Producing electricity directly from sun light using organic photovoltaics (OPVs) is an important step away from fossil fuel energy sources. OPV efficiency must increase in order to become economically viable. There are four fundamental processes within every OPV that need improvement:
Photon absorption – absorb light from the sun to form an excited molecular state (exciton)
Charge separation – transfer an electron from the excited molecule to a nearby, unexcited molecule
Charge transport – provide a pathway for the electron and hole to quickly diffuse apart
Charge collection – have that pathway end at an electrode where the electron (hole) may be collected and used to do work
Our work focuses on improving all four of these processes. Our primary research thrust involves designing and synthesizing covalently bound electron donor-acceptor molecules which 1) strongly absorb solar radiation, 2) efficiently separate charges intramolecularly, 3) readily assemble to form charge conduits perpendicular to the molecule which 4) extend to both electrodes.
Through these activities, we established various strategies to organize large numbers of molecules into supramolecular structures that can bridge length scales from nanometers to macroscopic dimensions. Additionally, we are actively developing new non-fullerene electron-accepting molecules for OPV applications by multichromophore approaches. By time-resolved spectroscopies in conjunction with advanced structural characterization techniques, we developed fundamental understanding of the interplay between molecular structures, mesoscale architecture, unproductive pathway, and charge generation efficiency.
We also explore the less common photophysical process to harness the process of singlet fission, where a singlet exciton spontaneously decays to a pair of triplet excitons, as a novel way of efficient exciton generation. By interrogating the ultrafast singlet fission processes of several designer molecules in both the solution and multicrystalline solid states, we have established the equisetic role of interchromophore geometry and the degree of charge-transfer character in this multiexciton-coupled process.
Recent Group Publications:
1. Singlet Fission in Covalent Terrylenediimide Dimers: Probing the Nature of the Multiexciton State using Femtosecond Mid-Infrared Spectroscopy. Chen, M., Bae, Y.J., Mauck, C.M., Mandal, A., Young, R.M. and Wasielewski, M.R., (2018). Journal of the American Chemical Society. DOI: 10.1021/jacs.8b04830
2. Charge Separation Mechanisms in Ordered Films of Self-Assembled Donor–Acceptor Dyad Ribbons. Logsdon, J. L., Hartnett, P. E., Nelson, J. N., Harris, M. A., Marks, T. J., & Wasielewski, M. R. (2017). ACS applied materials & interfaces, 9(39), 33493-33503. DOI: 10.1126/sciadv.1701293
3. Direct Observation of a Charge Transfer State Preceding High Yield Singlet Fission in Terrylenediimide Thin Films, E. A. Margulies, C. E. Miller, L. Ma, E. Simonoff, G. C. Schatz, and M. R. Wasielewski, J. Am. Chem. Soc. 139, 663-671 (2017). DOI: 10.1021/jacs.6b07721
4. G-quadruplex organic frameworks. Wu, Y.L., Horwitz, N.E., Chen, K.S., Gomez-Gualdron, D.A., Luu, N.S., Ma, L., Wang, T.C., Hersam, M.C., Hupp, J.T., Farha, O.K. and Snurr, R.Q., (2017). Nature chemistry, 9(5), 466. DOI: https://doi.org/10.1038/nchem.2689
5. Influence of Anion Delocalization on Electron Transfer in a Covalent Porphyrin Donor–Perylenediimide Dimer Acceptor System. Hartnett, P. E., Mauck, C. M., Harris, M. A., Young, R. M., Wu, Y. L., Marks, T. J., & Wasielewski, M. R. (2017). Journal of the American Chemical Society, 139(2), 749-756. DOI: 10.1021/jacs.6b10140
6. Small molecule acceptor and polymer donor crystallinity and aggregation effects on microstructure templating: Understanding photovoltaic response in fullerene-free solar cells. Eastham, N.D., Dudnik, A.S., Aldrich, T.J., Manley, E.F., Fauvell, T.J., Hartnett, P.E., Wasielewski, M.R., Chen, L.X., Melkonyan, F.S., Facchetti, A. and Chang, R.P., (2017). Chemistry of Materials, 29(10), 4432-4444. DOI: 10.1021/acs.chemmater.7b00964