides in its excited state. This method can also monitor the degree of chiral emission, which is a property of great current technological interest, from the first few femtoseconds out to nanoseconds, ideally. One might expect the degree of chirality of the emission to change if the initial excitation energy “moves” to a different region of the molecule that is more helical or less helical. If this energy transfer leads to increased chiral emission, this may suggest that the final, lowest energy geometrical state of the system is optimal and could drive the next iterations of molecular design.

Measurements of chirality are intrinsically challenging as it is a subtle effect even in the ordinary absorption spectrum, let alone in transient spectroscopy. Though our first attempts weren’t super conclusive, we will continue to optimize the methodology and the sample properties. For example, we can explore alterations in the polymer structure a bit more. Currently, the structure of the molecule makes it quite flexible—solvents will fold and unfold it, temperature will fold and unfold it. I think we may want something with a bit more rigidity for an enhanced chiral response. We need to re-optimize the structure, which could happen before I next come to CFN.

The polymer we have been studying in this application is a member of the furan family of organic compounds that can be derived from biomass feedstocks, such as residues and waste from agriculture and forestry, algae, and industrial/urban waste. The polymers the Noonan group has been making are based on the furan main molecular chain, with different side chains that affect the geometry. The first one they made had an alkyl side chain, and that molecule essentially self-destructed on exposure to light. However, when they put an ester group on there instead, this improved the photo-stability tremendously. We wouldn’t have been able to study its properties using transient absorption otherwise.

What are the advantages of conducting research at CFN and the other NSRCs?

These days, almost nothing is measured using just a single technique. There’s not any one approach that gives you the complete answer that you are looking for. So being able to study a sample in different ways, all in one sitting, is important. In my research, the results can be very sensitive to how the sample is prepared, how the thin film is made, etc. If you are trying to take a measurement later on with a different sample, you immediately run into the question of whether you really are studying the same thing as before. It’s a real time saver to be able to use multiple techniques at one time, and the NSRCs offer this.

At our university, we lack ultrafast methodologies for studying samples and optical spectroscopy capabilities in general. Therefore, I really value having access to these tools at CFN, as well as being able to interact with folks who understand what I need. These researchers and technicians have become a community that I don’t have at my home institution, and those discussions carry over to what I’m doing in my lab at Carnegie Mellon. So, the advantages of CFN go beyond the availability of the instrumentation—the value is also in the scientific prowess of the staff, who are knowledgeable about the technology and the research. It speeds up the overall research process.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

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