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
license in:
M. Oehzelt, N. Koch, G. Heimel:
Nature Communications 5, 4174 (2014).
full-text
