Energy level engineering of thieno[3,4-b]pyrazine based organic sensitizers for quasi-solid-state dye-sensitized solar cells

Thieno[3,4-b]pyrazine based organic sensitizers with cyanoacrylic acid (FNE64 and FNE66) or carboxylic acid (FNE65 and FNE67) as anchoring groups have been designed and synthesized to optimize the lowest unoccupied molecular orbital (LUMO) level of panchromatic sensitizer FNE32 with an identical auxiliary electron acceptor. Their absorption, electrochemical and photovoltaic properties are systematically investigated. It is found that sensitizers FNE65 and FNE67 with carboxylic acid as the anchoring group possess more negative LUMO levels as compared with sensitizers FNE64 and FNE66 with cyanoacrylic acid as the anchoring group. In addition to tuning the LUMO level of thieno[3,4-b]pyrazine based organic sensitizer FNE32 by attaching different anchoring groups, the number of the bridged benzene ring has been optimized and two types of organic sensitizers with D–A–π–A (FNE64 and FNE65) and D–π–A–π–A (FNE66 and FNE67) frameworks have been comparatively investigated. Upon the incorporation of an additional phenylene unit into sensitizers FNE64 and FNE65 with the D–A–π–A configuration, sensitizers FNE66 and FNE67 with the D–π–A–π–A structure exhibit unusual hypsochromically shifted absorption maxima. Moreover, the LUMO levels of the thieno[3,4-b]pyrazine based sensitizers FNE66 and FNE67 are lifted up, which results in a dramatically improved driving force for the electron injection from the excited dye molecules to the conduction band of the titania semiconductor. Consequently, the power conversion efficiency of the quasi-solid-state dye-sensitized solar cell (DSSC) based on FNE66 increases by 95% in comparison to that for the quasi-solid-state DSSC based on panchromatic sensitizer FNE32 whose photo-response range is over 900 nm. Our findings will facilitate the understanding of the crucial importance of energy level engineering and present a way for tuning the LUMO levels of organic sensitizers at a fixed electron acceptor.