Organic sensitizers with different thiophene units as conjugated bridges: molecular engineering and photovoltaics

Three structural modifications with incorporation of alkyl, alkoxy and vinyl bond into the skeleton of thiophene bridge in D-π-A featured organic sensitizers are specifically developed for insight into their influences on photophysical, electrochemical as well as photovoltaic properties in nanocrystalline TiO2-based dye sensitized solar cells (DSSCs). The insertion of vinyl bond into the conjugation bridge leads to the molecular planar configuration, and the conjugation bridge of 3,4-ethylenedioxythiophene (EDOT) is prone to positively shift its highest occupied molecular orbital (HOMO). The electrochemical impedance spectroscopy (EIS) results indicate that the grafted long alkyl chain onto thiophene is favorable to suppress dye aggregation when adsorbed onto TiO2 film and modification on interface of TiO2/dye/electrolyte, resulting in a relatively high open-circuit voltage (Voc). Under optimized conditions, dye LS-4 bearing hexylthiophene as the conjugation bridge shows a relatively high overall conversion efficiency of 5.45%, with a photocurrent of 11.61 mA cm–2, Voc of 744 mV.