Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - SFB 951 - HIOS

 Colloquium of the SFB 951
'Hybrid Inorganic/Organic Systems for Opto-Electronics'

 

Thursday, February 23 at 4:00 pm
Department of Physics, Humboldt-Universität zu Berlin, Newtonstraße 15, Room 1´201: Christian-Gerthsen-Hörsaal

Organic molecules can affect the surface energetics of inorganic semiconductors [2] and the interface energy level alignment of semiconductor structures, such as solar cells.[3] While initial effects on actual junction devices were modest,[4] advances in our understanding and in experimental abilities led to much increased effects, which are relevant for many device structures [5]. Our work on Si [6] led to the ability to achieve both chemical and electronic surface and interface passivation, which then allowed construction of near-ideal (S=0.9) Si/Hg diodes, of MIS devices without effective Insulator, and of inversion Si solar cells. Having a semiconductor (SC) instead of a metal as electrode might seem to complicate controlling and understanding the mechanisms that govern charge transport through junctions. However, with a sufficiently electronically passivated interface the asymmetry of metal/molecule/SC junctions, with respect to charge carrier type, provides unique information on the energy levels involved in transport, beyond what is possible when using only metal electrodes. Thus, I-V data give unequivocal evidence that transport in Si/alkyl chain/metal (Hg, Au) junctions is controlled by |E_F – LUMO|, rather than by |E_F – HOMO| as commonly assumed. This result agrees with complementary UPS/IPES measurements on samples without top contact, which also show our conclusion to be valid for Au/alkyl/Au junctions.

[1] Work done, at the Weizmann Inst., by Rotem Har-Lavan and Omer Yaffe, with L. Segev, L. Kronik, A. Vilan, and at Princeton Univ., with Y. Qi, A. Kahn.
[2] G. Ashkenasy et al., (2002) Acc. Chem. Res., 35, 121 –128
[3] D.Gal et al., (1997) Proc. Ind. Acad. Sci. Chem. Sci. , 109, 487-496
[4] A. Vilan et al., (2000) Nature, 404, 166-168
[5] D. Cahen et al., (2005) Adv. Funct. Mater 15, 1571-1578
[6] A. Vilan et al., (2010) Adv. Mater., 22, 140–159

The functionality of devices based on HIOS is crucially influenced by the crystal quality and interface morphology. In-situ X-ray scattering provides this structural information on the atomic / molecular scale, and, importantly, can be performed in real-time to study the time evolution of the (non-equilibrium) growth process. While there have been some recent theoretical and experimental advances in our understanding of molecular orientation on inorganic substrates [1] and the diffusion of molecules across step edges [2], the experimental understanding is still lacking and parameters such as energy barriers for step edge diffusion are known only for few selected molecules. Here we will present recent synchrotron data for DIP on ZnO, DIP on SiO2 , and C60 on mica and show that both the position of molecules within the 1^st, 2^nd, … or n^th layer, as well as the typical size / distance of molecular islands can be determined in real-time. This combined measurement of anti-Bragg oscillations [3] for layer number and small angle scattering is experimentally challenging, but will make it possible to determine energy barriers for each successive molecular layer during growth. The extension of the technique to HIOS structures, as well as the connection to Monte-Carlo and molecular dynamics simulations for rational design of HIOS will be discussed.

[1] F. Della Sala, S. Blumstengel, and F. Henneberger, “Electrostatic-Field-Driven Alignment of Organic Oligomers on ZnO Surfaces,” Physical Review Letters, vol. 107, no. 14, pp. 1-5, Sep. 2011.
[2] G. Hlawacek, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl, and C. Teichert, “Characterization of step-edge barriers in organic thin-film growth.,” Science, vol. 321, no. 5885, pp. 108-11, Jul. 2008.
[3] S. Kowarik, A. Gerlach, S. Sellner, F. Schreiber, L. Cavalcanti, and O. Konovalov, “Real-time observation of structural and orientational transitions during growth of organic thin films,” Physical Review Letters, vol. 96, no. 12, p. 125504, 2006.