Conjugated Polyelectrolyte-Sensitized TiO<sub>2</sub> Solar Cells: Effects of Chain Length and Aggregation on Efficiency
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Abstract
Two sets of conjugated polyelectrolytes
with different molecular
weights (<i>M</i><sub>n</sub>) in each set were synthesized.
All polymers feature the same conjugated backbone with alternating
(1,4-phenylene) and (2,5-thienylene ethynylene) repeating units, but
different linkages between the backbone and side chains, namely, oxy-methylene
(-O-CH<sub>2</sub>-) (P1-O-<i>n</i>, where <i>n</i> = 7, 9, and 14) and methylene (-CH<sub>2</sub>-) (P2-C-<i>n</i>, <i>n</i> = 7, 12, and 18). They all bear carboxylic acid
moieties as side chains, which bind strongly to titanium dioxide (TiO<sub>2</sub>) nanoparticles. The two sets of polymers were used as light-harvesting
materials in dye-sensitized solar cells. Despite the difference in
molecular weight, polymers within each set have very similar light
absorption properties. Interestingly, under the same working conditions,
the overall cell efficiency of the P1-O-<i>n</i> series
increases with a decreasing molecular weight while the efficiency
of the P2-C-<i>n</i> series remains constant regardless
of the molecular weight. Steady state photophysical measurements and
dynamic light scattering investigation prove that P1-O-<i>n</i> polymers aggregate in solution while P2-C-<i>n</i> series
are in the monomeric state. In P1-O-<i>n</i> series, a higher-molecular
weight polymer results in a larger aggregate, which reduces the amount
of polymers that are adsorbed onto TiO<sub>2</sub> films and overall
cell efficiency