Research

Organic semiconductors

Structural, electronic, and optical properties of polymers and molecular crystals have been a central topic of the group during the last few years. The studied materials are the polymers poly-acetylene, poly-paraphenylene, poly-thiophene, poly-paraphenylene-vinylene, and poly-fluorene, as well as molecular crystals like the oligo-phenylenes, the oligo-acenes, and the oligo-thiophenes. This kind of semiconductors are being used in opto-electronic devices like light emitting diodes, lasers, field effect transistors, flexible panel displays, and photo-voltaic cells. Our research includes several areas, which are the role of excitonic effects in the optical spectra, pressure studies, and the investigation of organic surfaces and interfaces, which have started recently.

The latter project (FWF-S9714) is part of the National Science Network Interface Controlled and Functionalised Organic Films. Therin we focus on surface energies of organic molecular crystals as well as on the adsorption of organic molecules onto metal surfaces. We currently also evaluate the role of van der Waals intertaction for these properties.

Optical absorption in organic semiconductors is strongly influenced by electron-hole correlations. We calculate optical absorption spectra including excitonic effects by solving the Bethe-Salpeter equation for the electron-hole correlation function. Using this approach, we obtain the exciton binding energy for oligophenylens, -thiophenes and -acenes as a function of molecular length and pressure. It turns out that the exciton binding energy is crucially influenced by intermolecular interactions leading to a drastic reduction of excitonic effects in the 3D environment as compared to isolated molecules or polymers [1]. Similarily, the electron-hole interaction is weakened by applying pressure as shown for anthracene [2], where the nature of the lowest absorption feature also depends on the polarization of the incoming light. Moreover, it is found that an increase in the molecular length [3] leads to a decrease of the exciton binding energy [4]. This work has been summarized in a recent review article [5].

The pressure study of anthracene mentioned above is based on the pressure-dependent crystal structure [6], which was investigated in a collaborative project (FWF-P14237) with experimentalists. The pressure effects on the structural and electronic properties have also been calculated for the oligo-phenyls [7].

Selected publications:

[1] P. Puschnig and C. Ambrosch-Draxl, Phys. Rev. Lett. 89, 056405 (2002). pdf
[2] K. Hummer, P. Puschnig, and C. Ambrosch-Draxl, Phys. Rev. Lett. 92, 147402 (2004). pdf
[3] K. Hummer and C. Ambrosch-Draxl, Phys. Rev. B 72, 205205 (2005). pdf
[4] K. Hummer and C. Ambrosch-Draxl, Phys. Rev. B 71, 081202(R) (2005). pdf
[5] K. Hummer, P. Puschnig, S. Sagmeister, and C. Ambrosch-Draxl, Mod. Phys. Lett. B 20 261-280 (2006).
[6] K. Hummer, P. Puschnig and C. Ambrosch-Draxl, Phys. Rev. B 67, 184105 (2003). pdf
[7] P. Puschnig, K. Hummer, C. Ambrosch-Draxl, G. Heimel, M. Oehzelt, and R. Resel, Phys. Rev. B 67, 235321 (2003). pdf