Abstract
In this feature article, we discuss the key aspects of solid-state
dye-sensitized solar cells (SDSC) and propose different concepts
based on extensive studies carried out in our group to improve their
performance. The influence of compact TiO2 layer, novel donor-antenna
sensitizing dyes, nature of nanocrystalline-TiO2 layers and solid-state
organic hole conductors on the performance of SDSC is discussed in
this article. Both preparation and thickness of the compact TiO2
layer were optimized using spray pyrolysis. The studies revealed
that an optimum film thickness of 120-150 nm of compact TiO2 yielded
the best rectifying behavior and SDSC performance. The influence
of three different mesoporous titania films, obtained from three
different titania nanocrystals, prepared by sol-gel, thermal, and
colloidal-microwave process, was also studied and discussed here.
The TiO2 layer with the optimum pore volume and pore diameter (similar
to 44 nm) displayed the highest efficiency and IPCE in SDSC. The
importance of pore size rather than high surface area for filling
the mesoporous layer with solid-state hole conductor became evident
from this study. A series of heteroleptic Ru(II) complexes carrying
donor antenna moieties, namely, triphenylamine (TPA) or N, N'-bis(phenyl)-N,
N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD), were synthesized
and applied in SDSC. These novel donor-antenna dyes revealed spectacular
performances of power conversion efficiencies in the range 1.5-3.4\%,
as measured under AM 1.5 spectral conditions. This was attributed
to highly efficient light harvesting of these novel dyes and the
improved charge-transfer dynamics at TiO2-dye and dye-hole conductor
interfaces. Different low molecular weight and polymeric triphenyldiamines
were synthesized and utilized as hole-transporting layers (HTL) in
SDSC. Different studies showed that low molecular TPDs displayed
better efficiency than polymeric counterparts due to their improved
filling into the pores of nc-TiO2 layer. Another interesting study
revealed that an optimum driving force in terms of HOMO-level difference
between the dye and HTL decides charge carrier generation efficiency.
Recently, novel hole conductors with spiro-bifluorene-triphenylamine
core for transporting holes and tetraethylene glycol side chains
for binding lithium ions were synthesized and applied in SDSC. This
work clearly emphasizes that Li+-salt is required at the TiO2/dye
interface as well as in the bulk of HTL. It was also found that the
addition of about 5-20\% of these Li+-binding hole conductors and
higher Li-salt (N-lithiotrifluoromethane sulfonamide) concentrations
improved the SDSC performance. An improvement of about 120\% in the
solar cell efficiency as compared to the reference cells was achieved
with an optimum composition of Li+-binding hole conductor and Li-salt.
(c) 2007 Elsevier B. V. All rights reserved.
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