Conjugated Polyelectrolyte-Sensitized TiO₂ Solar Cells: Effects of Chain Length and Aggregation on Efficiency

Abstract

Two sets of conjugated polyelectrolytes with different molecular weights (Mn) 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-CH2-) (P1-O-n, where n = 7, 9, and 14) and methylene (-CH2-) (P2-C-n, n = 7, 12, and 18). They all bear carboxylic acid moieties as side chains, which bind strongly to titanium dioxide (TiO2) 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-n series increases with a decreasing molecular weight while the efficiency of the P2-C-n series remains constant regardless of the molecular weight. Steady state photophysical measurements and dynamic light scattering investigation prove that P1-O-n polymers aggregate in solution while P2-C-n series are in the monomeric state. In P1-O-n series, a higher-molecular weight polymer results in a larger aggregate, which reduces the amount of polymers that are adsorbed onto TiO2 films and overall cell efficiency.