HOPV12 - Blogs

4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 282
C141 - A Joint Experimental and Theoretical Approach for Probing Interfaces in
Quantum Dot Sensitized Solar Cells
Johannes T. Margraf a , Vito Sgobba a , Tim Clark b , Dirk M. Guldi a
a, Friedrich-Alexander-Universität Erlangen-Nürnberg, Department of Chemistry and Pharmacy & Interdisciplinary Center for
Molecular Materials (ICMM), Egerlandstraße 3, 91058 Erlangen, DE
b, Computer-Chemie-Centrum, Nägelsbachstraße 25, 91058 Erlangen, DE
Quantum dot sensitized solar cells (QDSSCs) are currently emerging as a promising
candidate for a cheap and efficient third generation photovoltaic technology owing to their
potentials with respect to tackling typical shortcomings of dye-sensitized solar cells (DSSCs)
ranging from lifetimes to light-harvesting.[1] Nevertheless, their efficiencies are still below 5%.
A likely rationale is that optimum cell architectures including linker, electrolyte, counter
electrode, and cell assembly process, have not been found yet.
In the current work, we have pursued of a joint experimental and theoretical study to shed
light on the nature of the linker to govern the QDSSCs features. To this end, we compared
QDSSCs, in which either cysteine (Cys) or mercaptopropionic acid (MPA) were employed as
linkers. In accordance with recent results, we have found higher photocurrents for Cys based
QDSSCs at comparable QD loadings.[2] This fact was further corroborated by using periodic
DFT calculations of linkers adsorbed onto a TiO2 anatase (101) surface. Important is the
zwitterionic adsorption geometry of Cys, which was found to be significantly more stable than
a regular geometry, in which a TiO2 surface O-atom is protonated. In fact, the zwitterionic
geometry results in a favorable dipole moment at the QD/TiO2 interface. Complementary IRspectroscopic
analyses confirm doubtlessly that the zwitterionic geometry is operative.
Impelled by aforementioned results, we adsorbed Cys from aqueous solutions that were
adjusted to a pH 6. The corresponding cells revealed efficiencies of 1.4%. Please compare this
to the best previously published Cys-based QDSSC, which reached efficiencies of around
0.61%.[2] Our findings indicate that the selected conditions correlate with the isoelectric point
of Cys and the point of zero charge of TiO2-anatase electrode. As a matter of fact, electrostatic
repulsions between the two are minimized. Furthermore, using aqueous solutions rather than
organic solvents allows employing higher Cys concentrations. As a consequence, an
amelioration of the overall device performance due to higher QD loading was achieved.
References
[1] Yang, Z.; Chen, C.; Roy, P.; Chang, H. "Quantum dot-sensitized solar cells incorporating nanomaterials". Chem.
Commun., 47, 9561-9571 (2011).
[2] Guijarro, N.; Shen, Q.; Giménez, S.; Mora-Seró, I.; Bisquert, J.; Lana-Villarreal, T.; Toyoda, T.; Gómez, R. "Direct
Correlation between Ultrafast Injection and Photoanode Performance in Quantum Dot Sensitized Solar Cells". J.
Phys. Chem. C, 114, 22352–22360 (2010).
© SEFIN 2012