HOPV12 - Blogs

4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 99
B52- Energy Level Alignment in Hole Transporting Molecular Layers studied with
Hard X-ray Photoelectron Spectroscopy
Rebecka Schölin a , Martin H. Karlsson b , Susanna K. Eriksson b , Johan Oscarsson a , Hans Siegbahn a ,
Erik M. J. Johansson b , Håkan Rensmo a
a, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
b, Department of Physical and Analytical Chemistry, Uppsala University, Box 259, SE-751 05 Uppsala, Sweden
Dye-sensitized solar cells, and their solid state equivalent, are nowadays a large research
field. For the solid state devices that uses a hole conducting molecule, or polymer, instead of a
liquid electrolyte, there are though still some work before they can compete in efficiency with
the liquid analogues. Important for the photocurrent and photovoltage in the solar cell is the
relation between the energy levels in the different materials. Oxide material, dye molecule
and/or hole conductor can be changed in order to manipulate the energy alignment. Other
ways are by using different additives, dopants or small dipole molecules, to name a few.
This study shows how direct measurements of the energy levels and their alignment can be
performed using X-ray photoelectron spectroscopy (XPS). In focus of the study is hole
conducting molecules, such as spiro-OMeTAD and P3HT and how changes of the position of
the energy levels in these materials can be achieved with additives or dipole molecules. By
measuring photoelectron spectroscopy using X-rays with high energy, so called hard X-rays
(the technique is called HArd X-ray PhotoElectron Spectroscopy, HAXPES), it is possible to
study buried interfaces in a nondestructive way. HAXPES is also convenient for measurements
where one would like to minimize direct spectroscopic effects from surface structures which
otherwise could be a reason for energy level shifts seen in normal XPS. Measurements have
been performed at the synchrotrons BESSY II in Berlin (Germany) and MAXlab in Lund
(Sweden).
The convenience of using hard X-rays was shown in a study of P3HT/dipole molecule/ TiO2
interfaces where direct measurements of the energy levels in the interfaces were achieved (1).
Measurements of spiro-OMeTAD with and without the additive Li-TFSI showed that the Li-salt
shifts the Fermi level towards the HOMO in spiro-OMeTAD. The effect of adding Li-TFSI is
hence the same as p-doping of the molecule. This could be confirmed with absorbance
measurements where a contribution from oxidized spiro-OMeTAD could be observed. By
varying the photon energy of the X-rays, it could also be concluded that there is a
concentration gradient of Li-TFSI in the spiro-OMeTAD film, with a larger amount of salt closer
to the surface of the molecular film. In the solid state dye-sensitized solar cells, this means that
the spiro-OMeTAD is oxidized in the presence of Li-TFSI, and that the concentration of Li-salt is
as highest at the interface between the hole transport material and the counter electrode.
References
[1] Johansson, E. M. J.; Schölin, R.; Siegbahn, H.; Hagfeldt, A.; and Rensmo, H. "Energy level alignment in
TiO2/dipole-molecule/P3HT interfaces". Chemical Physics Letters 515, 146-150 (2011).
© SEFIN 2012