Influence of π-conjugation units in organic dyes for dye-sensitized solar cells

Two organic dyes with the general structure donor-conjugated chain-acceptor (D-pi-A) have been investigated as sensitizers for nanocrystalline TiO2 solar cells. The electron donor and acceptor groups were pyrrolidine and cyano acrylic acid, respectively. The conjugated chain of 2-cyano-3-{5-[2-(4-pyrrolidin-1-ylphenyl)vinyl]thiophen-2-yl}acrylic acid contains one phenyl ring and a thiophene unit and is therefore denoted PT, while for 2-cyano-3-{5 -[2-(5-pyrrolidin-1-ylthiophen-2-yl)vinyl]thiophen-2-yl}acrylic acid the phenyl ring is replaced by a second thiophene unit (TT). Solar-to-electrical energy conversion efficiencies under simulated AM 1.5 irradiation (1000 W m(-2)) of 2.3% were obtained for solar cells based on PT but of less than 0.05% for those based on TT. The reasons for the dramatic difference of the efficiencies were analyzed. Photoinduced absorption measurements revealed that the TT dye was not properly regenerated by redox electrolyte after electron injection. This sluggish regeneration is probably due to the 0.3 V less positive HOMO level for TT dye compared to the PT dye, resulting in a lower driving force for regeneration of the oxidized dye by iodide in the electrolyte. In addition, regeneration of the oxidized TT dye and electron injection from the excited TT dye may be poor due to formation of dye aggregates/complexes, as FT-IR measurements show an excess of not properly and/or unidentate bound TT dye molecules instead of bidentate bound PT dye molecules. The results highlight that small structural change of dyes results in significant changes in redox energies and binding features, affecting dramatically the performance of these dyes in dye-sensitized solar cells.