energy level-alignment

at electrode-organic interfaces

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organic semiconductor density of states controls the energy level alignment at electrode interfaces

Modern display technology - technology that you have heard of and that you have likely seen, either on your mobile phone or as your TV-screen - largely relies on the unique properties of a special kind of organic molecules. These glow if electrical current is passed through them. To do that, however, the fundamental carriers of electrical charge first have to pass from the external, metallic wiring onto the molecules and they don't do so easily. Minimizing charge-carrier injection barriers between metallic electrodes and organic semiconductorsis is thus critical for optimizing the performance of organic (opto-)electronic devices. In this research paper, we implemented a detailed numerical model that is capable of quantitatively reproducing the experimentally observed barriers. Covering, furthermore, the full phenomenological range of different qualitative regimes within a single, consistent framework we continuously connect the limiting cases described by previously proposed models and, thereby, resolve conflicting views in literature. Our results highlight that the distribution of energies that charge carriers can have in the organic semiconductor as key factor. The shape of this distribution and, in particular, its tails into the "forbidden gap" - a range of energies that charge carriers in semiconductors simply cannot have due to quantum-mechanical weirdness - is found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt steps at the very interface to extended regions of smooth variation. Our finding thus finally allow rationalizing device designs that have been established through decades of costly and cumbersome trial and error. More importantly, however, the predivtive power of our model now enables the knowledge-based desing of future technology.

This work was published under the Cc by nc sa license in:
M. Oehzelt, N. Koch, G. Heimel:
Nature Communications 5, 4174 (2014). full-text

wider impact

link     check out our project on organic-organic interfaces

  further reading

K. Akaike, N. Koch, G. Heimel, M. Oehzelt
The Impact of Disorder on the Energy Level Alignment at Molecular Donor-Acceptor Interfaces,
Advanced Materials Interfaces 2, 1500232 (2015). link

K. Akaike, N. Koch, M. Oehzelt
Fermi level pinning induced electrostatic fields and band bending at organic heterojunctions,
Applied Physics Letters 105, 223303 (2014). link

H. Wang, P. Amsalem, G. Heimel, I. Salzmann, N. Koch, M. Oehzelt
Band-Bending in Organic Semiconductors: the Role of Alkali-Halide Interlayers,
Advanced Materials 26, 925 (2014). link

calculations support measurements:

S. Winkler, P. Amsalem, J. Frisch, M. Oehzelt, G. Heimel, N. Koch
Probing the energy levels in hole-doped molecular semiconductors,
Materials Horizons 2, 427 (2015). link

H. Mendez, G. Heimel, S. Winkler, J. Frisch, A. Opitz, K. Sauer, B. Wegner, M. Oehzelt, C. Rothel, S. Duhm, D. Tobbens, N. Koch, I. Salzmann
Charge-transfer crystallites as molecular electrical dopants
Nature Communications 6, 8560 (2015). link

A. Opitz, A. Wilke, P. Amsalem, M. Oehzelt, R. P. Blum, J. P. Rabe, T. Mizokuro, U. Hormann, R. Hansson, E. Moons, N. Koch
Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge redistribution
Scientific Reports 6, 21291 (2016). link

I. Salzmann, G. Heimel, M. Oehzelt, S. Winkler, N. Koch
Molecular Electrical Doping of Organic Semiconductors: Fundamental Mechanisms and Emerging Dopant Design Rules
Accounts of Chemical Research 49, 370 (2016). link

  press coverage (english) link (english) link (english) link (german) link (german) link (german) link (finnish) link (deutsch) link (english) link (german) link (german) link (german) link (english) link (german) link (german) link (german) link (german) link (german) link (german) link (german) link (german) link (english) link (english) link (english) link (english) link (english) link (english) link (english) link (english) link (deutsch) link (deutsch) link (english) link (deutsch) link (english) link (deutsch) link (deutsch) link

  others using our model

G. Horowitz
Validity of the concept of band edge in organic semiconductors
Journal of Applied Physics 118, 115502 (2015). link

T. J. Whitcher, W. S. Wong, A. N. Talik, K. L. Woon, N. Chanlek, H. Nakajima, T. Saisopa, P. Songsiriritthigul
Investigation into the Gaussian density of states widths of organic semiconductors
Journal of Physics D: Applied Physics 49, 325106 (2016). link

S. Beck, D. Gerbert, T. Glaser, A. Pucci
Charge Transfer at Organic/Inorganic Interfaces and the Formation of Space Charge Regions Studied with Infrared Light
The Journal of Physical Chemistry C 119, 12550 (2015). link

J.-P. Yang, W.-Q. Wang, F. Bussolotti, L.-W. Cheng, Y.-Q. Li, S. Kera, J.-X. Tang, X.-H. Zeng, N. Ueno
Quantitative Fermi level tuning in amorphous organic semiconductor by molecular doping: Toward full understanding of the doping mechanism
Applied Physics Letters 109, 093302 (2016). link

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