Significance of π-bridge contribution in pyrido[3,4-b]pyrazine featured D-A-π-A organic dyes for dye-sensitized solar cells

The 3,4-ethylenedioxythiophene (EDOT) pi-bridge is one of the most commonly used building blocks for sensitizers in dye-sensitized solar cells (DSSCs). We investigated its influence on the molecular structure, the photophysical and electrochemical properties compared to a cyclopentadithiophene (CPDT) pi-bridge in two pyrido[3,4-b]pyrazine featured D-A-pi-A dyes SH3 (CPDT pi-bridge) and SH4 (EDOT pi-bridge). Surprisingly SH4 with EDOT as a pi-bridge exhibited not only poor absorptivity but also inferior photovoltaic performance. On the contrary, SH3 achieved more than 5% power conversion efficiency under standard AM1.5G illumination at 100 mW cm(-2) when employed in both solid state and liquid state DSSCs. Theoretical calculations suggested a significant twist in the molecular configuration between EDOT and carboxylic acid. We attributed it to the interaction between the O atom of EDOT and the H atom of alkene as well as the repulsion between the O atoms of EDOT and the carbonyl group of the carboxylic acid, which could retard the intramolecular charge transfer process. Consequently, this rotation in the molecule decreases the molar extinction coefficient and increases charge recombination. Electrochemical impedance spectroscopy results showed enhanced charge recombination in DSSC devices based on SH4, undermining the charge collection efficiency and the power conversion efficiency compared to SH3. Herein the detrimental effect of tilting the dye structure is isolated from the other characteristics of the dye, showing its importance as a general design strategy for new dyes.