Exploration of novel concepts for organic photovoltaics
Final Report Abstract
The work carried out within our Research Fellowship explored novel concepts and possibilities for the use of organic semiconductors in photoactive systems and devices. We all know the key device applications in the field of organic electronics: organic light-emitting diodes, organic solar cells, and organic transistors. In this fellowship, I tried to motivate my work based on two questions: 1) How can novel organic material classes and device concepts lead to a higher performance of existing technologies? 2) For which applications beyond the known can organic semiconductors be used, at best in environments where especially organic compounds are superior to other materials due to their inherent properties? In difference to inorganic semiconductors, organic molecules used in electronic devices and systems form amorphous layers, where long distance coupling and coherence cannot be found. Here, the fundamental excitation upon photon absorption leads to a very localized state, often treated as quasi-particle and named "exciton". Due to the high degree of localization, exchange interactions induce a significant energetic splitting between singlet and triplet excited states. This so-called singlet-triplet splitting is specific for every organic molecule and can be seen as part of the molecular fingerprint that defines its functionality. I used this singlet-triplet splitting as a starting point to derive research projects within this fellowship. Below, I want to highlight two major research topics of my past work. Singlet exciton fission: Typically, excitons can only be exchanged in between the singlet and triplet manifold via intersystem crossing. However, if the energetic splitting between singlet and triplet states is as large as the triplet energy itself, a multi-exciton-generation process called singlet fission can occur very rapidly and efficiently. Here, the singlet exciton instantaneously splits into two child triplet states of half the energy. This process can be used for spectral conversion where, in contrast to other methods, the thermalization losses during the conversion can be minimized. To point out one research highlight, we used this process in pentacene to realize the first solar cell that surpasses the one electron per photon limit of a singlet junction cell in the visible part of the solar spectrum. Biluminescence: As mentioned above, the singlet and triplet states of an organic molecule are distinct in energy. Fluorescence takes place from the singlet state to the molecules ground state, on the other hand, the transition from the triplet state is spin forbidden, and thus the triplet is considered a "dark" state. However, I could show that the triplet state can be brought back to a luminescent state at room temperature, if the competition with non-radiative modes is effectively suppressed for the triplet such that the weak coupling between triplet excited and ground state is sufficient to obtain efficient luminescence – phosphorescence – from the triplet. I could demonstrate molecules where both states, i.e. singlet and triplet, participate efficiently in the luminescence transforming these molecules in dual state emitters, a characteristic I termed "bioluminescence".
Publications
- Phys. Status Solidi A 209, 2341 (2012)
S. Reineke and M.A. Baldo
- Appl. Phys. Lett. 103, 093302 (2013)
S. Reineke, N. Seidler, S. R. Yost, F. Prins, W. A. Tisdale, and M. A. Baldo
- J. Am. Chem. Soc. 135, 13326 (2013)
N. B. Shustova, A. F. Cozzolino, S. Reineke, M. A. Baldo, and M. Dinca
- Science 340, 334 (2013)
D. N. Congreve, J. Lee, N. J. Thompson, E. Hontz, S. R. Yost, P. D. Reusswig, M. E. Bahlke, S. Reineke, T. Van Voorhis, and M. A. Baldo
- Adv. Mater. 26, 1366 (2014)
N. J. Thompson, E. Hontz, D. N. Congreve, M. E. Bahlke, S. Reineke, T. Van Voorhis, and M. A. Baldo
(See online at https://doi.org/10.1002/adma.201304588) - Scientific Reports 4
S. Reineke and M. A. Baldo
(See online at https://doi.org/10.1038/srep03797)
