Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

Work progress and achievements during the first year
Introduction
The black dye, coded N749, is the actual best performing complex for dye-sensitized solar cell since
the report of 11.1 % photon-to-electron conversion efficiency announced by Sharp in 2006 [1]. This
champion cell was scored notably by focusing the effort in the light confinement of the photo-anode
and by introducing the concept of electrode haze as an indicator to increase Jsc. From a photophysical
point of view, absorption of the N749 holds the ability to cover the whole visible spectrum
range up to near IR and consequently deliver high current density that could attain more than
21 mA/cm². To date, no other dyes can challenge the N749 in terms of panchromaticity; however one
key of improvement is niche in the increase of the molar extinction coefficient of the complex so that
IPCE above 80 % can be achieved while decreasing the photo-anode thicknesses to reduce electron
motion length. Within this aim, a step forward recently came out from the design of a new branch of
heteroleptic Ru-based complexes lying upon enhanced �-conjugated thiophene ligands. This novel
series of sensitizers, coded CYC-B1 [2], CYC-B3, SJW-E1 [3], C101 [4] or C104 [5], exhibits a higher
molar extinction coefficient (�MLCT � 17000 L.mol -1 .cm -1 ) as well as a low-energy metal-to-ligand charge
transfer (MLCT) bands displaying improved red absorbance as compared to Z907 derivatives. The
preliminary reports in the literature showing their photovoltaic characteristics highlight already competitive
performances in the range or beyond the 10 % threshold conversion efficiency which therefore
pave the way towards new opportunities to record new achievements in DSSC. We get inspired during
this first year project by Han’s work showing the relevance of the photo-anode for high efficiency and
we combined such an approach with the use of these new thiophene-based sensitizer and notably we
focused on the most promising C101 dye in order to progress our groundwork state of the art and
meet this first stringent requirement of deliverable.
Results
1) New materials for photo-anode
EPFL has highlighted that the utilization of the dyesol paste coded DSL18NR-T as part of the transparent
layer of the photo-anode can enhance by around + 0.2 % the overall photon-to-electron conversion
efficiency in alternate to the standard EPFL 20 nm-based paste. Because the particles size,
morphology, active surface area can drastically modify the optical behavior and/or the transport properties
of the photo-anode, EPFL has established during this first year project a closed partnership with
Dyesol, Australia in order to pursue this improvement and notably by using different type of particles
and mixtures of big and small particles. Table 1 gathered the different type of materials considered to
date. The photovoltaic performance of these different products, used either as transparent layer or as
scattering layer, has been evaluated in combination with the C101 dye and volatile electrolyte Z960
(1M DMII (1,3 dimethyl-imidazolium iodide), 0.03M I2, 0.1M GuNCS (guanidinium thiocyanate), 0.5M
tert-butylpyridine, 0.05M LiI in 85:15 by volume of acetonitrile / valeronitrile).
Table 1: Characteristics of the Dyesol pastes
Morphology /
Particle size
DSL 18NR-T DSL 30NRD-T DSL 18NRtrans
Rods of 16 x 52
nm
Rods of 23 x 69 nm mixture of DSL
18NR-T with
rocky particles
of 120 – 320 nm
DSL 18NR-AO
Mixture of DSL
18NR-T with
spherical particles
of 200nm
Pore diameter 32 nm 31 nm 24 nm 25 nm
Film porosity 67 % 67 % 64 % 58 %
S(bet) (N2) 75 m²/g 68 m²/g 68 m²/g 57 m²/g
Figure 1 gathers the results of this survey. It shows the evolution of the photon-to-electron conversion
efficiency as a function of incident light intensity (A.M. 1.5G) and photo-anode structure (7.5-8 µm of
the first + 5-6 µm of the second layer). The results clearly emphasize on the significant role of the
photo-anode structure on the device behaviour since improvement by 0.6 % of conversion efficiency
has been obtained by changing from 7.5 µm thick 20 nm-based TiO2 layer + 5.5 µm 400 nm scattering
particles to a similar configuration made by DSL30NRD-T + DSL18NR-AO. The major improvement,
ca. + 0.4 %, originates from the replacement of the 400 nm-based scattering layer by a mixture of
Robust DSC, M. Graetzel, EPFL
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