HOPV12 - Blogs

4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 121
C20 - Modelling of Energy and Hole Transfer in Co-sensitized Dye-Sensitized TiO2:
Electronic structure, optical properties and FRET
Mariachiara Pastore, Filippo De Angelis
ISTM-CNR, Via Elce di sotto, 8, Perugia, 6123, IT
Dye-sensitized solar cells (DSSCs) 1 are attracting a wide-spread interest as low-cost
alternatives to conventional photovoltaics. A possible way to increase the power conversion
efficiency in DSSCs is to enhance the light harvesting in the near–infrared portion of the solar
spectrum by co-sensitization of TiO2 with organic dyes having high NIR absorption; this
strategy was recently investigated by Hardin et al. 2 employing a zinc naphthalocyanine-based
dye (AS02) and the Ru(II) C106 dye The main drawback of co-sensitization is the usually low Voc
due to higher recombination rates of NIR dyes. 3 Higher Voc can be obtained by insulating the
NIR dye from the oxide surface thus using it as an energy rely dye (ERD), able to absorb energy
and undergo Forster resonant energy transfer (FRET) to the sensitizing-dye (SD). To enhance
the FRET efficiency and reduce the voltage losses, a careful tuning of the optical and structural
properties of the NIR-ERD is required: it should have intense absorption and high
photoluminescence quantum efficiency, its emission spectrum should largely overlap the
absorption spectrum of SD, its HOMO should be lower than the redox mediator potential to
have fast regeneration and finally, it should intimately mix and interact with the SD. Motivated
by the work of Hardin et al. 2 and by the great interest in the design of new NIR-ERDs having
the proper electronic and structural properties, we carried out a computational investigation
of (AS02+C106) co-sensitized TiO2 models based upon DFT, Time-Dependent (TD) DFT and ab
initio MP2 calculations.
Figure 1 Model of AS02 and C106 co-adsorption on a grid of Ti atoms representing the TiO2 anatase (101) surface.
On the basis of our experience in the computational prediction of the optical properties of
organic 4 and inorganic 5 sensitizers and in the modeling of dye/TiO2 heterointerfaces 6-7 we
calculated the electronic absorption and emission spectra of the dyes in solution and grafted
to the titania surface, determined the preferred adsorption geometry of the AS02 and C106
dyes onto the semiconductor surface and the relative energy level alignments with respect to
the semiconductor CB. Employing a larger TiO2 model, we modeled the coadsorption of AS02
and C106 dyes, evaluating at MP2 level of theory the relative stability of various co-adsorption
patterns and then the associated optical response.
Owing to our high-level computational methodology, we are able to provide new insights
into the energy transfer mechanisms in co-sensitized solar cells, suggesting guidelines to
design new ERDs with extended NIR absorption and the proper energy level alignment to
suppress hole transfer processes.
© SEFIN 2012