Image courtesy of XY Zhu, Columbia University
Electron energy distribution as a function of time for pentacene (lower) and tetracene (upper) thin films (thickness ≥ 15 nm) with the energetic positions of the observed excitonic states - the singlet (S1), multiexciton (ME), and two triplets (2×T1) indicated. In both molecular systems, the multiexciton state (ME) is clearly visible.
Researchers have detected ultrafast formation and decay of a previously unobserved multi-exciton (ME) state in model organic photovoltaic systems (tetracene and pentacene). This ME state, generated from the absorption of a single photon, can efficiently transfer two electrons to an adjacent interface.
This multiple electron charge transfer process could be used for organic photovoltaic cells with efficiencies exceeding the single photon limit of 31%, a limit based on the assumption that each absorbed photon can generate at most one electron of current.
Multiple exciton generation (MEG) refers to the creation of two or more pairs of charge carriers (electron-hole pairs known as excitons) from the absorption of one photon. Although MEG holds great promise for improving the efficiency of organic solar cells, it has proven challenging to implement. Using a model system based on either pentacene or tetracene molecules deposited upon carbon fullerene bilayers, EFRC scientists have used femtosecond electron spectroscopy to directly observe a new multiexciton (ME) state ensuing from the absorption of a single photon in the molecular layer. Data for both systems indicate that the ME state can decay into two separate excitons and that one electron can be transferred into the fullerene layer from each exciton. For pentacene, two electrons can be directly transferred from the ME state to an adjacent fullerene layer on a sub-picosecond time scale, which is much faster than electron transfer from either of the two separate excitons from ME decay. In this mechanism, losses in photovoltaic efficiency due to unproductive decay or recombination of individual excitons can be avoided by directly extracting multiple electrons from the ME state at the fullerene surface. Investigation of these processes has generated a new set of design principles for harvesting energy through multiple exciton generation in molecular systems.
Director; Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control (RPEMSC) EFRC
DOE Office of Science, Basic Energy Sciences, Energy Frontier Research Centers (EFRC) Program. The Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control EFRC did the second harmonic experiments, interpretation, and theoretical exploration; the Center for Understanding Charge Separation and Transfer at Interfaces in Energy Materials (CST) EFRC developed the second harmonic generation techniques. Additional experimental contributions for this research in its earlier stages were supported by the National Science Foundation.
Wai-Lun Chan, John R. Tritsch and X. Y. Zhu. "Harvesting Singlet Fission for Solar Energy Conversion: One- versus Two-Electron Transfer from the Quantum Mechanical Superposition". J. Am. Chem. Soc. 134, 18295-18302 (2012). [DOI: 10.1021/ja306271y]
W.-L. Chan, M. Ligges, A. Jailaubekov, Loren G. Kaake, L. Miaja-Avila and X.-Y. Zhu. "Observing the Multi-Exciton State in Singlet Fission and Ensuing Ultrafast Multi-Electron Transfer". Science 334, 1541-1545 (2011). [DOI: 10.1126/science.1213986]
Wai-Lun Chan, Manuel Ligges and X. Y. Zhu. "The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain". Nat. Chem. 4, 840-845 (2012). [DOI: 10.1038/nchem.1436]
Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control (RPEMSC) EFRC
Center for Understanding Charge Separation and Transfer at Interfaces in Energy Materials (CST) EFRC
Collaborations, Non-DOE Interagency Collaboration