February 2013

Double the Charge from One Photon in Organic Photovoltaics

First observation of key intermediate state in the conversion of one photon to two electrons.

Click to enlarge photo. Enlarge Photo

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.

The Science

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.

The Impact

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.

Summary

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.

Contact

Xiaoyang Zhu
Columbia University
xyzhu@columbia.edu

James Yardley
Director; Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control (RPEMSC) EFRC
jy307@columbia.edu

Funding

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.

Publications

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/ja306271yExternal link]

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.1213986External link]

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.1436External link]

Related Links

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

Highlight Categories

Program: BES, EFRCs

Performer/Facility: University

Additional: Collaborations, Non-DOE Interagency Collaboration

Last modified: 12/5/2013 5:18:04 PM